Biennial fruits bearing or Irregular bearing or Alternate bearing is serious problems in fruit crops leading to considerable decline in their fruits production potentials. Alternate bearing means "a condition at which high or optimum fruit production in on year or higher and certain year bear little or no fruit (off-year).
Flower initiation is very important because it is the first step towards attaining fruit. Recent trials have clearly shown that while the extent (quantity) of flowering affects yields, time of flower emergence has a significant influence on time of fruit maturity.
Early flowering clearly resulted in early fruit maturity, therefore, one method to manipulate flowering is to use the plant growth regulators particularly the growth retardants like paclobutrazol. Paclobutrazol is a member of the triazole plant growth inhibitor group that is responsible restricts vegetative growth and induce flowering in biennial bearing several fruit crops.
Paclobutrazol (PBZ) is responsible for the induction and manipulation of flowering, fruiting and tree vigor in biennial bearing fruit crops. The application of paclobutrazol in the soil promotes flowering and increasing yield in many fruit crops. Besides reducing gibberellins level, paclobutrazol increases cytokinin contents, root activity, and C: N ratio, whereas its influence on nutrient uptake lacks consistency.
It also affects microbial population and dehydrogenase activity in soil. Paclobutrazol has been characterized as an environmentally stable compound in soil and water environments with a half-life of more than a year under both aerobic and anaerobic conditions. However, when it is applied in optimized rate the residual concentration detected will not be above quantifiable level (0.01 ppm) in soils and fruits.
It is extensively studied due to its high potential for controlling plant growth and development. PBZ which is also known as different commercial names such as pp333, cultar, bonzi; sadabahar, parley, clipper, etc, is an important growth retardant. Paclobutrazol has been effective in controlling the growth of a wide range of fruit crops by inhibiting both sterol and gibberellin biosynthesis.
The main effect of paclobutrazol in the plant is the inhibition of gibberellin biosynthesis, which reduces change in the sink source relationship by reallocating the carbohydrate source towards other organs of the plant than the shoot apex. Paclobutrazol is effective not only in flower induction but also in early and off-season flower induction in fruit crops.
The Mode of action of plant growth retardants such as paclobutrazol is highly specific to cultivar, rate of application, cultivar, developmental stages, and climatic condition. Paclobutrazol holds considerable promise in the manipulation of flowering, yield, and vigor in fruit crops. However, its high potency for harmful to nature, slow mobility in the orchard soil, persistence in soil and fruit over its long term use.
Plant growth retardants are being used widely in chemical manipulation of growth and development by modifying associated biochemical and physiological processes. Among them, paclobutrazol is considered as one of the most versatile plant growth retardant which restricts vegetative growth and induces flowering in many fruit crops like apple, pear, peach, citrus, mango, avocado, litchi, temperate fruits, and nuts.
Paclobutrazol (PBZ) is a triazole derivative with which plays an important role in regulating excessive vegetative growth, enhancing and advancing flowering, inducing early bearing, managing biennial bearing tendency, establishing a high-density plantation.
Why the application of paclobutrazol in fruit crops?
The biennial bearing is a very serious problem in fruit crop production. Because biennial bearing or alternate bearing in fruit crops is a major problem faced by fruit growers. This problem causes great economic loss to the growers. Irregular and alternate bearing problem-solve through paclobutrazol is commonly practiced by growers.
Paclobutrazol is a synthetic plant growth retardant, which has been used in fruit tree crops to control vegetative growth and to induce flowering. The alternate flowering in fruit crops might be due to improper orchard management practices, environmental factors, varietal character, Imbalance of hormone, etc.
Mode of action of paclobutrazol
Gas chromatography-mass spectrometry confirmed that PBZ was taken up by roots and translocate acropetally via the xylem to stems and accumulated in leaves. Paclobutrazol is having anti-gibberellins activity which blocks the conversion of ent-kaurene to ent kaurenoic acid in the gibberellins biosynthetic pathway, which is responsible for cell elongation and cell division and ultimately retards plant growth.
When gibberellin production is inhibited, cell division still occurs, but the new cells do not elongate. That results in the production of shoots with the same numbers of leaves and internodes compressed into a shorter length. Paclobutrazol treated trees show increased production of the hormone abscisic acid and the chlorophyll component phytol, which are beneficial to tree growth and health.
It also induces morphological modifications of leaves, such as smaller stomatal pores, increased number, and size of surface appendages, thicker leaves, and increased root density that may provide improved environmental stress tolerance and disease resistance and it also has some fungicidal activity due to its capacity as a triazole to inhibit sterol biosynthesis.
The amount of paclobutrazol residue left in the soil or plant parts would appear to depend on the methods of application, doses and the crop.
Guideline for use of paclobutrazol on fruit crops
- Paclobutrazol should be applied to healthy trees.
- Tree base should be kept weed-free before and after its application.
- Adequate moisture in the soil at the time of application and for the following 40-50 days is essential.
- Paclobutrazol treated trees start producing inflorescence in 3-4 months of application. This flush of inflorescence as well as new vegetative flushes and flowers should be regularly protected from the attack of insects, pests, and diseases.
- Paclobutrazol treated trees are expected to bear a good crop every year. Therefore, twice the recommended dose of fertilizers and organic manures should be fed to the tree from the second year of paclobutrazol application.
Time of application of paclobutrazol in fruit crops
Usually, it is applied as a soil application in the month of September-November in the case of mango. The growth retardant paclobutrazol is applied after harvest as a soil application or foliar application. Paclobutrazol is applied as Cultar @ 23 SC to the tree basin soil by making a 15 cm deep and 30 cm wide-furrow at a radial diameter of 1 m from the tree trunk during last week of September in mango and Litchi.
Method of application of paclobutrazol
Paclobutrazol applied as
- Foliar application
- Soil application
- TSLP method
- Soil and plant trunk injection
- Incorporating in nutrient solution
Paclobutrazol must be applied directly to the soil due to its low solubility, long residual activity, and lack of efficient foliar uptake. Soil application rather than the foliar application of paclobutrazol has been found to be more responsive in suppressing the vegetative growth and enhancing the reproductive growth in mango trees.
Paclobutrazol is a potent inhibitor of gibberellin biosynthesis and can be applied as an overall spray, as a soil drench or by way of trunk painting; better results have been achieved when used as a soil drench, either in the root zone or the collar region of the tree. The required quantity is mixed in approximately 1 L of water and poured onto the soil around the trunk in a circular band.
Ankit Kumar Pandey1*, Deepak Maurya2, Kuldeep Pandey3, Rakesh Kumar Pandey3, and Ram Niwas4
1Department of Horticulture (Fruit and Fruit Technology), Bihar Agricultural University, Sabour, Bhagalpur, Bihar, India 813 210
2Department of Horticulture (Vegetable & Floriculture), Bihar Agricultural University, Sabour, Bhagalpur, Bihar, India 813 210
3ICAR-Indian Agricultural Research Institute, New Delhi, India 110 012
4Department of Plant Pathology, Bihar Agricultural University, Sabour, Bhagalpur, Bihar 813210, India