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  Flowering And Fruiting

Introduction > Self Incompatibility > Possibilities of Growing Regular Bearing Varieties > Breeding for Regular Varieties and Yield > Fruit Set > Irregular Bearing >


Ikisan - Flowering and Fruiting of mango Trees

Introduction

  • Flowering in mango is preceded by the differentiation of the flower bud in the shoots.
  • The period of differentiation is reported to be October- December, depending upon the local climatic conditions.
  • The time of flower-bud differentiation in mango is in the month of August under Punjab conditions which appears to be too early a period because there is no dormancy between the fruit-bud differentiation and inflorescence elongation.
  • In 'Baramasi', however, sometimes the critical time of differentiation is twice a year and in certain years it is only once.
  • Differentiation period is generally during May-June and September-October.
  • This appears to be its hereditary character.
  • The development of different organs in the mango flowers is in the following order.

Calyx, corolla

Stamen

Staminodes

Carpel and the disc
  • It takes about a fortnight for the tiny bud to develop and open into a flower.
  • Apart from the inherent character of the variety, the time of flowering in different regions is mainly governed by the local climatic conditions.
  • Flowering may start as early as November or usually during December in Rayalaseema (Andhra Pradesh) and the south Konkan on the West Coast of India.
  • In northern India mango flowers from February to March and the period of full bloom may be some time during March.
  • Thus under milder climatic conditions of the southern and western India, mango may start flowering from December itself, whereas under extreme climatic conditions of the north the flowering time is comparatively more precise and late (February- March).
  • Bloom period in eastern India is earlier than in the north.
  • In the Philippines too the mango flowers during December-January.
  • In Java (Indonesia) the local varieties flower from June to August and bear fruits to maturity during October-November.
  • Flowering period in Queensland (Australia) and South Africa is during June-July and August-September respectively, and fruits are available from November to January.
  • Thus flowering in mango is controlled by the climatic factors.
  • There are certain varieties of mango in India such as 'Rumani', 'Bangalora', 'Neelum' and' Alipasand' which put forth flower flushes twice or thrice a year.
  • This is observed particularly when these varieties are grown under Kanyakumari conditions.
  • 'Baramasi' is yet another erratic variety which may flower once, twice or thrice a year even under north Indian conditions.
  • In northern India the duration of flowering in mango is for about 20- 25 days.

 
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ikisan - Self Compatability of Mango Varieties

