Plant growth regulators (PGRs) are a class of synthetic or naturally extracted chemical substances with similar physiological functions to plant hormones. They can achieve goals such as increasing yield and improving quality by regulating the growth and development of plants. According to their mechanism of action and physiological functions, they are mainly divided into the following categories:
1. Growth promoters
Gibberellins (GA): such as GA3 (920) and GA4+7, which can break seed dormancy (such as potato seed soaking treatment), promote seedling growth, increase fruit setting rate (such as spraying during the flowering period of citrus) and induce seedless fruits (such as combined with cytokinins for seedless grapes). In rice hybrid seed production, it can make the flowering period of parents meet, increasing yield by more than 30%.
Auxin: including indolebutyric acid (IBA), naphthaleneacetic acid (NAA), etc. IBA has a significant effect in inducing rooting in tissue culture (the rooting rate of blueberry tissue culture is 90%) [3; NAA is used for fruit tree cuttings (the rooting rate of apple hardwood cuttings increased from 15% to 85%). Phenol compounds such as 2,4-D (low concentration to preserve fruit, high concentration to kill weeds) need to strictly control the concentration.
2. Growth retardants and inhibitors
Paclobutrazol (PP333): a triazole compound that shortens rice internodes by 40%-50% and improves lodging resistance by 3 levels by inhibiting GA synthase activity. After application to young apple trees, the growth of new shoots is reduced by 60%, promoting early fruiting. However, the soil residue period is as long as 2-3 years.
Chlormequat (CCC): a quaternary ammonium salt regulator that inhibits the elongation of internodes at the base of wheat (after spraying in the Huanghuaihai wheat area, the plant height is reduced by 15-20cm and the yield is increased by 8%-12%). It is also used for flower plant shape regulation (such as poinsettia and chrysanthemum).
3. Cytokinins
6-benzylaminopurine (6-BA), chlorfenapyr (CPPU), etc.: promote cell division and differentiation (regulate organ formation in tissue culture), delay aging (maintain chlorophyll stability), thin fruit and improve fruit shape. Zeatin has higher activity but its application is limited by cost.
4. Ethylene regulators
Ethephon: promote fruit ripening and coloring (such as tomato ripening), inhibit vegetative growth, and increase the proportion of female flowers (spray male to female in the "bisexual period" of melons).
Ethylene inhibitors: inhibit ethylene production by blocking ACC synthase and delay aging.
5. Other types
Abscisic acid (ABA): antagonizes GA, promotes fruit ripening and enhances stress resistance (such as drought resistance treatment of grapes).
Brassinolide: has both auxin and cytokinin functions, enhances photosynthesis, relieves drug damage, and resists cold and frost.
Note: In actual application, it is necessary to make reasonable choices based on crop types, growth periods and goals to avoid abuse (such as the risk of 2,4-D toxicity or paclobutrazol residue problems).
