Classification of Various Plant Movements

In this article we will discuss about the classification of various plant movements.

1. Autonomic Movements:

These movements are due to internal causes. For instance, the organism and reproductive cells move as a whole from place to place. Some of the common examples are ciliary movements.

2. Ciliary Movements:

The cilium is a widely occurring organelle of eukaryotic plant cells.

A cilium consists of two main interrelated and interacting parts:

(i) The axoneme, an assemblage of many tens of different polypeptide chains produced under the control of chromosomal genes and

(ii) The ciliary membrane. Cilia and eukaryotic flagella are identical organelles and both are found among plants. The plant cilia are powered by a sliding microtubule mechanism where ATP utilisation takes place. Ciliary movements are important in mobility or zoospores, mobile gametes and mobile algae.

3. Gliding Movements:

The term ‘gliding movement’ is used to describe active displacement of an organism in contact with a solid substrate, where there is no visible structure responsible for the movement and no change in the shape of the organism. The gliding mobility is readily exemplified in the genus Oscillatoria where most healthy trichomes display separately smooth and sustained rates of locomotion.

The mechanisms responsible for such mobility have been proposed as: osmotic forces; surface tension activities; slime secretion and contractible waves. The ability to glide allows the organism to change location in a habitat, and thus is of distinct survival value.

4. Amoeboid Movements:

These are seen in plasmodia of Myxomycetes which are naked masses of protoplasm. They move over the surface of the substratum by putting out pseudopodia. Recent studies have indicated that amoeboidal movement is ultimately dependent upon contraction of actomyosin which are probably controlled by the cell (organism) through the regulation of localized intracellular concentrations of calcium ions.

5. Cytoplasmic Streaming and Cyclosis:

Cytoplasmic streaming is known in great variety of cells of living organisms. In plant cells, different types of streaming, for example rotation, circulation and agitation are known. Rotation is the typical streaming pattern of the cells with a large central vacuole. Rotational streaming or cyclosis is massive flow of cytoplasm around the periphery of the cell.

The simplest example is the root hair, which does not contain chloroplast, here all the fluid endoplasm flows along the cell walls. In green cells, however, two types of streaming can be distinguished. In internodal cells of Characean algae the chloroplasts remain stationary in the rigid ectoplasm, whereas fluid endoplasm flows along the ectoplasmic region.

In leaf cells of Elodea or Vallisneria, in contrast the chloroplasts participate in the cytoplasmic streaming. However, molecular mechanism of movement probably is the same in all cases. The use of some specific inhibitors has shown that actomycin-like-proteins are involved in the movement mechanism.

In cyclosis the chloroplasts are moved passively by the streaming endoplasm, like stones in a river. Cytoplasmic streaming depends upon the supply of oxygen and streaming can be inhibited by uncouplers of oxidative phosphorylation such as dinitrophenol and azide.

Changes in the composition or concentration of ions in the external medium or in the cell sap, could modify the ionic conditions in cytoplasm and could effect the cytoplasmic streaming. Changes in the ionic gradient across the cytoplasm streaming. Changes in the ionic gradient across the cytoplasm are also involved in the effect of action potential on cytoplasmic streaming.

Different external factors affect the rate of cytoplasmic streaming. The rate increases with increasing temperature between 5° and 30°C. Light is known to stimulate cytoplasmic streaming and to cause induction of cyclosis of chloroplast. The orientation movement, as is known brings chloroplasts into a position in the cell that is assumed to allow optimal light absorption and optimal CO₂ fixation under changing external light conditions.

Physarum is another interesting case of cytoplasmic streaming since it shows unique streaming during its Plasmodium stage. The contents inside stream with great speed (i.e. 1.5 mm s-1). This is a cellular slime-mold. The direction of cytoplasmic streaming reverses periodically every few minutes in a shuttle or pendulum way.

This slime-mold contains proteins actin and myosin. These are similar to muscles analogs in physico-chemical features and also there is formation of functional actomyosin hybrids between Physarum and muscle proteins. In slime-mold these two components do not form orderly structures but can contract.

Actomycin seems to provide the motive force for shuttle streaming. On the other hand, oligomeric forms of myosin possibly had the capability of generating the sliding of actin filaments that would lead to contraction.

This slime-mold also contains proteins which are similar to tropomyosin-troponin complex of skeletal muscle. It is well known that in muscle, Ca²⁺ regulates the interaction of actin with myosin through the tropomyosin-troponin system. In Physarum also Ca²⁺ has been demonstrated to be required for shuttle streaming and contraction. In this slime-mold there is a vesicle system present which supplies Ca²⁺.

Apparently the regulation of streaming through a Ca- sensitive tropomyosin-troponin-like mechanism in this organism seems a great possibility. Like the non-muscle contractile systems, action in slime molds also appears to transform readily between the monomeric and polymeric forms.

Further polymerized action is essential for the activity of myosin and the polymerization state of actin also influences cytoplasmic viscosity and the differentiation of cytoplasm into endoplasm and ectoplasm. Actin polymerization also seems to be influenced by ATP, 5’—AMP and other factors either directly or through actin-binding proteins like β-actin-like protein.

In comparison to the cytoplasmic movement which has been described in the organisms lacking cell well, streaming movement in plant cells with rigid cell wall has also been reported. In such a system different types of streaming (e.g., rotation, circulation, agitation, etc.) are reported.

In root hair (which lack chloroplast), endoplasm flows along the cell walls and in green cells two types of streaming are found : endoplasm moves but chloroplasts remain stable and in some plants (e.g., Elodea) chloroplasts move with the cytoplasm. Though the two patterns differ yet they have common features and their molecular mechanism also may be the same.

6. Movement of Chloroplasts and Nucleus:

The plant cell organelles are capable of random movements within the cell and also of directed movements in response to internal and external stimuli. These movements are important in the cell function and some of these are: chromosome movements and vesicle translocation during endo- and exo-cytosis.