Self Incompatibility

  • Although some workers had suspected the existence of self-sterility in mango, it was not until some years later its existence in 'Dashehari' was reported.
  • Recently studies on 'Dashehari', 'Langra', 'Chausa' and 'Bombay Green' revealed that these are self- unfruitful.
  • Initial fruit set following self- pollination is negligible (0.0-1.68%) in these varieties compared with that after cross-pollination {64-234%).
  • The majority of the selfed fruit) lets drop within 4 weeks of pollination and none attains even ha1f- grown size.
  • An analysis of its causes reveals that the processes culminating in ovule fertilization are the same as after compatible cross- pollination.
  • The difference between self- and cross-pollination becomes apparent from the fifteenth day after pollination, when degeneration of the endosperm and the surrounding nucellus is widespread after selfing.
  • 'Himayuddin' as the pollinizer resulted in 50% more fruit set and increase in fruit size compared with that of the open-pollinated 'Rumani' fruits with 'Jahangir' as the pollen parent, however, fruit size was reduced.
  • This emphasizes the need for study on suitable pollinizers for mango cultivars.
  • 'Dashehari' is cross- incompatible with 'Chausa' and 'Safeda Malihabad' and it is cross- compatible with 'Langra', 'Rataul' and 'Bombay Green'.
  • 'Langra' is cross- incompatible with 'Alphonso', 'Bombay Green', 'Chausa'.
  • 'Fazli'. 'Rataul', 'Safeda Malihabad' and 'Surkha Burma'. 'Chausa' is cross- incompatible with 'Bombay Green' and 'Rataul' and it is cross- compatible with 'Langra' and 'Safeda Malihabad'.
  • They reported that 'Bombay Green and 'Dashehari' appear to be the best pollinizers for 'Dashehari' and 'Chausa' respectively.
  • Self-incompatibility in 'Dashehari', 'Chausa' and 'Langra' is also reported and also reported that 'Dashehari' was cross-incompatible with 'Chausa' and 'Safeda Malihabad' and it was cross-compatible with 'Bombay Green' and 'Rataul'.
  • These studies also revealed that 'Langra' and 'Safeda Malihabad' are compatible with 'Chausa', whereas 'Totapari' and 'Dashehari' appear suitable pollinizers for 'Langra'.
  • 'Bombay Green' was the best pollinizer for 'Dashehari'.
  • 'Neelum' is partially self- compatible whereas 'Mallika' is self-incompatible.
  • Fruit growth and development in mango fruit is characterized by sigmoid curve.
  • Our studies have shown that the period of stone hardening is directly associated with the tremendous decrease in growth rate.
  • Development of fruit in 'Langra' and 'Dashehari' starts in the last week of March and is completed by the end of second week of June.
  • The percentage increase in growth in 'Dashehari' and 'Sarnar Behisht Chausa' as expressed in terms of length, breadth and thickness is maximum in April, followed by May and March respectively. It is least in June.
  • However, maximum increase in weight and volume of fruits is recorded in May, followed by April and June respectively.
  • It is almost negligible in March. Many characters of the pericarp like cell size, laticiferous canals, intercellular spaces, etc., in different tissues of the fruit contribute to the increase in length, breadth, thickness and volume of the fruit, whereas increase in weight in the later stages is associated with the accumulation of starch grains in the cells.
  • Development of the seed is similar to that of the fruit.
  • Peak growth period of fruits is directly associated with the peak growth period of seed.
  • In mango the period of rapid growth is directly associated with the period of maximum activity of auxin and gibberellin like substances in the seed.
  • Our observations have shown that the size of seed also contributes to fruit growth.
  • The second period of rapid development of fruit may be due to rapid initiation of development of seed and decrease in the inhibitor content of the pericarp.
  • Further, the slowing of growth after 64 days in 'Dashehari' and 29 days in 'Chausa' may 'be due to the lignification and development of endocarp, as it results in competition for food substances in the formation of the endocarp and the fleshy part of the fruit .
  • In the later stages of the growth and development of mango fruit, the exocarp region develops into a leathery protective skin, the mesocarp into a fleshy and pulpy region and the endocarp into a hard, stony and non-edible region.
  • The laticiferous canals meant for storage of fluid, called mango chenp, secreted by a well-defined layer of secretary cells surrounding the canal, appear in the endocarp region at anthesis and in the outer regions 7 days after anthesis.
  • They originate schizogenously and grow throughout the period of fruit growth.
  • Percentage increase in the growth of these canals is found highest in March, followed by April, in all the regions of fruit.
  • Their number is highest in exocarp, followed by endocarp and mesocarp.
  • All the 3 regions of the fruit grow throughout the growth period of mango plant, but the percentage increase in the growth varies during different months.
  • The growth in the exocarp is due to cell division, increase in cell size and increase in size and number of laticiferous canals.
  • Cell division in this region continues up to 35 days in 'Dashehari' and 42 days in 'Chausa'.
  • Thereafter the growth is controlled by other factors such as cell size and size and number of laticiferous canals:
  • The increase in the thickness of the mesocarp region is initially by cell division, cell size and size of laticiferous canals.
  • But cell division Stops after 42 days in 'Dashehari' and 49 days in 'Chausa'; thereafter the growth-contributing factors are cell size, size of laticiferous canals and increase in intercellular spaces.
  • In the endocarp. region the increase in thickness is due to cell division, cell size and size of the laticiferous canals in the early stages.
  • But cell division stops after 4-5 weeks and further increase in growth is due to increase in cell size, size of laticiferous canals and increase in the reticulate fibrous structures.
  • The hardening of endocarp region starts 64 days after anthesis in 'Dashehari' and slightly later in 'Chausa'.
  • The cell division in the inner epidermis, however, continues for a longer period than in the main endocarp region.
  • The accumulation of starch grains adds to the increase in fruit weight in later stages.
  • The developing mango fruit is a rich source of many growth substances such as cytokinin, auxin and gibberellin.
  • Zeatin, zeatin riboside, N6-adenine riboside and an unidentified compound which appeared to be a cytokinin glucoside were isolated from extracts of immature mango ('Irwin') seeds.
  • Cytokinin concentration in the seeds increased till 28 days after full bloom.
  • The quantity of cytokinins in the pulp or fruit increased from 10 days after full bloom and was highest 50 days after full bloom.
  • Lack of pollination and fertilization results in the cessation of the active growth of the ovule, thereby eliminating the sources of these growth hormones for the enlargement of fruits.
  • Also, the occurrence of natural parthenocarpy has not generally been reported in mango.
  • Observations show that growth substances produced in the seeds are necessary for the development of mango fruits.
  • For the first time that parthenocarpy is possible in mango provided the young ovaries are supplied with kinetin in the initial stages and later supplemented with a mixture of auxin and gibberellic acid.
  • Fully developed parthenocarpic fruits were obtained by spraying N6 benzyladenine (250 ppm) at the time of anthesis and later on a combination of "Symbol" -naphthoxyacetic acid (10 ppm) and GA3 (250 ppm) at fortnightly intervals.
  • The fruits although small in size were superior in quality to the normal seeded fruits. Emasculated flowers were sprayed in early February with GA 100 ppm and NAA 50 ppm.
  • Set fruits were sprayed twice with NAA (40 ppm) on 29 February and 20 March.
  • Initial fruit set was 5.3% (highest) with GA compared with 0.7% for untreated controls.
  • Fruit retention was also highest with GA, but only 2 fruits matured out of 83 set initially.
  • These turned out to be parthenocarpic.
  • The fruit contained a small degenerate ovule at one end, with a flat and compressed stone weighing only 7 g compared with 148 g in normal fruits.
  • Fruit size and quality were, however, little affected.

