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Growth mainly refers to the quantitative increase in plant body such as increase in length of the stem and root, the no. of leaves, the fresh weight and dry weight etc. On the other hand, germination of seed, formation of flowers, fruits and seeds, emergence of lateral buds, falling of leaves and fruits are qualitative changes, referred to as development. Growth and development of the plant body are controlled by two sets of internal factors, namely, nutritional and hormonal. Nutritional factors supply the plant necessary mineral ions and organic substances such as proteins, carbohydrates and others. These constitute the raw materials required for growth. However, utilization of these substances for proper development of the plant is controlled by certain chemical messengers, called plant growth substances or plant growth regulators, which in minute amounts increase or decrease or modifies the physiological processes in plants.
The term plant growth regulators‘ is relatively new in use. In earlier literature these were mentioned as Hormones. ―Hormone is a Greek word derived from ―hormao which means to stimulate. Now the term phytohormone is used in place of plant hormone.
Plant growth regulators or plant regulators are the organic compounds other than nutrients which modify or regulate physiological processes in an appreciable measure in the plants when used in small concentrations. They are readily absorbed and these chemicals move rapidly through the tissues when applied to different parts of the plant. Plant hormones or phytohormones are also regulators but produced by the plants in low concentrations and these hormones move from the sit of production to the site of action. Therefore, the difference between the plant regulator and plant hormone is in that the former one is synthetic and the latter one is natural from the plant source.
The various types of growth regulating substances are:
- Auxins
- Gibberellins
- Cytokinins
- Ethylene
- Abscisic acid
Auxins, Gibberellins and cytokinins are Growth promoters and Ethylene and Abscisic acid are growth inhibitors.
Growth Retardants: These are chemicals which have common physiological effect of reducing stem growth by inhibiting cell division.
Growth regulating substances have many practical applications in horticulture and some of the most important uses are:
1. Propagation of plants: The most common use of plant regulators in horticulture is to induce rooting in stem cuttings and in air and soil layers.
Rooting of cuttings:
Certain kind of plants may not successfully root under normal condition and with the aid of plant regulators; they can be easily made to induce rooting. The most commonly employed growth regulators for rooting are auxins like IBA, IAA, IPA and NAA. Among these chemicals IBA is most ideally used since, it is the most effective one. Concentrations ranging from100-500ppm are used for long dip method of treatment of cuttings for 12-24 hours and high concentrations of 10,000 to 20,000 for quick dip method for a few seconds. The concentrations differ according to the type of cutting i.e. herbaceous, Semi-hard wood and hard wood cuttings. Applications in the form of dust as talcum preparation or in the form of a paste in lanolin are also used.
Layering:
Another usage of plant regulators in plant propagation is in aiding rooting of air layering. Layering is the practice of inducing rooting on shoots/stems while it is still attached to the parent plant. This is practiced in fruit trees like guava, pomegranate etc. The main principle of layering is that a part of the aerial portion of the intact plant is girdled. This results in severing of phloem. Consequently, hormones and food substances coming from the leaves accumulate above the girdled portion. When the ring of bark is removed from the stem, the growth regulators like IBA or IAA in power or in Lanolin paste is applied at the distal end of the bark-removed portion to promote root formation
Grafting and Budding:
Grafting of plants is a widely used horticultural practice of multiplying the desired genotypes in mango, citrus and others. For this, a portion of the plant is inserted in to another plant of the same species or some times compatible plants of different species or genera. There are mainly two types of grafting: bud grafting and scion grafting. Whatever may be the method employed, the principle remains the same. When the cambium of a stock plant comes into physical contact with the cambium of a scion both from new xylem and phloem simultaneously together. Consequently, these become united and grow as one plant. Since, auxins have the property of promoting cell division of cambium these are often employed. Before grafting, either stock or scion or both are dipped in auxin solution. This promotes an early union and consequently, a better success of grafted plants.
2. Control of flowering: The plant growth regulators are used for the regulation of flowering in certain crops. In pineapple flowering is irregular and harvesting becomes a problem and hence to regulate flower production, plant regulators are used. The treatment generally consists of pouring a required quantity of (50ml), the solution containing 0.25 to 0.5 mg of the chemical of NAA in the central core of plants. In recent studies, Cycocel and Alar at 5000ppm and Ethrel at 100-200ppm have been shown to induce flowering in mango during an off year. In Jasminum grandiflorum, the flowering period is extended by the application of Cycocel at 500ppm. Flowering can also be induced in certain vegetables such as radish, beet root and carrot with the application of GA.
2. Control of flowering: The plant growth regulators are used for the regulation of flowering in certain crops. In pineapple flowering is irregular and harvesting becomes a problem and hence to regulate flower production, plant regulators are used. The treatment generally consists of pouring a required quantity of (50ml), the solution containing 0.25 to 0.5 mg of the chemical of NAA in the central core of plants. In recent studies, Cycocel and Alar at 5000ppm and Ethrel at 100-200ppm have been shown to induce flowering in mango during an off year. In Jasminum grandiflorum, the flowering period is extended by the application of Cycocel at 500ppm. Flowering can also be induced in certain vegetables such as radish, beet root and carrot with the application of GA.
