फल फसलों में एकांतर वहन प्रवृत्तियां और उसका प्रबंधन

Alternate bearing means production of an excessive crop one year followed by little or no crop next year and Irregular bearing means production of a large crop one year followed by a lesser crop next year / or any irregular succession of crops differing  as to yields. This phenomenon in perennial fruit crops appeared to be governed by positive and negative regulatory factors and their interplay decided the fate of meristems under optimum environmental cues.

Flower initiation is very important because it is the first step towards attaining fruit.  Floral induction is considered to be the result of elevated levels of up-regulated florigenic promoter (FP) and down-regulated vegetative promoter (VP), primarily gibberellins, whereas the reverse condition promotes vegetative growth.

The production of vegetative shoots in place of reproductive shoots is due to the elevated level of gibberellin which is considered as a vegetative promoter. Paclobutrazol, a gibberellin inhibitor, reduces VP level and thereby increases FP/VP ratio which stimulates flowering shoots in weakly inductive shoots of fruit crops.

Mango is a tropical evergreen fruit crop having a strong tendency towards alternate or biennial bearing habit. One of the methods to manipulate flowering is through the use of synthetic plant growth regulator, Paclobutrazol and potassium nitrate. The early flowering considerably enhanced fruit maturity.

Alternate bearing is one of the major problems in mango production all over the world including India. Though several remedial measures have been suggested to overcome this problem none of these was successful until the advent of plant growth retardants.

In commercial mango plantations, it is essential to control the vegetative growth and canopy size to get regular and uniform flowering. Alternate bearing severity maximum in mango, all commercial varieties suffer and even some hybrids bred for their regularity in bearing show this tendency. In litchi few varieties show this tendency, Example- China variety

Alternate Bearing: economic impact

• Places great economic strain on fruit growing industry.
• Dependable markets and constant income cannot be established when fruit supply remains uncertain.
• Orchard inputs and labour requirements depend on crop size, so it cannot be predicted accurately for the alternate bearing crops.
• Fruit growers cannot remain financially secure with their income from such orchards.
• Detraction from the orcharding business.

Factors responsible for alternate bearing

There are several factors responsible for alternate bearing habit

Environmental Factor:        

• Heavy rain, frost, hail at blooming
• High velocity wind
• Suitable day-night temperature
• Insects and diseases
• Nutrient and water stress
• Excessive pruning / inappropriate plant architecture
• Impact of cultural practices
• Orchard floor management

Weather conditions impact flower and fruit development and can exacerbate the alternate bearing tendency of individual tress or whole orchards. For example, a severe frost during flowering would kill developing flower tissue and result in a very low fruit set as is happening in mango in subtropics. As a result of the low fruit set, and reduced use of stored carbohydrates at each individual shoot, the following year most shoots will produce flowers and set fruit.

If all outgrowths on a tree are synchronized to flower during the same year it will generate a whole tree, and even whole orchard, pattern of alternate bearing. Adverse climatic conditions like rain, high humidity and low temperature sometimes convert an on year into off year directly or by promoting the incidence of diseases like powdery mildew, anthracnose and poor fruit set.

Synchronization of alternation over wide areas often has been blamed on spring frost with apples, olives, pecans and mangos in cool areas. There are two major differences which exist between tropical and temperate deciduous horticultural trees when it comes to floral initiation.

Tropical tree species such as mango initiate flowering in response to an environmental stimulus, while the temperate deciduous species, such as apple, initiate flowers autonomously.

Genetic Factor:

• Varietal and rootstock inherent influences
• Sex ratio
• Identifying genes responsible for regular flowering add fruiting
• Studies are underway to coordinate the regulation among chromatin structure, transcription, regulatory genes for ‘on’ and ‘off’ mechanism and alternative splicing during vegetative to flowering and fruiting transition.

Physiological Factor:

• Tree / Shoot age and vigour
• C : N ratio
• Vegetative flushing
• Crop load
• Hormonal imbalance

In fruit plants, N and CHO reserves play important role in flower bud initiation, even if these do not form the primary cause of the phenomenon of biennial bearing. It is quite probable that an accumulation of these compounds may create favourable conditions for the synthesis and action of the substances responsible for flowering. Flowering time is largely affected by large chemical constituents such as plant hormones.

