Historically, plant growth regulators are substances which, when added in small amount, modify substantially the growth of plants,both stimulatory or inhibitory. They have great promise to increase the world food supply.
It has been reported that since 1940s, the natural and synthetic plant growth regulators have been used with increasing incidence to modify crop plants by changing the rate or pattern or both of their responses to the internal and external factors that govern development from germination through vegetative growth, reproductive development, maturity or aging and post-harvest preservation.
Uses of growth regulators in agriculture and horticultural crops influence regulation of plant metabolism, control of flowering, effects on fruit set and development, abscission, and the size of plant organs.
This history of plant growth regulator goes back long before the time of Christ when it was a common practice in the Middle East to place a drop of olive oil on figs to promote development. It is now known that heat and time caused the breakdown of the oil, releasing ethylene, which affected the development of fig. By 1930s, ethylene was shown to accelerate flowering in pineapple and soon acetylene was used commercially in Hawaii to force the initiation of flowering. During 1940s, auxins were also shown to produce this effect, and naphthaleneacetic acid (NAA) was the next forcing agent to become commercially used in pineapple.
Some of the important and commonly used plant growth regulators are: abscisic acid, alachlor, atrazine, benzyladenine, barban, chlormequat, cycloheximide, etc. About 60 plant bioregulators are now commercially available. Several among these have achieved considerable importance in agriculture. Particularly, effective uses have been made in fruit production. Important uses of growth regulators are in the cloning of cells in tissue culture.
Plant growth regulators include not only herbicides that destroy living plants, but those materials that prevent germination of seeds, stimulate rooting of cuttings, increase or retard the growth of plants, induce flowering, increase or even prevent the setting of fruit and increase the sugar of food and increase the sugar or food content of crops. The most prolific plant growth regulators, however, have been herbicides. Although, the auxins, the first growth regulators were isolated in 1928, it was not until 2,4-D (2,4 - dichlorophenoxyacetic acid) was introduced in 1944 that the growth regulators, especially the herbicides became important commercial agricultural chemicals.
One of the oldest commercial uses for plant growth regulators has been to initiate and/or accelerate the rooting of cutting plants. Probably the best and most commonly used chemical for this purpose is indole-3-acetic acid. Considerable work has been conducted on promotion of rooting, resulting in thousands of publications and several books on the topic.
One of the best recognized uses of plant growth regulators are in the suppression of sprouting of potatoes and onions by treatment with maleic hydrazide. This compound also was extensively used in the 1940s and 1950s to inhibit the growth of grasses in lawns, on golf courses, in parks and along avenues and ways. The next most widely used regulator was chlormequat or cycocel (CCC), which shortened considerably the height of cereal crop particularly wheat without changing the size and quality of ears. This dwarfing in height in wheat prevented lodging after rain and wind when the application of nitrogenous fertilizer is high.
The use of another important growth regulator, the ethephon, (2-chloroethylphosponic acid) an ethylene-releasing agent, to boost the yield of latex from rubber trees has become standard plantation practice. This compound increases the length of time for the flow of latex between tappings and thereby increases by as much as 100 per cent of the commercial dry rubber from commercially important tree varieties. The use of ethephon obviates frequent cutting of tree bark, thus helping preserve the life of the tree.
One of the most important commercial developments in recent years of plant growth regulators has been the use of chemicals for the ripening of sugar cane. A wide variety of chemical structures have been shown to increase the sucrose content of care at harvest. The ethylene producing compound etephon is used commercially on sugar cane in South Africa and Zimbabwe. The increased sugar yield produced by treatments with ripeners is 5- 25% depending on the variety of cane treated, and on prevailing weather and soil conditions.
Growth regulators used for various other reasons also can effect fruit quality. Daminozide was introduced for application on apple trees in the mid-1960s. Ethephon improves the red coloration of many apple cultivars and rapidly transforms green fruit to ripe fruit for early marketing. Both ethephon and daminozide (succinic acid-2,2-dimethyl hydrazine) have been used to advance the commercial harvesting of peaches.
Ethephon also promotes ripening of bananas, peppers, dates
and prunes. The effects of ethylene on flowering are promotion, inhibition and sex reversal. Synthetic growth regulators that cause development of fruits in plants include chlorophenoxyacetic acid and napthoxyacetic acid. These chemicals are most effective on fruits with many plants, e.g. tomato, squash, eggplant and fig. However, these chemicals are ineffective on peach, plum and other stone fruits. Many fruits that can be set by such hormonal compounds also can be set by gibberellins.
An extensive study has shown gibberellic acid to be the growth regulator of choice to improve fruit set of citrus. Application of aqueous sprays of GA (gibberellic acid) to entire trees in full bloom increases the set and the yield of seedless fruit of five self incompatible citrus cultivars seedless fruits, however are more likely to drop after initial fruit set than seedy fruits produced by cross-pollination. The control of abscission i.e. the separation or shedding of a plant pat such as leaf, flower, fruit or stem from the parent plant is extremely important in agriculture and horticulture. To ensure the most effective crop growth, leaves usually should be retained in a healthy, green stage.
On the other hand, to simplify the mechanical of certain crops, e.g. cotton, it is highly desirable to have the leaves removed. In tree crops that have a large number of fruit started, it sometimes is desirable to thin the fruit by using an abscission inducing compound, thereby increasing the size and quality of the remaining fruits.
During crop growth, the fruits should be retained on the tree for maximum development and maturity. Ethephon appears to do a now satisfactory job in the thinning of cherries than it does for peaches. Ethephon has been used effectively in the spraying of macadamia trees in Australia to induce nut drop. In California, the same chemical has been effective in permitting an earlier harvest of walnuts. Several types of chemical harvest aids are used on more than 3/4th of the cotton acrage in the United States.
Defoliants are the preferred class of harvest aids to remove leaves without drying them. Abscised leaves are green and moist and, unlike dried leaves, tend to fall free of the lint in the open bolls. Harvesting usually can be precisely scheduled after a defoliant is applied. Defoliants are used primarily where cotton is harvested by spindle pickers, which cannot operate at peak efficiency if plants are erect and have an abundance of green succulent foliage. The development of growth retardants has had a considerable impact on the production of certain crops, especially floral crops, e.g., chrysanthemum, poinsettia, and other decorative flowers. The most commonly used growth retardants are daminozide and chlorphonium chloride. Ancymidol and dikegulac sodium have been extensively investigated, particularly for poinsettias in the case of ancymidol and for the prevention of shoot elongation in junipers by dikegulac sodium.