Plant growth regulators are a class of substances with similar physiological and biochemical effects to plant hormones. They induce localized or transient fluctuations in cell or electrolyte exchange within plants through stimulation, inhibition, or hormone receptor blockade, thereby increasing or decreasing the state of a specific hormone in the plant. Plant growth regulators not only increase crop yield but also enhance stress resistance and improve quality, playing a significant role in extending product shelf life and in crop breeding development.
A. Categories of Plant Growth Regulators
Approximately 40 active ingredients exist in plant growth regulators, which can be used as single components or in combination. They can actively regulate many plant developmental processes, such as accelerating or delaying seed germination, breaking plant dormancy, stimulating or reducing stem elongation, inducing flowering and fruiting, reducing or increasing fruit yield, and accelerating or delaying senescence processes (including fruit ripening and leaf fall). Benefits in production include improved crop management, increased and ensured yield and quality of harvested products, and extended crop shelf life.
Based on their role in agricultural production, plant growth regulators can be classified into five categories: plant growth promoters, plant growth retardants, plant growth inhibitors, preservatives, and drought-resistant agents. They can typically be transported to the site of action within the plant. At very low concentrations, they can promote or inhibit certain stages of plant life processes, allowing them to exert their desired effects.
Growth Promoters: These improve crop growth efficiency and can also facilitate the transition from vegetative to reproductive growth, increasing fruit set and yield.
Growth Retardants: Their main function is to reduce new shoot growth, promote flower bud formation, improve fruit set, delay ripening, and increase firmness and storage resistance.
Growth Inhibitors: These can completely inhibit shoot tip growth and have a permanent inhibitory effect on crops. They promote the transition from vegetative to reproductive growth, reduce pre-harvest fruit drop, and accelerate ripening.
Preservatives: These have protective and therapeutic effects, inhibiting the respiration and cell proliferation of pathogens. They are mainly used for post-harvest treatment of fruits and vegetables.
Drought-resistant agents: Chemical mulches effectively prevent soil moisture evaporation, increase root water absorption, and effectively reduce transpiration. Water-retaining agents, mainly composed of superabsorbent polymers, are primarily used in seed coating, root dipping of transplanted plants, and seed and seedling preservation. They allow seeds or plants to absorb moisture from the surrounding soil and air, ensuring seed germination and the survival and growth of cuttings and transplanted crops in the early stages of drought. Fulvic acid is currently a major antitranspirant; spraying it before and after transplanting can shorten the seedling recovery period and improve survival rates.
B. Commonly Used Plant Growth Regulators and Their Functions
a. Auxins
Auxin promotes cell division, differentiation, and growth, increases fruit set, prevents fruit drop, induces adventitious root formation, promotes the formation and differentiation of the vascular bundle system in new roots, promotes taproot growth, and improves germination and survival rates. Entering the plant through the tender epidermis of leaves and branches, and seeds, it reaches its point of action with the flow of nutrients, and is widely used in the rooting of tree and flower cuttings.
b. Gibberellin
Gibberellin has a long-lasting effect and is more efficient, stable, and safer than other plant growth regulators. Spraying during the seedling stage promotes root development and prevents diseases; spraying during the growing season promotes stem and leaf growth and balances nutrition; spraying during the flowering period protects flowers and fruits, promotes fruit enlargement and a more aesthetically pleasing appearance, and can also effectively alleviate crop diseases.
c. Aminoethyl Ester
Aminoethyl esters are broad-spectrum plant growth regulators that promote cell division and elongation, promote the growth and differentiation of the growing point; increase chlorophyll content in the plant, thereby increasing the rate of photosynthesis; regulate the plant’s water balance, enhance the plant’s absorption of water and fertilizer, and improve the plant’s drought resistance and cold tolerance. It promotes seed germination, root development, and flower bud differentiation, leading to earlier flowering, earlier maturity, increased yield, improved quality, and delayed plant senescence.
d. Chlorpyrifos
Chlorpyrifos affects the development of young buds, accelerates cell mitosis, promotes cell enlargement, differentiation, and organ formation, promotes the growth of roots, stems, leaves, and fruits, promotes fruit setting, prevents fruit and flower drop, accelerates leaf drop, thus delaying leaf senescence in later stages, resulting in uniform fruit size, improved quality, and increased yield. At high concentrations, it can be used as a herbicide.
e. Sodium Nitrophenolate
Sodium nitrophenolate is a broad-spectrum plant growth regulator that promotes protoplast flow, increases cell vitality, accelerates plant growth and development, promotes the growth of roots, stems, leaves, and fruits, protects flowers and fruits, increases crop yield, and enhances crop resistance and other abilities. It has low toxicity, leaves no residue, and is pollution-free. It can be used alone or as a fertilizer additive, making it an excellent fungicide, synergist, and fertilizer.
