Physiological Effects of Growth Regulator

1. Auxins:-

Physiological Effects of Auxins:-

a. Tropism:-  Response or orientation of a plant to a stimulus that acts with greater intensity from one direction than another. Growth due to tropism is mediated by changes in concentration of the plant hormone auxin within plant cells.

i. Phototropism (Heliotropism):- The orientation of a plant in response to light. 

Positive:- Orientation towards the source of light.

Negative:- Orientation away from the source of light.

ii. Geotropism (Gravitropism):- The orientation of a plant in response to gravity. 

Positive:- Orientation towards the gravity.

Negative:- Orientation away from the gravity.

b. Root initiation:- Auxin promotes growth of lateral and adventitious roots.

c. Flower initiation:- Auxin promotes flowering.

d. Apical dominance:- It is the dominance of shoot apex over its lateral branches. Here shoot apex inhibit the growth of lateral axillary buds. It is caused by the apical bud producing IAA (auxin) in abundance. The IAA causes the lateral buds to remain dormant.

e. Cell enlargement:- Its main function is of cell elongation. Far-red light induced increased internode elongation is a result of both increased cell elongation and increased cell division.

f. Prevention of Abscission:- Natural auxins have controlling influence on the abscission of leaves, fruits etc.

g. Respiration:- According to French and Beevers (1953), the auxin may increase the rate of respiration indirectly through increased supply of ADP (Adenosine diphosphate) by rapidly utilizing the ATP in the expanding cells.

h. Vascular Differentiation:- Auxin induces vascular differentiation in plants.

Mode of Action of Auxin:- Auxin causes Rapid increase in cell wall Extensibility. Cell wall enlargement in plants involves two steps:

i. Osmatic uptake of water across the plasma membrane resulting in increased turgor pressure of the cell.

ii. Extension of cell wall in response to increased turgor pressure.

2. Cytokinins:-

Physiological effects of Cytokinins:-

i. Cell Division:- Cytokinins are essential for cytokinesis though chromosome dou­bling can occur in their absence. In the presence of auxin, cytokinins bring about division even in permanent cells. Cell division in callus is found to require both the hormones.

ii. Morphogenesis:- Both auxin and cytokinins are essential for morphogenesis or dif­ferentiation of tissues and organs. Buds develop when cytokinins are in excess while roots are formed when their ratios are reversed.

iii. Differentiation:- Cytokinins induce formation of new leaves, chloroplasts in leaves, lateral shoot formation and adventitious shoot formation. They also bring about lignification and differentiation of inter-fascicular cambium.

iv. Senescence (Richmond-Lang Effect):- Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients.

v. Apical Dominance:- Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. They thus act antagonistically to auxin which promotes apical dominance.

vi. Seed Dormancy:- Like gibberellins, they overcome seed dormancy of various types, including red light requirement of Lettuce and Tobacco seeds.

vii. Resistance:- Cytokinins increase resistance to high or low temperature and disease.

viii. Phloem Transport:- They help in phloem transport.

ix. Accumulation of Salts:- Cytokinins induce accumulation of salts inside the cells.

x. Flowering:- Cytokinins can replace photoperiodic requirement of flowering in certain cases.

xi. Sex Expression:- Like auxins and ethylene, cytokinins promote femaleness in flow­ers.

xii. Parthenocarpy:- Crane (1965) has reported induction of parthenocarpy through cytokinin treatment.

xiii. Stomatal opening:- It has been shown that an increased cytokinin concentration in xylem sap promotes stomatal opening.

Mode of Action of Cytokinins:-

> Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth. 

> Simple experiments support this theory. 

> When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier. 

> Applying auxin to the cut stem again inhibits lateral dominance.

3. Gibberellins:-

Physiological Effects of Gibberellins:- 

i. Elongation of intact stems:- Many plants respond to application of GA by a marked increase in stem length; the effect is primarily one of internode elongation.

ii. Dwarf shoots:- Besides general increase in stem length, gibberellins specifically induce inter­nodal growth in some genetically dwarf varieties of plants like Pea and Maize. It appears that dwarf- ness of such varieties is due to internal deficiency of gibberellins.

iii. Bolting:- Gibberellins induce sub-apical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants (e.g., Henbane, Cabbage) and root crops (e.g., Radish).

iv. Dormancy:- Gibberellins overcome the natural dormancy of buds, tubers, seeds etc., and allow them to grow. In this function they are antagonistic to abscisic acid (ABA).

v. Seed Germination:- During seed germination, especially of cereals, gibberellins stimulate the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases and pro­teases. The enzymes solubilise the reserve food of the seed. The same is transferred to embryo axis for its growth.

vi. Fruit Development:- Along with auxin, gibberellins control fruit growth and development. They can induce parthenocarpy or development of seedless fruits from unfertilized pistils, especially in case of pomes (e.g., Apple, Pear).

vii. Flowering:- They promote flowering in long day plants during noninductive periods.

viii. Vernalization:- Vernalization or low temperature requirement of some plants can be replaced by gibberellins.

ix. Application of gibberellins increases the number and size of several fruits, e.g., Grapes, To­mato; induce parthenocarpy in many species; and delay ripening of citrus fruits thus making storage safe.