Light intensity and direction both affect the intracellular distribution of chloroplasts in many cell types. The mechanism of movement is considered in terms of a photoreceptor-effector system assumed to comprise a method of sensing light direction and intensity, and actual movement system to change chloroplast distribution, and a transducing mechanism capable of regulating (Fig. 21-1) the movement system.

The relation between chloroplasts distribution and photosynthesis suggests that the displacement helps regulate light absorption in the cell. Since photosynthesis would reach the saturation point under strong light conditions for extra absorbed light serves no purpose and may have damaging effects on the cell.

If there occurs a chloroplast arrangement which decreases absorption it would serve an adaptive function (Fig. 21 -2). However, it has been calculated that the actual changes in light absorption due to chloroplast migration may be only as large as 20%. Thus the resultant small absorption changes are of doubtful adaptive significance.

There are some reports which show that at light intensities limiting for photosynthesis, large increases in photosynthesis were observed as the chloroplasts migrated to favourable position. However, the inverse in photosynthesis rate is much larger than can be explained by light absorption increase.

In summary, it can only be said that light absorbed by profile position of chloroplasts is utilised less efficiently than light absorbed by phase position chloroplasts. In case of these light induced chloroplast migrations (Fig. 21-2), the photosynthetic pigments, phytochromeor a presumed flavine pigment have been identified as photoreceptors.

A single plant cell may have several photoreceptors and even a single physiological response can be mediated by more than one photoreceptor.

In general, however, most chloroplast movement responses are sensitive to blue and UV light and thus seem to make use of presumed flavine photoreceptor. The flavinephotoreceptor and phytochrome photoreceptors responsible for chloroplast movement are not located on the chloroplast.

The external gradient created by the photoreceptor pigments can somehow alter the chloroplast distribution. The microfibrilactomycin system appears to be important in the light induced chloroplast migration responses. About the transduction mechanism very little is known. In addition to chloroplast movement, there are several examples of nuclear migration in plant cell development.

There are many examples of this type of movement. Formation of root hair and guard mother cells are preceded by an asymmetric division which involves nuclear migration. Another type of nuclear movement is post-mitotic nuclear migration.

This movement has been reported to be inhibited by colchicine. The microtubules are supposed to be involved in this movement process but very little is known about the detailed mechanisms.

7. Chemotaxis:

These movements are exhibited by bacteria and motile gametes. Bacteria show positive chemotaxis when they swim towards the source of food. They may show negative chemotaxis when they move away from the toxic compounds. The spermatozoids of many forms are attracted towards malic acid which is released in the archegonia.

This chemosensitive behaviour is widespread among bacteria and both multicellular and unicellular eukaryotes. Chemotaxis has both immediate and long term adaptive consequences. Chemotactic responses have been demonstrated in a wide variety of bacterial genera such as Bacillus, Escherichia, Proteus, Pseudomonas, Salmonella and Streptococcus. For any given species, the responses are specific to certain chemicals, but there is considerable variation between bacteria in different genera.

For example, phenol is a repellent to Salmonella but not to Escherichia. Regarding the chemical nature of chemotactic agents in bacteria the principle classes’ are sugars and amino acids. cAMP and other nucleotides have been found ineffective as chemotactic agents of E. coli. The role of the chemotactic agents is confined to the binding to a surface receptor.

It does not have to be either transported or metabolised to provide the stimulus. These chemotactic agents operate through gradient sensing mechanism. The chemo-receptors have been shown to be periplasmic binding proteins, associated with the outer surface of membrane and contained by the cell wall.

Among the eukaryotes the most extensively studied chemotactic response is in Chlamydomonas. Again in this case, the nature of receptor is proteinaceous.

8. Thermotaxis:

These movements are seen in flagellated algae e.g. Chlamydomonas and Volvox. If a beaker containing Chlamydomonas in cold water is gently warmed from one side, the algae will be seen to move towards the wann side. This is positive thermotaxis. However, with the increased temperature, the algae swim away from that side and show negative thermotaxis.

Similar types of thermotactic responses have been shown in case of bacteria especially in Spirillum and E. coli. The effects of temperature on mobility are complex, but in some way, they combine to give migration in thermal gradients, response which may well have evolved to ensure migration to a favourable temperature for growth.

The most well studied case of thermotaxis is slime mold Dictyosteliumdiscoideum. Some authors have suggested that a specific “Biothermometer” is active in such organisms and adaptation is then the ability of the organism to modify its biothermometer dependent upon the temperature of growth. The molecular basis of such proposed thermoreceptor is still uncertain.

9. Movements of Curvature:

The curvature movements are displayed by the organs of rooted plants. The movements may be due to growth when they are caused by unequal permanent growth on the different sides of the plant organ. The fast growing side becomes convex, while the slow growing side becomes concave.

Consequently, the organ tends to curve to the side where the growth is slower. These movements occur only in radial plant organs which are still growing. The curvature movement may also be caused by the changes in turgor on two sides of a plant organ. The curvature movements are of two types, e.g., autonomic and paratonic.

10. Autonomic:

These are of 3 types and in the following their brief account is given:

Nutation:

The nutation of stem apices is an autonomic movement. A growing stem tip does not remain in an absolutely vertical position but bends alternatively. Thus it exhibits a kind of nodding movements in the Iwo directions. This is because of greater growth of stem apex at one time on one side and a little later the growth passes to the opposite side.

This type of movement is called nutation. If the region of greater growth passes gradually around the growing point instead of alternating in two opposite directions, so that the stem apex moves spirally, as it elongates, the movement is called circumnutation. This movement is more marked in twining plants in which it greatly increases the chances of the shoot, coming in contact with a suitable support.

