Plant Growth Regulators Classified by Function
Plant Growth Regulators (PGRs) are synthetic chemicals that have the same physiological effects and similar chemical structures as endogenous plant hormones. Falling under the broad category of pesticides, PGRs are used to control plant growth and development. They include both synthetic compounds similar to natural plant hormones and hormones directly extracted from biological organisms.
PGRs can be broadly classified into three types based on their function:
First Category: Plant Growth Promoters
These promote cell division, differentiation, and elongation in plants. They enhance the growth of vegetative organs and reproductive organs, prevent fruit drop, encourage rooting and germination, and induce parthenocarpy. Their regulatory effects are similar to those of endogenous plant hormones such as auxins, cytokinins, or gibberellins. Common plant growth promoters include indole-3-acetic acid, indole-3-butyric acid, α-naphthaleneacetic acid, 6-benzylaminopurine, 4-chlorophenoxyacetic acid, and 2,4-dichlorophenoxyacetic acid.
Second Category: Plant Growth Inhibitors
These inhibit the growth of apical meristems and sprouting, eliminate apical dominance to increase lateral branches, and control weeds. The effects of plant growth inhibitors cannot be reversed by applying gibberellins. Many herbicides, when used at very low concentrations, can also function as growth inhibitors. Their regulatory effects are similar to those of endogenous plant hormones such as abscisic acid. Common plant growth inhibitors include maleic hydrazide, glyphosate, mepiquat chloride, chlorpropham, daminozide, and triiodobenzoic acid.
Third Category: Plant Growth Retardants
These inhibit the growth of sub-apical meristems, reducing internode elongation without suppressing apical bud growth. This results in shorter, thicker stems, increased leaf thickness, and higher chlorophyll content. Their action is primarily through regulating gibberellin synthesis, and their effects can be reversed by applying gibberellins. Common plant growth retardants include chlormequat, mepiquat chloride, paclobutrazol, daminozide, uniconazole, and trinexapac-ethyl.
Advantages of Plant Growth Regulators
Wide Range of Action and Applications: PGRs are suitable for almost all higher and lower plants in agriculture. By regulating physiological processes such as photosynthesis, respiration, nutrient absorption and transport, signal transduction, stomatal behavior, osmotic pressure, and transpiration, they control plant growth and development, improve plant-environment interactions, enhance crop resilience, increase yields, and improve product quality.
Low Dosage, Rapid Action, High Efficiency: Most crops only require a single application within the prescribed period each season.
Bidirectional Regulation: PGRs can regulate both external traits and internal physiological processes of plants.