Mode of Action of Gibberellins:-

> Gibberellins cause seed germination by breaking the seed's dormancy and acting as a chemical messenger. Its hormone binds to a receptor, and calcium activates the protein calmodulin, and the complex binds to DNA, producing an enzyme to stimulate growth in the embryo.

> Gibberellin appears to induce its effect on stem elongation by de-repressing negatively regulated genes Le., by deactivating or degrading the repressors of GA response so that GA induced genes are transcribed and stem elongation or growth occurs.

4. Abscisic Acid:-

Physiological effects of Abscisic Acid:- 

i. Seed and bud dormancy:- Its main function is to maintain seed dormancy. Abscisic acid induces dormancy of buds towards the approach of winter. Abscisic acid accumulates in many seeds during maturation and apparently contributes to seed dormancy.

ii. Senescence:- ABA acts as a general inducer of senescence (Thimann). The onset of senescence is correlated with stomatal closure. The ABA content of aging leaves increases markedly as senescence is initiated.

iii. Abscission:- It is the process of shedding old or unwanted organs such as leaves, flower, floral organs, and fruits. ABA promotes abscission through ethylene.  

iv. Flowering:- In long-day plants, the effect of gibberellins on flowering is counteracted by ABA, which accumulated in the leaves during the short winter days. This ABA acts as inhibitor of flowering in long-day plants. On the other hand ABA induces flowering in short-day plants.

v. Starch hydrolysis:- The GA-induced synthesis of a-amylase and other hydrolytic enzymes in barley aleurone cells is inhibited by abscisic acid. This inhibition can be reversed by increasing the amount of GA supplied.

vi. Geotropism:- ABA controls geotropic responses of roots. It stimulates positive geotropism in roots.

vii. Stress response:- It also close the stomata of the leaves in dry conditions. Hence it is also called stress hormone.

5. Ethylene:-

Physiological effects of Ethylene:- 

i. Fruit Ripening:- Its main function is to ripen fruits. The most commonly used chemical is called ethephon (2-chloro ethylphosphonic acid). It penetrates into the fruit and decomposes ethylene.

ii. Triple Response:- Ethylene  causes plants to have – 

    i. Short shoots:- Inhibition of stem elongation.

    ii. Fat shoots:- Stimulation of radial swelling of stems.

    iii. Diageotropism:- Increased lateral root growth and horizon­tal growth of stems with respect to gravity.

iii. Formation of Adventitious Roots and Root Hairs:- Ethylene induces formation of adventitious roots in plants from different plant parts such as leaf, stem, peduncle and even other roots. In many plants especially Arabidopsis, ethylene treatment promotes initiation of root hairs.

iv. Inhibition of Root Growth:- Ethylene is known to inhibit linear growth of roots of dicotyledonous plants.

v. Leaf Epinasty:- When upper side (adaxial side) of the petiole of the leaf grows faster than the lower side (abaxial side), the leaf curves downward. This is called as epinasty. Ethylene causes leaf epinasty in tomato and other dicot plants such as potato, pea and sunflower. Young leaves are more sensitive than the older leaves. However, monocots do not exhibit this response.

vi. Flowering:- Ethylene is known to inhibit flowering in plants.

vii. Sex Expression:- In monoecious species especially some cucurbits like cucumber, pumpkin, squash and melon; ethylene strongly promotes formation of female flowers thereby suppressing the number of male flowers considerably.

viii. Senescence:- Ethylene enhances senescence of leaves and flowers in plants. In senescence, concentra­tion of endogenous ethylene increases with decrease in conc. of cytokinins and it is now generally held that a balance between these two phytohormones controls senescence.

ix. Abscission of Leaves:- Ethylene promotes abscission of leaves in plants. Older leaves are more sensitive than the younger ones.

x. Breaking Dormancy of Seeds and Buds:- Ethylene is known to break dormancy and initiate germination of seeds in barley and other cereals. Seed dormancy is also overcome in strawberry, apple and other plants by treatment with ethylene. Non-dormant varieties of seeds produce more ethylene than those of dormant varieties.

Mode of Action of Ethylene:-

> At a cellular level, ethylene can inhibit or promote cell division. 

> It sometimes inhibits cell expansion. 

> In other circumstances, it stimulates lateral cell expansion. 

> The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of cells. 

> Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.