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Lecture notes on Plant Physiology. After reading these notes you will learn about: 1. Introduction to Plant Physiology 2. Need for the Study of Plant Physiology 3. Role of Water in Plant Physiology 4. Role of Hydroponics in Plant Physiology 5. Cell Wall of a Plant 6. Osmosis in Plants 7. Role of Transpiration-Recent Synthesis in Plants and Others.
Contents:
- Notes on the Introduction to Plant Physiology
- Notes on the Need for the Study of Plant Physiology
- Notes on the Role of Water in Plant Physiology
- Notes on the Role of Hydroponics in Plant Physiology
- Notes on the Cell Wall of a Plant
- Notes on Osmosis in Plants
- Notes on the Role of Transpiration-Recent Synthesis in Plants
- Notes on Factors Influencing Nutrient Uptake in Physiology of Plants
- Notes on Plants as Indicators of Mineral Nutrients
- Notes on the Physiology of Foliar Nutrition in Plants
- Notes on Physiology of Photosynthesis
- Notes on the Photorespiration and Plants
- Notes on the Physiology of Insectivorous Plants
- Notes on the Phenomena of Growth in Plants
- Notes on Plant Development
- Notes on Growth Hormones in Plants
- Notes on the Physiology of Flowering Plants
- Notes on the Physiology of Seed Development in Plants
- Notes on the Physiology of Fruit Development
- Notes on Aging and Senescence in Plants
- Notes on the Physiology of Plant Biotechnology
- Notes on the Relationship of Plant Physiology to Other Sciences
Note # 1. Introduction to Plant Physiology:
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In plant physiology, natural phenomena operating in the living plants and plant parts are studied. It is a discipline of botany where the structure of the cell, tissues and organs is associated with processes and functions. The different responses of organisms to environmental alternations and the resultant growth and development which are the outcome of such responses are also studied in plant physiology.
A process comprises a series of sequential events operating under natural conditions. Some of the most important processes operating in plants are stomatal mechanism; water and mineral absorption; photosynthesis, respiration, etc. A plant physiologist tends to understand, describe and explain such processes.
The natural activity of the cells, tissues, organs and organisms is referred to as their function. A plant physiologist must understand, describe and explain these functions as well. These functions are explained at the cellular and molecular levels. In plant physiology, an attempt is also made to study the factors which modify growth and development.
In other words, a plant physiologist must understand and explain how external stimuli and factors modify plant responses. In brief, in plant physiology, comprehensive information on the structure, processes and functions operative at the cells, tissues, organs is desired in order to explain the processes of growth and development in an organism.
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These days, a plant physiologist must have a sound knowledge of chemistry, biochemistry, physics, statistics and plant molecular biology. In fact, these sciences have contributed immensely in elucidating the functional and structural aspects of plants.
Indeed the development of these sciences has contributed to the evolution and discovery of several instruments and techniques which helped in the elaboration of plant anatomy. A sound knowledge of plant anatomy is essential for interpreting the functions and processes operating in a plant.
Several of the plant processes could only be revealed by the use of sophisticated instruments and techniques provided by the chemical and physical sciences.
The availability of the techniques of radioisotopes, antimetabolites, chromatography, scanning, transmission electron microscopy, and mass spectrometry has helped in the elaboration of several processes.
In fact, elaboration of cell structure has considerably aided in understanding plant structure in relation to function. The discovery in plants of lysosomes, peroxisomes and other organelles in recent years is the outcome of combined technology of biochemists, biophysicists and anatomists.
Clearly plant physiology is not a static discipline but shall always change and be enrichened by the discovery and usage of new methodology and instruments. This is also a reason why for one process divergent opinions exist. The observations are dependent upon techniques employed while the interpretations are dependent upon understanding of divergent chemical and physical phenomena.
Obviously a plant physiologist is always open to self or other corrections. It is an enlivened science which will change with time and be enrichened with more and increased experiences over the years. Take, for instance, the process of photosynthesis and even the discovery of photorespiration or the processes operating in C4 plants.
Since plants possess different patterns and habitats as compared to animals it is important to study plant physiology. Plants are also static and possess the capacity to manufacture their own food. Therefore, they possess the capacity to grow and add cells throughout their life span.
Clearly in plants organs develop, grow and die. The regulating systems controlling growth differ from those of animals. The control of plant development is the outcome of action of several hormones, environments and nutritional factors.
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Most of these factors interact with each other and cause effective growth of the plant. As compared to animals plants possess special features which point towards their unique physiological characteristics.
These have non-motile habit, autotrophy, and dependence on soil for mineral supply; possess several devices in the terrestrial habitat to protect against excessive evaporation, heating and transport of water from the soil to the plant apex. The presence of rigid cell wall, occurrence of circadian rhythms for several metabolic processes, development of devices to overcome drought, frost-injury, also add to the uniqueness of plants.
These characteristics are best studied and explained on the basis of physiological organization. A successful plant must have the ability to compete in its environments to its best advantage. Thus, plants have undergone physiological evolution and adaptation to the environments.
Note # 2. Need for the Study of Plant Physiology:
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The knowledge of plant physiology will help in forging several advances in agriculture, horticulture, forestry, plant pathology and other disciplines of botany. In fact, researches in plant physiology have been and are likely to contribute immensely to crop improvement. Increase in crop production is based on exploiting maximal levels or plant metabolic processes.
The production of new varieties and strains shall have to take into account the physiological attributes of basic material or genotypes. The control of soil fertility, overcoming presence of excessive salts in the soil through the knowledge of plant physiology has helped in increased crop production.
A basic knowledge of plant metabolic processes can help in the increase of photosynthetic conversion of solar energy for the production of food materials that are utilized by human beings.
Basic knowledge on nitrogen fixation will help in increased utilization of atmospheric nitrogen by different plant species. In the past few years several tissue culture techniques have been developed which have shortened the life cycle of several plant species, helped in raising plants from seeds with shrunken endosperm, and have increased our understanding of cell wall formation and mineral uptake.
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The detailed knowledge of plant hormones, their synthesis and mode of action, has considerably facilitated their application in checking water loss, manipulating growth and development of certain crops and improving the quality of food materials. The usage of certain hormonal weedicides has minimized the occurrence of weeds in the crop fields.
Crop yields have increased by the judicial usage of auxins, cycocel, gibberellins, phenols and aliphatic alcohols, etc. The regulation of flowering, seed formation and fruit setting has been controlled through the application of different hormones at the appropriate time of plant height and age. In recent years most of the breeding projects also seek the help of the plant physiologists.
Physiological processes play a significant role in several interactions between plant and animals. Man changes his environments deliberately or unintentionally and this change affects the physiological behaviour of plants.
The physiological reactions of plants due to their introduction and cultivation are an admitted fact. Increased urbanization, development of industry and excessive utilization of land has led to the modifications of the environments.
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As a result of these modifications there is a drastic effect on the accompanying fauna. For successful agricultural practices a sound physiological base is a must. Plant cultivation or agriculture was a crude art or a native effort and plant physiological studies have developed into a regular science. Modern understanding of the physiological mechanisms of growth and development is being increasingly exploited for increased quality and quantity of crops.
This insight has also increased the survival and or extending the range of desirable plants. Controlled fertilizer application and proper water management have been exploited to the best advantage of limited resources. Practices as crop rotation, green crop ploughing, usage of selective fertilizers, use of growth hormones, inhibitors, etc., are all based on our understanding of plant physiological concepts.
Recently computer technology has been used in aiding plant growth and manipulating plant responses to varied environments. With the growing population there are greater demands on production of various food crops.
Agriculture and agricultural produce are becoming industrialized and the role of plant physiologist is ever increasing. Environmental engineering in all aspects including usage of barren lands, growing increased number of crops, and exploitation of solar radiations by the existing and newly bred species shall involve plant physiologists.
In the coming years, teaching and research in plant physiology will occupy a pivotal place in our institutions. The search for deeper understanding and insight of how plants absorb water and minerals, utilize and conserve them will continue with added scientific techniques.
Note # 3. Role of Water in Plant Physiology:
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Of all the vital substances essential for plant growth and development, water is needed in enormous amounts. It is present throughout the plant body. Water constitutes more than 70% of the fresh weight of a plant and in some of the growing cells its level may be up to 90%. Water supply affects the growth rate of plants considerably.
(i) It is a major component of the plant body.
(ii) What is an essential solvent in which mineral nutrients are dissolved and translocated from the roots to the apex of the plant body. Minerals are also absorbed through water.
(iii) Large number of metabolic reactions take place in the water medium.
(iv) It maintains the structure of nucleic acids, proteins by supplying hydrogen bonding.
(v) Several processes like photosynthesis use water as a reactant of raw material. Thus formation of complex carbohydrates from the simple ones also involves the removal of water while the reverse reaction requires water as a reactant.
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(vi) This essential component is required to maintain the turgidity of the cell. Thus, it helps the cells to retain their tensile strength and provides proper shape to the cells.
Turgidity is essential for the opening of the stomata, and also activity of several organelles. In addition opening of flowers, folding of leaves occurs due to the changes in the turgidity of the cells.
(vii) Water also acts as a temperature buffer since it has an exceptionally high heat capacity for specific heat.
