IMPORTANCE OF PLANT GROWTH REGULATORS IN HORTICULTURE

IMPORTANCE OF PLANT GROWTH REGULATORS IN HORTICULTURE

The plant growth regulators are the organic chemical compounds, other than nutrients and vitamins, which modify or regulate physiological processes in an appreciable measure in the plant when used in small concentration.

They are known to influence fruit set, retention, yield and quality of horticultural crops. Fruit trees are considered high-value crops and even small modifications in production efficiency, product quality or enhanced cosmetic appeal have the potential to significantly increase product value.

Application of plant growth regulators results in better output as it improves the internal physiology of developing fruits to improve fruit set, reduce fruit drop and to amend various physiological disorders in order to improve quality and yield.

Content:

• Introduction
• Plant hormones, Plant growth regulators, Growth Retardants & Inhibitors
• Various uses of plant growth regulators
• Plant growth regulators used in Horticultural crops

Introduction

Plant growth regulators (PGRs) are natural or synthetic compounds that affect developmental or metabolic processes in higher plants, mostly at low dosages. They do not possess a nutritive value. They are used to regulate the growth and yield of plants.

PGRs are employed in horticulture mainly in floriculture, olericulture, pomology and viticulture to obtain specific advantages, such as decreased susceptibility towards biotic and abiotic stress, improved morphological structure, facilitation of harvesting, quantitative and qualitative increases in yield and modification of plant constituents. Salicylic acid and jasmonic acid induce defence reactions in fruits and vegetables that prevent fungal infection and induce resistance to low temperature injury.

Plant hormones and Plant growth regulators

Plant hormones or phytohormones are the organic compounds produced by plants which in low concentrations regulate plant physiological process. These usually move within the plants from a site of production to a site of action. It means they synthesized in one part of the plant and translocated to another part, where in very low concentrations causes a physiological response.

The plant hormones are identified as promoter (Auxin, Gibberellins and Cytokinin), inhibitors (Abscisic acid and Ethylene) and other hypothetical growth substance (florigen, flowering hormone etc.)

Growth regulators work similar to phytohormones but synthetic in nature. These are organic compounds which are active at low concentrations (1-10 ml) in promoting, inhibiting or modifying growth and development in plants.

The naturally occurring (endogenous) growth substances are commonly known as plant hormones, while the synthetic ones are called growth regulators.

Growth retardant

• The Growth retarding chemical or growth retardant slows cell division and cell elongation of shoot tissues and regulate plant height physiologically without formative effects.
• They do not affect the formation of leaves, flowers and fruits. g., AMO 1618, Phosphon-D, CCC (Cycocel), Chloromequat, Maleic hydrazide and Alar. CCC respond to increasing flowering in Grapes and Lemon.
• These do not occur naturally in plants and acts in retardation of stem elongation, preventing cell division.
• Plant growth retardants are defined as synthetic organic chemicals that cause a retardation of cell division steps in pathways of hormone biosynthesis without evoking substantial growth distortions.

Inhibitors

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

Phenolic inhibitors: Benzoic acid, Salicylic acid, Coumaric acid and Chlorogenic acid.

Synthetic inhibitors: E.g., Maleic hydrazide, Tri-Iodobenzoic acid (TIBA), SADH etc. An inhibitor from young leaves of Betula sps. prevent the growth of apical buds e.g., ABA and Dormin. SADH promotes flowering in Apple, Pear, Peach and Blurberry etc.

Various uses of plant growth regulators

1. Propagation of plants:

A number of plants are propagated by stem, leaf cutting and by layering. For promotion of rooting, the most commonly utilized hormone is IBA followed by NAA. Gibberellic acid causes inhibition of root formation in cutting. Cytokinins also help in quick and profuse root formation in cuttings and layers. By use of auxins, profuse root formation is observed in cuttings of Guava, Fig, Pomegranate, Crotons, Rose, Hibiscus, etc.

2. Seed germination:

Many seeds have natural dormancy which can be got over by dipping the seeds in auxins. Soaking seeds of French beans and Peas in 10-20 ppm solution of GA for 12 hours before sowing, significantly improves the yield and quality. Dipping Sweet potatoes in 5ppm GA solution for 5minutes before sowing increases sprouting and yield of potatoes.

3. Control of plant size:

In fruits and vegetables, application of higher doses of nitrogenous fertilizers spraying Cycocel (growth retardant), the superfluous growth of leaves is checked. By spraying 10 ppm solution of morphactin in potato, the growth of plant is reduced and thereby the size of tubers is increased.

The growth retardants are useful in checking the growth of hedges in ornamental gardens there by reducing the cost of trimming the hedges.

4. Regulation of flowering:

In Pineapple, due to later flowering the fruit get ready in rainy season. This deteriorates the quality of the fruit. This difficulty can be overcome by spraying 5-10ppm solution of NAA before flowering. Application of 100-200 ppm GA in Dahlia plants induces early flowering. Sometimes, it is necessary to delay flowering. E.g., crossing of varieties which do not flower simultaneously. Hence, the crossing becomes difficult.

5. Control of Sex expression:

In number of cucurbits, such as Ridge gourd, Bitter gourd, Watermelon, Cucumber and Pumpkins which have proportion of male flowers is more than female flowers. For better yield, it is necessary to increase the number of female flowers. This can be achieved by application of auxins which increases the number of female flowers and decreases the number of male flowers. The commonly used auxins are NAA and ethrel.

6. Control of fruit set and growth of fruit:

Spraying NAA, TIBA, and PCPA on flowers increases the fruit set. Dipping of grape bunches (young fruits) in GA solution increases the berry size in Thompson seedless grape.

7. Control of fruit drop:

In Nagpur Santra, the fruit drop can be controlled by spraying 10-20 ppm NAA or 10 ppm 2,4-D after fruit set. The fruit drop in mango can be controlled by these two auxins.

8. Thinning of fruits:

Sometimes it is necessary to thin the fruits so as to bring a balance between the supply of nutrients and development of fruit. In such cases spraying with mild solution of ethrel or morphactin reduces the fruit load by 25-30 per cent.

9. Early ripening and development of fruit colour:

If the fruits could be brought in the market in early part of the season, they fetch good price. Spraying with 2, 4,5-T and B-9 hastens maturity of apples by 1-4 weeks.

10. Prevention of sprouting:

In potatoes and onions, after harvest, in storage, the buds start sprouting which makes them unfit for cooking. Spraying of malic hydrazide (MH) solution before storing, prevents sprouting and these can be stored safely for 6 months.

11. Control of weeds:

The conventional method of controlling the weeds is to remove them by uprooting manually. Successful control of weeds is obtained by spraying 2,4-D in many crops.

Plant Growth Regulators used in Horticultural crops

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PLANT GROWTH REGULATORS- CLASSIFICATION & FUNCTIONS