Role of plant growth regulators in Vegetable Crops Credit Seminar on Role of plant growth regulators in Vegetable Crops Navjot Singh H-10-54-M

Plant Growth Regulator ? 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.

Classification of PGR’s Sr. No. Growth regulator Example 1 Auxins IAA, IBA, NAA, 2,4-D 2 Gibberellins Gibberellic acid 3 Cytokinins Kinetin, Zeatin 4 Ethylene 5 Dormins Abscissic Acid 6 Flowering hormones Florigen, Anthesin, Vernalin 7 Miscellaneous natural sunstances Cyclitols, Vitamins, Phytochrome, Traumatic substances etc. 8 Phenolic substances Coumarin 9 Synthetic growth retardants CCC, Phosphon D, Morphactins, Maleic hydarzode (MH) etc. 10 Miscellaneous synthetic substances Synthetic auxins, synthetic cytokinins etc.

Plant Growth Promoters Plant Growth inhibitors Major classes of PGR’s Auxins Gibberellins Cytokinins Ethylene Abscisic acid Plant Growth Promoters Plant Growth inhibitors

Plant Growth Promoters Auxins The word Auxins has been derived from a Greek word auxein- “to grow/increase”. It was first isolated from human urine. These are generally produced by the growing apex of stem and roots of the plants.

Indole-3-Acetic acid (IAA) Types of Auxins Natural Synthetic Indole-3-Acetic acid (IAA) IBA, 2,4-D, NAA

Pinching promotes the lateral bud initiation Functions of Auxins 1. Apical dominance Terminal buds prevent the development of lateral buds on plant stem. Pinching of terminal bud stops flow of Auxins down the stem and allows side shoots to develop. Pinching promotes the lateral bud initiation

2. Root initiation and development Applied on cuttings to stimulate root growth and development.

3. Phototropism Auxins are responsible for plants bending towards light. They move down towards shaded side of the stem and cause cells to elongate.

4. Parthenocarpy Auxins induces parthenocarpy i.e., formation of seedless fruits without fertilization

5. Inhibition of abscission layer Formation of an abscission layer at the base of petiole or pedicel results in shedding of leaves, flowers or fruits. Auxins inhibit abscission, as they prevent the formation of abscission layer.

6. Used as weedicide Many synthetic auxins are used as selective weed killers and herbicides. 2, 4-D is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants.

7. Flowering Foliar spray of NAA and 2, 4 - D induces flowering in many crop plants.

8. Storage Auxins such as NAA is used to prevent the sprouting of potato tubers. Hence, increases the storage life of the produce.

Gibberellins Second most important growth Hormone. Gibberellins are named after the fungus Gibberella fujikuroi , which causes rice plants to grow abnormally tall (Kurosawa et al., 1930). Are produced in the shoot apex mainly in the leaf primordial (leaf bud) and root system, hence they translocates easily in the plant in both directions. More than 60 types of Gibberellins are known today.

Functions of Gibberellins 1. Stem elongation Gibberellins cause internodes to stretch in relation to light intensity. Less is the light intensity more will be internode length and vice versa. Stimulate stem growth through cell elongation and cell division.

3. Seed germination and Seedling growth Used commercially in stimulating seed germination and seedling growth.

4. Seed dormancy GA3 is used to break the seed dormancy of freshly harvested seeds in many vegetable crops such as potato and lettuce. Sprouting in Potato

Cytokinins They were first isolated from coconut milk. Miller, Skoog and their coworkers isolated the growth factor responsible for cellular division from a DNA preparation calling it as Cytokinins (1950). They are synthesized in root apex, endosperm of seeds, young fruits, where cell division takes place continuously.

Functions of Cytokinins Promotes cell division, cell enlargement and cell differentiation (used in tissue culture). They prevent aging of plants. They inhibit apical dominance and help in growth of lateral buds. Therefore it is also known as anti-Auxins.

Plant Growth Inhibitors Ethylene Ethylene is a colourless gaseous hormone Found in ripening fruits, flowers and leaves and nodes of stem. Synthesis of ethylene is inhibited by carbon dioxide and requires oxygen. 

Functions of Ethylene It induces ripening of fruits. Promotes abscission and senescence of leaf, flowers etc. In cells it only increases the width not the length.

Abscisic Acid It is also known as dormins, which acts as anti-Gibberellins. It is synthesized in leaves of wide variety of plants. Responsible for closing stomata during drought conditions, hence acts as plant stress hormone.

Functions of Abscisic Acid It induces dormancy of buds and seeds as opposed to Gibberellins, which breaks dormancy. Involved with leaf and fruit abscission (fall). It inhibits seed germination and development. ABA also plays important role in controlling stomata opening and closing.

Relative concentration of some plant hormones Plant part Auxins Gibberellins Cytokinins Abscisic acid Shoot tip +++ + Young leaves Elongating stem ++ Lateral buds Flower and fruits Developing seeds Mature leaves Lateral shoot Mature stem Root Root tip +++ High concentrations ++ Medium concentration + Low concentration

Commercial Use of PGR’s in Vegetable crops

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 has been 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 Seed Dormancy is the main problem in 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.

