1. Other Plant Hormones

2. 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.

3. 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)

4. 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.

5. 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.

6. Promoting Cell Division • Cytokininspromote 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)

7. 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).

8. 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

9. 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.

10. 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.

11. Ethylene Gas • It increases the sugar content in sugar cane. • It promotes flowering of pineapples. • It accelerates the aging of tobacco leaves.

12. Florigen • The flowering hormone. • This has not been isolated but there is evidence that there is a hormone which induces flowering.