Epinasty and hyponasty are two terms used to denote, respectively, the more rapid growth of the upper and lower sides of an organ. These are nastic movements, found in leaves, buds, petals, etc. If the upper (inner) surfaces of leaves in a bud grow more than the lower surface (epinasty), the bud opens.

If the growth of the lower (outer) surface is more (hyponasty), the bud remains closed. The fronds of a fern are circinately coiled up in the bud and unfold and straighten as the upper side (inner side of the coil frond) grows more rapidly than the lower side.

Autonomic Curvature Movement of Variation:

In Desmodiumgyrans, the telegraph plant, there is a ternate compound leaf. The terminal leaflet is large and the two lateral leaflets are relatively small. The lateral leaflets show peculiar rhythmic movement during the day time. They move up and then back and then move down on the lower side and then back again to the original position. Each movement which is often jerky and is completed in two minutes.

The paratonic movements are either growth or movements of variation. The movements are of two types Tropisms produced by directive stimuli that affect the whole plant uniformly and non-directional which do not determine the direction of the response, e.g., changes in humidity of the medium, intensity of illumination, etc.

Tropic Movements (Tropisms):

These are chiefly of the following types:

a. Phototropism:

When a growing plant is illuminated by a light from one side, it bends towards the source of light (Fig. 21-3). The bending of the plant is caused by cells elongating on the shaded side at a much greater rate than the cells on the illuminated side.

This differential growth response of the plants to light is called phototropism. Phototropism is attributed to unequal distribution of auxins; the higher concentration of the hormone being on the shaded side. This unequal distribution of auxin could be accomplished by one of the three ways, e.g., light-induced inactivation of auxin, lateral transport of auxin or inhibition of basipetal transport of auxin (Fig. 21 -4).

Phototropism involves the change in growth direction of the organ, caused by the redistribution of growth; in most cases as curvature develops the convex side elongates more rapidly than the concave.

Phototropism includes only those bending movements associated with growth; the synthesis of new cellular material: phototropism includes movements towards the light source (positive) and away from it (negative). Phototropism is based on a change in the spatial distribution of growth, but this change may occur in different ways. This phenomenon is observed in both unicellular and multicellular organisms.

It is observed that two such diverse systems have Phycomycessporangiophores and Avena coleoptiles have very nearly identical action spectra. This detailed correspondence strongly suggests that both organisms have the same phototropic photopigment and this is probably a flavine. There is no sufficient evidence regarding the role of phytochrome in phototropism.

Phototropism of coleoptiles and perhaps some other multicellular organs as well can be described in the terms of Cholodny-Went theory. This theory holds that the differential growth which underlies phototropic curvature is caused by the establishment of a lateral concentration gradient of a growth hormone or auxin, across the coleoptile.

The light intensity gradient across the apex lead, in some unspecified way through the establishment of such an auxin gradient. Recent experiments have shown that the auxin gradient is established by the transport of mechanism rather than mechanisms of auxin synthesis or destruction.

When unilateral light stimulus is given to corn coleoptiles and subsequently after stimulation ¹⁴C- IAA is applied still the distribution of ¹⁴C-IAA is differential in illuminated and shaded sides, respectively. This shows that light clearly does not act on the applied IAA itself but rather sets up in the tissue and asymmetry or polarity, which subsequently moves the IAA laterally.

Regarding the effect of light on polar auxin transport it may be added that if the transport of auxin through a tissue such as coleoptile is polarized, it is due to a polarity within the individual cells that makes up this tissue. The mechanisms by which light intensity gradient alters such transport are basically mechanisms by which the polarity of each cell is altered.

Experiments involving the direct measurement of the distribution of radioactivity in Avena coleoptiles to the apices of which ¹⁴C-IAA had been applied revealed that there is inhibition of longitudinal IAA transport relative to dark controls.

The mechanism of this photoinhibition is not clear although one theory attributes this inhibition to the formation of an IAA-oxidation product 3-metphylene-oxo-indole (3-M) by the action of light on a flavine sensitizer. It was observed that 3-M enhances the uptake of IAA by coleoptile sections but inhibits its release.

(In view of this it has been suggested that 3-M acts by inhibition of the enzymatic action of IAA from the cell membrane). The above theory of photoinhibition of auxin transport is based on experiments on corn coleoptile but the results of experiments with Avena do not bear out the contention that the inhibition is due to the formation of 3-M. Clearly, further experiments are required to get a generalised picture.

In some phototropically active fungi where action spectra for different responses are identical to that for phototropism, a flavin attached to a protein (flavoprotem) appears to be the sole pigment involved. Following absorption of light, the flavoproteins become oxidized by reducing a type cytochrome in the plasma membrane.

However, no information is available on the regulation of auxin migration towards the shaded side of a coleoptile tip by the photoreceptor. Recent studies have shown that light does not destroy auxin. It has been demonstrated that full sunlight is clearly not essential.

When exposed to as little as 10⁻¹¹ einsteins of blue light per cm² of tip area, marked bending of coleoptile was noticed. In fact there are first positive and second positive responses. Apparently, phototropism is due to second positive responses. Experiments conducted during the past few years have demonstrated that in dicot stems and coleoptiles, the primary cause of phototropism is attributed to auxin migration in response to blue light.

Further young leaves and rarely mature leaves also, are the source of auxin and, therefore, control the phototropism. In a young seedling the curvature of the hypocotyl is controlled by light absorbed by the cotyledons. In some instances the light absorbed by the stem itself may control its curvature. In summary, it may be stated that leaves must be fully exposed to maximum sunlight to avoid formation of leaf mosaics.

In brief, compared with the scheme of Cholodny-Went which emphasized the unequal or asymmetric distribution of auxin on the two surfaces, the recent concept centers around the lateral transport of auxin.

Earlier asymmetric distribution of auxin was attributed to three factors: photo-destruction of auxin on the illuminated side, increased synthesis of auxin on the dark side, and lateral transport of auxin from the light side to the dark side.