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. 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. GA3 (10-25 ppm), IAA (100 ppm) and NAA (100 ppm) when sprayed at 2-4 leaf stage in cucurbits, then they have been found to increase the number of female flowers. Whereas, GA3 (1500-2000 ppm), silver nitrate (300-400 ppm) and Silver thiosulphate (300-400 ppm) sprayed at 2-4 leaf stage induces male flower production in cucurbits.

Hybrid seed production 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 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. GA3 at 100 mg/l can also be used for inducing male sterility in pepper.

Hybrid seed production….conti Use of ethephon (100 ppm) has been used for producing 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, silver nitrate at 500 mg/l  has been reported to be effective 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 such as PCPA (20-25 ppm) and 2,4,-D (1-5 ppm), Kinetin (5 ppm), NAA (10 ppm) and GA3 (10 ppm) have been reported to enhance fruit set under both normal and adverse weather conditions, when applied at flowering stage in tomato, brinjal and chillies.

Parthenocarpy Plant growth regulators helps to stimulate the fruit development without fertilization (parthenocarpy). 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. GA3 (100 ppm) also used to develop parthenocarpic fruits in cucumber.

Fruit Ripening Ethephon, an ethylene releasing compound, has been reported to induce ripening in tomato and pepper. Field application of  ethephon (1000 ppm) 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 ppm has also been reported to induce ripening in mature green tomatoes.

Fruit Yield Tomato: Soaking of seed in GA3 (5-20 ppm) and CIPA (10-20 ppm), 2,4-D (0.5 ppm) have been reported to improve fruit yield in tomato. Brinjal: Soaking of seedlings roots in NAA (0.2 ppm) and ascorbic acid at (250 ppm)  have been reported to produce higher fruit yield. Chillies and Peppers: Foliar sprays  of  GA3 (50 mg/l) at fruit setting  or planofix (NAA 10 ppm) double sprays (at flowering and 5 week later) decreased flower shedding and gave better fruit yield in chillies.

List of plant growth regulators and their important uses in vegetable crops Conc. (mg/l) Method of application Crops Attributes affected Cycocel (CCC) 250-500 Foliar spray Cucurbits, tomato, okra Flowering, sex expression, fruit yield P-Chlorophenoxy Acetic acid (PCPA) 50 Tomato Fruit set and Yield Ethephon (CEPA) 100-500 Cucurbits, okra and tomato Flowering, fruiting, sex expression and yield 2000 Post- harvest Tomato, chillies Fruit ripening Gibberellic acid (GA) 10 Water melon, tomato Sex expression, fruiting , yield Indoleacetic acid (IAA) 10-15 Okra, tomato, brinjal, Seed germination, fruit set and yield Contd…

Naphthalene acetic acid (NAA) 0.2 Seedling roots Tomato, brinjal, onion Growth and yield 10-20 Foliar sprays Chillies and tomato Flower drop, fruit set and yield 25-30 Seed/ foliar okra ,Tomato, brinjal, onion, cucurbits Seed germination, growth and yield Naphthoxyacetic acid (NOA) 25-100 Tomato, okra Germination, growth and yield Silver nitrate 500 Foliar spray Cucumber Induction of male flower in gyn, lines Silver thiosulphate 400 - Musk melon 2,3-5, tri-iodobenzoic acid (TIBA) 25-50 cucurbits Flowering, sex expression and yield Tricontanol 2 Chillies and peas Fruit set and yield Source :Chadha and Kalloo,1993

Precaution in Growth Regulator Application Growth substances should be sprayed preferably in the afternoon. Avoid to spray in windy hours. Spray should be uniform and wet both the surface of leaves. Add surfactant or adhesive material like Teepol, Tween-20 are Gum with growth substances @ 0.5 – 1.0 ml/l solution. Use growth substances at an appropriate stage of plant growth is of great importance. Contd…

Chemical should be completely dissolved before use over plant. Use always fresh solution of chemicals. Solution should always be prepared in distilled water only. Fine spray can be ensured by hand automizer. It is most economical and effective method of spray. Wash the machine/pump after each spraying. Repeat the spray with in eight hours if chemical is wash out due to rain.

Constraints in the use of growth regulators The difference in sensitivity of each plant species or even cultivars to a given chemical treatment prevent easy predication of the biological effects. The cost of developing new plants growth regulator is very high due to which they are very much costly. Screening for plant growth regulatory activities entails high costs and is very much difficult. Some synthetic plant growth regulators cause human health hazards e.g. dominozide. Contd…

Lack of basic knowledge of toxicity and mechanism of action. Inadequate market potential. Lack of support from agricultural researchers in public and private sectors. Difficulty in identification of proper stage of crop at which the growth regulators should be applied.

Conclusion and Future thrust Most of the biological processes associated are polygenic, so gene transfer may be difficult and hence the use of PGR’s may be beneficial for short imperatives. PGR’s provide an immediate impact on crop improvement programmes and are less time consuming. Applications of PGR’s must lead to quantifiable advantages for the user. PGR’s must be specific in their action and toxicologically and environmentally safe. Contd…

Industries involved in development of PGR’s should be well informed about the latest scientific development in production of PGR’s. Plant growth regulators should be recognized as more than academic curiosities. They are not only interesting but profitable to use to grower, distributor and manufacture. There is need for some plant growth regulators which can inhibit photo- respiration in plants. More research is needed to develop simple, economical and technical viable production systems of PGR’s.