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Other Plant Hormones Gibberellins • There are now more than 60 known gibberellins. • These cause rapid growth of the internodes on stems. – The internode is the region between the points where the leaves are attached. – Dwarf plants are often short because they do not produce enough gibberellin to form long internodes. Gibberellins • These hormones are produced in young leaves and buds. • They cause flowering in long-day plants even when they are kept in short-day conditions. • They cause flowering in biennials that normally need a period of chilling (vernalisation) Gibberellins • They help to mobilise the endosperm (food reserve) in cereal seeds. • They promote the germination of a wide variety of seeds that would otherwise be hard to germinate. • They are becoming increasingly important in industry – e.g. they increase the size of seedless grapes when sprayed on them. The Cytokinins • These are produced mainly in the roots • They regulate the process of cell division. • These hormones are found in coconut milk, fruits, seeds and roots. Promoting Cell Division • Cytokinins promote cell division in intact plants and tissue cultures. – Relatively high conc of auxin to cytokinin produces roots in a tissue culture. – Higher proportions of cytokinins to auxins produces buds and leaves. – Roughly equal concs make callus tissue (undifferentiated tissue) Cytokinins • Cytokinins slow down the process of aging (senescence) in plants, if applied to leaves it can prevent the yellowing of mature leaves in autumn and their dropping (abscission). Abscisic Acid • In general, this functions as an inhibiting hormone acting against auxin, gibberellins and cytokinins. • It plays a role in abscission. • Dormant buds have relatively high levels of ABA which drop as spring approaches • Some dormant seeds also contain relatively high concs of ABA Abscisic Acid • It also seems to have a role in helping plants to withstand drought conditions, by affecting transpiration and closing guard cells of the stomata. Ethylene Gas • “one rotten apple spoils the barrel” • A ripening apple gives off ethylene gas that ripens adjacent fruit. • Commercially ethylene gas is important: – It influences the ripening of fruit – It helps the harvesting of berries, causing them to drop. – It helps the rubber in rubber trees to flow for longer. Ethylene Gas – It increases the sugar content in sugar cane. – It promotes flowering of pineapples. – It accelerates the aging of tobacco leaves. Florigen • The flowering hormone. • This has not been isolated but there is evidence that there is a hormone which induces flowering.