The recent studies have brought out the exact location of the photoreceptors. Some investigators also suggest that the geotropic and phototropic stimuli are mediated by the same mechanism. However, the explanation may not be that simple as suggested.

b. Geotropism:

Growth movements in response to the gravitational pull caused by earth are instances of Gravitropism. These movements may occur towards or away from the gravity. Accordingly they are referred to as positively gravitropic and negatively gravitropic respectively. While stems and flowers are negatively gravitropic, the roots are positively gravitropic.

Primary and secondary roots vary in their response to earth’s gravity. Such an adaptation helps them to exploit the soil resources more efficiently. Branches of a stem, stolons, rhizomes are plagiogravitropic in response to gravity since they grow horizontally.

In comparison, both stem and roots are vertically oriented and exhibit orthogravitropism. Any hypothesis must account for gravitational stimulus causing geocurvature, and also for the differential behaviour of root and shoot. Apparently, gravity must be affecting an organ by causing movement in one of its component parts.

It has been suggested that there are particles called statoliths which respond to gravity by shifting to the lower side and it is their movement which results in causing gravitropism (Fig. 21-4A, B). Amyloplasts are implicated to play an important role and move to the bottom of the cells experiencing gravity.

Cells depleted of starch exhibit very slow gravitropism. The area of detection of gravity and response are several layers apart. In a coleoptile kept horizontally, the tip is mainly responsible for the detection of gravity while the cells below respond by differential growth. Thus, upward curvature is caused. In the cells of the two zones, amyloplasts show specific pattern of settling on the bottom of the cells.

In a root the main area of detecting gravity is the root cap while the curvature is caused in the elongating cells behind the root cap. In root where the root cap is removed, the curvature is absent. However, with the regeneration of a new root cap within a few days, the curvature is restored. It may be added that root cap abounds in amyloplasts.

In dicot stems, gravity is detected near the apex and in some cells which are rich in amyloplasts. These cells form a continuous layer around the vascular bundles and referred to as starch sheath.

The curvature is caused in the elongating parenchyma cells of the cortex external to the starch sheath. Similarly meristematic cells of shoot tips and also the young leaves are essential for gravitropism. These tissues are the source of IAA and gibberellins which are essentially required for the cell elongation.

In the monocot stems, curvature is due to two kinds of tissues: growing region of the stem itself at the base of the hollow internode (e.g., in Panicoideae) and collenchyma and parenchyma cells rich in amyloplasts at the base of the cylindrical leaf sheath surrounding the stem.

These gravitropically sensitive cells are sometimes called a pulvinus. The precise mechanism of displacement of amyloplasts causing curvature of a stem or root is still obscure.

Cholodony-Went hypothesis accounts for the gravitropism based on differential auxin accumulation lower side, e.g., in a stem, accumulates more. However, such a differential accumulation causes differential response in the stem and root. Some workers have linked amyloplasts distribution with the polarized auxin transport from the base of each cell towards the lower side of the organ.

Statoliths displacement may also be causing differential distribution of other growth regulators. Based on the Herman Dolk’s technique for the collection of auxin in agar blocks and replacing the cut coleoptile tip with such a block, differential auxin migration to the lower and upper halves is demonstrated.

The differential differences are attributed to migration rather than synthesis or destruction. Thus in a dicot stem there is comparatively high auxin accumulation on the lower side.

Subsequent workers suggested the role of gibberellins in the Geotropism of stems. Lower side of the stem had gibberellin level much higher than the upper side of a horizontally placed stem.

Based on the observations that the lag time preceding stimulation of stem growth by GA are as long as hour, their role in causing geotropism has been questioned. Further, in grass internodes, gravitropism was not caused by lateral transport of a growth regulator though elongating cells themselves detect and respond to gravity.

Compared with Cholodony-Went hypothesis based on high auxin accumulation on the lower side, recent investigations have revealed that some growth inhibitors accumulate in the lower half of a horizontal root cap.

This inhibitor moves basipetally out of the cap cells and lessen growth of the cells in the elongating zone. Added evidences have revealed that inhibitor is produced in the root cap and is transported to the elongating zone. Henry Wilkins and his associates have identified this inhibitor as probably ABA.

c. Graviperception in Higher Plants:

The gravity-dependent orientation of plant organs has several chains of reactions and can be divided into perception, transduction and response. On the whole, plant organs possess an extreme sensitivity towards mass acceleration.

The cell structures participating in the perception of stimulus have very little space and time in which to transform the physical into the physiological signals. The site of perception possibly lies in statoliths, e.g., amyloplasts, etc. (Fig. 21-4C). The site of perception may be primary root tip or coleoptile tip or grass node.

The statolith function of amyloplasts is now well established. In general the following three steps may be suggested:

(i) The new distribution of the statoliths after gravity-induced translocation.

(ii) The sliding of statoliths along the cytoplasmic structures during translocation in the intracellular stimulus.

(iii) The critical factor is also the change in amount and direction of pressure exerted on the original site of sedimentation, i.e., on a sensitive structure. The above three points are the three models which have favours and disfavours. The final proof is still awaited. Of the three, the third model is most compelling.

The gravi-sensitivity of the plant seems to lie in the sensitivity of bio-membranes to pressure.

Excellent reviews on root gravitropism have been published by Audus (1975), Juniper (1976), and Torry (1976), where involvement of different hormones is discussed.

There also appears some evidence that the photocontrol mechanism for inhibitor synthesis develops in the root apex following the removal of the cap. At any rate the root cap even though small and sophisticated organ, has the ability to produce and regulate the transport of regulatory compounds which control the direction of the root growth.

d. Thigmotropism or Haptotropism:

Thigmotropism is a growth curvature movement which occurs in response to the stimulus of contact or friction. This is exhibited pronouncedly by the tendrils of vine, Passiflora etc. In Passiflora, young tendril is spirally coiled, with the lower side outwards. Subsequently, it straightens up and its tip nutates in the air.

