Five major types of hormones regulate plant growth and development
Plant hormones exert control by affecting cell division,
elongation, and differentiation
1- Auxins
2- Cytokinins
3- Gibberellins
4- Abscisic acid
5- Ethylene
Auxin
Auxin stimulates the elongation of cells in young shoots
1- Auxins are a class of chemicals that promote seedling
elongation
2- The apical meristem of shoot tips is a major source of IAA
synthesis
3- In moderate concentrations, auxin promotes cell elongation
in stems
4- In high concentrations, auxin reduces cell elongation in
stems
5- Auxins affects cell elongation in roots at lower
concentrations
Auxin has a number of effects other than elongation
- Promotes
growth in stem diameter
Promotes growth of
fruit-
- Can induce
fruit development without pollination
Cytokinin
1 Cytokinins
stimulate cell division (cytokinesis)
2- Cytokinins are produced in actively growing organs or
tissues as roots, embryos and fruits.
3- Cytokinins and auxins
interact to control apical dominance:
- Auxins inhibit axillary bud growth (reducing lateral
branching)
- Cytokinins counter the action of auxin by promoting
axillary bud growth
- The ratio of auxins to cytokinins controls axillary bud
growth
Gibberellins
Gibberellins
affect stem elongation and have numerous other effects
1-
They are plant hormones that promote stem and leaves elongation by increasing
cell division
2-
They promote fruit development and seed germination
3-
Enhance the action of auxin
Ethylene
1-
Ethylene triggers fruit ripening and other aging processes
2-
Plants produce ethylene in response to stress as mechanical pressure, injury,
infection and drought or flood
3-
Ethylene promotes leaf abscission in fall by breaking down cells at the base of
the petiole
4-
Growers inhibit ethylene production using CO2 to inhibit ripening in
stored fruits
Abscisic
acid (ABA)
Abscisic
acid inhibit many plant processes:
1-
ABA is a plant hormone that slow or inhibits growth
2-
High concentrations of ABA promote seed dormancy
3-
Ratio of ABA to gibberellins controls germination
4-
ABA influences plant water relations
-
Accumulation of ABA in wilted leaves promotes stomatal closure
-
ABA produced in roots can signal low soil moisture conditions and triggers plants to conserve water by
closing stomata.
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