Plant Physiology: Study Notes

The below mentioned article provides an introduction on the plant physiology.

Understanding of the processes by which plants keep alive and grow is the aim of plant physiology.

Thus plant physiology deals with the life processes of the vegetable organism. Any chemical or physical change occurring within a living plant cell or any interaction between the organism and its environment would be considered as a physiological process.

Such classifications of the plant and its activity into morphology, histology, physiology, ecology, etc., followed in text-books are rather arbitrary and are adopted for mere convenience. In fact, all of them are interrelated. Thus plant physiology cannot be studied separately without consideration of the other aspects of living organism as a whole.

The study of the life process of a plant evidently pre-supposes a thorough under­standing of the structures of the organisms, the environment in which it lives, its here­ditary make-up, etc. Structures and functions are so intimately related that it is impossible to separate one from the other.

A plant grows—so the processes concerned in its growth are certainly dynamic. The problems in plant physiology are concerned with how’s and- why’s and what’s. What is the mechanism of absorption of water, gases and solutes by the plant from its surroundings?

Why does a plant flower only at certain periods in its life cycle or under certain environmental conditions? How do the plants prepare their own food? The explanations of these and many such questions and interrelationship of one with the other give us an insight into the co-ordinated growth of the plant as a whole.

From the earliest times to the eighteenth and nineteenth centuries, and even up .to comparatively recent times opinion was almost universal and unanimous that living organisms possess and operate through some intangible and unknown forces which distinguished them from the non-living.

There are, however, few advocates of such ‘vitalistic’ theories at the present time. The most widely held assumption now is that the living organisms, whether plants or animals, also operate following the same physico- chemical principles that are true in the non-living world.

The behaviour of the living organism does not differ in many respects from the non-living bodies. Weight, rigidity, elasticity, conductivity of heat and electricity, etc., are certainly the properties of a solid non-living body and the plants and animals in spite of large amounts of water in them, are as a rule solid and show the same properties of a solid body.

What then distinguishes the living from the non-living and can we attempt at a definition of life? Is it the capacity for reproducing its kind accompanied by relatively slow and small transformations of energy which is the biochemical criterion of life? A candle flame may be said to be ‘living’—it has got a shape or form, it can reproduce its kind—you can get one flame from the other.

The transformation of energy in such a flame is, however, too rapid and if such quick transformations of energy occurred in the living body, the whole thing would go into a flame just like the candle.

The virus nucleoprotein which is responsible for many animal and plant diseases, shows one of the fundamental properties of the living body in that it can be made to reproduce its kind when it thrives in the host body (virus restricts the normal activity of the host cells and compels those cells to produce more of virus particles-—virus DNA or RNA nucleo­protein is incapable of dividing by itself outside a host cell).

But the virus particles do not show even the minutest trace of transformation of energy to maintain their existence and self-duplication (they do not respire) which is an important manifestation of life from a purely biochemical point of view.

Moreover, virus protein can be isolated and crystallised in the pure State; so where does virus protein take its position in the ‘ladder of life’—from the inanimate to the animate world? Certainly virus protein and the other nucleoproteins in the chromosomes of the nucleus, e.g., genes, are the missing links in the biochemical evolution of life from the inorganic to the living organic world.

Can we then not attempt at a definition of life in the light of the great advancement of know­ledge in physical sciences during the last half of a century? We prefer the definition of life as given by John Perret as consistent and quite in keeping with the trend of development of modern science.

Perret’s definition of life as “a potentially self-perpetuating open system of linked organic reactions, catalysed stepwise and almost isothermally by complex and specific organic catalysts which are themselves produced by the system” certainly satisfies to a large extent the phenomenon and characteristics of a living organism.

It can be further clarified if we learn to recognise that the laws of non-living matter merge continuously and perhaps smoothly in those governing the living. Potentially self- perpetuating—it is always capable of reproducing its kind—it may not be doing so all the time and thus perpetuate itself.

The most important parts in the definition are the linked system of reactions and the specific enzymes. The linked system as such obeys the laws of chemistry, but it is probable that the system of reactions having a dynamic stability may adjust themselves to small but limited changes.

The reactions are catalysed step by step by enzymes almost isothermally, i.e., without any large absorption or libera­tion of energy as heat; thus the temperature of the medium or the body in which the reactions take place does not’ show any large or rapid fluctuations in temperature as long as the reactions are taking place, i.e., as long as the organism is said to be living.

Once the catalysts or enzymes have been produced by the system of reactions itself, the chemical laws governing the activity of enzymes take on the aspect of a biological law of adaptation, i.e., the living organism can adapt itself to a change in the surrounding environment in which it lives.

It is also true that the enzymes, to be effective agents in the linked system of re­actions, must have physical characteristics such as size and coherence between the molecules due to opposite electric charges, which is only possible in very large or macro- molecules of polymers (compounds formed from polymerisation of smaller molecules).

Among such polymers, the large and complex molecules of proteins formed by peptide linkage of comparatively smaller molecular units of amino acids occupy a pre-eminent position. We know, however, that many organic molecules exist in both left-handed and right-handed forms (d and I forms), yet most of these molecules are found in only one of these forms in living organisms.

