The plant growth regulators are small simple molecules of diverse chemical composition. They are also called as plant hormones or phytohormones. They produce in one place and transported to other place for its function.
Auxin:
Auxin was first isolated from human urine. The term auxin is applied to the indole 3 acetic acid and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growth apices of the plants from where they migrate to the regions of their actions. Auxins like IAA and Indole butyric acid have been isolated from plants. NAA(naphthalene acetic acid) and 2,4 D(2,4 dichloro phenoxy acetic acid) are synthetic auxins.
Role of auxins
- They initiate the formation of roots in cutting stem
- It promotes the flowering
- It promote abscission in mature and older leaves but prevent falling of fruit and leaves at early stages.
- It shows the apical dominance-It inhibit the growth of plants as lateral side.
- It induce parthenocarpy in fruit.
- It acts as weedicide helps in eradication weeds.
Gibberellins
Gibberellins are another kind of growth promoter. There are more than 100 Gibberellins discovered so far and named as GA1, GA2,GA3 and so on. However GA3 is most commonly occurring Gibberellin in plant.
Role of Gibberellins
They have the capacity to increase the intermodal region of plants. They delay senescence. It promote the bolting in beet cabbage, etc. It breaks up the dormancy period in seeds.
Cytokinin
Cytokinin have specific affects an cytokinesis and were discovered as kinetin from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. Related compound Zeatin is found in corn-keneis and coconut milk.
Natural cytokinin are synthesized in regions where rapid cell division occurs. It helps to produce leaves, chloroplast in leaves lateral shade growth. It helps in overcome the apical dominance. They promote nutrient mobilization.
Ethylene
It is a simple gaseous PGR. It is synthesized in large amount by tissue undergoing senescence and ripening fruits. Influence of ethylene on plants includes horizontal growth of seedlings swelling of the axis and apical hook formation in dicot seedlings. It promotes senescence and abscission in plants fruit ripening etc.
Abscisic acid
It acts as general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibit seed germination. It stimulates the closer of stomata in epidermis and increase the tolerance of plants to various kinds of stresses. Therefore it is also called as stress hormone.
Photoperiodism
Flowering in certain plants depends not only on a combination of light and dark exposures but also their relative duration. This response of plants to the period of day/night is termed as photoperiodism. The sites of perception of light/dark/ duration of leaves.
It has been hypothesized that there is a hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photo period. The significance of photo periodism is in regulating flowering in plants. Flowering is an important step towards the seed formation. Thus photoperiodism has an important role in evolution.
Vernalization
There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed as vernalisation. It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity. Vernalization refers specially to the promotion of flowering by a period of low temperature.
Some important food plants wheat, barley rye have two kinds of varieties-winter and spring varieties. The spring variety normally planted in the spring and produce flower and grain before the end of the growing season. Winter varieties however if planted in springs would normally fail to flower or produce mature grain within a span of a flowering season. Hence they are planted in autumn. They germinate and over winter come out as small seedlings resume growth in the spring and are harvested usually around mid summer.