This is an autonomic movement. The apical part of the tendril is very sensitive, when rubbed or touched by a solid body it bends towards the stimulated side. The side which comes in contact with the support does not grow. It becomes the inner side.

However, the growth of the outer side remains normal or is accelerated and the tendril coils around the support. The bending thus caused brings a fresh part of the tendril in contact with the solid object and so the stimulation is continued.

The coiling around a support is a growth movement. When the apical part of the tendril has coiled around a twig, a secondary reaction of the tendril occurs. The basal part of the tendril twists spirally in the reverse direction from the apical coil and the stem of the climber and the support is drawn close to each other.

The perception of contact stimulus is attributed to the presence of special tactile pits on the tendrils in some cucurbitaceous plants. These are un-thickened areas in the outer walls of the epidermal cells.

Their protoplasm is close to the surface and is readily irritated by contact with the solid objects. Thigmotropic responses are induced only when the actively growing tendrils come in contact with the solid objects having rough surfaces.

Thigmotropic responses are also seen in roots which are negatively thigmotropic. While encountering solid objects e.g. stones, etc. they bend away and this enables them to avoid obstacles in the soil.

The mechanism of thigmomorphogenesis is thermochemical and endergonic in nature. Jaffe (1976) has indicated that rapid changes in the electrical resistance of the internodal tissue suggest that there are probably electrochemical events taking place that are closely linked to the primary mechanism of action.

Further, a desensitization of the responding system occurs during the first few minutes after stimulation. Also ethylene is an important part of thigmomorphogenetic mechanism. Thus exogenous application of an ethylene precursor mimicks the response to rubbing on the gross morphological level as well as on the ultrastructural level.

e. Heliotropism:

The leaves of some plant species exhibit striking movements to keep the leaf perpendicular to incident sunlight at all times of the day (diaheliotropic movements), while the leaves of other species show fixed leaf orientation during the day.

Diaheliotropism is an adaptive advantage to a plant because it allows the leaf to experience high solar irradiance and more quantum available for maximal photosynthesis.(Rama Das, 1984). In response to water stress some of the plants show paraheliotropic leaf movements in which the leaf movements are oriented parallel to the incident sunlight.

In brief diaheliotropism has a tremendous impact on the photosynthetic rates because this enables the leaf to experience high solar irradiances.

f. Hydrotropism:

The growth movement in response to variations in the moisture contents is called hydrotropism. Roots are positively hydrotropic. It can be shown by germinating seeds in moist saw-dust contained in a sieve. The emerging roots grow vertically downwards due to gravity until they come out of the pores at the bottom.

If the sieve is inclined at an angle to the vertical, the roots will bend towards the outer surface of the sieves in search of moisture and may, then, re-enter through other pores into the sieve. This also shows that roots are more positively hydrotropic than gravitropic.

g. Georeaction in Papaver Flower Stalk:

The curvature of the flower stalk of Papaver is caused by differential cell elongation of opposite sides of the stalk. The general assumption is that the differential cell elongation is caused by a difference in cell wall loosening between the two sides.

The weight of the flower bud in causing the positive gravitropic behaviour is also involved. Flower bud of Papaver contains auxin and the regulator of stalk movement translocated from the flower bud to the stalk most probably involves auxin.

Nothing precisely is known regarding the role (s) of other hormones. Most recent studies indicate that the flower bud appears to play a dual role in the flower stalk movement, causing nodding of the newly formed flower stalk with its weight and supplying the stalk with growth substances which regulate growth and tropistic movements of the flower stalk.

There appears to be some participation of the plant growth substance produced in the flower bud and transported to the stalk during the curvature and gravitropic straightening periods.

11. Nastic Movements:

The movements induced by stimuli that have no definite direction, resulting in particular orientation of the plant, are termed nastic movements. These movements occur in plant organs which have dorsiventral symmetry. The stimulus is due to the variation in the intensity of some external factors such as temperature, light, etc..rather than the direction of the factor.

The response is determined by the structure of the plant part and is always the same whatever the direction from which the stimulus preceded. In the nastic movements, the asymmetry in the response does not result from a gradient in the light stimulus. Instead the asymmetry is developed because of the differential flow of ions initiated by the stimulus in and out of specialised cell.

The direction of nastic movements is determined by the anatomy of the plant. Such movements fall into three categories; rhythmic leaf movements in nyctinastic plants; more rapid seismonastic movements that occur in a limited number of nyctinastic species, and the thigmonastic or thigmotropic curling of thread-like appendings in certain vines.

These movements depend upon differential turgor changes in key cells in the organ of curvature which may be the pulvinus of a nyctinastic plant or the tendril of the vine. These three types of movements vary considerably in rate, reversibility and molecular control mechanisms. Nyctinastic and seismonastic movements in plants with pulvini are completely reversible, while tendril curling involves permanent changes in cell size and structure.

It is thought that transmembrane fluxes of K⁺, CP, Ca²⁺, FT, sugars and tannin-like compounds are responsible for such movements. The activity of auxin, ethylene and possibly other hormones as well may also play important role. Following is the brief discussion on different types of nastic movements.

12. Nyctinastic Movement:

Plants whose leaves or leaflets assume a vertical orientation (close) in darkness are called nyctinastic. Such leaves normally assume a horizontal orientation (open) upon illumination (Fig 21-7).

These movements are not purely photonastic but are partially autonomic. For example, leaves in many nyctinastic plants do not remain open or closed for more than 12 hours, oscillating between these two states with a circadian period in the absence of environmental disturbances; such movements are called circadian.

The capitula of Calendula officinalis and many other flowers open on exposure to light and close when darkened. In some acacias movements are photonastic and the leaflets undergo folding when the leaf is darkened.

