Plant hormones-Abscisic acid & Ethylene

plant growth inhibitors

1. PLANT HORMONE ( ABSCISIC ACID & ETHYLENE ) BY A.MANOJ KUMAR (A9PBTOO4) II M. Sc (BIO-TECHNOLOGY)

2. PLANT HORMONE:- Definition :- ” ’ Plant hormones (or plant growth regulators , or PGRs) are internally-secreted chemicals in plants that are used for regulating the plants growth . According to a standard definition , plant hormones are signal molecules produced at specific locations, occur in low concentrations , and cause altered processes in target cells at other locations. Growth in plants is regulated by a variety of plant hormones, including auxins , gibberellins , cytokinins , and growth inhibitors, primarily abscisic acid and ethylene

3. Growth inhibitors Growth inhibitors of various types have been identified in plants. The best characterized one is abscisic acid , which is chemically related to the cytokinins . It is probably universally distributed in higher plants and has a variety of actions Another growth inhibitor is ethylene , which is a natural product of plants, formed possibly from linolenic acid (a fatty acid) or from methionine (an amino acid).

4. 1. ABSCISIC ACID It is also called stress hormone because the produc­tion of hormone is stimulated by drought, water logging and other adverse environmental conditions. Abscisic acid is known as dormin as it induces dormancy in buds, underground stems and seeds. Its other names are abscissin II and inhibitor B. Abscisic acid is a mildly acidic dextrorotatory cis sesquiterpene growth hormone which functions as a general growth inhibitor by counteracting other hormones ( auxin , gibberellins, and cytokinins ) Functions of Abscisic Acid: 1. Bud Dormancy: Abscisic acid induces dormancy of buds towards the approach of winter .

5. 2. Seed Dormancy: It is mainly caused by abscisic acid. Dormancy allows seeds to tolerate desiccation and extremes of temperature better. The buds as well as seeds sprout only when abscisic acid is overcome by gibberellins. Because of its action in inducing dormancy, abscisic acid or ABA is also named as dormin . 3. Stoppage of Cambium Activity: Formation of abscisic acid stops mitosis in vascu­lar cambium towards the approach of winter. 4. Abscission: Abscisic acid promotes abscission of flowers and fruits. 5. Leaf Senescence: Its excessive presence stops protein and RNA synthesis in the leaves and hence stimulates their senescence (leaf fall is actually promoted by ethylene).

6. BUD DORMANCY BUD DORMANCY CAMBIUM LEAF ABSCISSION

7. 6.Transpiration During desiccation and other stresses, abscisic acid is rapidly synthesised . The inhibitor causes closure of stomata and hence prevents transpiration. 7. Resistance: Abscisic acid increases resistance of plants to cold and other types of stresses. It is, therefore, also known as stress hormone. 8.Starch Hydrolysis Abscisic acid inhibits gibberellin mediated amylase formation during germination of cereal grains. 9. Flowering: In small quantities, abscisic acid is known to promote flowering in some short day plants, e.g., Strawberry, Black Currant. 10. Parthenocarpy : ABA has been found to induce parthenocarpic development in Rose.

8. FLOWERING

9. 2.Ethylene: It is a gaseous hormone which stimulates transverse or isodiametric growth but retards the longitudinal one. Businessmen dealing with storing and shipping of fruits had known quite early that a rotten or ripe fruit could trigger early ripening of other fruits present nearby. Cousins (1910) found that ripe oranges produced a volatile substance that hastened ripening of unripe bananas nearby.

10. Functions of Ethylene: 1. Growth: Ethylene inhibits longitudinal growth but stimulates transverse or horizontal growth and swelling of axis. 2. Gravity: It decreases the sensitivity to gravity. Roots become Apo-geotropic while stems turn positively geotropic. Leaves and flowers undergo drooping. The phenomenon is called epinasty . Seedlings develop tight epicotyl hook. 3. Senescence: It hastens the senescence of leaves and flowers. 4. Abscission: Abscission of various parts (leaves, flowers, fruits) is stimulated by ethylene which induces the formation of hydrolases . 5. Apical Dominance: Ethylene promotes apical dominance and prolongs dormancy of lateral buds.

11. 6. Breaking of Dormancy: It breaks the dormancy of buds, seeds and storage organs. 7. Abscisic Acid: It seems that formation of abscisic acid in the leaves under conditions of water stress is mediated through ethylene. 8. Growth of Rice Seedling: Ethylene promotes rapid elongation of leaf bases and internodes in deep water rice plants. As a result leaves remain above water. 9. Root Initiation: In low concentration ethylene helps in root initiation, growth of lateral roots and root hairs. This increases the absorption surface of the plant roots.

12. BREAKING OF DORMANCY GROWTH OF RICE SEEDLING

13. 10. Fruit Ripening: It aids in ripening of climacteric fruits and dehiscence of dry fruits. Climacteric fruits are fleshy fruits which show a sudden sharp rise of respiration rate at the time of ripening (respiratory climacteric). They are usually transported in green or unripe stage. Ethylene is used to induce artificial ripening of these fruits, e.g., Apple, Mango, Banana, etc. 11. Flowering: It stimulates flowering in Pineapple and related plants as well as mango though in other cases the gaseous hormone causes fading of flowers. This helps in synchronizing fruit set. 12. Sex Expression: Like auxins and cytokinins , ethylene has a feminizing effect on sex expression. The genetically male plants of Cannabis can be induced to produce female flowers in the presence of ethylene. The number of female flowers and hence fruit is enhanced in monoecious plants like Cucumber.

14. DELAY OF FRUIT RIPENING

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