controlling plant growth

• auxins are plant hormones which are responsible for regulating plant growth
• abscisic acid inhibits bud growth. high levels of auxins in the shoot may keep abscisic acid high in the bud. when the source of auxin is removed, the abscisic acid levels drop and buds grow
• Cytokinins promote bud growth. by directly applying cytokinin to buds, it is possible to override the apical dominance effect. high levels of auxin make the shoot apex a sink for cytokinins produced in the roots. most of the cytokinins goes to the shoot apex, when the apex is removed then cytokinin can spread evenly around the plant.
• gibberellins are the plant hormones which are responsible for the control of stem elongation and seed germination.
• gibberellins also promote seed germination. when the seed absorbs water, the embryo releases gibberellin which travells to the aleurone layer in the endosperm region of the seed. the gibberellin enables the production of amylase, which can break down starch into glucose. this provides a substrate for the embryo and so it grows- the glucose is also used for protein synthesis.