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What is a plant Growth Regulator:
Ø A growth regulator is An organic compound ,Can be natural or synthetic. It modifies or control one or more specific physiological processes  within a plant but the sites of action and production  are different. If the compound is produced within the plant it is called a plant hormone. (E.G. Auxin, Which regulates the growth of longitudinal cells involved in bending a stem of a plant one way or the other).
                           A large number of chemicals tend to increase the yield of  certain plants such as tomato and okra . Both internal plant hormones and lab created hormones are called plant growth  regulators.
                         Substances applied externally also can bring about     modifications  such as improved rooting of  cuttings, increased rate of ripening, easier separation of fruit from the stem. Etc. Hormones usually move within plant from a site of  production  to site of action.
They are five classes of polytohormones:

   Auxins
   Giberellins _GA  _                                           Pl.Growth Promotors
   Cytokinins _Kinetin, Zeatin   

   Abscisic acid _                                                Pl.Growth inhibitor
   Ethylene_
Fl.hormone: Florigene, Anthelin, Vernalin

Misc.nutral substances: Cyclitols, Vitamins, phytochrome, Transmetric substances.

Phenolic  substance                :  Coumarin,
Synthetic Growth retardents             : CCC, Phosphon D morphactins,  MH etc

Figure 1
Auxins:
v These are Organic substances  which at low concentration ( less than 0.001 m) Promote growth along the longitudinal axis, but auxins also influence a wide range of growth and development response.
v Gibberellins:
                 These substances are having gibbane ring skeleton capable of producing the same physiological response as gibberellic acid. It was first isolated from the soil borne fungusGibberella fujikuroi.
                  The gibberellins are phyto hormones which are active in regulating dormancy, flowering, fruit setting, and stimulating germination of seeds and extending growth of shoots.
Cytokinins:
                These are substances composed of hydrophilic group of higher specificity (adenine) and one lipophilic group without specificity.The cytokinins form a group of  plant hormones having similar effects as those  of Ga3 in breaking the dormancy of a wide range of seeds and in increased fruit set.
                 These hormones mainly stimulate cell divisions and prevent chlorophyll degradation.
 Absecisic acid (ABA):
Ø                ABA is a naturally occurring sesquiterpene which regulate plant and metabolism in various ways and have been detected in nearly all plants.
                It is involved in the abcission of plant organs, retardation of vegetative buds, regulation of fruits repining and generally in reduction of growth.

v Ethylene:
     It is the only gaseous hydrocarbon hormone which plays an important role in the ripening of fruits, inhibition of root growth, abscission and growth processes.
              Unlike the other hormones, ABA  and  ethylene are not discovered through any interaction with fungi.
Role of plant Growth Hormones in vegetable Production:
v The role of plant regulators in various physiological and biochemical processes in plants is well known Growth regulators are known to affect.
§  Seed germination
§  Seed dormancy
§  Vegetative growth
§  Nodulation
§  Tuberization
§  Fruit ripening and yield.
                    These can also be used for producing polyploidy and male sterility in order to overcome inter-Species incompabilities and for producing hybrid seeds.
Seed Germination:
                 Pre-showing treatment of seed with growth regulators has been reported to enhance seed emergence.
Ø In tomato, higher germination with GA3 at 0.5 mg/l, and 2,4-D at 0.5 mg/l is reported.

Ø Soaking of seeds in ethophon at 480 mg/l for 24 h improved germination in muskmelon, bottle gourd, squash melon and watermelon at low temperature. 

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