3. Fruit set: Various growth regulators like IAA, IBA, IPA, NAA, 2, 4-D, 2, 4, 5-T and GA have been found to improve fruit set in many crops. Among these chemicals 2, 4-D and NAA (Planofix) have been found in general to be most effective in increasing the fruit set. The optimum concentrations for this purpose are 10-20 ppm of auxins and 10-100ppm of GA in different crops. Spraying the flower cluster thoroughly 4-6 days after full bloom with 100 ppm GA increased the fruit set in grape. It has been found that in chillies spraying of Planofix @ 1ml in 4.5 litres of water at 60th and 90th day after planting is beneficial for good fruit setting.
4. Fruit drop: Losses resulting from pre-harvest drop of fruits have long been a serious problem. When the growth regulators have been put in to use in apples and pears, pre- harvest fruit drop can be checked by the application of 2,4-D and 2,4,5-T effectively. Pre harvest fruit drop in citrus is controlled with 2,4-D at a concentration of 20ppm2,4-D, 10-15ppm of NAA and 2,4,5-T at 15 to 30ppm at pea stage and marble stage and 2,4D at 20ppm and 2,4,5-T at 10-15ppm in mandarins. At 10ppm and NAA at 20ppm have effectively prevented fruit drop in mango. Application of planofix containing NAA at‖ pea seed ―and‖ marble‖ size of the fruits completely controlled early fruit drop in Guava.
5. Parthenocarpy: Partenocarpic fruit set could be induced in a no. of vegetables like cucurbits, bhendi, brinjal, chillies and tomato and fruits like guava, straw berry, citrus, watermelon etc.IAA,IBA,NAA,NOA,NAD,2,4-D,IPA and GA are effective in different plants. Application of GA at 100 ppm induced complete seedlessness in grape varieties Viz., Anab-e-shahi,Pachadraksha etc.The problem of development of seeds in Poovan variety of Banana in Trichy area of Tamilnadu is controlled by application of 2,4-D at 25ppm in the bunches when the last hand is opened.
6. Fruit ripening: The plant growth regulators can be employed to hasten or delay fruit ripening. Plant growth regulators like 2, 4, 5-T at concentrations of 25 to 100ppm has been found to hasten the ripening in some varieties of plums and peaches. In banana ethrel treatment at 2500ppm induces ripening in 24 hours.
Application of 2, 4-D at 16ppm delays ripening in Washington navel oranges. In Calymirna fig maturity and ripening of the fruit is greatly hastened by spraying 2, 4, 5-T, while in apples in addition to this B-Nine also hastens ripening by about 1-4 weeks. Ethephon has been shown to hasten ripening in grapes. In tomatoes all fruits on a plant won‘t mature and ripen at a time .This is a serious disadvantage for mechanical harvesting .Ethephon applied 1-2 weeks before harvest promotes degreening and ripening of tomatoes. Application of smoke is commercially employed to hasten and ripen bananas, the active ingredient responsible being ethylene. Ethyphon is also employed for degreeing and colour development of harvested fruits.
Application of 2, 4-D at 16ppm delays ripening in Washington navel oranges. In Calymirna fig maturity and ripening of the fruit is greatly hastened by spraying 2, 4, 5-T, while in apples in addition to this B-Nine also hastens ripening by about 1-4 weeks. Ethephon has been shown to hasten ripening in grapes. In tomatoes all fruits on a plant won‘t mature and ripen at a time .This is a serious disadvantage for mechanical harvesting .Ethephon applied 1-2 weeks before harvest promotes degreening and ripening of tomatoes. Application of smoke is commercially employed to hasten and ripen bananas, the active ingredient responsible being ethylene. Ethyphon is also employed for degreeing and colour development of harvested fruits.
. 7. Fruit size and quality: Increase in berry size in Anab-e-shahi,Kismis and Bhokri varieties was reported when GA was applied at 40ppm at bud and flower stages. Higher concentrations resulted in the increase in the length of berries.
8. Sex expression: Plant regulators can be employed to modify the sex expression in crops. In cucurbitaceous vegetables the production of male flowers will be always more in number than the female flowers and this sex ratio can be narrowed down by the application of ethrel at 100 to 250ppm, if sprayed four times at weekly intervals commencing from 10 to 15 days after sowing. This growth regulator not only increases the number of female flowers to male flowers, but also produces female flowers at earlier nodes. Application of GA, the sex ratio is shifted towards maleness in several cucurbits.
Certain plant regulators are employed to induce male sterility in crop plants, so that such male sterile plants can be used as a female plant in the hybridization work. This process dispenses the expensive work. Complete male sterility in bhendi can be obtained by spraying with 0.4% of MH. A single spray one week before floral bud initiation offers male sterility for 10 days and a subsequent spray at floral initiation extends the effect to 22 days.