• The use of these substances for proper plant growth is controlled by certain “chemical messengers” called plant growth substances or plant growth regulators, small amounts of which increase or decrease or modify physiological processes in plants.

Phytohormones

• These are hormones produced by plants that in low concentrations regulate the physiological process of the plant.
• They usually move from the production site to the worksite within the plants.

Plant Growth Regulators

• These are organic compounds other than nutrients, which in small amounts promote, inhibit or modify any physiological process in the plant. Or it can be defined as any organic compound that is active at low concentrations (1–10 ml) in promoting, inhibiting, or modifying growth and development in plants.
• Naturally occurring (endogenous) growth substances are commonly referred to as plant hormones, while synthetic ones are called growth regulators.

Plant Hormones

• It is an organic compound synthesized in one part of the plant and transferred to other parts to cause a physiological reaction in very low concentrations.
• Plant hormones are recognized as stimulators (auxin, gibberellin, cytokinins), inhibitors (abscisic acid and ethylene) and other hypothetical growth substances (florigen, death hormone, etc.).

Auxin

• Auxin is a Greek word derived from auxin which means to increase. It is a general term for chemicals that usually stimulate cell elongation by loosening the cell wall. Similarly, auxins also affect a wide range of growth and developmental responses.
• Chemical isolation and characterization were described by Kogi et al. (1934). Auxins are the first identified hormones with IAA being the major naturally occurring endogenous auxin in plants and crops.
• In addition to IAA, there are three other compounds in plants that are structurally similar 4, chloro indole acetic acid (CIAA), phenylacetic acid (PAA), indole butyric acid (IBA), give the same reaction as IAA.

Site of Auxin Synthesis

Auxin is synthesized in the stem tip and young tissues and moves mainly down the stem (basipetal movement) i.e. from the shoot tip to the root.

Synthetic compounds are classified into five major categories:

1. Indole Acid
2. Naphthalene Acid
3. Chlorophenoxy acid
4. Picolinic acid.

Role of Auxin

1. Cell division and enlargement: AA + GA, eg – cambial increase in diameter.
2. Tissue Culture: Shoot multiplications (IBA and BAP), callus growth (2, 4-D), root multiplication IAA and IBA (1-2 mg).
3. Breaking dormancy and apical dominance (inhibition of lateral buds): NAA
4. Shortening internodes: (NAA) dwarf fruit branch in the apple tree.
5. Rooting of cutting: (10-1000 ppm-NAA, IAA, Phenyl acetic acid)
6. Prevent Lodging: NAA develops woody and erect stems.
7. Prevent Abscission: Premature leaf, fruit, and flower drop (NAA, IAA and 2,4-D).
8. Parthenocarpic fruit: Grapes, Banana, and Orange (IAA).
9. Flower Initiations: Pineapple uniform flowering and fruit ripening (NAA) and delay flowering (2, 4-D).
10. Weed Iradication: 2, 4-D.

Gibberellins

• It is an active principle isolated from the soil-borne fungus Gibberella fujikuroi.
• The concentration of GA₃ is usually highest in immature seeds, up to 18 mg/kg fresh weight in Phaseolus species, but decreases rapidly as seeds mature.
• In general, roots contain higher amounts of GA₃ than shoots.
• Gibberellins have also been found to be effective in overcoming the dormancy of both buds and seeds.

Role of Gibberellins

1. Cell elongation or cell division: synthesis in the leaf and shoot elongation (IAA + GA₃) by influencing cell elongation or cell division or both.
2. Increases metabolic activity: Promotes growth and height by mobilizing reserved food material, increasing root activity and kinetin production in the root-transferred to the growing bud.
3. Shoot elongation: GA₃ spray increases the height of the seedlings.
4. Delayed ageing: Increase photosynthesis and protein synthesis to reduce abscission.
5. Increase cambial growth and differentiation: induce flower and fruit set (IAA+GA₃).
6. Dwarf plant (genetically) to normal height: GA₃.
7. Promote flowering in long-day plants: Substitute for long-day conditions and cold treatment (vernalization).
8. Induction of parthenocarpy in grapes: Three physiological events: rachis cell elongation, flower thinning, and berry growth.
9. Breaking of dormancy and leaf expansion.

Cytokinins

• The first endogenous cytokinin called zeatin was isolated from maize kernels.
• Germinating seeds, roots, sap streams, developing fruits and tumor tissue is rich in cytokinins.
• Cytokinin-treated seeds germinated better in the dark than untreated lettuce seeds.
• Similarly, cytokinins interact with gibberellins to effectively break the photo dormancy of celery (Apium graveolens) seeds.

Synthetic cytokinins: Kinetin, Benzyladenine, and Ethoxy ethyladenine.

Role of cytokinin

• Cell division, elongation, and enlargement.
• Tissue culture morphogenesis.
• Initiation of flowering and fruit development.
• Overcoming apical dominance.
• Breaking dormancy.
• Delay aging.
• Improves N₂

Ethylene

• Neljubo (1901) is credited with identifying the active growth-regulating component of a luminous gas in the form of ethylene.
• Ethylene is naturally produced in plants in sufficient quantities to exert regulatory effects and can be considered a plant hormone.
• Ethylene may also be active in reducing secondary dormancy. (Ross, 1984).
• Some synthetic chemicals such as ethrel, ethephon, chloroethyl phosphonic acid (CEPA) have been reported to release ethylene when used on plants.