(viii) Water molecules have the unique property of adhesion and cohesion and thus these processes keep the water molecules together. This property helps in upward movement of water in the plant body.
(ix) The elongation phase of cell growth is mostly dependent on water absorption.
(x) Water is also a metabolic end product of respiration.
(xi) Plans absorb enormous quantities of water and simultaneously lose greater amounts of water through transpiration.
Note # 4. Role of Hydroponics in Plant Physiology:
‘Hydroponics’ involves soil-less growth and refers to the raising of crop plants, vegetables and flowering plants in well-balanced nutrient solutions in the absence of soil. The plants are grown in large shallow pots which are full of nutrient solutions. The tubs or tanks are covered with wire-netting to provide support to the seedlings. The solution is aerated at regular intervals by means of an inlet tube (Fig. 9-2).
Sand culture technique is also employed. Except for Hydroponics have certain advantages like: possibility to provide desirable raising crops in green houses, these techniques are not economically feasible at present, nutrient environment; regulation of pH of the nutrient solution conducive to specific crops; it is possible to replace the nutrient solution periodically; it is possible to control pathogens which are otherwise present in the soil; it is feasible to regulate aeration at regular intervals: in situations where equipment is automatic much cost on labour can be saved; there is no need to undertake tillering weeding; uniform plant growth can be easily achieved and nutrition budget can be easily regulated to meet the changing pattern of plant growth and maturity.
However, hydroponics have some disadvantages also and these are: limited plant yield and production; the system is only suitable to more specific and highly valued crop species; considerable technical experience and skill is needed to handle hydroponics; each crop demands specific modifications, and lastly several of the crop plants like potatoes, sweet potatoes, carrots may even change their shape and size.
Despite its several disadvantages and practical difficulties, large scale commercial hydroponics growth is carried out in many floricultural and horticultural species. In several of the European countries it has been possible to obtain high yields in radish, tomato, lettuce, cucumbers, etc.
Note # 5. Cell Wall of a Plant:
Plant cells have the outer hard covering which is known as cell wall. It consists of three layers-middle lamella, primary and secondary walls.
The middle lamella is a common layer of the two adjacent cells and is amorphous and colloidal in nature.
It consists of pectic compounds, chiefly calcium and magnesium pectate. Sometimes lignin and hemicellulose also occur in the middle lamella.
The primary wall is made up of cellulose and pectic compounds along with hemicellulose and other polysaccharides (Plate 1). Rarely these walls are also lignified but usually they are elastic and highly hydrated.
The cell wall formation is initiated in the late telophase of mitosis. The tubular fragments of the endoplasmic reticulum migrate to the equatorial region during the telophase and participate in the formation of the cell plate or middle lamella which cements the two adjoining cells due to their calcium pectate composition with greater binding property.
Pectic compounds and hemicellulose play an important role in the initial stage of cell growth.
As the cell matures secondary wall starts developing on the inner surface of the primary wall. The cell wall thickens as layers of cellulose are laid down by the cytoplasm.
The wall becomes plastic and the cell growth ceases. Secondary wall gives the plant cell its structural independence and mostly contains cellulose and other polysaccharides including hemicellulose.
It is less hydrated and hence more dense. The secondary wall is mostly mechanical in function and is usually distinguishable in three layers (S1, S2, and S3).
Of these, the S3 layer is also referred to as the tertiary wall. In some plant species the secondary wall is made up of several layers which are designated as S1, S2 … Sn etc. It is a permanent layer i.e. it cannot be extended by growth.
Cytoplasmic ground substance (cytosol) is not a constant material and its consistency changes from time to time.
Its main component is water and its percentage varies from 90 (parenchyma cells) to 4-5 (in seeds; spores and pollen).
The cytosol contains several living (mitochondria, plastids, ribosomes, lysosomes, etc.) and non-living particles (fat globules, aleurone grains, organic acids, pigments, fat globules, etc.).
Cytosol varies enormously in the same cell at different times and variation also exists from plant to plant.
Note # 6. Osmosis in Plants:
Take a container and separate it into two parts by a differentially permeable membrane. Add pure water in one of the parts of the beaker and a sugar solution in the other. On the side having pure water the water potential (Ψ) is higher than that on the other side. The sugar molecules cannot diffuse across their differentially permeable membrane, though water can.
Water will diffuse from the side where it is pure i.e. having higher Ψ to the side where sugar solution is present i.e. the side with lower Ψ. This diffusion of water across a differentially permeable membrane from a region of higher potential to a region of lower potential is called osmosis.
In the plant system different biological membranes are present which are differentially permeable. These are plasmalemma, tonoplast and the membrane which surrounds organelles. Because of their physical or chemical nature water molecules pass easily through these membrances.
On the other hand, molecules of different solute dissolved in the water either cannot penetrate or do so slower than water molecules. There are also membrances which are permeable or semipermeable depending upon whether both solute and solvent molecules together or only solute molecules can pass through them respectively.
The osmosis in biological systems was regarded as a process of diffusion but recently it has been shown to occur through mass flow. More recent studies have also shown that some of the living membranes in plants are traversed by continuous aqueous channels whereas others are not.
Therefore, osmosis may occur by mass flow only in some of the membranes. It is uncertain whether the porosity of the membrane is species specific or is dependent upon other factors. Possibly the age of an individual cell may also affect its porosity. Osmosis is important in the absorption of water in higher plants (Fig. 6-1).
It has a special role to play in the transport of water into plant cells. The phenomena of plasmolysis depend upon osmosis. We have observed that water moves under the influence of imbibition and thus water potential Ψ) must be affected by these forces. Matric potential (ΨM) is the term commonly employed to account for all the forces causing imbibition or holding the water in a matrix.
The potential of water in a matrix e.g. in a soil, colloid etc., can be explained as follows:
Ψ = Ψπ + Ψp + ΨM
Note # 7. Role of Transpiration-Recent Synthesis in Plants:
Transpiration is completely essential to the life of a land plant since it helps in absorption of CO2 from the atmosphere. It seems that stomata were evolved to accomplish CO2 absorption and transpiration was a consequence of it. The fact that stomata remain closed at night, their role in oxygen absorption does seem feasible.
This is also supported by the concentrations of oxygen and CO2 in the general atmosphere. However, during the day time when the green leaves are undergoing photosynthesis, oxygen released is diffused out of stomata. However, in reality stomata are primarily evolved to enact the role of CO2 absorption. The general argument against the necessity of transpiration could be advanced since several of the alpine plants could be successfully grown when completely submerged.
Admittedly transpiration as such may not be all that useful to a plant but it is a necessity which has turned into an advantage. Thus, several of the plant species could grow without transpiration but transpiration confers several advantages. For a long time it was assumed that transpiration contributed towards the transport of minerals.
Recent studies have shown that there is a kind of circulation of minerals in the plant. Using tracer elements it has been demonstrated that water moves from assimilating organs to the using organs via phloem tissue; even in the absence of transpiration, and this water returns via xylem tissue. Clearly, transpiration does not appear to be essential for the movement of minerals within the plant.
The general inference is that in the presence of transpiration, there is mineral absorption from the soil and then transport it in the various parts of the plant. Experiments in greenhouse grown tomato plants revealed that calcium transport required an active transpiration stream.
Note # 8. Factors Influencing Nutrient Uptake in Physiology of Plants:
1. Light:
Plants growing in bright light show rapid solute uptake than those grown under weak light. This may be due to high photosynthesis rate and greater supply of sugars to enhance respiration in roots. In green algae and some water plants light directly affects mineral uptake. Here light energy is converted into chemical form e.g., ATP which helps in ion uptake.
2. Temperature:
High temperature up to 40°C increases solute uptake. The slow growth of plants growing in cold climate is due to slow uptake of solutes. High temperature also stimulates respiration of roots which may be aiding ion uptake. Temperature above 40°C affects respiration and thus uptake of solutes is slowed down.
3. pH:
pH 4-9 affects maximal plant growth. When the pH of soil is from 5-7 the growth is maximal. Thus some plants prefer to grow in alkaline soils where as other species grow best in acidic soils. There are several ways in which pH affects growth and uptake of solutes. For instance pH influences the ionic charge and affects the phosphate uptake.
The low availability of iron and other elements may be due to slow growth of plants at a high pH. When pH is low hydrogen ions usually decrease the cations absorption whereas anion absorption is enhanced.
4. Aeration:
Roots must have sufficient quantity of oxygen available for absorbing solutes. Plants growing in waterlogged soils have slow growth since aeration is poor. There is also a possibility of high CO2 level about the roots. Roots aerated with oxygen show increased absorption of potassium.
5. Nutrients Level of Plant:
If plant roots have enough amounts of specific solutes then absorption of that solute is comparatively slow as compared with the roots which are deficient. The level of elements affects the uptake of solutes mechanisms. There are also reports available where abundance of one ion influences the uptake of another.
Thus when the level of nitrogen was high plant was able to absorb more of sulphate and phosphate. When phosphate was high then the absorption of nitrogen was also stimulated. Apparently some interaction between nitrogen and phosphate and nitrogen and sulphate occurs inside the plant.
