Fruit drop occurs when fruit has less auxin as compared to stem
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Auxins and Gibberellins are two major hormones that affect the growth of plants. They are effective at low concentrations. They are produced in one area of the plant and transported to other parts where they have an effect.
Auxins, e.g IAA
Indole Acetic Acid (IAA) is a produced at the growing points – root tips, shoot tips and meristems
IAA stimulates cell division, elongation and differentiation
High levels of IAA stimulates shoot growth and inhibits roots growth. This allows IAA to be used as a selective herbicide. It causes the plant shoots of broad-leaved plants to grow so rapidly that it exhausts the plant, whilst having little effect on narrow leaved plants
Low levels of IAA promote the growth of adventitious roots in cuttings and can therefore be used as a rooting powder
IAA is destroyed by light and this leads to the phototropic effect (plants moving towards the light). IAA on the side nearest the light is destroyed and therefore the cells do not undergo as much division and elongation, whilst those on the opposite side are experiencing the effects of IAA causing the apparent bending of the plant
IAA effects on other plant organs
IAA produced in the apical buds can inhibit the growth of lateral (side) buds. Apical dominance can be prevented by removing the apical bud. This allows the side buds to develop and the plant will become bushy.
IAA produced by developing seeds causes the fruit coat to develop from the ovary wall.
Low levels of IAA cause the formation of an abscission layer, which leads to leaf drop.
Gibberellins, e.g GA
Gibberellic Acid (GA) overcomes dormancy in seeds and buds. It induces the production of \alpha-amylase by the aleurone layer in barley seeds.
Gibberellic Acid, GA, indclsing the production of amylase in a the aleurone layer of a barley seed.
GA can also be used to grow dwarf plants to normal height. Cell elongation is promoted and the internode length is increased.
A dwarf plant and a normal plant. The normal plant has a longer internode marked.