In studying the complex life activities of plants throughout their growth cycle, researchers have discovered that key stages such as rooting, germination, growth, flowering, and fruiting are controlled by both genetic factors (genes) and regulated by plant hormones. Plant hormones, also known as endogenous hormones, are organic substances secreted by various organs of plants in small quantities but with significant effects. Once formed in specific organs, they act locally or are transported to other parts of the plant to perform their physiological functions.
Currently, five major classes of plant hormones that regulate growth and development have been identified. These include auxins, gibberellins, ethylene, cytokinins, and abscisic acid. Additionally, brassinosteroids, which possess functions of all five major hormone classes, have been recognized as the sixth class of plant hormones by the International Society of Plant Physiology, as appointed by the United Nations Food and Agriculture Organization (FAO).
The Six Major Plant Hormones:
Auxins
Auxins were among the first plant growth regulators used in agriculture. The most commonly applied auxins in production are indolebutyric acid, naphthalene acetic acid, and 2,4-D. These hormones are insoluble in water but can dissolve in organic solvents such as alcohols, ketones, and ethers. Auxins primarily promote organ growth, prevent organ abscission, aid in fruit setting, and induce flower bud differentiation. In horticulture, they are used to promote rooting in cuttings, prevent flower and fruit drop, encourage fruit set, control sex differentiation, alter stem angles, and induce flowering in pineapples.
Drawback: At lower concentrations, they promote growth, but at higher concentrations, they inhibit growth.
Gibberellins
Gibberellins are a group of 121 known compounds, all derivatives of gibberellic acid. The most common gibberellin products available are GA3, GA4, and GA7, derived from large-scale cultivation of genetically diverse gibberella fungi. Gibberellins are soluble in organic solvents like alcohols, acetone, and acetic acid. They are mainly used to elongate stems, replace low-temperature long-day conditions, promote leaf enlargement and lateral branch growth, encourage male flower formation, initiate seed germination, stimulate parthenocarpy, and improve fruit formation.
Drawback: Excessive use can cause accelerated cell elongation, leading to reduced quality by diluting nutrients, thickening fruit skins, and diminishing flavor.
Ethylene
Ethylene is a gas at room temperature, which makes it difficult to apply directly. Therefore, ethylene-releasing agents like ethephon are used. Once absorbed by plants, these agents release ethylene. Common ethylene-releasing agents include ethephon and cycocel. Ethylene is mainly used to induce fruit ripening, promote flowering and female flower differentiation, stimulate abscission, and increase secondary metabolite production. Ethylene inhibitors, such as aminoethoxyvinylglycine (AVG), aminooxyacetic acid (AOA), silver thiosulfate (STS), and silver nitrate, are used to suppress ethylene production, reduce fruit drop, inhibit post-harvest ripening, and extend the shelf life of fruits and cut flowers.
Drawback: Excessive residue levels, shortened storage life, and potential negative impacts on production and yield.
Cytokinins
Cytokinins primarily promote cell division. These are typically derivatives of adenine. Common synthetic cytokinins include kinetin, chlorbenzuron, triadimefon, and thidiazuron. The most widely used cytokinins in production are kinetin and 6-benzylaminopurine (BAP), which are typically dissolved in alcohol and then diluted with water. Cytokinins are essential for promoting cell division, inducing bud differentiation, stimulating lateral bud development, removing apical dominance, preventing organ senescence, increasing fruit set, and improving fruit quality.
Drawback: Overuse can lead to malformed fruits and shorter storage life.
Abscisic Acid (ABA)
Abscisic acid is a growth-inhibiting plant hormone known for inducing leaf abscission. It also promotes bud dormancy and the formation of tubers in potatoes, and it has a suppressive effect on cell elongation. The most common synthetic ABA compounds include S-ABA, paclobutrazol, and daminozide. Abscisic acid primarily accelerates organ abscission, inhibits plant or organ growth, induces dormancy, closes stomata, and enhances plant resistance to stress.
Drawback: It can inhibit growth if used excessively.
Brassinosteroids
Brassinosteroids regulate and balance the distribution of gibberellins, cytokinins, ethylene, abscisic acid, and auxins within plants. They promote healthy, vigorous plant growth, enhance productivity, delay aging, extend the harvesting period, and improve yields.
Plant hormones are essential organic substances secreted by various plant organs, regulating crucial processes such as germination, growth, flowering, and fruiting. These hormones are controlled by genetic factors and can be transported within the plant to influence physiological functions.
These hormones are critical in agricultural applications for controlling plant growth and enhancing crop productivity, though their proper use is necessary to avoid negative side effects on plant quality.