6. Growth:
Rapidly dividing and growing cells accumulate more of ions. On the other hand mature cells accumulate fewer amounts of ions. In the young cells the proteins provide a binding site for the several cations whereas other compounds are rapidly changed in order to become a part of protoplasm. A specific relationship definitely exists between the age of cell and solute uptake.
Note # 9. Plants as Indicators of Mineral Nutrients:
Plants are used in prospecting minerals in three ways mapping the distribution of indicator species by studying physiological and morphological changes in plants growing on high mineral soils; and by chemical analysis of the metal content of the plant. Members of Caryophyllaceae and Labiateae and some mosses grow on copper rich soils.
In Wales (UK) copper depositions were found where plants like Armeriamaritima, Polycarpeaspirostylis, Bulbostylisbarbata, etc., were growing. The ‘copper mosses’ led to the discovery of copper deposits in Sweden. High zinc bearing soils have been found where plants of Violoalutea, Armeriahalleri, etc., were growing.
Crotalaria cobalticola and Silenecobalticola are indicators of cobalt. Polygala baumii and Mechoviagrandiflora inhabit soils which are rich in manganese. In Russia Juncus, Salsolanitraria were used as indicators of soils rich in boron. Similarly Yucca species are reliable indicators of granite and silica.
Artemrsia indicates the occurrence of sand deposits. Same is indicated by the distribution of some members of Liliaceae and Polygonaceae which can tolerate high sulphur contents in the soils. Some plants are indicators of gypsum. While exploring oil in the Caspian sea halophytes like Salsolanitraria were used to locate salt domes.
The absence of vegetation is also used as an indicator in the exploration of minerals. For instance, in Congo soils rich in copper (8-14%) show no tree growth. The quality of particular mineral is also related with changes in the morphology of the species.
Thus when zinc is present in high percentage the petals of Papaver macrostomumbecome incised. The dwarfness of the species and a change in its height or form is also related to a mineral gradient concentration.
In summary, plant analyses have also helped in the discovery of zinc, tin, tungsten, arsenic, copper and vanadium. Further amount of metal content in the plant has helped in providing indications on its distribution in the soil e.g., manganese, chromium, cobalt and molybdenum.
Note # 10. Physiology of Foliar Nutrition in Plants:
In recent year’s variety of chemicals in solution form are directly sprayed on the leaves of the crop plants since they are readily absorbed and utilized more efficiently. Based on this observation mineral nutrients are also supplied to the plants as foliar sprays.
This practise has several advantages; it reaches the desired tissues quickly, the growth and development are accelerated, the process is highly advantageous in crop plants which are tolerant to aerial spraying.
This technique is of immense use where plants have thick cuticle or waxy cuticle layer. Several instances may be mentioned where foliar sprays have been employed with advantage to overcome deficiency of some micronutrients. It is also possible to supply macronutrients during critical growth periods when it is not possible to supply fertilizers in the soil.
In plants where it takes long time for the applied nutrients to reach the place of its need this method is also useful. This is particularly true of annual crops which are growing fast in specific periods. It is also an economical means of fertilizing some crops. There could be lot of saving both on the material and finances especially when u is desired to use micronutrients.
Several companies are manufacturing products which employ a particular formulation of growth substances, enzymes, amino acids chelated micronutrients (Zn, Fe, Mn, Bo, Mo) complex combined with ethoxylatesiloxane derivatives that increase nutrients and water retention. These products are available in several combinations and are produced by different companies.
They are soluble in water and are sprayed on the foliage to provide an effective method of applying nutritional support to plants. The complexes of this kind are especially useful when responses are demanded within short growing seasons.
The foliar penetration of the nutrients begins with the absorption of bio-chemicals through the leaf trichomes during early growth periods when the waxy cuticle concentration is low and non-limiting. Radioisotopic studies have revealed that micronutrients supplied as foliar sprays to trees, potatoes, corn, barley, rice and wheat were well absorbed and translocated to different organs of the plant.
Also when the soil application of these micronutrients was compared with the foliar sprays, a less time lag with foliar feeding system was observed. The penetration of the micronutrients may also occur through stomata or even diffuse through cuticle.
Sometimes there are wounds, breaks, sutures in the leaves and these micronutrients may enter through them. The role of ectodesmata in the translocation of micronutrients applied as foliar sprays also seems feasible. Foliar uptake may also be accompanied by leaching of some nutrients also. Once leached they may be reabsorbed by the roots and used by the plant.
The effect of foliar spray treatments of low doses of phosphorus has been shown in several crop plants. The studies were conducted on cereals, oil-seed plants and vegetables, etc.
From these investigations it becomes apparent that considerable quantities of phosphaticfertiliser could be saved by spraying the nutrient at the flowering or juvenile stage. Some authors have added nitrogen to the phosphatic spray and when sprayed on the leaves of wheat, protein content of the grain increases.
Note # 11. Physiology of Photosynthesis:
Synthesis of compounds with the aid of radiant energy especially in plants are known as photosynthesis.
1779―Jan Ingen-Housz concluded that O2 production occurred within few hours of photosynthesis and secondly it was involved during the day time. Plants which were green only used CO2 of the atmosphere.
He also proposed the following equation:
1782― Jean Senebier demonstrated the essentiality of CO2 to produce O2 by the green plants.
1804―Nicholas Theodore de Saussure showed importance of water in photosynthesis. Light was required for the evolution of O2.
1837―Dutrochet demonstrated the importance of chlorophyll in photosynthesis.
1842―Mayer described sun as the source of energy.
1845― Liebig indicated the involvement of CO2 in the formation of organic compounds.
1864― Sachs reported formation of carbohydrates in photosynthesis. He used half- leaf experiment for this.
1905― Blackman proposed Law of Limiting factors.
1923― Warburg used Chlorella as a basic system for studying photosynthesis.
1924― CB van Niel showed that some bacteria use H3S instead of H2O for photosynthesis.
1939― Robert Hill performed first experiment to demonstrate that separate light reaction was localized in chloroplasts.
1940-50― Melvin Calvin and Andrew Benson unravelled the outlines of CO2 fixation into carbohydrates in photosynthesis. Calvin used 14CO2 in his experiments.
1941― S. Ruben and M. Kamen used 18O2 to confirm that O2 produced during photosynthesis was derived from H2O.
1950― Ochoa and Vishniac showed that NADP+ could substitute as the hydrogen acceptor in the Hill reaction.
1954―Kortschak described the formation of C4dicarboxylic acids.
1954―Daniel Arnon made discovery that light and of dark reactions photosynthesis were separable.
1957― Emerson and his associates discovered Emerson effect.
1960― Woodward confirmed structure of the chlorophyll-α and synthesized it in the laboratory.
1960― Hill and Bendall showed two-step electron in transport system-photosynthesis.
1962― Fujita and Hattori investigated the effect of the various factors onformation of phycobiliproteins in algae.
1962― Jagendorf introduced the concept of two-state phosphorylation.
1966― Hatch and Slack extended the work of Kortschak.
1967― Arnon proposed two-photosystem scheme of photosynthesis.
1968-69― Laetsch described ultrastructure of bundlesheath and mesophyllchloroplasts.
1973― Rouhani and his associates proposed pathways in CAM plant Sedum.
1980― GJ Lorimer showed that rubisco uses CO2 as a substrate and activator.
Note # 12. Photorespiration and Plants:
Under normal atmospheric conditions, when the O2 is 21%, the rate of photorespiration in Helianthus. Leaves is nearly 17% of gross photosynthesis. Every photorespired CO2 needs an input of two O2 molecules. This implies that the true rate of oxygenation nearly 34% and the ratio of carboxylation to oxygenation is about 3 to 1. The ratio of oxygenation and carboxylation depends on the relative levels of O2 and CO2 since both gases compete to bind to the active site on Rubisco. When the O2 level declines, the relative level of carboxylation increases till at zer O2, photorespiration is also zero. Contrarily, increase in the relative levels of O2 or decreases in CO2 shifts the balance towards oxygenation, Enhancement in oxemperature also favours oxygenation due to decrease in solubility of gases though oxygen solubility is less affected than carbon dioxide. Therefore oxygen will inhibit photosynthesis.
Measured by net CO2 Reduction in Plants with Photorespiration:
With photorespiration energy costs are also associated and same is true of glycolate pathway. Thus, the energy expended (ATP, NADPH) in glycolate pathway following oxygenation is nearly equal to that expended for the reduction of one CO2 during the PCR cycle. There is net loss of carbon. Photorespiration apparently is expensive and inefficient process with regards to energy and carbon. It is reasonable to pose a question regarding existence of this wasteful process in plants.
The answer to this question is highly complicated and several ideas have been advanced. One view is that oxygenase function of Rubisco is inescapable since it evolved during period of earth when CO2 level in the atmosphere was high and oxygen was low. Both oxygen and CO2 react with the enzyme at the same active site and oxygenation needs activation by CO2 and same is true of reverse.
It is suggested that photosynthesis led to accumulation of oxygen in the atmosphere, but high built up of the oxygen was not accompanied by the loss of bifunctionality of the Rubisco. So to say C3 plants created problems for themselves-generating oxygen which was a competent inhibitor of carbon reduction. Thus, oxygenase part of Rubisco is an hangover of the past but has no role.
