Coordination in Plants
Plants are devoid of nerves and muscles. They are fixed. Even then they show movements though locomotion is absent. These movements may be directional (tropic) or non-directional (nastic). growth or turgor-based. The movements are quite slow in most of the cases. It may take a few days to notice the change in position or movement. However, some turgor-based movements are quite fast and easily discernible.
Turgor-Based Plant Movements
The plants have certain areas which have special cells that can shrink or swell with loss or gain of water. They show reversible movements, e.g., opening or closing of stomata, sleep movements of legume leaves, drought induced rolling of some grass leaves. Sleep movements of legume leaves are called nyctinasty. Drought related rolling of grass leaves is called hydronasty.
Immediate Response to Stimulus
If you happen to touch a pinnule of Mimosa pudica (Sensitive Plant, Touch-me-not Plant, Shame Plant, Shy Plant) all the pinnules fold upwardly, the pinnae come together and the whole leaf droops down. The place of stimulus is away from the area of movement. Further, it goes beyond the area of sensitization to other pinnae as well as to the whole leaf. Naturally the stimulus travels from the area of contact to the area of response. This passage must be cell-to-cell like an electrical chemical called turgorin. The differential movements are due to special cells at the base of pinnules, pinnae and petiole that shrink on activation and later recover after about 10 minutes.
As the movement in Mimosa is non-directional, it is called nastic movement. The movement is due to touch or shock. It is, therefore, known as haptonasty or seismonasty.
Growth-Based Plant Movements
They are plant movements caused by differential growth or unequal growth in different parts of the organ. This results in bending and other types of movements. Growth movements are of two types, nastic and tropic. Nastic movements are non-directional. They are determined by structure of the organ and not the direction of the stimulus, e.g., opening of flowers due to greater growth or epinasty on the upper surface of sepals and petals. Tropic movements occur in cylindrical organs. The direction of movement is related to the direction of the stimulus. Tropic movements are also called curvature movements.
Differences between Nastic and Tropic Movements
|Nastic Movements||Tropic Movements|
|1. Direction. It is non-directional, i.e., not related to the direction of stimulus.|
2. Organ. The organ showing nastic movement is either flat or asymmetrical.
3. Stimulus. It is multilateral or diffused.
|1. It is directional and is related to the direction of stimulus.|
2. The organ showing tropic movement is generally cylindrical.
3. It is unilateral.
Depending upon the stimulus, tropic movements are of many types-phototropism, geotropism, hydrotropism, chemotropism, thigmotropism.
- Phototropism. It is tropic movement in response to unidirectional exposure to light. Stems generally bend towards the source of light. They are called positively phototropic. Leaves lie at right angles to the direction of light. They are diaphototropic. Roots are either neutral or negatively phototropic.
In stem the stimulus of light is received by stem apex. Leaves are essential for it. The region of response is zone of elongation present below the apex. Blue light is effective in causing phototropism. The photoreceptor is a chemical called phototropin. The movement is caused by decreased availability of auxin on the illuminated side and more on the shaded side. The shaded side grows more resulting in bending movement.
In root where negative phototropic movement is observed, more auxin on the shaded side causes inhibition of growth (opposite to that of stem). Sunflower heads perform sun tracking due to positive phototropism while plants growing in the open do not show bending movements as auxin diffuses uniformly in the growing region.
Demonstration of Phototropism
Fix a wire mesh over the mouth of a conical flask full of water. Place one or two freshly germinated bean seeds on the mesh. Take a large cardboard open on one side.
Place the flask in the cardboard kept near a window so that seedling receives light from one side only. Observe after 2-3 days. The shoot has bent towards the source of light while the root has bent away from it. It shows that stem is positively phototropic while root is negatively phototropic.
Now rotate the flask by 180° and observe after 3-4 days. The shoot and the root have developed curvatures and new direction of bending by the young shoot towards the light and young root away from light.
- Geotropism. It is tropic movement or growth movement of curvature which occurs in response to the vector of gravity. Stems are generally negatively geotropic while roots are positively geotropic. Their branches are plagiogeotropic or lie at an angle to the direction of gravity Stimulus of gravity is perceived by stem apex, stem nodes and root cap. It is because of the negative geotropic nature of stem and presence of receptor region in the nodes that the lodged shoots become vertical, Negative geotropism helps shoots to properly expose themselves to sunlight. Positive geotropism of roots helps them to fix the plant in the soil and absorb water as well as minerals from the same.
Demonstration of Geotropism
Place a potted plant horizontally on the ground. Provide support to the pot to prevent rolling. Regularly water the plant. Observe after a week. The apical part of the shoot has bent upwardly while the roots have bent downwardly. It shows that stem is negatively geotropic while the root is positively geotropic.
- Hydrotropism. It is tropic or directional growth movement of curvature which develops due to unilateral exposure to water. Hydrotropism is mostly seen in roots which are positively hydrotropic. Positive hydrotropic movement of roots is stronger and even overcomes their positive geotropic response.