Biennial bearing problem appears to be closely associated with the fruit development process and the inhibitory influence of the developing fruits on vegetative growth. Higher levels of auxin like substances and an inhibitor (similar to ABA) and lower levels of Gibberellin (GA3) like substances are vital for a florigenous shoot. There is inverse relationship between the level of endogenous inhibitor in the shoot and vegetative growth. Higher inhibitor content promotes flowering.

Fruit Tree Growth Pattern  

Growth Pattern of fruit tree in fruit trees usually takes place in many flushes, which vary in different parts of the country location. Early initiation and growth cessation, followed by a definite dormant period, will help the shoots to achieve the proper physiological maturity that is necessary for the initiation of the fruit-bud. The shoot maturity prerequisites for floral induction in mango and litchi. 

In litchi plant generally produces 3-4 flushes annually:

1^stflush: Mid of July (30 days after pruning)

2^ndflush: 3-4^th week of August

3^rdflush: November

4^thflush: Mixed flush during January

In mango flushing is marked by the emergence of new shoots from the old shoots terminals. The major flush occurs once the harvesting of fruits is done. The flushing takes place in order to enlarge tree size and for producing leaves. The leaves are used for the process of photosynthesis that results in the production of carbohydrates which is used in the fruit development later.

The mango tree produces more shoot flushes than it requires especially in the favourable conditions of high temperature and plenty of water and nutrients. The proper control of the growth of shoot flushes can leads to better yields of fruits.

Crop Load Factor:

Fruiting is an exhausting process and the number of fruits retained till harvest is a varietal feature. The total number of fruits that are harvested is important because of their deleterious influence on the production of new shoots for the next season and their subsequent fruit-bud differentiation.

The fruit load appears to be one of the main conditioning factors for ‘on’ or ‘off’ year in fruit crops. This hypothesises, that if fruit load is more, then it blocks the recognition of flowering inductive signal. It prevents the emergence of inflorescence and bud break.

Measures of the alternate bearing problem

Smudging

• Very old practice
• Burn a smudge pot or other smoke-producing device in mango orchard in early spring, before flowers form on the tree.

Application of KNO₃ and Ca (NO₃)₂

• KNO₃ – It contains 13% nitrogen and 44% potash. Use @ 1-2% solution in September - October. Two sprays at 2-4 days interval.
• Ca (NO₃)₂– It contains 19% calcium and 15% nitrogen. Use @ 2-3% solution in September - October. Two sprays at 2-4 days interval.

De-blossoming and thinning of fruits

• NAA has a strong de-blossoming activity 200 - 400 ppm
• Use Ethephon @1500 - 1800 ppm.
• Cycloheximide @250 -500 ppm
• Involves high risk as sometimes there may be less setting than expected.
• NAA has a strong thinning activity at 100 -200 ppm.
• Thinning activity is strongest when fruit diameter is between 10 and 15 mm, and progressively milder when applied earlier.
• Thinning activity becomes less certain as fruit diameter exceeds 17 mm, and fruits become less sensitive to NAA when diameter exceeds 20 mm.
• Benzyladenine (BA) is a synthetic cytokinin also known for strong chemical thinning efficacy at 75-175 PPM.
• Ethylene directly stimulates fruit abscission as well as flower formation. Ethephon (e.g., Ethrel®) is used for fruit thinning at 400 - 600 PPM. Effective up to 25 mm fruit size.
• Choice of materials, rates used, timing and spray application method all influence the outcome.
• Deblossoming: Deblossoming of the panicles with NAA @ 200ppm during ‘on’ year. NAA has been used with success on mandarin-type fruits

Manage crop load

• Where there is established biennial bearing behaviour the objective should be to bring crop loads down to within 15-20% of final crop load within 6-8 weeks time.
• Both fruit thinning and shoot pruning are utilized as a crop load management practice.
• Pruning removes bearing surface (fruit buds) and stimulates vegetative growth from remaining buds.
• Early fruit thinning is the most beneficial strategy for enhancing fruit size and quality in the current season and controlling the crop load on the tree.