On the other hand, inefficiencies due to photorespiration if any, are not severe. Some workers believe that plants have turned photorespiration to its metabolic advantage. Thus glycolate pathway acts as a scavenger. For two turns of the cycle, two molecules of phosphoglycolate are formed by oxygenation. Of the four carbon atoms, one is lost as CO2 and three are returned to the chloroplast.
Glycolate pathway thus recovers 75% of the carbon that could be lost through glycolate pathway, otherwise. Amino acids like serine, glycine are also produced and are used in other biosynthetic pathways. The third groups of workers suggest that photorespiration acts as a safety valve in circumstances that need dissipation of excess excitation energy. In some situations, in the absence of O2 and CO2, normal light, the rate of photosynthesis declines. Photorespiration could save the leaf in situations when excess of oxygen is present.
The CO2 generated through photorespiration protects the plants from photo-oxidation damage by operating electron transport chain continuously. In situations when the plant has high light and low carbon dioxide, this could be of immense ecological significance. In conditions when the plant experiences water stress and the moisture in the surroundings is low, stomata close and this could be a protective measure.
Continued attempts are being made to suppress/inhibit photorespiration through the use of chemicals or genetically, to increase yield. Efforts are also on to identify species with Rubisco having low affinity for oxygen. To date such efforts have not yielded substantial success in this direction.
Obviously success in increasing photosynthesis and improving crop productivity has to be sought in other ways. Thus, a mechanism of concentrating CO2 in the photosynthetic cells could be a way to suppress photorespiration losses and improve the overall efficiency of carbon assimilation.
Note # 13. Physiology of Insectivorous Plants:
Several of the plant species inhabit marshy and swampy soils which are deficient in nitrogen. They are otherwise autotrophic but are dependent upon the nitrogen requirement through several insects. They break animal proteins and use the nitrogen rich compounds thus produced.
In tissue culture studies conducted recently in Utricularia by Mohan Ram and his students it has been proved that these insectivorous plants do not necessarily require animal proteins as source of nitrogen.
Several of the insectivorous plants are known and these include Drosera, Dionaea, Nepenthes, Utricularia sp. In all these plants the leaves or their parts are diversely modified to prey upon the different types of insects and trap them. Through the enzyme(s) action these insects are decomposed and used as nitrogen source. The products of digestion are absorbed by the plant.
These plants possess several specialized trichomes which increase the ability of the plant to trap insects. These trichomes possess proteolytic enzymes and possibly were evolved from hydathodes. In the leaves of Pinguicula two types of glands are present (Fig. 16-1) and these are either sessile or stalked. The stalked glands have a large basal cell, stalk and columellar cell with a head of 10-13 cells.
On the head drop of mucilage oozes out which helps in the capture of the insect. On the other hand the sessile glands have basal and columellar cells bearing a distinct head of 2-8 cells. The lateral walls of columellar cell is cutinized and thus resembles endodermis. The mature secretory cells of all the insectivorous plants have wall ingrowths resembling transfer cells.
These are well developed on the radial walls of both the glands. The heads of the trichomes have no well-developed cuticle and therefore secretion can easily pass through it. The glandular trichomes show high activity of several hydrolases including esterase, acid phosphatase and ribonuclease.
It has been shown that the chief site of activity of these enzymes was in the anticlinal or radial walls of the head cells. It is suggested that enzymes are moved in the radial walls and then through a poorly developed cuticle through the pores outside. If the sessile glands of Pinguicula are stimulated secretion begins within one hour and within two to three hours it oozes out.
This secretion has a detergent characteristic and possibly wets the exoskeleton of the insect. The glands of insectivorous plants are interesting since in them the secretion products move out of the cuticle and the digested products move in the glands simultaneously.
In Drosophyllum also both types of glands are present. The mucilage is, however, secreted by the outer stalked or tentacle glands. These glands are rich in dictyosomes. In fact during the period of secretion these glands abound in dictyosomes.
The mucilage formation is respiration dependent and temperature also affects the rate of secretion. The closure of the leaf is shown to be brought about by electrical signals which originate in the multicellular sensory hairs. In Drosera the large tentacles on the leaf may be regarded as large stalked glands which are not wholly epidermal in origin.
These glands have multicellular stalks and have a bundle of tracheids and a head with 2-3 cells. The cuticle of these cells is perforated. Plasmodesmata are present between different cells in the head. Transport of calcium has been traced in these glands. Through series of experiments it has been demonstrated that digestive material can pass to the inner side against the direction of flow of mucilage and digestive enzymes.
The situation in pitcher plant (Nepenthes sp.) is very interesting. The pitchers in this plant possess different kinds of glands. These glands are similar in structure but differ in location and function. There are ‘alluring glands’ which are multicellular and are present on the under surface of the lid of the pitcher. These glands secrete nectar. On the inner wall of the pitcher several digestive glands are distributed.
These glands have digestive and absorptive functions. They are more in distribution towards the base of the pitcher. These glands also bring about a short distance transport of the material from the pitcher cavity to the tissue. These glands also secrete several hydrolases like acid phosphatase, esterase and ribonuclease, etc. The pitchers may have as much as one litre of digestive fluid.
In Utricularia the leaves are modified into bladder-like traps and trap the insects in several ways. On the outer side of the bladders there are sessile and stalked glands which secrete mucilage and nectar in order to attract insects.
The inner wall of the bladder is lined with sessile glands which extract water from the contents and cause tension in the bladder. Consequently water stream alone with the insects is moved in. The door of the bladder is guarded by hair which open towards the inner side. The trapped insects are not allowed to escape.
Cuscuta (Cuscutaceae) species are among the best known angiosperm parasites and about 180 species are reported in the genus. The parasite joins the host plant through a wedge-shaped physiological bridge called haustorium.
In recent years development, ultrastructure and physiology of haustorium have attracted much attention. Recent studies have shown that from the distal end of the haustorium hyphae like structures develop which penetrate the individual cells the host cortex or pith.
These are search or contact hyphae. Of the two types, search hyphae grow inter and intracellularly. Tripodi (1967) has studied details of the haustorium and also changes in the host cytoplasm through electron microscope. This author reported that Golgi vesicles increased in the host cytoplasm.
Dorr (1969) has reported several plasmodesmatal links between the protoplasts of host and parasites. She has also reported hand like furrow at the tip of the hypha where it attaches to host’s sieve tube membrane. Cuscuta is unique in developing a highly specialized contact with the host phloem. Recent studies by Malik and Komal (1979) have shown that formation of haustoria is sequential and is regulated by cellular relations within the host.
Haustoria fail to penetrate the secondary tissues, especially those which are lignified. There are four stages in the developmental sequence of the Cuscutahaustorium and these are dependent upon at least two conditions.
The first is the initiation of haustorialprimordium within the prehaustorium and the second is the physical and chemical environment existing in the immediate surroundings of developing haustorium. The haustorialprimordium has a programmed set of requirements and these must be met with to affect hasutorium formation.
The initiation of the haustorium does not require the presence of host and that haustorial extensions and basal xylem formation can be induced by specific chemical stimulants or even physcialcondtions. Wolswinkel (1979) has reviewed the transport of assimilates and minerals and the role of phloems unloading in the parasitic relationships.
Though Cuscuta is traditionally regarded as classical example of total parasite which is unable to grow autotrophically, some reports are available in recent years which suggest the possibility of photosynthesis by the Cuscuta vines.
In a recent review paper Malik and Singh (1980) have discussed the biochemical aspects of parasitism by Cuscuta and discussed the available literature on the physiology of host-parasite relationships. In the haustorium diverse types of digestive enzymes are reported. Most of these hydrolases are not excreted enmasse into the host tissues.
Where host-parasite tissues did not contact no hydrolases were localized. Assumingly haustorial cells contained lysosomes and the latter fused with the cell membranes and released their contents into the host cells. Hydrolases act upon the host cells plasmalemma and destroy its semi-permeability attributes.
As a result its turgor pressure is disturbed. When followed by the mechanical force of the haustorium, these host cells get distrupted and are crushed due to the pressure and hence their cytoplasm is released. The space thus made is occupied by the tip of the haustorium which subsequently grows within the host tissues.
Apparently both enzymatic and mechanical processes were involved in facilitating the entry of the haustorium in the host tissues. Major function of the cells in the tip of the haustorium is to cause digestion and disruption of the host cells. Thus weakening of surrounding cells enzymatically appears to be an essential precondition for the entry and pentration of the haustoria.
There are several semi-parasites like Loranthus, Viscum, Arceuthobium, etc. Most of these species grow on the tree branches and possess green leaves. Viscum species have a dichotomously branched shoot bearing green leaves. Each branch produces a wedge shaped haustorium which penetrates the host tissues.
There are also secondary haustoria which establish contact with the xylem of the host. It draws water and minerals from the host tissue. Haustoria are also regarded as sucking roots. Loranthus species are also referred to as Dandropthae sp.
They are semi-parasites growing on the branches of mango, dalbergia, fig and other species. Here also primary haustoria penetrate the host tissue and obtain water and mineral supply whereas leaves of the parasite are green and manufacture food material themselves.
