Plant Growth Substances

1. Plant Growth Substances

2. Introduction Like those of animals, the activities of plants’ bodies are tightly controlled by a range of hormones. These hormones move about the plant by translocation in the phloem

3. Auxins Produced in the tips, or apical meristems’ of shoots and roots (‘coleoptiles’) Increase the ‘plasticity’ of cell walls Redistribution in the phloem away from light causes phototropism Promote growth by cell elongation in shoots while having the opposite effect in roots

4. Other effect of the auxins… Apical dominance By inhibiting the development of lateral bud, auxins promote tall, narrow growth preventing ‘bushiness’.

5. Other effect of the auxins… Apical dominance Formation of lateral roots Primary growth is inhibited in roots by auxins while the production of adventitious (lateral) roots is promoted

6. Other effect of the auxins… Apical dominance Formation of lateral roots Abscission Probably by causing the secretion of ethene, auxin promotes the later stages of leaf and fruit fall

7. Other effect of the auxins… Apical dominance Formation of lateral roots Abscission Fruit development Produced in the developing seed, auxins stimulate the development of fruits

8. Gibberellins (GA) GA is produced in the tips of leaves and also in roots, buds and seeds. GA breaks seed dormancy by promoting germination It stimulates the secretion of hydrolytic enzymes in the seed which causes the liberation of stored food molecules

9. Gibberellins (GA) GA is produced in the tips of leaves and also in roots, buds and seeds. GA breaks seed dormancy by promoting germination GA stimulates fruit development GA causes stem elongation (by increasing internodal distance) GA can mimic the effects of long days

10. Cytokinins Remember: cytokinesis = cell division Produced in the meristems, cytokinins promote cell division They promote growth of lateral buds They delay senescence Help break seed dormancy

11. Abscissic acid Found throughout the plant abscissic acid promotes the abscission of leaves, fruits and flowers Abscissic acid also… promotes seed dormancy causes stomatal closure during drought stress promotes senescence in fruit

12. Ethene The only gaseous hormone, it is moved by diffusion around the plant rather than translocation Ethene is produced throughout the plant Stimulates the final stages of fruit development = ripening It also stimulates fruit and flower fall.

13. Interactions between hormones Synergism is when two hormones act together to bring about an effect Antagonism is when two hormones counteract each other and reduce each other’s effects