What are the plant growth regulators?
It is very easy to say that the growth and development in plants is under the control of extremely minute quantities of hormones produced within the plant itself. Several synthetic compounds have been shown to promote similar activity. Some of these compounds are extensively employed for promoting rooting in stem cuttings, inhibiting sprouting of tubers and bulbs in storage, controlling the development, maturation, ripening and falling of fruits, and selective eradication of weeds in cultivated fields. Those compounds are called plant growth regulators.
The main uses of synthetic growth regulators in agriculture and horticulture are as follows:
Some varieties of ornamental plants and fruit trees possess certain desirable characters, but when raised from seeds they do not breed true to type. If these plants are vegetative propagated, all features of the parent plant are maintained in the progeny. The plants thus raised also show uniform vigor. Cuttings of several plants when put in soil do not form roots. In such cases, growth hormones play as plant growth regulators and play an important role. Auxins hasten initiation of roots, and increase the number of roots on the cuttings, which help in establishing the cuttings to vigorous new plants quickly. IBA and NAA are more effective in this respect.
Breaking of dormancy
Gibberellins help in breaking seed dormancy. In several crops like bean, pea, barley, wheat, etc. the emergence of seedlings is hastened by 2-4 days by the application of GA3. Low temperature requirement for germination of grapes, apple, cherry, peach, etc. can be overcome by treating the seeds with gibberellins. GA3 is used for promoting uniform sprouting of potato tubers. Ethylene and chlorohydrin are also applied in place of GA as plant growth regulators for breaking dormancy.
Inhibition of sprouting
Sprouting of potato tubers in storage can be prevented by the application of plant growth regulators. NAA is very effective if the whole tuber is dipped in aqueous solution of auxin. Maleic hydrazide sprayed 1-2 weeks before the harvest prevents sprouting of bulbs in storage. Such bulbs do not sprout for nearly six months. Maleic hydrazide is also used to put off sprouting of some vegetables like carrots and sugar beets in storage.
Control of flowering
Plant growth hormones also induce flowering. Many cultured varieties of pineapple promptly flower when 0.5 mg aqueous solution of NAA is applied to the plant. The flowers bloom concurrently and thus all fruits become ready for harvest at the same time. In many woody plants, vegetative growth is aggressive to flowering and in these plants flowering is usually promoted by reducing vegetative growth by pruning branches or withholding irrigation. . In such plants new vegetative growth can be checked by spraying auxins and growth retardants. Application of NAA to litchi and B-nine to apple, pear, etc. reduces vegetative growth and promotes flowering. In sweet potato, flowering can be induced by spraying 2, 4-D. Besides promoting flowering, growth regulators are also widely used to delay or prevent flowering. In strawberry, this is vegetative propagated by runners, suppression of flowering leads to better runner formation. Similarly, crossing of varieties which do not flower simultaneously, delay of flowering in early varieties by the use of growth regulators makes possible simultaneous flowering for breeding purposes.
Control of sex expression
Sex expression, that means the ratio of female to male flowers can easily be controlled by the application of plant growth regulators. Application of auxins increases the number of female flowers and decreases the male flowers in cucurbits. Ethephon also induces femaleness in cucurbits. The increase in the number of female flowers results in a corresponding increase in the number of fruits.
Parthenocarpic fruit set
Naphthoxy acetic acid and BPA effectively induce parthenocarpic fruits. Application of 2 to 10 ppm of BPA to grapes at full bloom is very effective in getting large sized seedless fruits.
Control of abscission
Application of plant growth regulators, development of very large number of fruits on a plant affects their size and quality and thus promoting abscission at an early stage is advantageous. Thinning in grapes has been achieved by spraying NAA at the time of fruit set. Spraying gibberellin before the opening of flowers in grapes also results in less compact bunches. Ethephon and morphactin have also proved useful in this regard.
Application of plant growth regulators like cytokinins and growth retardants is quite effective in delaying degradative processes and retaining freshness for a longer duration of leafy vegetables like cabbage, lettuce, green onion, asparagus, brussel sprout, etc.
Weeds are unwanted plants competing with crop plants. In cereal crops usually broad-leaved plants (dicotyledons) occur as weeds. 2,4-D is a very effective selective herbicide and its spray easily kills broad-leaved weeds without injuring the crop.