In Punjab and United Province one of the most prominent root parasites is Orobanchecernui and other species. This plant infests the roots of Brassica and Solanaceous members. The tall shoots arise from the roots and bear pink flowers and white scaly leaves. Similarly species of Rafflesia are also specialized total root parasites. Members of family Santalaceae are partial root parasites.
Note # 14. Phenomena of Growth in Plants:
The dictionary meanings of growth are many the advancement towards or attainment of full size or maturity; development; a gradual increase in size, etc. In plant physiology arbitrarily we separate the concepts of growth and development, using the term growth to denote increase in size, thus not considering any qualitative concepts such as full size or maturity which are clearly unrelated to the process of increase.
The first requirement for studying growth is a suitable method of measuring it. Growth can be measured as an increase in length, width or area; often it is measured as an increase in volume, mass or weight (either fresh or dry weight). Each of these parameters describe something different and in a growing organism, there is rarely a simple relationship among them. The simplest definition of growth is an increase in size.
Since size is a synonym for volume, this means that:
Growth mark = Vt – Vo
and growth rate = Vt – Vo/t
where Vt = volume at end of time t and
V0 = original volume at time zero.
The ideal method of measuring growth would, therefore, be to determine the volume of the plant part, but this is usually difficult to do with any degree of accuracy.
In case of growing leaves the area is more readily determined:
Growth = St − So
where St = surface area at end of time t
So = original surface area.
In case of stems and roots, a simple measurement is length:
Growth = Lt— Lo
Where Lt = length at end of time t
Lo = original length,
The easiest and the most accurate measurement, however, is usually the weight of the plant or plant part and hence it is represented as:
Growth = Wt− Wo
where Wt = weight at end of time t
Wo= original weight.
The measurement of the weight may lead to some complicating factors. It must be decided whether fresh or dry matter is a more correct measurement of growth; for example, during seed germination there is an initial uptake of water which is not accompanied by any significant growth as we normally consider it.
There is an increase in fresh weight as well as volume but not in dry matter. Subsequently the seedling increases enormously in length but there is a net decrease in dry matter. Of course, growth by any reasonable definition has occurred.
On the other hand during mid-winter, when the tree has stopped growing, there is a slow increase in the dry matter because of accumulation of reserve food materials. Dry matter can, therefore, be used as a parameter for the measurement of growth only when it does not include any significant alterations in plant reserve food materials.
It is a satisfactory measure of growth of seedling only if the endosperm or cotyledonary leaves are not included. In other words, weight can only be used as a measure of growth if W < V.
There seems to be no acceptable solution to this problem presently because growth, development and simple changes in size frequently overlap and can occur separately or together in any combination.
Some of the physiologists have suggested that the best definition of growth is the self- multiplication of the protoplasm, living material itself. The germinating seeds convert much of their stored food materials into more functional compounds within the protoplasm of the growing and newly formed cells. Although this definition appears to be appealing yet it is not very practical. The problem arises while considering how one could measure the functional protoplasm in a plant.
Thus, perhaps, if we sampled the organism, count the cells, and try to estimate the rate of cellular increase this would provide more meaningful definition. Since the growth of a plant is initiated in the meristems, the cellular changes in these tissues might throw some light on the process. The three main changes in the complete development of a cell are cell division, cell elongation and cell differentiation.
The growth of a plant part may result from either cell division or cell enlargement. However, it must be realized that even growth resulting from cell division is possible only because the daughter cells enlarge to the size of the mother cell before they divide again. Cell enlargement must not include any simple turgor-induced changes in cell volume if it is a true cell growth.
Consequently, the growth of a plant or plant part is an increase in size caused by cell enlargement when measured by cell constant positive cell turgor. In the simplest instances of cell growth there is an uptake of water, producing a turgor pressure to overcome the force of attraction between the particles in the cell wall.
Consequently, wall stretches to become thin and the osmotic pressure drops because of water uptake which dilutes the cell content. The consequent increase in cells water potential soon brings end of water uptake (and, therefore, growth). Under normal conditions, cells are capable to increase in size more than 15-folds.
In these cells, as water enters during enlargement solutes are also absorbed, and cell maintains a sufficient turgor pressure to continue stretching its wall. Besides absorption of water and solute, other changes occur during cell growth.
The new wall material must be laid down due to an insertion of new microfibrils between the old wall components that have become separated, i.e., wall thick, opposition (a deposit of new material on the inner surface of the wall) occurs during cell maturation.
Biochemical studies have revealed that cell wall formation is accompanied by many biochemical changes which include synthesis of proteins, nucleic acids, phospholipids, multiplication of organelles and utilisation of energy as ATP, etc.
The concept of growth in cellular terms can be summarized as follows:
Kinetics of Growth:
Many investigators have measured size of an organism and have plotted the data as a function of time. Growth of a plant or plant part characteristically passes through stages represented by an S- shaped curve. Size versus time plot is the most direct way of handling growth data.
We can also plot the longrithum of the size as a function of time. In a size plot the units are length, volume or weight but in the rate curve (Fig. 19-1) the units are length, volume or weight per unit time (Fig. 19-2).
Growth curves plotted in any of the two ways have two clearly distinguishable phases (Fig. 19-2). The first is rapid or exponential (a) phase during which the rate curve as well as size curve is increasing. In the last phase, the size continues to increase but more slowly so that the rate decreases (c).
The first phase has been called logarithmatic or exponential phase and the last phase is referred to as senescence or decreasing phase. In many examples of growth curve, another phase is also clearly evident. This is a phase of minimum growth rate or the linear phase (called by Sachs, the grand period of growth), and it lies between the logarithmatic phase and the senescence phases.
Blackman (1919) put forward the idea that growth of a plant could be represented by the equation
Wt = Wo.ert
where W1 is the final size (weight, height, etc. after time t), Wo is the initial size at the beginning of the time period, r is the rate at which plant substance is laid down during time t, and e is the base of natural logarithms. It should be noted that r is the relative growth rate as discussed above.
Black-man pointed out that this equation also describes the manner in which money placed at compound interest increases with time, and the term compound interest law is used to describe such phenomena. Banks usually apply compound interest quarterly or annually so that the increase in amount occurs at a jump. With plants or other biological systems, compound interest is applied continuously, and size increases as a smooth curve.
From the same equation, it is also seen that the size of an organism (Wt) depends on the initial size (Wo). It has been shown that seedling growth in seeds of different sizes follows such an expectation, with the large seeds giving a large plant. This is true, however, only during the early stages of growth, and if t is large, then the final size of plants from small and/or large seeds frequently are equal.
From this equation it is seen that plant size also depends on the magnitude of r, the relative growth rate. Blackman suggested that r might be used as a measure of the ability of a plant to produce new plant material and called r the efficiency index. Plants with a high efficiency index could be expected to perform better than those with a low efficiency index.
While r does differ among plant species, it is not constant during the life of aslant. Furthermore, all parts of a plant are not equally involved in synthesizing new plant substances; some of the material goes for storage or is catabolized. The usefulness of the efficiency index as a predictive factor in evaluating performance or yield is limited.
Note # 15. Plant Development:
Plant development comprises a series of events. Some are repetitive whereas others occur only once in the life of the plant. These include flowering or seed germination. Divergent types of controls are operative during the initiation of these events. In the chapter on Physiology of Flowering we shall discuss the role of day length in controlling flowering in plants.
Plant seems to possess the potentiality to measure the length of light and thus flowering occurs at the proper time and season of the year as determined by the night length, specific to that season. Some plants must experience cold treatment before they can flower.
This process checks premature flowering in such species. The possible mechanism through which a plant is able to perceive, measure and react to light and temperature.
Similarly processes like seed germination and leaf abscission are regulated by external factors. For instance seed germination is dependent upon water absorption and provision of proper physiological conditions whereas leaf abscission is mostly influenced by physiological and environmental factors.
Plant behaviour in terms of its development is rhythmic. Thus some flowers open in the morning while others do so at night. Similar aspects are exhibited by the leaves. Photosynthesis and respiration and flowering are rhythmic changes which are dependent upon alternation of day and night.
Plants have a built in clock which can measure time and rhythm. Though several models have been advanced yet the nature and mode of regulation of plant behaviour remains conjunctural.
Note # 16. Growth Hormones in Plants:
In intact plants endogenous hormones must move from the site of synthesis to several other locations where they direct growth, differentiation and morphogenesis. Understanding the direction and pattern of their movement is highly important in elucidating their mechanism of control. Most information is based on exogenous applications and then tracing them in different parts of the plant.
Auxins:
Using isolated sections as a system one can estimate uptake, distribution and movement of auxin. Uptake is measured by comparing the loss and gain of auxin by the gainer and the donor. Because of the complexity of the system it is difficult to infer that uptake from the donor is equivalent of uptake by a polar transport system.
Uptake due to diffusion, utilization, etc. is difficult to ascertain presently. The available data show that transport is polar because growth causes a sink for auxin. Auxin can also pass through cytoplasm and pass across the cell membrane into extracellular space before passing to the adjacent cell. Plasmodesmata are also likely to participate in transport.