f. Ethylene
Ethylene is the simplest olefin. It exists in plants in very low amounts and is a natural plant metabolite. It can break the dormancy of plant seeds and seedlings, induce the production of adventitious roots and root hairs, slow down plant growth, promote fruit ripening, accelerate leaf senescence, inhibit flowering, and in monoecious plants, alter the sex and differentiation direction of flowers in the early stages of flower development.
g. Cytokinins
Cytokinins are sometimes isolated from corn or other plants, and sometimes synthesized artificially. They are generally produced in the roots of plants and are a class of substances that promote cytoplasmic division and lateral cell elongation. They are plant hormones that promote the differentiation and growth of various plant tissues and have a synergistic effect with auxins. They also have effects such as breaking apical dominance, inhibiting stem elongation, inhibiting chlorophyll decomposition, breaking the dormancy of plant seeds and seedlings, promoting chloroplast development, and providing cold resistance.
h. Abscisic Acid
Abscisic acid, also known as abscisic acid or dormancy hormone, is named for its ability to promote leaf abscission and is widely distributed in higher plants. Besides inducing leaf abscission, it can regulate seed and embryo development, induce bud dormancy, inhibit cell elongation, cause stomatal closure in plant leaves, and promote tuber formation in potatoes. Furthermore, it can increase plant resistance to stress, offering protection against salt, heat, and cold damage.
i. 2,4-D
2,4-D promotes plant growth, prevents flower and fruit drop, and induces parthenocarpy at low concentrations. Low-concentration 2,4-D aqueous solutions treated with tomato flowers can prevent flower and fruit drop, form seedless fruits, and promote fruit growth. High concentrations of 2,4-D can be used for flower and fruit thinning and weed control.
Naphthaleneacetic acid promotes rooting of cuttings, prevents abscission, and promotes parthenocarpy. Soaking current-year green leaf cuttings in a suitable concentration of NAA solution can significantly improve rooting rates. Additionally, naphthaleneacetic acid (NAA) also improves transplant survival rates and promotes graft healing.
k. Plant Growth Factor
Plant growth factor is an antibiotic that inhibits cell mitosis, suppresses apical growth, promotes axillary bud growth, dwarfs plants, and inhibits seed germination. It is also commonly used to create dwarf bonsai trees.
l. Maleic Acid
Maleic acid hydrazide, commercially known as chlormequat chloride, has the opposite effect of auxins. It inhibits bud germination and stem elongation, causing plants to prematurely end growth, promoting maturity, and prolonging dormancy. It is commonly used to prevent premature sprouting in potatoes, onions, and garlic.
m. Chlormequat Chloride
Chlormequat chloride has the opposite effect of gibberellins, inhibiting elongation in various plants. It is mainly used in barley, wheat, and rice to make plants sturdier and reduce lodging. Chlormequat chloride is also used to inhibit excessive vegetative growth in cotton, improve plant shape, and increase yield.
n. Mepiquat chloride
Mepiquat chloride has a similar growth-inhibiting effect to chlormequat chloride, but its effect is longer. It is used to prevent lodging in wheat and to inhibit excessive vegetative growth and boll shedding in cotton.
o. Paclobutrazol
Paclobutrazol’s physiological effect is to weaken vegetative growth, inhibit new shoot sprouting, and accumulate assimilates. It is beneficial for flower bud differentiation. In peanut cultivation, excessive vegetative growth often leads to lodging, resulting in reduced yield. Spraying the leaves with a paclobutrazol solution during the most vigorous period of stem and leaf growth can make the plant shorter and more compact, with darker and thicker leaves, promoting the transport of more photosynthetic products to the underground fruiting organs, thus increasing peanut yield.
p. Paclobutrazol
Paclobutrazol is an inhibitor of GA. The main function of paclobutrazol is to dwarf the plant and promote lateral growth, resulting in thicker stems, upright leaves, increased tillering or branching, and earlier flowering and peak production in young plants. q. Ethephon
Ethephon decomposes and releases ethylene at pH values greater than 4.1. Its effects are similar to those of vinyl acetate, promoting fruit ripening, inducing female flower formation in cucumbers, and stimulating the excretion of secondary metabolites.
It is worth noting that when used according to the dosage, timing, and method specified on the plant growth regulator registration and approval label, it generally does not pose a threat to human health. However, in actual production, improper use can lead to excessively rapid crop growth, inhibited growth, or even crop death. Furthermore, it can negatively impact the quality of agricultural products and human health.
