Abscisic acid (ABA) also known as dormin is a growth inhibitor found in a wide variety of plants. It was first isolated by Cams and Addicot in 1963.
As these substances induced dormancy, they were named as dormin. Ohkuma etc. Isolated an inhibitor from cotton fruits and named it as abscisin II. Later on it was found that abscisinll and dormin were identical.
Abscisic acid opposes the action of auxins, gibberellins and cytokinins. It is produced mostly in mature leaves and translocated through phloem and xylem to shoot tips.
1. It accelerates abscission and senescence in many plants such as cotton.
2. It induces dormancy in buds and seeds and inhibits seed germination in ash and Lettuce.
3. It inhibits IAA-induced growth in oat coleoptiles and in many test conditions GA- induced growth is also inhibited by ABA.
4. It inhibits GA-induced enzyme synthesis in barley aleurone layers.
5. Regulates growth and ripening of fruits.
6. Responsible for geotropism in certain roots. Ethylene (ETH)
Ethylene (H2C=CH2) is the only gaseous plant hormone synthesized in very minute quantities in ripening fruits, leaves, flowers, stems, roots, tubers and seeds. Its recognition as a natural plant hormone was confirmed by Pratt and Goeschl in 1969. In plants, ethylene is synthesized from the amino acid methionine and diffuses from one part to other.
1. It induces ripening of fruits, rooting in certain plants and post harvest maturation of certain fruits like apples, pears, tomatoes, oranges etc.
2. Stimulates seed germination and also flowering in certain plants (Pineapple).
3. Promotes leaf abscission while causes enormous growth of auxilary buds.
4. It induces root hair formation and epinastic and geotropic movements in plants. Other plant hormones