However, recently Galston and his colleagues have used the electron probing device to detect a substantial movement of potassium ions from the upper to the lower side of the pulvinus and back in bean plants.

This movement of potassium ion causes large changes in the osmotic potential of the motor cells of the pulvinus, thus affecting the rise and fall of leaves. Their studies further indicated that sugars were not involved since they accounted for a small fraction of the osmotically active substances in the motor cells.

It is also believed that auxins also contribute towards this response. For instance, it is suggested that pulvinus produces large amounts of IAA during the day and this is transported primarily to the lower side of the petiole.

As a result the potassium ions move to the area of high auxin and then water enters the lower side of the pulvinus. Thus leaf rises. At night the auxin is reduced and the action is reversed. When auxin was applied to the upper or the lower side of the pulvinus, there was falling or rising of the leaf, respectively.

It is generally believed that nyctinasty in the plants is timed by an internal rhythm. Further analysis have shown that such an effect is mediated by phytochrome.

In another set of experiment when the isolated pulvinus was obtained it was found to be reactive to light and also underwent redistribution with regard to potassium and turgor reactions in a fashion as it happens in natural conditions.

From this experiment it may be concluded that gulvinus has the apparatus for the reaction to potassium and also light conditions. There is a good possibility to assume that phytochrome affects permeability of the membranes and this in turn causes redistribution of potassium and as a result the osmotic turgor changes and movements occurred.

The adaptive value of the horizontal day time position which orients the leaves for maximum absorption of sunlight is obvious, but the function of the sleep movement is less apparent. Some authors have suggested that nyctinastic movements which bring paired leaflets together in a way that their upper surface is shaded by each other to prevent moonlight from interfering with fine measurement.

In photoperiod sensitive plants of very low intensities of light have been found to influence time measurement in some plants. In addition to these leisurely circadian movements, rapid or seismonsatic movements also occur in Mimosa and a few other species when plants are stimulated mechanically, that is, by shaking or touch.

13. Seismonastic Movements:

A number of plants including Mimosa pudica, Biophytumsensitivum and Cassia ficulata can perform rapid seismonastic movements in addition to the slow nyctinasticmovements. These movements occur by mechanical stimulus and also by completely different stimuli such as electrical or temperature shocks or chemical treatment or high intensity of light. Independent of the stimulus phase rapid movements are all called seismonastic.

In Mimosa pudica in the ‘sleep position’, the leaflets are folded and the whole leaf droops down. This position is normally assumed during the evening.

The plant subjected to the stimulus or similar check also shows same effect. The movement is rapid and within a few seconds of the stimulation the whole set of leaves will have assumed the sleep position. Movements which occur as a consequence of shock stimulus are referred to as seismonastic movement.

Movement of leaves, leaflets is caused by pulvinus which is made up of parenchymatous cells to form a swollen structure at the base of a leaf, etc. Motor cells are located on opposite sides of the pulvinus and it is in these that water moves in or out causing expansion or contraction of portion opposite to the pulvinus (Fig. 21-7, 9).

Then the leaf moves up or down. The water moves in due to osmotic potential. In all probability potassium ions move in or out due to ATP driven transport mechanisms. The movement of organ is thus caused by different stimuli which also affect potassium transport.

Mechanism:

It is assumed that the seismonastic movements are caused by a change in the turgor of the upper and lower half of the pulvinus part of petiole. When examined histochemically, the lower half of the pulvinus is seen to be made up of thin- walled cells with large intercellular spaces whereas the upper half has cells which are comparatively thick walled with a fewer intercellular spaces.

On stimulating, cells of the lower half lose water and pass it on to the intercellular spaces. As a result their turgor falls. This is accompanied by an increase in the permeability of their membranes and decrease in the osmotically active substances of these cells, which move from vacuole into the cytoplasm.

Cells of the upper half become more turgid by absorbing water from the intercellular spaces of the lower half. Consequently the upper half of the pulvinus presses down the lower flaccid half and as a result the leaf droops down.

In due course cells of the lower flaccid half gradually re-absorb water from the intercellular spaces and become turgid and the leaf returns to its normal position after a while. Evidently these changes are reversible.

Yet another theory is based on the presence of small vacuoles in the cytoplasm of turgid pulvinus cells. In the seismonasty they disappear. It is as if contents of vacuole are ejected out to cause water loss.

These is yet another suggestion that pulvinus activity is based on the hydration and dehydration of proteins. Consequently cell colloids contract or expand depending upon the availability of water. ATP provides the needed energy during the process.

In earlier years, J.C. Bose assumed the presence of a nervous system in plants and through this the stimulus was transmitted. Recent studies have provided some evidence on the transmission of action potential through the xylem. Further, hormones also stimulate an electric potential in the tissues of Mimosa.

These is also a view put forth that the stimulus may be propagated through successive loss of turgor in the motor cells. In conclusion, it may be mentioned that not one but Fig. 21-9A. Seismlnastic movement in Mimosa pudica.several explanations may have to be taken into A mere touch has caused folding of the leaflets and account to explain the process. drooping of branch on right hand side.

Seismonastic Movements and Recovery:

Some of the species like Mimosa pudica, Cassia ficuculata, Biophytumsensitivum, etc. perform rapid seismonastic movements in addition to the slow movements. These movements take place at the 1°, 2°, 3° pulvini of Mimosa; the paired pinnules and pinnae fold together as during the sleep movements, while the petiole droops so that it is oriented in an almost vertical direction.

The moment the plant of Mimosa is shaken, movements begin within a wink of an eye. The petiole returns to the non-stimulated state within 10-15 min. It is interesting to mention that a leaf, electrically stimulated to droop every 20 or 30 min during light-dark cycles or continuous light always returns to the same angle as a non-stimulated control.

The rate of recovery is increased by exposure to white light, if the plant is put in dark, or by an increase in temperature.

