AGRITEK

Sustainabble Solutions

Plant Growth Regulators

Plant Growth Regulators are defined as small, simple chemicals produced naturally by plants to regulate their growth and development.

Characteristics

Plant Growth Regulators can be of a diverse chemical composition such as gases (ethylene), terpenes (gibberellic acid) or carotenoid derivates (abscisic acid). They are also referred to as plant growth substances, phytohormones or plant hormones. Based on their action, they are broadly classified as follows:

• Plant Growth Promoters – They promote cell division, cell enlargement, flowering, fruiting and seed formation. Examples are auxins, gibberellins and cytokinins.
• Plant Growth Inhibitors – These chemicals inhibit growth and promote dormancy and abscission in plants. An example is an abscisic acid.

Note: Ethylene can be a promoter or an inhibitor, but is largely a Plant Growth Inhibitor.

Effects

• Promote flowering in plants like pineapple.
• Help to initiate rooting in stem cuttings.
• Prevent dropping of fruits and leaves too early.
• Promote natural detachment (abscission) of older leaves and fruits.
• Control xylem differentiation and help in cell division.

Gibberelins

These phytohormones are partly responsible for cell division and the elongation of stems and other tissues. They were discovered by Japanese researchers studying a disease in rice. The disease caused recently germinated seedlings to acquire a yellowish colour and the stem to elongate excessively, leading to the death of the plant. The researchers discovered that these symptoms were caused by the Gibberella fujikuroi fungus. This fungus produces a large quantity of these phytohormones which are introduced into the host plant.

Since then, various types of gibberelins have been discovered and isolated. These were given successive numbers as they were discovered; GA1, GA2, GA3, etc. GA3 is gibberellic acid.

Gibberelins are synthesised mainly in meristematic organs and developing tissues.

Functions of gibberelins

Seed germination. In seeds, some of the gibberelins combine with glucosides, and become inactive in this form. During germination, enzymes destroy this combination and the gibberelins are unlocked and activated. This stimulation of germination has been demonstrated in a number of experiments which showed how the application of gibberelins accelerated the germination of lettuce seeds. It was also shown that exposure to light accelerated the germination of lettuce seeds. Later studies showed that light accelerates the transformation of the gibberelins from the inactive conjugated form to the active form.

Sex expression. In species with unisexual flowers – that is, separate male and female flowers, either on the same plant (monoecious) or on different individuals (dioecious) – gibbberelins appear to have a regulatory effect on sex expression. For example, the application of gibberelins in female asparagus plants produces the appearance of male and hermaphrodite flowers. By contrast, gibberellin application in maize plants produces the appearance of female flowers in the tassels (masculine inflorescences).

Abscisic acid (ABA)

As the name indicates, this hormone is directly implicated in the senescence and abscission of leaves, flowers and fruits. It also affects the latency of some seeds.

As in the case of ethylene, this phytohormone induces the expression of resistance genes for a number of stresses. One effect of ABA is to produce stomatal closure during drought conditions, thus preventing dehydration in the plant.