Plant Growth Regulators

Plant Growth Regulators


What is a Plant Growth Regulator?

A growth regulator is

  • An organic compound,
  • Can be natural or synthetic,
  • It modifies or controls 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).

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. A large number of chemicals tend to increase the yield of certain plants such as corn and sugarcane.

Both internal plant hormones and lab created hormones are called plant growth regulators.

Hormones usually move within plant from a site of production to site of action.

There are five classes of phytohormones:

  • Auxins,
  • Gibberellins,
  • Cytokinins,
  • Abscisic acid and
  • Ethylene.

Auxins:

These are organic substances which at low concentration (less than 0.001 m) promote growth along the longitudinal axis, but auxins also_

  • Suppress the auxillary buds and this results in apical dominance.
  • Used as selective herbicides when applied at higher dose. e.g. 2,4-D
  • Induce rooting in cuttings. e.g. IBA
  • Help in the development of fruits in the absence of pollination called parthenocarpic fruits.
  • Increase no.of female flowers and inhibit male flower production in monoecious vegetable.
  • Induce flowering and enhance fruit set.
  • Control pre-harvest fruit drop and increase yield.
  • Overcome cross imcompatibility in breeding programmes.

Gibberellins:

These substances are having gibbane ring skeleton capable of producing the same physiological responses as gibberellic acid. It was first isolated from the soil borne fungus Gibberella fujikuroi.

The gibberellins are phytohormones which are active in regulating dormancy, flowering, fruit setting, and stimulating germination of seeds and extending growth of shoots.

  • Overcome genetic and physiological dwarfism.
  • Break the dormancy of seed, buds and leaf.
  • Induce parthenocarpic fruit set.
  • Increase vegetative growth and yield.
  • Cause leaf expansion to harvest more sunlight for photosynthesis.
  • Increase activity of α-amylase enzyme which help in increasing the TSS.
  • Substitute long day requirement of plants and help long day plants to flower in short day.
  • Sex expression: Promote maleness at higher concentration ( 1500 ppm ) therefore, used to maintain gynoecious lines of cucumber in hybrid seed production.

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 GAs in breaking the dormancy of a wide range of seeds and in increased fruit set. These hormones mainly stimulate cell divisions, deley leaf senescence and prevent chlorophyll degradation.

Absecisic acid (ABA):

ABA is a naturally occurring sesquiterpene ( C15H24 ) which regulate plant growth and metabolism in various ways and have been detected in nearly all plants. It is involved in the abscission of plant organs, retardation  of vegetative buds, regulation of fruits repining and generally in reduction of growth.

Stomatal regulation: Moisture stressed plants produce ABA which facilitates stomatal closure and help in maintaining cell turgidity.

Ethylene:

It is the only gaseous hydrocarbon hormone which plays an important role in the ripening of fruits, inhibition of root growth, abscission and other growth processes. Unlike the other hormones, ABA and ethylene are not discovered through any interaction with fungi.

  • Regulates ripening process of fruits.
  • Induce early flowering and anmass ripening.
  • Induce formation of abscission layer based on auxin : ethylene ratio.

Role of Plant Growth Hormones in Vegetable Production:

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.

In tomato higher germination with GA3  at 0.5 mg/l, and 2,4-D at 0.5 mg/l is reported.

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

Seed Dormancy : Main problem has been potato where freshly harvested tubers fail to sprout before the termination of rest period. Chemicals which have been reported to break the rest period are GA, ethylene chlorhydrin and thiourea.

The treatment which has been used for breaking of dormancy in potato comprise the vapour treatment with ethylene chlorhydrin (1 liter per 20 q) followed by dipping in thiourea  (1% sol.) for 1h finally in GA (1 mg/l) for 2 seconds.

Lettuce is another vegetable in which treatment with GA has been reported to break seed dormancy induced by high temperatures.

Flowering : Induction of flowering in plants which otherwise fail to flower has also been reported with the use of various plant growth regulators. Application of GA at 50 mg/l to young leaves of non- flowering varieties of  potato, when floral buds had just  formed, resulted in flower induction in all varieties. MH delayed flowering in okra . GA has been reported to induce early flowering in lettuce.

Sex Expression : The treatment with growth regulators has been found to change sex expression in cucurbits, okra and pepper.

Gametocides : Some plants growth regulators possess gametocidal actions to produce male sterility which can be used for F1 hybrid seed production. The chemicals which has been reported to show good performance are MH at 100 to 500 mg/l in egg plant, okra, peppers and tomato, GA3 in onion, 2,3- dichloroisobutyrate (0.2 to 0.8%) in egg plants, muskmelon, okra, onion, root crops, spinach and tomato and TIBA in cucumber, egg plants, onion, and tomato. GA at 100 mg/l can also be used for inducing male sterility in pepper.

Hybrid Seed Production : Growth regulators can be used as an aid in hybrid seed production. Use of ethephon has been used for producting temporary female lines in some cucurbits. Successful F1 hybrid in Butter-nut squash has been made by using female line produced with ten weekly sprays of ethephon Plant growth regulators have also been used for maintenance of gynoecious lines. In cucumber, GA3  sprays have been made to induce staminate flowers in gynoecious lines. Silver nitrate at 500 mg/l  has been reported to be as effective as GA3  in inducing male flowers on gynoecious lines of cucumber . However, in muskmelon foliar sprays of Silver thiosulphate at 400 mg/l was found best for induction of male flower on gynoecious lines.

Fruit Set : Poor fruit set is a major problem in tomato, brinjal and chillies which is frequently caused by adverse weather conditions during flowering. Plant growth regulators have been reported to enhance fruit set under both normal and adverse weather conditions.

Parthenocarpy : Studies on use of growth regulators in many vegetables have shown an increase in the fruit size with their applications. The role of plant growth regulators in fruit development can also be seen from the fact that  with their help it is possible to stimulate fruit development without fertilization (parthenocarpic). In brinjal, application of 2,4-d at 0.00025% in lanolin paste to cut end of styles or as foliar sprays to freshly opened flower cluster has been reported to induced parthenocarpy.

Fruit Ripening : Ethephon, an ethylene releasing compound, has been reported to induce ripening in tomato and pepper. Field application of  ethephon at 1000 mg/l at turning stage of earliest fruits induced early ripening of fruits thus increasing the early fruit yield by 30-35%. Post-harvest dip treatment with ethephon at 500-2000 mg/l has also been reported to induce ripening in mature green tomatoes .

Fruit Yield :

Tomato : Soaking of seed in NOA at 25-50 mg/l, GA at 5-20 mg/l and CIPA at 10-20 mg/l, 2 ,4-D, 0.5 mg/l or thiourea at 10-1 M have been reported to improve fruit yield in tomato.

Brinjal : Soaking of seedlings roots in NAA at 0.2 mg/l and ascorbic acid at 250 mg/l  has been reported to produce higher fruit yield.

Chillies and Peppers : Foliar sprays  of  GA at  50 mg/l at fruit setting  or planofix (NAA 10 mg/l) double sprays (at flowering and 5 week later) decreased flower shedding and gave better fruit yield in chillies.

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