Plant growth regulators refer to non-nutritive chemicals synthesized artificially (or extracted from microorganisms) and applied externally to plants to regulate plant growth and development. There are many kinds of plant growth regulators, but they can be roughly divided into the following categories according to their sources, modes of action and application effects: 1. Auxin Auxin is the earliest growth regulator used in agriculture. Indopropionic acid and indolebutyric acid were the earliest used. They all have indole rings similar to indole -3- acetic acid (IAA), but the side chain lengths are different. Later, it was found that compounds with naphthalene ring instead of indole ring, such as α -naphthylacetic acid (NAA) and compounds with benzene ring, such as 2,4-dichlorophenoxyacetic acid (2,4-d), also had similar physiological activities to indoleacetic acid. In addition, naphthoxyacetic acid (NOA), 2,4,5-trichlorophenoxyacetic acid (2,4,5-trichlorophenoxyacetic acid, 2,4,5-t), 4- iodophenoxyacetic acid (4- iodophenoxyacetic acid, trade name of chanbufalin) and their derivatives (including salts). At present, IBA, NAA and 2,4-Didi are the most widely used in production. They are insoluble in water and soluble in organic solvents, such as alcohols, ketones and ethers. The main physiological function of auxin is to promote the growth of plant organs, prevent organs from falling off, promote fruit setting and induce flower bud differentiation. In horticultural plants, it is mainly used for cutting and rooting, preventing falling flowers and fruits, promoting fruit, controlling gender differentiation, changing branching angle and promoting pineapple flowering. 2. There are many kinds of gibberellins, including 12 1, all of which are derivatives with gibberellin as the skeleton. Commercial gibberellin is mainly obtained by large-scale culture of genetically different generations of Gibberella clones, and its products include gibberellic acid (GA3) and the mixture of GA4 and GA7. There are also some compounds that do not have the basic structure of gibberellin, but also have the physiological activity of gibberellin, such as coumarol, kauri acid and so on. At present, GA3, also known as 920, is insoluble in water, soluble in organic solvents such as ethanol, acetone and glacial acetic acid. It is relatively stable under low temperature and acidic conditions, and it is ineffective when neutralized with alkali. Attention should be paid to its preparation and use. The main physiological functions of gibberellin are to promote cell elongation, prevent delamination, break dormancy, break dormancy of tubers and bulbs, and also induce flowering, increase fruit-setting rate and parthenocarpy rate of some plants, and increase the proportion of male flower differentiation. 3. Cytokinin Cytokinin is a kind of plant growth regulator which mainly promotes cell division, and all of them are derivatives of adenine. Common synthetic cytokinins are kinetin (KT), 6- benzyladenine (BA.6-BA) and tetrahydropyranyl benzyladenine (PBA). Although some chemicals have no adenine structure, they also have the physiological function of cytokinin, such as diphenyl uea. Kinetin and 6- benzyladenine are widely used in horticultural production. When in use, it is dissolved with a small amount of alcohol and then diluted with water. Kinetin is easy to be destroyed in acidic solution, and a small amount of alkali should be added during preparation. The main physiological functions of cytotaxonomy are promoting cell division, inducing bud differentiation, promoting lateral bud development, eliminating apical dominance, inhibiting organ senescence, increasing fruit setting rate and improving fruit quality. Ethylene is inconvenient to use because it is gaseous at room temperature. There are various ethylene generators commonly used, which can release ethylene in plants after being absorbed by them. Ethylene generators include ethephon (CEPA), Alsol, CGA- 1528 1, ACC, cyclohexylimine, etc. Ethephon is the most widely used in production. Ethephon is a kind of strong acid substance, which has corrosive effect on skin and metal containers, especially when it meets alkali, it will produce flammable gas, so special attention should be paid to safety issues when using it. The main function of ethephon in production is to ripen fruits, promote flowering and female flower differentiation, promote shedding and promote the secretion of secondary substances. Ethylene inhibitors, such as aminoethoxyvinylglycine (AVG), aminooxyacetic acid (AOA), silver thiosulfate (STS), silver nitrate (silver nitrate), etc. In production, it is used to inhibit the production or function of ethylene, reduce fruit shedding, inhibit fruit ripening and prolong the fresh-keeping life of fruits and cut flowers. 5. Growth inhibitors and growth retardants Growth inhibitors are growth regulators that inhibit the growth of plant apical meristem, which can slow down cell division and inhibit elongation and differentiation, but can promote the differentiation and growth of lateral branches, destroy apical dominance, increase the number of lateral branches, and make plant morphology change greatly. Some growth inhibitors can also make the leaves smaller and affect the development of reproductive organs. External use of auxin can reverse this inhibitory effect. The common growth inhibitors are triiodobenzoic acid (TIBA), morphine peptide and gentamicin (MH). Growth retarder is a kind of growth regulator that inhibits the growth of sub-apical meristem. It shortens the internodes of plants and makes the plants compact and short, but it does not affect the growth of apical meristem, the development and number of leaves and the development of flowers. The elongation of subapical meristem cells is mainly caused by gibberellin, so exogenous gibberellin can reverse this effect. Common growth retardants are chlormequat chloride (CCC), auxin (Pix), paclobutrazol (PP333), uniconazole (S-3307), Bijiu (B9) and so on. 6. Other types of growth regulators Some newly discovered and synthesized plant growth regulators have different action modes or mechanisms from the above-mentioned regulators, and their properties have not been fully understood, so they are temporarily classified as one category. Such as zearalenone, oligosaccharin and triacontanol.