Recovery also depends upon respiration, K⁺ and other osmotically effective substances by abaxial motor cells. The promotive effects of light and temperature are most reasonably interpreted in terms of the need for ATP for ion uptake. Abe and Oda (1976) determined that cells on both adaxial and abaxial sides of the pulvinus are excitable with intracellular electrodes. However, cells on the lower surface have five times more action potential than the upper surface.

Recent studies as reviewed by Haupt and Feinlaib (1979) show that both seismonastic and rhythmic movements depend upon changes in cellular K⁺ and its accompanying ion Cl⁻ and other substances that affect motor cell water potential. However, turgor regulation differs in the two movements types. Ca²⁺ provides a common link between the two types.

Rhythmic changes in membrane transport or permeability could lead to all of the following oscillations: K⁺ uptake through the plasmalemma, membrane potential, sucrose-mediated H transport, amplitude of seismonastic leaf drop and rate of recovery.

Action potential in plants is because of transient efflux of K⁺ from excitable cells as in petiole of Mimosa. There is K⁺ loss during the pulvinar action potential. However, K⁺ loss from cells on the abaxial side of the pulvinus is far more massive than usually during transmission of an action potential, and recovery takes place in longer time than usual.

Several workers draw a parallel between motor cells of Mimosa and contractile vacuoles of animals. Action is invariably present in living systems and is usually in close association with cell membranes. It has been suggested that when the motor cells of Mimosa are stimulated, actomyosin-like contractile system might alter the conformational aspect of proteins in the tonoplast membranes and thus membrane permeability is increased. There is thus K⁺ loss along with other osmotic agents.

Several points support this:

(i) Protamine which interferes with contractile proteins and their linked ATPase activity close Mimosa pinnules without contact stimulus and inhibits recovery following mechanical stimulation;

(ii) ATPase activity has been detected histochemically in pulvini;

(iii) Electron micrographs reveal that fibrils attached to the tonoplast fragment after stimulation and the tonoplast collapse; and

(iv) Ca²⁺, which activates contractile, proteins in animal systems, migrates from the tannin vacuole to the central vacuole after Mimosa has been mechanically stimulated. Recent workers have shown that Ca²⁺ migration stimulates a contractile process in Mimosa which then leads to rapid increase in membrane permeability and K leakage followed by loss of turgor and leaf movement.

In a recent study attempts have been made to measure ion and saccharide concentrations in the upper and lower parts of the laminar pulvinus of the primary leaf of Phaseolus vulgaris in relation to the circadian movement.

Concentration of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, organic acids, NO₃⁻ , H₂ PO₄⁻, fructose and fructose yielding saccharides in the pulvinus were 75-120, 0.3-0.7, 5-8, 6-12, 40-60, 60- 73, 19-35, 2-9 and 1 mM, respectively, and the osmotic pressure of the pulvinus was considered to be due to these ions.

The cell volume in the expanding part was larger than that in the contracting part. The change of the cell volume altered the molar concentration in the cell sap and therefore the amount of solutes actually transported from the upper to the lower part and vice versa was estimated from the concentrations expressed in moles per gram of dry weight.

The results from these experiments indicated that K⁺, Cl⁻, organic acid (or H⁺), and NO₃⁻ moved from the upper to lower parts or vice versa in the pulvinus in relation to its deformation, keeping the electroneutrality among these ions, whereas Ca²⁺ and Mg²⁺ did not move.

The difference in the K⁺ concentration between the upper and the lower parts when the leaf was up or down amounted to 30% of the whole osmotic pressure. It was concluded that the endogenous clock controlled unequal distribution of K⁺, CP, organic acids and NO₃⁻ in the pulvinus could be the force for the circadian leaf movement.

The pulvinus is an open system which is multicellular i.e. it consists of many cells. When the ions move from the upper to the lower side of the pulvinus or vice versa, they pass through some of the cell layers or at least through or across the cells on both sides of the xylem.

The cell, if the carrier or pump is localized in the plasmalemma, should have two kinds of carriers or pumps; one transports the ions into the cell from the adjacent cells and out of the cells to the adjacent cells, and the other transports the ions, carbohydrates, amino acids and other substances from the xylem and phloem and excretes some ions and metabolites into the xylem and phloem.

14. Thigmomorphogenesis:

N.J. Jaffe of Ohio University, Athens (USA) reported that mechanical stimulation, especially rubbing, of the internodes caused well marked responses to the given stimulus. For instance, the rubbed stems were short, stocky and elongated slower than the unrubbed controls. He called such developmental responses to mechanical stress as thigmomorphogenesis.

In nature bending of stems, trampling of plants especially grasses, winds, etc. cause such plant responses. Plants protected from strong winds produced elongated internodes and were tall. The author also suggested that rubbing of plants or plant parts by the animals as well as farm machinery also caused same effect.

However, once the stimulus was removed stem attained original state of growth. Recent studies have indicated several different types of thigmomorphogenetic responses. The changes in growth pattern are usually attributed to alternations in the pattern of growth regulators.

There may be general decrease in IAA and gibberellin or stimulation of ethylene production. Enhancement in ABA production or availability has also been suggested. In addition rate of photosynthesis also decreases. Several suggestions have been made regarding changes in the pattern of growth regulators.

One of the possibilities includes decrease in electrical resistance within seconds of rubbing, followed by a gradual rise to the normal state. Thus, changes in the production of growth regulators by alterations in the availability of enzymes concerning formation of specific growth regulators or their precursors may be affected.

15. Reception and Transduction of Electrical and Mechanical Stimuli:

Like animals, plants are able to perceive and respond to environmental stimuli. However, plants have not developed a specialised sensing apparatus equipped with a standard type of receptor cell and a nervous network to process stimuli.

Instead, they employ diverse types of receptive structures including, in a few cases, a receptor cell. In spite of this, the causative chain of plant movement can be analysed in terms of following three indispensable steps.