Demonstration of Hydrotropism
Take a sieve. Fill it with moist saw dust and support it on the sides over wooden blocks. Place a few soaked germinating seeds in the sawdust. Sprinkle water over the sawdust. Within 1-2 days radicles will be seen coming down from the pores of the sieve and hang in the air. However, within a few hours the radicle will be found to bend back and enter the wet saw dust showing that positive hydrotropism of roots is stronger than that of their positive geotropism.
- Thigmotropism. It is tropic or growth movement of curvature which occurs in tendrils and twiners in response to stimulus of contact. As a tendril or twiner comes in contact with a support, it encircles the same. The reason is less growth in region of contact and more growth on the free side. Here, contact causes less auxin production and hence less growth.
- Chemotropism. It is tropic or growth movement of curvature which occurs in plants in response to chemical stimulus. The best example is the passage of pollen tube through the style and inside the ovary towards the ovule where synergids are sending out chemical signals.
Chemical Coordination in Plants
Plants do not have nerves and muscles for coordinating their activities. Instead, they have chemical messengers or hormones. Plant hormones or phytohormones are non nutrient diffusible chemical substances which are able to control various activities of plants like growth, differentiation, movements, development and other physiological processes. For this the target or effector cells possess receptors for picking up the hormones, Plant hormones are also called plant growth regulators (PGRS) as they function through promoting or inhibiting growth. Major plant hormones are auxins, gibberellins, cytokinins (growth promoters), abscisic acid (growth inhibitor) and ethylene (both promotion and inhibition).
They are weakly acidic organic substances having unsaturated ring structure which promote cell enlargement of shoots in the concentration range of 10-100 ppm, that is inhibitory to growth of roots. Natural auxin is indole 3-acetic acid or IAA. Some other examples are IBA, NAA, 2, 4-D, 2,4,5-T. IAA is synthesized inside shoot tips, young developing leaves and seeds.
- Cell Enlargement. Auxin promote cell growth at a concentration of 10 ppm in stem and 0.0001 ppm in root.
- Apical Dominance. Axillary buds do not sprout near the apical bud due to secretion of auxin by it.
- Prevention of Abscission. Auxin prevents premature falling of leaves and fruits.
- Movements. Phototropic, geotropic and other plant movements are caused by differential distribution of auxin.
- Root Formation. Root formation on stem cuttings is stimulated by auxin like NAA and IBA.
- Tissue Culture. Auxin promotes callus formation in tissue culture. Alongwith cytokinin it promotes differentiation of callus into plantlet.
- Fruit Growth. Auxin promotes fruit growth, sweetening of fruit and even parthenocarpic development of fruit.
- Weedicides. In higher concentration 2,4-D and 2,4,5-T are used as weedicides.
They are mildly acidic tetracyclic organic substances which bring about cell elongation of leaves, stem and fruits. They are formed by the young leaves of buds, root tips and developing seeds. Functions
- Growth. Gibberellins promote growth in leaves, stems, internodes and fruits. They are used to increase yield of fruits and sugar in sugarcane.
- Overcoming Dormancy. Dormant buds and seeds sprout in the presence of gibberellins.
- Flowering. They can replace cold and long day requirement for flowering in some plants.
- Parthenocarpy. Like auxin gibberellins can induce formation of seedless fruits. 5. Overcoming Dwarfism. Genetically dwarf plants grow to normal size on application of gibberellins.
They are mildly alkaline aminopurine or adenine derivatives which promote cell division in plants, e.g., zeatin (natural), kinetin (synthetic). Cytokinins are synthesized in root tip and endosperm of developing seeds.
- Cell Division, Differentiation and Morphogenesis. Cytokinins are essential for cell division, cell differentiation and morphogenesis.
- Prevention of Senescence. Aging of leaves, marketed vegetables, fruits and cut flowers can be prevented by application of cytokinin.
- Accumulation of Nutrients. Cytokinins improve yield and quality of fruits by their stimulation of nutrient flow into them.
- Resistance. They enhance resistance of plants to environment stresses and diseases.
- Apical Dominance. Application of cytokinins overcomes apical dominance. The nearby axillary buds sprout.
Abscisic Acid (ABA)
It is mildly acidic organic substance which functions as a general growth inhibitor. Abscisic acid is able to counter growth promoting activity of auxins, gibberellins and cytokinins. It helps the plant to overcome stress conditions mostly through induction of dormancy. Therefore, it is known as stress hormone as well as dormin.
- Controlled Growth. By counteracting the effect of other hormones, abscisic acid or ABA controls growth.
- Wilting. Under conditions of stress, it causes wilting and senescence of leaves.
- Abscission. It promotes abscission of flowers and fruits.
- Dormancy. Abscisic acid causes dormancy of buds and seeds.
- Transpiration. It checks transpiration by causing closure of stomata. Because of this, abscisic acid can also be used as antitranspirant.