Control of Apical Dominance
Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds
If the terminal bud is removed, plants become bushier.
Anti-Aging Effects
Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues.
Cytokinins
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Gibberellins
Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination.
Stem Elongation
Gibberellins stimulate growth of leaves and stems.
In stems, they stimulate cell elongation and cell division.
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Fruit Growth
In many plants, both auxin and gibberellins must be present for fruit to set.
Gibberellins are used in spraying of Thompson seedless grapes.
Germination
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate.
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Effects of gibberellins on stem elongation and fruit growth
Gibberellin-induced stem
growth
(b) Gibberellin-induced fruit
growth
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Mobilization of nutrients by gibberellins during the germination of seeds such as barley
Gibberellins (GA)
send signal to
aleurone.
Aleurone secretes
-amylase and other enzymes.
Sugars and other
nutrients are consumed.
Aleurone
Endosperm
Water
Scutellum
(cotyledon)
Radicle
1
2
3
GA
GA
-amylase
Sugar
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Brassinosteroids
Brassinosteroids are chemically similar to the sex hormones of animals.
They induce cell elongation and division in stem segments.
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Abscisic Acid
Abscisic acid (ABA) slows growth.
Two of the many effects of ABA:
Seed dormancy
Drought tolerance.
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Seed Dormancy
Seed dormancy ensures that the seed will germinate only in optimal conditions.
In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold.
Drought Tolerance
ABA is the primary internal signal that enables plants to withstand drought.
Abscisic Acid = ABA
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