Plant growth regulators usually are defined as organic compounds, other than nutrients, that in small concentrations, affect the physiological processes of plants. In practical purpose, they are defined as either natural or synthetic compound that is applied directly to plant to alter its life processes/structure in some beneficial way so as to enhance yield, improve quality and facilitate harvesting. When herbicides are applied to induce a specific beneficial change, then they are considered as plant growth regulators. If the compound is produced by the plant it is called a plant hormone.
Important Roles of Plant Growth Regulators
- Germination: Germination is the beginning of the growth of a plant from a previously dormant seed which contains the embryo. Dormancy is the phase in which biological activities are at rest/suspended; dormancy can be broken by external application of PGRs like auxins and GAs.
- Rooting: Root promoting chemicals are referred to as “rooting hormones” they are often used on stem cuttings using different methods. Auxins are used as rooting hormones.
- Callus Induction: In plant tissue culture, cytokinins and auxins are used for callus induction from explants.
- Growth Inhibitor: Inhibit means to „stop‟ the growth. It stops apical growth and promotes lateral bud growth, which results in branched and more compact plants with an increased number of flowers and fruits. These chemicals are also used for long storage of suckers.
- Growth Retardant: Retard means to „slow‟ the growth. These chemicals regulate shoot growth of plant resulting in a sturdier and more compact plant with improved color.
- Thinner: Thinning is the process of reducing denseness of flowers/fruits, which helps to increase fruit size, quality and maintain tree structure. To maximize crop quality and yield, the load must be estimated for the optimal crop, the maximum number of fruits to be retained after thinning.
- Chemical Pruning: Pruning is a process of restricting of plant height. It is also known as pinching or suckering. Pruning promotes the growth of new branches. Generally, growth inhibitors are used for chemical pruning.
- Defoliant: Any substance or mixture of substances intended for causing the leaves or foliage to drop from a plant, with or without causing abscission. Defoliation is the method of treatment that causes only the leaves of a plant to abscise or fall off. These are applied to cotton to improve and facilitate mechanical harvesting.
- Desiccant: Any substance or mixture of substances intended for artificially accelerating drying of plant tissues. Desiccation is the method of treatment that rapidly kills the leaves, which are used for the purpose similar to the uses of defoliants, but desiccant causes green foliage to lose water; it is a hastened drying process that results in the removal of leaves.
- Fruit Ripener: Ripening is the final stage of fruit development, which involves series of physiological and biochemical events mainly to change color, texture, and aroma and flavor that makes fruit attractive & tasty. Fruit ripeners are the substances which can hasten ripening process and artificially ripen fruits for commercial purpose
Auxin, or indole 3-acetic acid, was the first plant hormone discovered and contains an indole ring much like the melatonin hormone of animals.
Auxin is known to stimulate cell elongation and cell division, differentiation of vascular tissues, root initiation, and lateral root development. Auxin can also mediate the bending responses to light and gravity, and within the apical bud, it suppresses the growth of lateral or axillary meristems. It can delay senescence, and interfere with leaf and fruit abscission. It can induce fruit setting and delay ripening in some fruits. It can also stimulate the production of another plant hormone, ethylene.
Functions of Auxin
The following are some of the responses that auxin is known to cause,
• Stimulates cell loosening, expansion, and elongation
• Initiation of adventitious roots on stem cuttings
• Lateral root development in tissue culture
• Stimulates differentiation of phloem and xylem
• Stimulation of abscission (young fruits) or delay of abscission
• Stimulates cell division in tissue culture in combination with cytokinins
• Mediates the tropistic response of bending in response to gravity and light
Gibberellic acid (GA) was first discovered from fungi that can stimulate plant cell elongation and cause significant and “leggy” growth of rice plants. GA is a series of 136 diterpene compounds that contain 19 or 20 carbons in four or five ring systems. These are named for the order in which they were discovered (GA1, GA2, etc.). The other functions of GA, as mentioned previously, are in general antagonistic to the actions of ABA. For example, ABA promotes seed dormancy, while GA is required in most cases to break seed dormancy. The actions of GA on barley germination have been well studied where it has been shown that GA promotes expression of the a-amylase genes required to break down starch in barley aleurone, an important process in the grain-malting business. GA also plays a prominent role in stimulating flower development under long days.
Functions of Gibberellins
Physiological processes stimulated by active gibberellins depend on their types.
• Stimulate cell elongation in stem
• Breaks seed dormancy in some plants which requires light for germination
• Stimulates alpha-amylase production in germinating cereal grains
• Stimulates bolting/flowering in response to long days
• Delay senescence in leaves and also in citrus fruits
• Induces maleness in dioecious flowers
• Play a role in the development of seedless fruit (parthenocarpic).
Cytokinin is generally considered the second most important plant growth-regulating hormone, following auxin. Cytokinin is similar to adenine and was first discovered in 1941 as the active component in coconut milk that promoted the growth of plant cells in tissue culture. Cytokinin can promote cell division and shoot growth and can delay senescence.
Functions of Cytokinins
Physiological processes stimulated by cytokinins,
• Stimulates cell division
• Activates metabolite attraction (sink effect)
• Retardation of senescence (at low concentrations)
• Induction of apoptosis (at high concentrations)
• Stimulates morphogenesis in tissue culture
• Stimulates the growth of lateral buds and leaf expansion by cell division &
• Stimulation of chlorophyll synthesis that causes the conversion of etioplasts
• into chloroplasts
• Enhance stomatal opening in some plants
Ethylene, a hydrocarbon gas, is a very simple molecule that is best known for its stimulation of fruit ripening and promotion of the seedling triple response. Indeed, people of ancient cultures understood the actions of ethylene and could burn incense in a closed room to stimulate fruit ripening. The triple response of seedlings is a specific developmental program wherein an apical hook forms in the shoot, and the root becomes thicker. These adaptations may increase survival under certain conditions. In addition, ethylene can stimulate the release of dormancy, adventitious root formation, flower opening, and flower and leaf senescence.
Functions of Ethylene
Ethylene is known to affect the following plant processes,
• Induce shoot and root growth and also differentiation
• Stimulates the growth/release of dormancy
• Stimulates leaf and fruit abscission
• Stimulates Bromeliad flower induction
• Induction of femaleness in dioecious flowers
• Stimulates flower senescence, leaf senescence, and flower opening
• Stimulates fruit ripening
Abscisic acid (ABA) was first identified in a search for an abscission-promoting hormone. This is not the function of ABA, and as noted earlier, it functions in promoting dormancy and in sensing drought and other stresses. ABA is derived from mevalonic acid and carotenoids and is thus similar in structure to the developmental factor from animals called retinoic acid. Transport of ABA can occur in the vascular tissues. ABA stimulates closure of the stomatal pore and can inhibit shoot growth. In seeds, it promotes dormancy and stimulates the production of seed storage proteins. It is mostly antagonistic to gibberellic acid (GA) and can inhibit the response of grains to GA.
Functions of Abscisic Acid
The physiological responses made by abscisic acid,
• Stimulates the closure of stomata
• Stress responses especially to water deficiency
• Induces seed and bud dormancy
• Induces seed to synthesize, storage proteins
• Inhibits shoot growth but does not have much effect on roots
• Inhibits the synthesis of alpha-amylase stimulated by gibberellins
• Induce some effects on induction and maintenance of dormancy