Stimulus reception ” stimulus transduction ” physiological response.

Stimulus Reception:

It denotes the stimulus specific physical or physico-chemical interaction of the stimulus and the living matter: a sensor is activated by the physical or chemical energy of the stimulus. Stimulus reception may occur independently of metabolic activity.

Stimulus-Transduction:

It concerns strictly with the control of a specific cellular (biophysical, biochemical) process by the activator sensor. The transduction besides sensory transducer mechanisms includes signal transmission and dose response relationships.

16. Physiological Response:

This implies the initiated or modified plant movement or developmental process. In plant cell or tissues, electrical potential differences are mostly generated by membranes, a notable exception being Donnan potential.

A membrane potential is assumed to arise from:

(i) Passive ion diffusion in parallel with,

(ii) ATP- driven electrogenic ion pump, probably proton export in the plasmalemma.

In association with plant movements and morphogenesis there occurs change of membrane potential in time and membrane potential gradients (Bioelectric fields). A cell membrane may faction as an electrosensor i.e. it may sense changes of its ionic milieu.

The plasmalemma of many plant cells has been shown to sense and transduce endogenous and applied electrical signals into localized cell growth. Effective electrical signals are direct or alternating electric fields or ion concentration gradients established across developing cells.

In brief, it may be mentioned that plants have not developed a standard type of sensory cell with an amplitude modulated receptor potential, generating frequency modulated trains of action potential, the only known exception being the mechano-sensory cell of the Droseraceae (a family of carnivorous plants) noteably in Dionaea.

In higher plant organs, some cells are equipped with axillary sensory devices to facilitate stimulus reception; namely statoliths. Furthermore, elaborate distropic system for photoreception is found throughout the plant kingdom.

However, no cytoplasmic structure has been identified to serve as a stimulus transducer for the plants most relevant environmental signals; gravity, non-photosynthetic light and mechanical forces. In case of Dionaea, the ER complex in the receptor cell is the suspected sensory structure. Following are the description of the rapid movements of carnivorous plants.

A. Venus Flytrap:

In several of the flytrap plants there exist indented cells which are the mechano-transducers of the traps. In these cells the ER is whorled around a vacuolar structure. In Drosera the initial rapid thigmonastic bending of a marginal secretory tentacle towards the centre of the leaf begins with a non-propagated slow polarization of some cells in the tentacle head.

This putative receptor potential then generates trains of action potential. It has also been suggested that sensory cells are located not far away from the stalk. The neck cells appear to be indented cells.

These indented cells are mechanotransducer of the venus flytrap. Electron microscopy has revealed a special feature of these cells; the endoplasmic reticulum is arranged in a whorled fashion around a vaculoar structure and is located in the cell’s basal portion. This points to a functional asymmetry known in animal receptor cell.

B. The Utricularia bladder:

It is an aquatic plant where leaf and leaf segments are modified into small bladders (Fig. 21-10). Recently Sydenham and Findlay (1975) have described the details of setting the bladder and making it as an effective trap. The bladder is set by the export of solutes chiefly Na⁺, K⁺ and CI and of water from the lumen of the bladder. Bladder decreases in volume and the hydrostatic pressure of the lumen drops down.

The salt concentration in the bladder is high. Thus there is movement of water from outside to inside of the bladder. Based on ion flux and voltage measurements, the above authors have located a respiration dependent electrogenic.

CI pump in the wall cell plasmamembranes facing the lumen. The two armed hair cells which are non-cutinized cells inside the bladder near the mouth take up the fluid from the lumen. In the set bladder, the trap door opens towards the inner side if one of the two hairs on the door is touched. The site of mechanorception is not known.

C. The Sun-dew tentacles:

In Drosera, when an insect alights on one of the marginal tentacles of its leaves the latter is stimulated. As a result, the tentacles bend towards the centre of the leaf due to a growth curvature in the basal part of its stalk. Here the direction of movement is determined by the properties of the tenacle. It is a nastic movement and causes the insect to come in contact with the smaller tentacles situated in the centre.

Soon after these stimulated tentacles transmit the stimulus to other marginal tentacles which also bend over the insect from all the directions. The latter movements are haptotropic because they bring the tentacles towards the leaf centre from wherever the stimulus proceeds.

The Drosera tentacles also show chemonastic or even chemotropic curvature movements when stimulated by ammonia salts, phosphates or drops of water having proteins. Haptonastic curvatures are also induced in the leaves of Dionaea. When one of the six sensitive hairs, on the upper surface of the leaf is stimulated by contact with the body of an insect, the two halves of the leaves close rapidly on the midrib which acts like a hinge.

D. Hygroscopic movements:

Movements which occur in the non-living parts of a plant as a result of imbibition or loss of water are called hygroscopic movements. These are either xerochasy or hydrochasy.

Xerochasy:

Such a movement is caused by a loss of water from the dead tissues, as in the dehiscence of pods or capsules, dehiscence of fern sporangium. In Stipa grass the awn is hygroscopic and undergoes twisting movement. Thus the seed is pushed into the oil.

Hydrochasy:

Such a movement is brought about by the imbibition of water by the dead tissues e.g. the movement of peristome teeth of moss capsule and the elators of Equisetum. They open out when dry. Similarly, capsules of Impateins burst open on absorption of water.

Chemotropism:

Chemotropism is a growth movement in response to stimulating effects of chemical compounds. Pollen tubes are positively chemotropic. They grow through the style and reach the egg apparatus under the influence of chemical substances including phytohormones.

Many fungal hyphae show positive chemotropism towards sugars, etc. They are however, negatively chemotropic to the products of their own metabolism and to acids and alkalies. In case of pollen tubes growing through the style different phytohormones and some ions play an important role.

Role of calcium as a chemotropic factor has been well identified in Antirrhimum pollen. Some studies have implicated the role of gaseous hormone ethylene in determining chemotropic responses.