The proteins certainly represent, among others, any easily built-up set of molecules having just the properties needed to open the way for an evolving metabolism in contrast to a metabolism of a low order of efficiency, being merely repetitive. No wonder then that the viruses and genes which occupy a hypo­thetical position between living and non-living, are largely proteinaceous.

The first organic molecule to achieve characteristics of living must have been a protein. And what can be said with assurance is that there is actually a unique and nearly ubiquitous compound, with the empirical formula, H₂₉₆₀O₁₄₈₀C₁₄₈₀N₁₆P₁.₈S  called living matter!!  The moment the protein molecule was formed, the chain of reactions leading to the evolution of life, as we know, was surely and steadily moving towards its ultimate goal.

The presence of mostly one form of organic molecules, either d or I and the remarkable basic similarity among all forms of life, however, support a strong presumption that only once did a living molecule originate from non-living matter.

Again, all amino acids, the alphabet of protein structure which is ultimately responsible for the specificity of variability of living matter derived from proteins, are a-amino acids (except glycine which is not optically active).

This fact strongly suggests the ‘common origin of all living matter on the earth. And for life to originate, an atmos­phere without oxygen is necessary. Further spontaneous regeneration of life seems impossible once the green plants evolved which put oxygen into the atmosphere.

It is true that the complexity of many of the physiological processes taking place in the living world sometimes eludes our grasp but that should not be the reason to endow living organisms with some intangible and incomprehensible varieties of energy.

Attempts should be made to explain them by the easily and truly recognisable physico- chemical laws based on experimental evidence and logical deductions. A knowledge of fundamental principles of physics and chemistry is, therefore, absolutely essential for a clear understanding of the physiological processes and this cannot be overstressed.

It is impossible to learn even the rudiments of plant physiology without being familiar with the underlying principles of physical sciences and organic chemistry.

With unimportant exceptions, it is indisputable that the basis of all life on the earth is photosynthesis by green plants, a process that involves both physics (in the fixation of solar energy) and chemistry (in the union of CO₂ and water to form carbohydrates and more complex biochemical compounds).

The relation of plant physiology to agricultural sciences is intimate and profound. The plant world not only supplies us with all our food requirements but also supplies the raw materials for many of our basic industries.

An industrial civilisation such as ours not only requires a wide variety of plant products but also naturally demands and expects certain standards of quality. In order to maintain or to improve this standard, cultiva­tion of plants has become a skilled occupation requiring a thorough understanding of the physiological processes taking place in plants.

The application of the principles of plant physiology is certainly helping to solve the various problems with which the ordinary farmers, the fruit grower, the horticulturist, the cotton and tea planters, the foresters and all others who cultivate plants are confronted daily.

The foundations of plant physiology may be said to have been laid with the publi­cation in 1727 of Stephen Hales’ ‘Vegetable Staticks’. Long before that, however, many books on agricultural practices employed have been published and there was an exten­sive literature on agriculture in Roman times.

Man’s conscious interest in plants must have begun long before any recorded history and agriculture that is the cultivation of economically important plants for food and clothing was a highly developed art ever since man begun living in social groups and started his long progressive march towards the control of the environment around him.

The books and learned treatises published on agricultural practices mostly of course contained speculations based on observations through ages by men close to the soil but without any critical experimental evidence. Some of these ingenuous speculations have stood the test of time while others have been proved to be entirely erroneous.

Thus Palissy’s speculation in 1563 on the agricultural practices employed in Italy at that time about the importance of burning unused wheat straws in the field when growing this crop for second year in succession was fundamentally correct and must have inspired other investigators who came later in the field on the realisation of the importance of manures to crop yield.

In the theories propounded by Van Helmont (1577) and great Francis Bacon water was considered as the sole nutrient for plants; the purpose of grow­ing them in soil was only to keep them upright and protect them from excessive cold of heat! This, of course, as we all know, is largely false.

Plant physiology as a science made very little progress between the time of the publication of ‘Vegetable Staticks’ in 1727 and the announcement of discovery of photosynthesis by Joseph Priestley in 1781. What was the reason of so little interest in plant physiology shown by the naturalists between 1727-81, a period of about 50 years?

It was certainly to a large extent due to the pio­neering work on classification of plants by the great Linnaeus. Naturalists of that period were inspired by the work done by Linnaeus (his rather pompous statement, “God created plants and Linnaeus classified them” was illuminating showing the spirit of the man and of the age in which he lived) and almost all of them were busy in identifying, checking and classifying plants according to the new system of classification based on binomial nomenclature.

The advent of plant physiology as a science certainly coincided with the great advance the pneumochemists made in the latter half of the eighteenth century—rightly called the ‘age of enlightenment’—in the discovery of various gases and in the deter­mination of the composition of air.

The names of Joseph Priestley who discovered oxygen and photosynthesis in green plants, Jan Ingen-Housz, Senebier, de Saussure, Boissingault, Liebig among others must be honoured as the founders of modern plant physiology.

By careful experiments, precise reasoning and an innate capacity of grasping the truth, these pioneer plant physiologists have left a blazing trail for future investigators in this field, as in all others, to expand and enrich the realm of human knowledge.