Commercial application of Plant growth regulators in Agriculture and Horticulture

A) Auxins

a) IBA (@250 ppm) and NAA were found to increase root development in the propagation of stem cuttings.

b) 2,4-dichlorophenoxy acetic acid (2,4-D) stimulates excessive uncontrolled growth in broad leaf plants for which it is used as a herbicide.

c) Application of NAA (Napthalene Acetic Acid) reduces flower and fruit drop in Mango.

d) NAA application brings uniform flowering and fruit set by inducing ethylene formation in Pineapple.

e) NAA application @ 10-100 ppm during fruit setting period controls boll shedding
in cotton crop.

B) Gibberellins:

a) GA is used extensively on seedless grape varieties to increase the size and quality of the fruit. Pre- bloom spray of 20 ppm induces rachis of the fruit cluster to

elongate. This creates looser clusters that are less susceptible to disease during the growing season.

b) GA is used to increase the yield of barley malt and to decrease the time required for this process to occur. Application of GA to germinating barley supplements the endogenous content of this hormone and accelerates the production and release of hydrolytic enzymes. They can easily degrade the stored carbohydrates.

c) Foliar spray of GA3, at 100 ppm during panicle initiation stage enhances the panicle exertion and increases seed weight and yield in hybrid rice.

d) GA has also has been used to control flower sex expression in cucumbers and squash. GA application tends to promote maleness in these plants.

e) Gibberellic acid is also applied to citrus crops, though the actual use depends on the particular crop. For example GA3 is sprayed onto oranges and tangerines to delay or prevent rind-aging, so that fruit can be harvested later without adverse effects on rind quality and appearance. For lemons and limes, GA3 synchronizes ripening and enhances fruit size.

f) Gibberellic acid is used extensively to increase the sucrose yield of sugarcane. Sugarcane, a normally fast-growing C4 member of the Poaceae (grass) family, is sensitive to cooler winter temperatures, which reduce internode elongation and subsequent sucrose yield. The adverse effects of cooler temperatures can be counteracted by the application of GA3.

C) Ethylene:

a) Because ethylene regulates so many physiological processes in the plant development, it is the most widely used plant hormones in agriculture. Auxins and ACC can trigger the natural biosynthesis of ethylene and in several cases are used in agricultural practice.

b) Because of its high diffusion rate, ethylene is very difficult to apply in the field as a gas, but this limitation can be overcome if an ethylene releasing compound is used. The most widely used such compound is ethephon or 2chloro ethyl phosphonic acid (CEPA) (trade name: ethrel).

c) Ethrel @ 100-250 ppm sprayed at 2-3 leaf stage induce femaleness in cucumber and melons.

d) It helps in degreening of citrus and banana which increases its market acceptability.

e) Storage facilities developed to inhibit the ethylene production and promote preservation of fruits have a controlled atmosphere of low 02 concentration and low temperature that inhibits ethylene biosynthesis. A relatively concentration of CO2 (3-5%) prevents ethylene action as a ripening promoter.

D) Other growth regulators:

o AMO 1618 (a quaternary ammonium salt) is used in the cultivation of ornamental plants and causes a bushy shape and a sturdy growth of the treated plants.

o paclobutrazol: Reduces the problem of biennial bearing in Mango.

o Mepiquat chloride, Chlormequat chloride (Cycocel) : used in ornamental plants for shorter internodes and thicker stems (used in poinsettias), it also prevents lodging and increases tillering in cereals.

o Malichydrazide (MH): prevents premature sprouting of onion and potato.

o 2,3,5-T or Triiodo benzoic acid (TIBA): Increases flowering in chrysanthemum.