We know that plant coordinate their response against the environmental stimulus using hormones. These hormones are called plant hormones or Phytohormones.Plant hormones or phytohormones are naturally occurring organic chemical substances present in plants that bring about control and coordination of various activities in them. They are, therefore, also known as plant growth substances.
These phytohormones are synthesized in minute quantities in one part of the plant’s body and these simply diffuse to another part where they influence specific physiological processes.
There are five major types of phytohormones or plant growth substances that are involved in the control and coordination of plants:
- Abscisic acid
- Ethene (ethylene)
Auxins are synthesized at the shoot tip and help the cells to grow longer. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light. Thus, the plant appears to bend towards the light.
Gibberellins– These promote cell enlargement and cell differentiation in plants in the presence of auxins. These also promote growth in stems and fruits. These also induce parthenocarpy (i.e. the formation of seedless fruits without fertilization) in many plants.
Cytokinins– These promote cell division and occur in greater concentration in areas of rapid cell division such as in fruits and seeds. They help in breaking the dormancy in seeds and buds; These delay the ageing in leaves and also promote the opening of stomata.
Abscisic Acid – It promotes the dormancy in seeds and buds and thereby inhibits growth. It also promotes the closing of stomata and thus effects wilting of leaves. It also promotes the falling of leaves (abscission)
Ethene or ethylene- It promotes growth and ripening of fruits. It helps in breaking the dormancy in buds and seeds. It stimulates the formation of separation layer (abscission zone) in leaves, flowers and fruits. It promotes yellowing of leaves
The duration of sunlight also regulates the germination of seeds and flowering in plants. The length of the day during which the sunlight is available to the plants is called photoperiod. Plants respond to photoperiod with the help of a special blue-green pigment called phytochrome pigment. It is present in very small amounts. The effect of photoperiod on the germination of seeds and flowering in plants is called photoperiodism.
Thus, plant hormones (Phytohormones) and phytochrome pigment together are involved in the control and coordination of plant responses.