 
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Ikisan - Possibilities of Growing Regular-Bearing Mango Varieties

Possibilities of Growing Regular-Bearing Varieties

  • In mango most of the commercial varieties of northern India are biennial in bearing habit.
  • These include 'Langra', 'Bombay Green' (Sarauli'), 'Samar Behisht Chausa' and 'Dashehari'.
  • Some of the south Indian varieties show a pronounced tendency towards regular bearing, e.g. 'Neelum', 'Bangalora' and 'Totapari Red Small'.
  • However, most of the mango varieties are biennial bearers.
  • 'Neelum' and 'Bangalora' are the commercial varieties of south India, which bear regularly even under north Indian conditions.
  • However trees of 'Neelum' instead of being vigorous or semi-vigorous just as under south Indian conditions, become dwarf in north India.
  • Regular bearing is particularly responsible for this dwarfing effect.
  • Although inferior in fruit quality, 'Neelum' and 'Bangalora' can be recommended for obtaining regular fruiting and for making fruits available late in the season.
  • 'Neelum' being dwarf can specially be recommended for planting in kitchen yards.
  • Some varieties of West Bengal such as 'Himsagar', 'Anupam' and 'Safdar Pasand', which show comparatively less biennial bearing, need popularization in areas where these can suitably be grown.
  • Performance of these varieties under Delhi conditions has not been satisfactory .
  • 'Mallika' and 'Amrapali' the mango hybrids evolved at the IARI, should find due place in commercial plantings.

 
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Breeding of regular varieties of Mango

Breeding for Regular Varieties

  • In none of the fruits exist a better scope for breeding a commercially acceptable regular-bearing variety as in mango.
  • However, attempts so far made in this direction have been sporadic.
  • The basic drawback has been raising of very few hybrids and taking a large number of objectives in a single cross-combination.
  • At Sabour (Bihar) hybrid 'Mahmud Bahar' was evolved by crossing 'Bombai' and 'Kallapady', which was a regular bearer.
  • But nothing more was heard of this hybrid.
  • The work at the Division of Horticulture, IARI, on a large-scale hybridization in different cross- combinations such as 'Dashehari' X 'Neelum', 'Neelum' x 'Chausa', 'Neelum' X 'Langra', 'Dashehari' x Totapari Red Small', 'Dashehari' x 'Bangalora' etc. showed yielding encouraging results.
  • The characteristic regular-bearing habit of 'Neelum' was inherited by quite a few hybrids which have superior fruit.
  • A hybrid that shows shoot growth on the fruited twig in the same season of fruiting and such growths are able to differentiate flower buds in the following season can be termed a potentially regular-bearing hybrid.
  • Inheritance of this essentially regular- bearing characteristic has considerably cut short the time involved in assessment of the hybrids in their bearing tendency.
  • Thus by raising a large number of hybrids of desirable combinations, it would be possible to evolve a regular-bearing hybrid.
  • Apart from good soil management, the only other means of reducing losses from alternate bearing seems to be by breeding varieties that tend to bear annually.
  • Thanks to the new hybridization technique perfected at the Division of Horticulture, IARI, large-scale hybridization is now possible in mango.
  • With the discovery of self-incompatibility in the commercial varieties of mango, these need to be caged only with the desirable regular parent and by introducing houseflies, the most important pollinator in mango, crossing is effected without resorting to hand-pollination.
  • