Dr. C.M. Ayyub, Sana Saleem, Alqa Yousuf (IHS, UAF)
Plant hormones
Plants produce some natural substances in their tissues particularly growing points, these are called plant hormones. These are produced in minute amount in one part of the plant and then these hormones transferred to their required location to regulate the activities of plant. These are also called phyto-hormones.
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Plant growth regulators (PGRs)
Hormones are synthesized by plants itself but some time they need to apply exogenously. But when we talk about PGRs, these are synthetic or artificial products, which produce reaction almost identical to natural substances when applied to plants. The use of PGRs is much easy to apply so this is useful for farmers. Plant hormones and plant growth regulators are classified in to five categories i.e. auxin, cytokinins, gibberellins, ethylenes, abscisic acids. A lot of mechanisms have been proposed but still its actual mode of action is not yet found. A lot of PGRs are available in the markets.
Auxin
The natural form of auxin is indole-3-Acetic acid and synthetic form is IBA (indole-3-butyric acid), 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthaleneacetic acid). This compound gives positive effect on formation of bud, stimulating root growth and development, induce flowering, inducing the formation of parthenocarpic fruit and prevent from abscission and sprouting. It is also use for eradication of weeds.
Cytokinins
The common synthetic cytokinins are kinetin, zeatin and benzyladenine. It is also known as anti-auxins because they help in growth of lateral buds and inhibit apical dominance. The function of cytokinins includes cell division (pea), cell enlargement (pumpkin) and differentiation. They help to break seed dormancy (lettuce), induce flowering and prevent aging of plants.
Gibberellins
It is the second growth regulator. Gibberellins or gibberellic acid help in seed germination and seedling growth, stem elongation, flowering and sex expression (maleness), fruit setting, promote bolting and to break seed dormancy. They have lot of thypes.
Ethylene
It is a colorless gaseous hormone. In cells it increases the width only not the length. It causes inhibition of root growth and stimulate the formation of adventitious roots. It induces ripening of fruits (tomato). It stimulates senescence and accession of leaves and also helps in induction of femaleness (cucumber, squash and melon). Now a day ethylene generators are available for artificial ripening of fruits.
Abscisic acid
This hormone produced in response to stress. It has direct effect on seed dormancy and also acts as inhibitory chemical compound. It is responsible for closing of stomata and inhibits protein formation and activity of mRNA. It also promotes abscission of leaves and fruits. It also plays vital role in triggering the defensive mechanisms of plants as it signals the onset of stress. Its high concentrations in the plants normally causes plants harmful effects leading to die.
Role in vegetables
Potato
In potatoes 250 ppm ethrel foliar spray increase yield as well as diameter of shoot, plant height and number of tubers per plant. Ethylene, GA3 and thiourea (1% aqueous solution for 1 hour) break dormancy in potatoes. Soaking tubers in 10 ppm to 15ppm GA3 for 10 to 20 minutes breaks dormancy in potato. Foliar application of 50mg/l of GA3 induce flowering in all potato varieties when apply on young leaves. At 60 days after transplanting application of GA3 increase height of plant while late application induce maximum sprouted tubers and increase its physiological age.
Tomato
Plant growth regulators increase yield of tomato plant and also effect on its growth parameters. Pre sowing treatment with 100ppm NAA, IAA and IBA enhance seed germination in tomato. 300 ppm GA3 sprays use for earlier flower production and increasing number of fruits per plant. It also increases yield in tomato. Seed treatment with 2,4-D @ 2-5 ppm induce parthenocarpy in tomato and gives early fruit setting. In normal or even adverse condition fruit setting can be enhanced in tomato by application of 5 ppm kinetin, 10 ppm NAA or GA3 or 2,4-D (1-5 ppm). Post-harvest treatment with ethephon at 500-2000 mg/l induces ripening in mature green tomatoes. 10 days before transplanting 60 ppm GA3 spray increase yield per hectare of variety Roma.
Cucurbits
Cucurbits are group of summer vegetables belongs to cucurbitaceae family. Growth regulators change sex expression in cucurbits, pepper and okra. 100 ppm IAA or NAA and 10-25 ppm GA3 when apply on 2-3 leaf stage it will increase female flowers in cucurbits, while 1500-2000 ppm GA3 increase male flowers production. GA3 and NAA (20 ppm+100 ppm respectively) increase vine length and number of leaves per plant. Same application gives maximum yield in cucumber. In bottle gourd fruit setting can be enhanced by foliar application of 400 ppm maleic hydrazide or 50 ppm triodobenzoic acid at 2 to 4 leaf stage.
Onion
Gibberellic acid use in high concentration (2%) acts as gametocide when it is applied at beginning of bolting process. For reducing physiological weight loss and to avoid spoilage dip roots in 100 ppm NAA. Ethylene and CCC (Cycocel) application increase shelf life of onion and delay sprouting of bulbs.
Chilies
In normal or even adverse condition fruit setting can be enhanced in chilies by application of 5ppm kinetin, 10 ppm NAA or GA3 or 2,4-D (1-5 ppm). Ethephon @ 150ppm increases number of fruits per plant. 4ppm concentration of 2,4-D also increase shoot dry weight, number of seed per fruit and number of fruits per plant. 60ppm concentration of NAA increase flower number per plant, seeds number per fruit and fruit yield per plant in capsicum.
Okra
20 ppm IAA and NAA application increase seed germination in okra. Application of 150ppm GA3 reduced number of days for first harvesting. All yield attribute such as number of pod per plant and pod length increases by application of thiourea @ 500ppm. GA3 @ 50 ppm improved seed quality related characters such as pod weight, 100 seed weight.
Cabbage
Maximum profitable yield can be obtained by 120 ppm GA3 spray in cabbage. By using 80 ppm concentration of NAA plant height, number of leaves per plant, head weight and yield increases. Combine application of IAA 10 ppm and GA3 70 ppm increase plant height, crud diameter, number of leaves, leaf length and yield per hectare. Higher concentration of NAA (140 ppm) enhances yield and vegetative parameters in cauliflower.
Brinjal
Application of 2,4-D at 2-5 ppm in lanolin paste induce parthenocarpic fruits in brinjal when apply to freshly opened flower clusters. In normal or even adverse condition fruit setting can be enhanced in brinjal by application of 5ppm kinetin, 10ppm NAA or GA3 or 2,4-D (1-5 ppm). Higher yield can be obtained by 4 ppm 2,4-D spray. Higher fruit yield and number of fruits per plants can be obtained by 40 ppm NAA application.
All physiological process occurs in crop plants such as growth, development, formation of roots and shoots, sprouting, flowering and ripening are regulating by plant growth regulators. These PGRS are also useful to attain maximum yield and to improve product quality attributes in vegetable production.
Conclusion
These are being applied in commonly now a day, but these is need to research out which method should be used for its application. Some PGRs are effective in soil amendment some show best results with some foliar applications. It is also of much important to check its combine effect. It is notable to see its weather these are effective alone or with some other chemicals. The stage of plant at application is also a important. The dose we apply paly vital role because they become useless at high concentration or even toxic. Moreover, it is utmost desire to synthesize all PGRs in Pakistan for both commercial and analytical grades. So that it must be cost effective as some PGRs are so costly.
