Plant hormones or phytohormones are substances produced by plants and that act in the regulation of their development and growth.
The function of hormones is to act as "chemical messengers" between cells, tissues and organs of higher plants.
They have action even in small amounts.
Hormones act in specific places, aiming to trigger an action.
Generally, they are carried to their place of action by the xylem and phloem. However, they can also act in the same place where they were produced.
The main plant hormones are: auxins, gibberellins, cytokinins, ethylene and abscisic acid.
Auxins
At auxins they constitute the first class of plant hormones discovered.
Auxins are produced at the ends of grass coleoptiles and at the tips of the stems of various plants. As well as in the meristems of young leaves, of fruits and seeds.
In general, they act in the development of the lateral gems, tropisms and in the development of fruits.
Its characteristic action is cell elongation and expansion, promoting the growth of roots and stems.
However, this condition is determined by the amount of hormone. At high concentrations, they inhibit cell elongation.
The movement of auxin is called unipolar because it is unidirectional, from the apex of the meristems towards the base of leaves, stems and root tips. This type of transport requires energy and is not influenced by gravity.
Indoleacetic acid (IAA) is the most common natural auxin found in vegetables.
Also read about Phototropism and Geotropism.
Gibberellins
Gibberellins are produced in apical meristems of the stem and root, in young leaves, in the seed embryo and in the fruits.
The class of gibberellins control several aspects of plant growth and development. They act on stem elongation, on the growth of roots and fruits and on the germination of seeds.
The young plant embryo produces gibberellins that stimulate the seed to synthesize digestive enzymes. These enzymes degrade organic molecules stored in the endosperm. As a result of this degradation, sugars and amino acids are released into the embryo.
Currently, there are more than 137 types of gibberellins. The best known is gibberellic acid.
Cytokinins
Cytokinins are abundant in places with high cell proliferation activity, such as germinating seeds, developing fruits and leaves, and root tips.
In association with auxins, they act in cell division and control of apical dominance. In this case, their relationship is antagonistic, with auxin inhibiting the growth of lateral buds, while cytokinin promotes this growth.
Cytokinins are also responsible for slowing down plant aging.
Ethylene
Ethylene is the only gaseous plant hormone. It is a colorless gas.
It is produced in different parts of plants and probably diffuses into spaces between cells.
Its main action is to induce the ripening of the fruits.
abscisic acid
Abscisic acid is produced in leaves, hood and stem. It is produced in the roots and transported via the xylem.
Abscisic acid is an inhibitor of plant growth. It is responsible for blocking plant growth during winter.
It also acts on seed dormancy, preventing them from germinating prematurely.
Read too:
plant histology
meristems
Exercises
1. (UFF) A bunch of green bananas was divided into two parts: the first was placed in a bag with little air and the second was exposed to the air. After a few days, it was found that the fruits placed in the bag ripened more quickly. This happened as a result:
a) the decrease in the partial pressure of O‚ that stimulates the release of auxins;
b) the increase in the partial pressure of CO‚ that stimulates the release of gibberellins;
c) the release of a gaseous hormone;
d) the release of abscisic acid, a hormone of a protein nature;
e) the decrease in the action of auxins that are activated by light.
c) the release of a gaseous hormone;
2. (PUC-RS) The tropisms observed in higher plants are growths induced by plant hormones and driven by environmental influences. The curvature of the stem towards the light and of the root towards the ground are typical examples of positive phototropism and geotropism, respectively.
Such movements occur as a result of the differential concentration of phytohormones such as ________, in the different structures of the plant. High rates of this phytohormone, for example, ________ cell growth, which ________ the curvature of the stem towards light.
The terms contained in the alternative complete the text above, respectively:
a) cytokine - promote - induce
b) auxin - induce - provoke
c) gibberillin - inhibit - prevent
d) auxin - block - inhibit
e) cytokine - prevent - block
b) auxin - induce - provoke
3. (UFRN) When the plant is pruned, usually the lateral buds develop because:
a) cytokinin production increases, especially in pruned branches.
b) the plant is stimulated by the ethylene released by the injured region.
c) the plant starts to produce gibberellic acid, in order to have leaf abscission.
d) loss of apical dominance reduces auxin concentration.
d) loss of apical dominance reduces auxin concentration.
4. (UFPI) Auxins are:
a) plant hormones that control fruit development.
b) contractile proteins found in animal cells.
c) animal hormones responsible for secondary sexual characters.
d) epidermal structures that control the entry and exit of gases from the plant.
e) chemical mediators of nerve synapses.
a) plant hormones that control fruit development.