In most studies verification of 1AA movement has been done using 14C-IAA. Relation between transport and concentration of IAA and also data on velocity, specificity, polarity and inhibition of auxin transport have been obtained. Evidences for irreversible IAA mobilization during transport have been provided.
Several of the auxins and other related compounds, e.g., 2, 4-D, TIBA and substituted benzoic acids inhibit basipetal IAA movement. Maximum transport of IAA occurs when respiration is high. With respiratory inhibitors the transport rate is also inhibited. Plants exposed to ethylene for several hours before the transport decreaes transportation several-fold.
Auxin moves polarly in inernodes, petioles, hypocotyls, coleoptiles, leaves and roots. In roots polarity may be opposite of shoot. Here acropetal movement is more than basipetal. Light either promotes or retards basipetal auxin movement and is closely associated with the nutritional status of the tissue.
In many tissues the degree of polarity changes during growth and development. In brief polar transport is of general interest and may be related to the morphological polarity of plants and may be a vital factor in regulating growth and differentiation of different organs.
Gibberellins and cytokinins:
Gibberellins (14C-labelled) are appreciably stable in plants and tracers types have been used to follow their distribution. Its movement to a mature leaf is localized. When applied to mature leaves it moves into immature leaves.
Endogenously gibberellins do not move in a polar fashion. The occurrence of gibberellins in bleeding sap suggests that they are supplied by the roots via the transpiration to the stem and mature leaves. Then they enter phloem and move to the immature shoot parts.
Movement of cytokinins in plants was believed to be restricted. In some systems its distribution seems similar to that of auxin. Available evidences have shown that considerable amount is synthesized in roots. Suggestively roots might supply cytokinins to the leaves. Cytokinins have also been sampled from bleeding sap of several species.
Recent evidences have also shown interaction of different hormones in affecting their movements. Kinetin, for instance, enhances basipetal movement of IAA.
ABA:
It has been shown to be translocated through the phloem and xylem and movement could occur from mature leaves to shoot tips and roots. In plants under stress condition ABA level rises in xylem and phloem exudates. General feeling is that the significance of ABA transport may be greater after a period of water stress than during the stress period, especially when the stress is severe.
ABA can also move basipetally from the root cap to the root meristems. The precise site of synthesis of ABA in the roots is obscure but will presumably affect the direction of its movement in the roots.
Note # 17. Physiology of Flowering Plants:
Flower formation is a transitional phase in the life cycle of a plant. It is of immense importance for perpetuation and origin of variability in the next generations. Flower initiation takes place by the transformation of vegetative apex into a reproductive structure.
It signifies a transition from vegetative to the floral state. The shoot meristem is reduced and is also induced to develop sepals, petals, stamens and carpels in place of leaves. The pattern and timing of flower initiation vary from species to species. Thus, the flowers may be terminal, axillary, single or grouped into an inflorescence. A plant must attain a specific state of ‘ripeness to respond’ before it flowers.
Once this stage is reached, then it can be induced to flower. Thus by and large, two phases are recognized at this point. These are flower induction and flower differentiation. Both the processes are subjected to variable and diverse controls. The induction of flowering implies whereby all the cells of the shoot meristem instead of giving rise to leaves are turned towards the formation of floral organs.
Once such a switch over has been accomplished then the differentiation steps leading to flower formation follow. Of the two phases, the former has attracted the attention of several plant physiologists for a long time. One simple reason could be that once the induction of flowering has been accomplished, the differentiation would follow automatically.
With experience it is now widely accepted that during flower induction a plant must be subjected, for specific period of time, to an external condition e.g., light, temperature or even some chemicals, etc.
This time interval is generally referred to as induction period and the external conditions as inductive-conditions. Similarly, the external conditions under which a given plant continues to grow vegetatively is called non-inductive conditions.
The two main inductive conditions (i.e., light and temperature), which induce flowering. The flowering response to day length is called photoperiodism while low temperature treatment is called vernalization.
A large number of plant species undergo a state of vigorous vegetative growth and during this period, they cannot be induced to flower. This is called a juvenile phase. It is as if such species must attain ‘ripeness-to-flower’ even though suitable conditions are provided. On the contrary, several grass species like Loliumremotum, L. temulentum can be made to flower at any stage of growth.
The duration for ripeness-to-flower varies considerably. It may be a few days (Xanthium), a few weeks (Lunaria) or even several years {Mains, Citrus). In such instances, the plant must develop specific number of nodes and also minimal number of leaves before flowering is induced. It is this difference in the number of leaves which is critical for different species to flower at variable times.
In a longitudinal section of the vegetative shoot apex, a central dome or corpus is seen along with one or several layered tunica. Corpus is differentiated into central mother cell zone, flank meristem and rib meristem. During the transition of vegetative apex into reproductive primordium, several histological and biochemical changes occur.
One of these is an increase in the mitotic activity between rib meristem and central mother zone. Cells derived from this zone become central core of floral primordium. The reproductive shoot is an enlarged structure with several cells. Such a transformation also induces synthesis of nucleic acids.
Through the usage of antimetabolites i.e., inhibitors of nucleic acids synthesis, the suppression of such transformation is clearly made out. In Chenopodium there is a rapid increase in RNA content following photoperiodic induction. Similarly, with radio-active precursors of RNA an enormous amount of incorporation was observed in Lolium following inductive long-day conditions.
Murret (1977) has reviewed on the environmental interaction and the genetics of flowering. In brief, it may be said that before the initiation of flowering, there is reprogramming of genetic activity in the shoot apices. It is generally assumed that stimulus induces some of the passive genes blocks.
The latter are activated and these undertake synthesis of new messenger RNA; such genes are activated floral genes. The RNA thus synthesized controls flowering through the synthesis of new enzyme systems.
It is also believed that the flowering stimulus is a transmissible molecule which has the potentiality to combine with and inactivate the repressor i.e., floral gene repressors. Moreover, DNA may also be exerting a significant regulatory role through nucleotides. Genes may influence flowering through enzyme synthesis, membrane and transport phenomenon, energy supply, switching and regulator)’ mechanism, etc.
In tomato, flower differentiation is solely determined by genotype of the plant. In several plant species in addition to genetic make-up, day length, intensity and quality of light, temperature and mineral nutrition also influence flower formation enormously. In fact, the response of plants to the factors mentioned above is an adaptive property for their perpetuation during unfavourable conditions.
Note # 18. Physiology of Seed Development in Plants:
Seed develop from an ovule after fertilization. It consists of an embryo, endosperm (sometimes absent), seed coat or testa. It is a basic unit of dispersal. In cereals the seed coat is fused with fruit wall and the composite structure is called grain.
Seeds have been a source of food in most parts of the universe and are valued for their chemical composition, nutrition and changes during storage and germination. Factors related to their storage and germination are extremely vital to the plant breeders. Plant physiologists have investigated the effects of different temperatures, moisture, oxygen concentration, light, etc. on seed germination and seedling emergence.
Much emphasis is laid upon high-quality seed having excellent genetic potential and good germination and vigorous seedling growth. Recently techniques are employed to raise healthy and vigorous seeds to obtain vigorous seedlings.
Several hormones and chemicals are used to improve the oil, protein and other economic attributes of seeds of divergent varieties. Seed storage and raising quality seeds are attracting added attention. There is growing introduction of physiological and biochemical techniques in elucidating the processes linked with seed development and seed germination.
Embryo is a connecting link between two generations of a plant and provides a continuity of genetic material. The nutrition in the initial stages of embryo development comes from cotyledons or endosperm and later on embryo attains independence.
The chief role of testa is to protect the embryo against several environmental factors. It may also be modified to affect seed dispersal. In the preceding pages we have already summarized the stages in the seed development.
Composition of endosperm is highly variable. In a coconut endosperm nearly 85% of its dry weight is made up of carbohydrates especially glucose. Most part is sorbitol, myo-inositols and seyllc-inositol. They are chief source of carbon for protoplast and other metabolites synthesis. Some may be involved in wall formation. Inositol is converted to phytin when the seed enters the maturation phase. In cereals phytin is stored in aleurone tissue.
Phytin is a rich source of stored inorganic ions which are made available to the developing embryo during seed germination. Endosperm also abounds in free amino acids, proteins, amides and nucleic acids. They support the growth of the embryo. Coconut milk also contains gibberellins, indole acetic acid, cytokinins, etc.
Apparently endosperm has a complex of compounds and some play an important role in the regulation of plant growth. The contents of endosperm are possibly derived from synergid, antipodal cells and contents obtained from the surrounding cells in the central cell.
Embryo of the developing seed encounters several chemical and physical environmental factors. Of the two, chemical factors have been comparatively well understood while nothing much is known regarding physical factors.
A perusal of the available literature shows that morphological changes during embryogenesis and seed formation have been investigated in several plant species. A series of alternations in the activities of organelles is seen in the developing embryo.
This is especially true of dictyosomes, ER, plastids, mitochondria, ribosornes, etc. There are also changes in the biochemical constituents including nucleic acids, proteins, starch, several enzymes and phytohormones.
Biochemical changes involved in the development of okra seeds after anthesis have been studied. Much of the available information pertains to maize, pea and some other species. In maize the chief reserve component of the grain is endosperm. Endosperm development precedes embryo development.
