Authors: Amrendra Kumar, S.D. Pandey, Vishal Nath, R.R. Rai and R.K. Patel
National Research Centre on Litchi, Mushahari, Muzaffarpur, Bihar
Plant growth regulators (PGRs) are organic compounds other than nutrients supplying either energy or mineral elements that, in small amounts, promote, inhibit, or otherwise modify any physiological processes in plants. The term PGR includes naturally occurring plant growth substances or phytohormones, as well as synthetic compounds, or chemical analogs.
There are five well-established categories of "classical" phytohormones, namely:
1. Auxins,
2. Gibberellins,
3. Cytokinins,
4. Abscisic acid
5. Ethylene.
More recently, several other compounds that can regulate various facets of plant growth and development have been described, such as oligosaccharins, brassinosteroids, salicylates and polyamines, the list is likely to grow; with new PGRs waiting to be discovered. The availability of sophisticated methods for the identification and quantitative measurements of PGRs, hormone mutants, and powerful toots of molecular biology has greatly enhanced our understanding of their regulatory role in plant growth and development.
Since the 1940s, both natural and synthetic growth regulators have been used in agriculture and horticulture with increasing incidence to modify crop plants by controlling plant developmental processes, from germination through vegetative growth, reproductive development, maturity, senescence (or ageing), and post harvest preservation etc.
Auxins like IAA, IBA, NAA, 2,4-D, 2,4,5-TP, 2,4,-DP etc. plays important role in
1. Propagation,
2. Stimulation of fruit set
3. Chemical thinning,
4. Prevention of fruit drop
5. Herbicidal action, etc.
Gibberellins: Uses
1. Increasing fruit size
2. Stimulating fruit set,
3. Effect on fruit ripening,
4. Controlling flower bud production,
5. Controlling environmental stress etc.
Cytokinins like Zeatin, kinetin, Benzyl adenine (Pro-Shear), Tetrapyranylbenzyladenine (Accel) etc. plays role in
1. Cell division,
2. increase internal bud differentiation, branching, and growth
3. morphogenesis etc.
Abscisic acid: Uses:
1. Stomata closure,
2. Cold hardiness,
3. Delay flowering,
4. Induces storage protein in seeds etc.
Ethylene: Uses:
1. Promote fruit ripening,
2. Release from dormancy,
3. Promotes abscission,
4. Increase latex flow,
5. Delay flowering etc.
Jasmonates: Uses:
1. Inhibit growth and seed germination,
2. Promote senescence, abscission, tuber formation, fruit ripening, pigment formation, tendril coiling etc.
Brassinosteroids: Uses:
1. Promote stem elongation,
2. Inhibit root growth and development,
3. Promote ethylene biosynthesis and epinasty etc.
Flowering in litchi starts from later part of January or early February and fruits ripen in May-June. In litchi, the inflorescence is a much branched panicle, normally arising terminally from the previous season's growth (6-8 month old). Panicle may be pure or mixed. The different litchi cultivars show variations in their flowering and bearing habits and may accordingly be classified as regular, irregular, shy bearing etc. the flowers are mainly staminate, hermaphrodite and pseudo-hermaphrodite. The first flowers to pollen are males, followed by hermaphrodites functioning as females and pseaudo-hermaphrodites functioning as males. It has been was observed that if new flushes appear in autumn due to late rain, these shoots usually fail to bear flowers. Treatments with auxins and some other chemicals were found effective in inducing flowering. Two sprays with 100 ppm NAA, 1% potassium nitrate or 0.05% ethrel during of October and November or cincturing of shoots during these months found effective to promote flowering.
At Pantnagar, maximum reduction in the seasonal shoot growth and panicle length and higher fruit set, fruit yield was obtained with the treatment of Paclobutrazol (PP 333) @ 5ml/m canopy diameter application at 90 days before the expected date of flowering. Spraying of NAA has been also found to enhance the fruit set, fruit retention and fruit weight.
It is also reported that growth substances like NAA, PCPA, GA BA and CCC (Cycocel) proved beneficial in minimizing the drop and cracking and enhancing quality of litchi fruits. The minimum fruit drop was recorded with 20 ppm PCPA. Fruit cracking was least with 20 ppm NAA. The maximum fruit weight and highest aril percentage were obtained with 100 ppm GA. Fruits sprayed with 500 ppm CCC had the highest total soluble, total sugars and ascorbic acid contents, but had the least acidity. Fruit set, fruit and aril weights, fruit volume and pulp percentage, total soluble solid, total sugar content and fruit production significantly increased with NAA and 2, 4-D. Application of Cycocel @1000 ppm advanced ripening by 5-7 days. Foliar applications of NAA at 40 or 50 ppm is recommended to promote fruit set and reduce fruit drop.
Abscission of young fruit is a common phenomenon. Two synthetic auxins (2,4,5,-TP and 3,5,6- TPA), as formulated in two commercial products (Tipimon and Maxim, respectively), were found in Israel to consistently and significantly reduce litchi fruitlet abscission and increase yield in the three commercial cultivars Mauritius, Floridian, and Kaimana. Both auxins are now being used routinely in commercial litchi orchards in Israel. In adapting this treatment to other litchi growing regions in the world, it should be emphasized that timing of the spraying is crucial and may be different for other cultivars. There is need to conduct some comprehensive trials in our country also on different cultivars to realize the potential of these and other auxins. Evidences suggest that auxins inhibit the action of the hydrolytic enzymes poly galacturonase and cellulose, which are responsible for the degradation of the cell wall and the middle lamina in the abscission zone.
Gibberellins have been found to have a number of beneficial effects on fruits in storage, So this may be tried to enhance the shelf life of litchi fruits.
Conclusion
The foregoing account shows that the use of PGRs is very important in modern fruit cultivation. However, since most PGRs are synthetic compounds there is always a danger of their removal from use. There is tremendous scope and potential of PGRs in litchi to increase the production by enhancing the female flower in the panicle and reducing the fruit drop.
Table 2: Summary of role of plant growth regulators
|
Role | PGRs | ||||
| Auxin | Cytokinin | GA | Ethylene | ABA | |
| Extension growth | + | + | - | - | |
| Cell division | + | + | - | - | - |
| Lateral bud | - | + | - | - | - |
| Development | - | + | - | - | - |
| Leaf growth | |||||
| Length | + | ||||
| Breadth | + | ||||
| Chlorophyll | + | ||||
| Vein elongation | + | ||||
| Leaf Movement | + | ||||
| Root information | + | ||||
| Abscission | + | + | |||
| Maleness | + | ||||
| Femaleness | + | ||||
| Fruit growth & Development | + | + | + | + | + |
| Flower | + | + | + | ||
| Dormancy (Seed/bud) | + | + | + | ||
| Senescence | + | ||||
| Tropism | + | + | + | ||
+ promote,-Inhibit
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