Role of Ethylene

1. Breaking dormancy.
2. Induce ripening of fruits.
3. Induce abscission of leaves.
4. Inhibit elongation and lateral bud growth

Growth Retardant

• A growth retardant is a chemical that slows down cell division and cell expansion of shoot tissues and physically controls plant height without formative effects.

Examples: AMO 1618 (2-isopropyl-4-dimcthylamine-5-methyphenyl-1-piperidine-cai’boxylate methyl chloride), Phosphon-D, CCC (2-chloroethyl tri- methylammonium chloride), Chloromequat and Alar.

• They do not occur naturally in plants and act to slow stem elongation, inhibit cell division.
• Plant growth retardants are defined as synthetic organic chemicals that cause the retardation of cell division steps in the pathway of hormone biosynthesis without causing substantial growth abnormalities.

Inhibitors

• They suppress the growth of plants.
• These are phenolic inhibitors, synthetic inhibitors, and abscisic acid (ABA).

Phenolic inhibitors: Examples Benzoic acid, Salicylic acid, Coumaric acid and Chlorogenic acid.
Synthetic inhibitors: Examples Maleic hydrazide, Tri-Iodobenzoic acid (TIBA), SADH (succinic acid-2-2-dimethylhydrazide), etc.

• An inhibitor from the young leaves of Betula species inhibits the development of apical buds. Example ABA and Dormin.

Role of abscisic acid (ABA):

1. Preventing elongation.
2. Induce dormancy.
3. Delay in germination.
4. Stop the development process.

Methods of Application

Growth regulators can be implemented in various ways such as

1. Spraying method.
2. Injection of solution into internal tissues.
3. Root feeding method.
4. Powder form.
5. Dip the cutting in the solution.
6. Soaking in dilute aqueous solution.

Various Uses of Plant Growth Regulators

Propagation of Plants

• Many plants are propagated by the stem, leaf-cutting, and layering. To promote rooting, the most commonly used hormone is IBA followed by NAA.
• Gibberellic acid inhibits root formation in cuttings. Cytokinins also help with rapid and abundant root formation in cuttings and layering.
• With the use of auxin, abundant roots are formed in the cuttings of guava, fig, pomegranate, croton, rose, hibiscus, etc.

Seed germination

• Many seeds have a natural dormancy that can be removed by immersion in auxin.
• Soaking the seeds of french bean and peas in the 10-20ppm solution of GA3 for 12 hours before sowing significantly improves the yield and quality.
• Dip the sweet potato in 5ppm GA3 solution for 5 minutes before sowing to increase tuber germination and yield.

Plant size control

• In fruits and vegetables, spraying of Cyclocel (growth retarder) in the application of high doses of nitrogenous fertilizers prevents unnecessary growth of leaves.
• Spraying of 10ppm solution of Morphactin in potato reduces plant growth and increases the size of tubers.
• Growth retardants are useful in inhibiting the growth of hedges thereby reducing the cost of pruning.

Flower regulation

• Due to later flowering in pineapple, the fruits are ready in the rainy season. This deteriorates the quality of the fruit. This problem can be overcome by spraying a 5-10 ppm solution of NAA before flowering.
• Application of 100-200 ppm GA₃ in dahlia plants induces early flowering.
• Sometimes, it is necessary to delay flowering. Example hybridization of varieties that do not bloom together. In this case, crossing becomes difficult.

Control of sex expression

• In cucurbits such as cucumber, bitter gourd, watermelon, sponge gourd, and pumpkin, the proportion of male flowers is higher than that of female flowers. It is necessary to increase the number of female flowers for better yield. This can be achieved by using auxin which increases the number of female flowers and reduces the number of male flowers. Commonly used auxins are NAA and ethereal.

Control of fruit group and fruit growth

• Spraying of NAA, TIBA, and PCPA on flowers increases fruit set.
• Immersion of bunches (young fruit) in GA3 solution increases berry size in Thompson seedless grapes.

Fruit drop control

• After fruit set in Nagpur orange, fruit drop can be controlled by spraying 10-20 ppm NAA or 10 ppm 2,4-D.
• Fruit drops in mango can be controlled by these two auxins.

Fruit thinning

• Sometimes fruit thinning is necessary to strike a balance between nutrient supply and fruit development.
• In such cases, spraying of a mild solution of ethrel or morphactin reduces fruit load by 25-30 percent.

Early ripening and fruit color development

• If the fruits can be brought to the market early in the season, they fetch a good price.
• Spraying of 2,4,5-T and B-9 results in early apple maturity by 1-4 weeks.

Germination prevention

• In potatoes and onions, after harvesting, in storage, buds begin to sprout, making them unsuitable for cooking.
• Spraying of maleic hydrazide (MH) solution prior to storage inhibits germination and can be stored safely for up to 6 months.

weed control

• Successful control of weeds is achieved by spraying 2,4-D in many crops.