Endosperm becomes solid after 15- 20 days of pollination and embryo attains full development after 45 days of pollination. Most of the seed weight comprises endosperm and in the mature seed the water content is low i.e. at maturity the total water content of the seed is less than 10%.
During grain development, nitrogen content increases in the endorsperm and embryo. In the endosperm the protein synthesis takes place in the two phases. The first phase occurs during peak period of endosperm formation and also involves rapid nucleic acid biosynthesis. The second phase of protein synthesis takes place after 35 days of pollination and is confined to both endosperm and embryo.
During the second phase there is spurt in the storage proteins. Most of the RNAs breakdown in other molecules. As the seed undergoes dehydration, the protein synthesizing machinery is broken down. Maize seeds also contain lot of starch and its maximal synthesis occurs during 20 days after pollination and there is decrease in the soluble sugars. Endosperm has very little amount of fat.
Taking data on metabolites in conjunction with enzymes it is clear that seed development is a highly complex and intricate process. Zygote contains all the information on the types and sequence of cells and tissues to be formed during seed development. The environmental factors facilitate the operation of such genes.
In the mature seeds there is loss of water. Maximum water loss is in the seed coat and in this layer protective layers are also synthesized. Endosperm and cotyledons also lose water and there is accompanied formation of carbohydrates, fats, proteins, phytin, etc.
Water is also very low in the embryo and its cytoplasm lacks vacuoles. This process helps the seeds to withstand low temperature, attack of fungi and other pathogens, avoid accumulation of toxins and decrease the respiration.
The vagaries of weather at the time of crop ripening have compelled the breeders to dry the seeds in order to lessen the moisture contents. These seeds can be stored at low temperature for a long time in a viable state. Dry seeds have embryo at a low metabolic state but remains alive.
Given appropriate conditions, it sprouts and produces seedling. The length of viability varies and is usually referred to as life span. The length of viability varies from a few days to several thousand years (e.g. Lupinus arcticus, viable for 10,000 years). Seeds of subtropical and tropical regions are viable for a short period.
Considering the economical importance of seeds, several seed laboratories and seed technology sections in institutions and universities have been established. In the laboratories concentrated studies on seed development, storage, germination, and viability are investigated. Methods on seed storage to maintain their viability have been developed.
Figure 24-1 shows transients in oil, protein and starch during different stages of peanut seed development. The seed development is studied from 5 days after podding to maturity. The expression of the storage material is on % dry weight and as mg per 100 seed basis. Note initial increase in starch followed by a decline. High protein content in the initial stages is followed by an abrupt decline.
Contrarily oil increases continuously. Mature seeds contain high oil, low starch and proteins, on DW basis.
Note # 19. Physiology of Fruit Development:
As the seeds are developing the surrounding tissue of the ovary also grows, matures and undergoes several physiological changes. To begin with, the ovary enlarges as in drupes or even the accessory tissues like thalamus (e.g., in apple) may also grow. By and large, most of the physiological studies have been confined to fleshy fruits, possibly because of their economical importance.
In a majority of cases, the growth curve of the fleshy fruits (e.g., tomato, apple, pear) is sigmoid or of double sigmoid type (e.g., grape, plum, apricot). In the latter type, there are two growth periods and these are separated by a period of lesser growth. Of the two growth periods, the second one is characterized by an active growth of endosperm and embryo.
The reasons for the period of lesser growth are not fully understood. Figure 19-1 shows two types of growth curves. One is S-shaped and the other shows a plateau (—— ) in size increases during cell enlargement phase. These growth curves are seen in plum, peach, etc. Grapes also show similar type of two- phase growth curves.
Various embryological studies have shown that fruit development in several species is associated with the pollen germination on the stigma and subsequent growth of the pollen tubes. In recent years, it has become increasingly evident that pollination was essential for the proper growth of the fruits.
Further, the developing seeds must be present to achieve normal fruit formation. Late Prof. J.P Nitsch, a French plant physiologist, performed interesting microsurgical experiments in strawberry.
In general, it is reasoned out that germinating pollen provides auxin to the pistil and in the latter its contents progressively increase. Moreover, young developing seeds are also rich in auxin. Nitsch demonstrated that the removal of seeds retarded the fruit development.
However, spraying of overies with auxin did overcome such a retardation. In case of parthenocarpic fruits, seeds do not develop but abound in immature ovules.
Here the ovary develops normally even though there is no pollination and/or fertilization. Parthenocarpy is widely known in grapes, banana, pineapple, etc. Several cucurbits and solanaceous fruits can be made parthenocarpic by excluding pollination and spraying with auxin or even GA3.
Figure 25-1 also shows different stages of fruit development e.g., initiation, anthesis, pollination, fertilization, growth and maturation. These stages are accompanied by cellular and metabolic activity. Also it is evident that early stages show cell division and the synthesis of new protoplasm and there is high synthesis of cytokinins and gibberelins.
Metabolites are translocated from the leaves, stem, root, etc. during the developmental phase. Beginning with zygote formation, there are two aspects of fruit development e.g. ‘pre-bloom’ and ‘post-bloom’. Fruit growth is measured in terms of volume, fresh weight, or dry weight.
Pre-bloom phase is characterized by the initiation of flower primordia while post-bloom occurs after anthesis and consists of pollination, fertilization, fruit growth and maturation. There is enormous amount of cell division and enlargement and cytokinins play a significant role.
However, cell division is immediately replaced by cell enlargement and IAA, GA contribute significantly in this phase.
In order a fruit attains the edible state, it passes through different stages e.g., cell divisions, cell enlargement, maturation and ripening. In the initial stages, there is rapid division of cells and this is followed by enlargement of each cell.
Then the cytoplasm of the cells shifts to the periphery and vacuoles thus created are filled with sugars. Subsequently there is increase in starch. At this stage fruit is mature and then the process of ripening begins. The last stage is senescence when the fruit becomes susceptible to physiological disorders and infections.
Two fruit types occur in abundance (berry and drupe) and they develop from a single ovary. In drupes (e.g., coconut, mango, etc.) endocarp becomes hard. Sometimes several ovaries of one flower develop into compound fruits e.g. in respberry, strawberry, etc.
In general several metabolites have been analysed in the developing fruits. In addition pigment and flavour changes have also been traced.
In the following a brief account is given:
(i) Carbohydrates and nitrogenous materials:
In almost all the young fruits, there is abundance of chloroplasts which help in the synthesis of sugars. These sugars are required for growth and development of fruits. In addition, much of the needed carbohydrates come from the leaves.
A similar situation prevails with regard to nitrogen compounds. The carbohydrates accumulated in the young fruit may act as substrate for respiration, produce organic acids or may participate in the biosynthesis of fats, starch or even cellulose. The sour taste of many young fruits is due to the abundance of citric acid and malic acid.
However, with the maturity of the fruit, their level decreases. Different fruits abound in different organic acids. The accumulation of organic acids may also be accomplished by other physiological mechanisms to be discussed elsewhere. Carbohydrates and nitrogen material is used in protein synthesis of fruits.
As the fruit continues to grow, there is active protein synthesis accompanying it. In most of the fleshy fruits starch content also goes up. During the ripening phase, there is a decrease in the starch level. The type of sugar(s) prevalent in a fruit varies with the species. In mature fruits of oranges, grapes, etc., organic acids decrease and sugars increase but this is not true of lemon.
(ii) Pigment changes:
In a maturing fruit, chloroplast is replaced by chromoplast and carotenoids. The colour of a mature fruit depends upon the types and level of carotenoids. When exposed to light, anthocyanins also change.
(iii) Flavouring compounds:
Basically, the flavouring compounds are aromatic esters, carboxyl compounds, alcohols, etc. The flavour of banana, however is due to amyl acetate. The smell of citrus fruits is due to various terpenoids, coumarins, etc. In any case, flavour and smell of fruits is an important commercial characteristic.
The chemical composition of mature fruits varies. Fruits like apple, dates, mango, bananas have abundant carbohydrates while those of olive, avocado store fats. In citrus fruits organic acids accumulate. These are citric acid, malic acid (apple) or tartaric acid (grapes). In general, fruits have low amount of proteins.
Note # 20. Aging and Senescence in Plants:
Like other living things, plants also grow old and die. Comparatively plants have long life and their life span varies from a few days to several thousand years. The process of aging in plants has not received much attention that it deserves.
However, one thing is amply clear that information has to be sought at the cellular and subcellular levels. Several workers equate aging and senescence for the same process. Aging is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant.
If the above definition of aging is taken as such this would imply all chemical and structural changes, which occur during the life span of a plant or its organs. At any given time a plant comprises living, dead and actively dividing cells. Therefore, it is difficult to study the process in whole plant. On the contrary the process is best studied in cotyledons, petals, leaves, etc.
With aging most of the metabolites like, proteins, amino acids and amides decrease. When studied at the molecular level, it has been observed that with aging some proteins decrease, others disappear, while still others persist. In other words, the pattern of proteins distribution in all likelihood reflects differences in metabolic activity.
Similarly tissues of different age may possess different enzymes. Differences in regard to nucleotides, carbohydrates, lipids, ions and phytohormones concentration may also be observed with again. The growing, young leaves act as a sink and receive metabolites from the older leaves. Such a transport system and its implications on aging are not well understood.