Ringing or girdling

• Removal of a ring of bark about 2-3mm wide and 2-3mm deep around scaffold limbs or secondary branches.
• Only half of the total branches are girdled in one year. The next half next year or in third year.
• Reduce annual shoot growth to 30-50% depending on timing and severity of the process.
• Have a powerful, direct effect on stimulating return bloom by reducing endogenous gibberellins supply from the roots and raising photosynthates concentrations in upper portion of the tree.
• Only healthy vigorous trees are girdled.

Root pruning

• Root pruning is a good option for managing excess shoot growth during “off” year.
• It has a direct effect on annual shoot growth extension which terminates vegetative growth for terminating buds to form new flower buds.

Using plant growth regulators

• Application of paclobutrazol @3.0 ml per meter canopy diameter through soil drenching during September is found to induce flowering during ‘Off’ year.
• Paclobutrazol checks Gibberelic Acid biosynthesis, increases cytokinin level, chlorophyll content, improves mineral uptake and carbohydrate gradient of the entire plant system.
• The low GA levels in the shoots result in the accumulation of total non-structural carbohydrates, primarily starch, in the leaves and buds. These series of events eventually lead to the formation of floral initials.
• Use of chemicals like paclobutrazol: Hormonal regulation of flowering is one of key mechanisms responsible for induction of flowering in fruit trees, which is based on a balanced ratio of hormonal synthesis and transport to the terminal shoots. Paclobutrazol (PBZ) treatment is a technological intervention that suppresses gibberellins and promotes flowering in mango under influence of various hormones Paclobutrazol checks.

Growing regular bearing varieties

• Planting of fairly regular bearing varieties like in mango: Arka Anmol, Arka Punit, Amrapali, Banganpalli, Bangalora and Neelum, Mallika, Arka Aruna, Ambika, Ratna, PKM 1, Mahmood Bahar, Prabha Sankar,  Sundar Langra(RM1), Alfazli (RM2),  Jawahar, Neeludin, Neelgoa, Neeleshan, Nellphonso,  AU Rumani are suggested for getting regular fruits.

Timely harvest

Early harvesting: It was shown that removal of fruits from ‘On’ year crop enhanced the return bloom (Shalom et al., 2014). Similar techniques have been used with mango and apples

Dwarfing rootstocks

Use of rootstocks- Response of mango varieties at different height of grafting on rootstock. Similarly, effects of clonal rootstocks on Hass avocado on alternate bearing has been found.

Proper upkeep and maintenance of orchards

• Careful timing of fertilizer and irrigation application.
• Fertilize and water mango/ litchi trees correctly to ensure they are healthy and well nourished.
• Weed removal /Floor management- avoid soil compaction
• Ensure proper pest management practices particularly for mango hopper, powdery mildew, and litchi mite
• Spraying with Carbaryl (0.25 per cent) or Diazinon (0.1 per cent) for mango hopper and wettable sulphur @2g/ lt of water for powdery mildew. Dicofol @3ml/ lt to control mite in litchi.
• Regulated Deficit Irrigation (RDI) in particular to reduce vigour and stimulate return bloom

Create the right canopy

• While excess tree vigour is a problem the fruiting canopy should not be too small for the well developed root system.
• Such trees usually have numerous high vigour branches responsible for their shading problem and unfruitfulness.
• The main objective of pruning is to develop the tree to carry sufficient crop load to soak up the photosynthates produced in the branches. The canopy must have sufficient leaves to support a good crop load.
• Selective complete removal of these over vigorous branches together with minimal pruning of those remaining branches is necessary. 
• Prune immediately after harvest.
• Pruning: Removing bearing surface (fruit buds) stimulates vegetative growth from remaining buds. This promotion of vegetative growth inhibits many of the remaining buds from becoming flora

Authors

Ankit Kumar Pandey¹, Sakshi Shastri², Deepak Maurya³, Ram Niwas⁴ and Kuldeep¹

¹Department of Fruit & Fruit Technology, Bihar Agricultural University, Sabour, Bhagalpur 813210 (Bihar)

²Department of Agricultural Extension, Indira Gandhi Krishi Vishwavidyalaya, Raipur, C. G, India,-492 001

³Department of Horticulture (Vegetable & Floriculture), Bihar Agricultural University, Sabour, Bhagalpur, Bihar, India 813 210

⁴Department of Plant Pathology, Bihar Agricultural University, Sabour, Bhagalpur, Bihar 813 210, India

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