The contents of starch decrease rapidly in the isolated leaf and its ribosomes as well as chloroplast also degenerate. Instead large drops of lipids appear in the cytoplasm and vacuoles also degenerate. Gradually the cells become empty. During the last phases of senescence, the tonoplast and the cell membranes of the cytoplasmic compartments undergo lysis. These processes are observed in all groups of plants.
In the aging leaves, there is a loss of ATP synthesis and they also exhibit a decrease in the potency of chloroplasts. Consequently the amount of photosynthetic products also decrease and the carbohydrate catabolism is activated.
In some leaves undergoing senescence several hydrolases like β- glucosidase, α- galactosidase, cellobiase increase. The decomposing products in the senescening leaf are transported to the young leaf. In tobacco leaves, the activity of β-1, 3-glucanase is several fold more than that in non-senescing leaves. On the other side several proteolytic enzymes increase.
Using antimetabolites it has been demonstrated that the increase is due to de novo synthesis of the enzymes. Senescence is accompanied by decrease in the specific proteins depending upon the age and the specific tissue. The decrease in the proteins is species specific and is due to the reduced system for protein synthesis.
Similarly activity of RNase also goes up and thus there is a decrease in the RNA content of senescing leaves. Indeed several nuclease increases in the senescing tissues and these are purine specific, phosphate specific, sugar unspecific nucleases. In old plant tissues these enzymes are synthesized at a very high rate.
The synthesis of nucleases is stimulated by the accumulation of ATP. Recent studies have shown that there is very little change in RNA composition produced in the senescing tissue e.g. in barley leaves. In the leaves nearing senescence, DNA degenerates completely. Evidently DNase is synthesised newly and in abundance.
In spinach leaves undergoing senescence, several nucleotides are incorporated into DNA without de novo synthesis. Such an incorporation could be due to a repair mechanism needed for the broken DNA strands. In some species, even chromatin has been shown to change its properties. This may be taken to mean that chromatin material decreases its transcriptional activity in the senescing cells.
Besides other factors, light plays a very vital role in controlling senescence. Further this light effect is also mediated by phytochrome. In Anagalis arvensis SD conditions maintained a high RNA content, rapid growth and profuse root formation.
On the contrary, LD conditions inhibit these parameters and quicken aging. In Marchantia thaili subjected to one hour white light had reduced senescence. The same effect could be caused by 5 min exposure to RL per day. Further the white light effect could be reversed by FR. In the aging plants role of light in decreasing senescence becomes less pronounced.
Several growth hormones affect senescence. In general, cytokinins retard it. This endogenous factor reduces nuclease and also slows down the degeneration of chloroplasts. In the senescing tissues there is a general decrease in the cytokinin level. On the contrary, ethylene and ABA promote senescence.
When the endogenous level of ABA was measured in the rose varieties, those with short-lived petals had the highest amount of ABA than with long-lived petals. In the cut flowers, the amount of ethylene increase with aging. Similarly the peel of citrus contains several neutral inhibitors besides ABA. As the age of the citrus fruit increases the level of ABA also goes up.
Further, the presence of ethylene promotes the level of ABA in the senescing tissues. The increase in ABA, respiration, ethylene increase in the senescing tissues cause shifts in the metabolic pathways especially photosynthesis. Protein synthesis and synthesis of endogenous factors like GA. ABA has also been shown to increase the level of nucleases.
Recent investigations have revealed that some of the amino acids like glycine, alanine, serine, etc. promote aging whereas arginine inhibits senescence. It is assumed that two amino acids in all probability activate operons for the two proteases.
Senescence is also known to be promoted by some sugars e.g. glucose while fructose and sucrose act in a lesser degree. These carbohydrates do not act osmotically and their precise mode of action is not well understood.
Several of the synthetic growth-regulating substances also cause senescence. These are morphactins, α-NAA, etc.
Electron microscopic studies of the cells undergoing senescence have revealed that most of the organelles disrupt quite early in senescence. Mitochondria, chloroplasts and ribosomes are very sensitive and are most affected. Treatment of leaves with kinetin prolongs their disorganization.
Several theories have been advanced regarding aging and senescence. In general they are included under four categories.
A brief account of these is given below:
(i) Toxic substances. One assumption is that with the onset of senescence there is accumulation of toxic and deleterious substance in the cells.
(ii) Loss of essential metabolites. It is also viewed that senescence causes gradual depletion of essential metabolites needed for the plant growth and metabolism.
(iii) Wear and Tear. With the beginning of senescence there is loss of activity and, therefore, cells undergo wear and tear due to disintegration of organelles.
(iv) Genetic damage. Some individuals accumulate mutated genes which cause deleterious effects at specific age of the organ/plant.
None of the above hypothesis is fully accepted and is with pitfalls. Several attempts to isolate toxic substances during senescence have produced information on secondary products but these do not induce senescence.
During growth and development plant parts leach several substances which are inorganic and organic molecules. Some workers believe there is something like ‘juvenility factor’ which keeps the plant in the vegetative state.
In summary it may be stated that not one but several factors may be causing or senescence.
Note # 21. Physiology of Plant Biotechnology:
The discipline of biotechnolgy employs divergent disciplines e.g. biological, chemical and engineering to develop the use of biological organisms in agriculture and industrial processes. In fact living organisms are used to produce substances for human consumption.
The subject of biotechnology has been in vogue for hundreds of years and production of beer, wine, cheese, and bread was accomplished through the activity of microorganisms.
By the end of 19th century chemicals like acetone, acetic acid and ethanol were produced through microbial fermentation. Around middle of 20th century microorganisms were employed for solid waste treatment including sewage.
However, by and large microorganisms were used for the preparation of foods. The advent of agriculture rested on several principles of plant breeding and various plant species were raised through selection, hybridization or both.
The process of crop breeding is time consuming and laborious and raising of varieties requires nearly a decade or so. Then came the advent of molecular biology and advances in DNA technology which led to the isolation and transference of gene between widely divergent organisms, creating new combinations of characters which was impossible in nature.
Initially developed in lab, rDNA is being translated in reality under field conditions, and new technology is being exploited increasingly by industry, where the economic potential is enormous. The role of plant physiologist is clear and immense.
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Be it improving the yield traits, enhance resistance against pests or herbicides, increase micro-propagation or produce pharmaceuticals and several other secondary metabolites from plant cell culture. Plant biotechnology involves the manipulation of biochemistry, physiology, and pattern of development.
We shall briefly describe the followings:
i. Tissue culture, protoplast fusion and recombinant DNA (rDNA) as basic techniques pertinent to the revolution and exploitation of plant biotechnology
ii. Selected instances of plant biotechnology that demonstrate the role and potential for plant physiology contributing to the biotechnology revolution.
Note # 22. Relationship of Plant Physiology to Other Sciences:
Since physiology deals with the functioning of organisms, it involves the chemical and physical changes associated with the living organisms and which can be explained on the basis of known physical and chemical laws and principles.
Hence the knowledge of physics and chemistry is essential for the understanding of physiological processes. Physics is concerned with the various manifestations of matter whereas chemistry covers the manifestations involving the exchange or sharing of electrons, that is, chemical reactions.
Biology is, therefore, more of complexity in the living system having originated out of physical and chemical manifestation and, therefore, the terms Biophysics and Biochemistry are now becoming increasingly important.
The understanding of the organic chemistry will help in the isolation and identification of the chemical compound occurring in plants and that of the plant biochemistry in identifying the chemical transformations carried out within the plant.
It is fashionable to link a physiological process with the gene operation. In recent years genetical concepts in physiology have taken a significant place. The science of genetics is concerned with the outward manifestation of the genes, and the geneticist unfolds the mechanisms of inheritance and gene action and in this respect, he completely overlaps the interest of the physiologists.
Environments play a key role in the expression of the physiological behaviour of plants beginning with seed germination to the completion of the life cycle. The environment triggered responses of physiological events are, therefore, many and each one of them has a specific link with the regulation of endogenous levels of growth hormones or growth regulators. Ecology is a discipline which deals with the study of interrelationship of organisms and environment occupies a significant place in the field of physiology.
Most scientists include it in a separate category as environmental physiology but it is not an appropriate division as all the physiological regulations are environment triggered. The most complex ecological organizations, the ecosystems, do not primarily refer to the structural adaptations and reactions to environments but most significantly to the functions of the ecosystems.
However, besides several disciplines of science involved in the understanding of physiological events, there are several other branches of botany like systematics, morphology, anatomy and cytology which need to be studied. Plant physiology, on the other hand, contributes to the descriptive plant sciences by aiding in the interpretation of form of plant parts in terms of functions.
Traditionally, plant physiology centred around characterizing physiological processes as some type of response of a plant. This is linked with a specific metabolic process controlled by specific enzymes or proteins. Recently many attempts are being made to characterize the genetic basis of the metabolic processes that control physiological phenomena.
More recently, it has been feasible to study the regulation of expression of specific genes. Thus, basic concepts in plant physiology have helped in unlimited replications of plants (tissue culture) and to study, physiological phenomena by identifying the genetic regulatory mechanism (Molecular Genetics).
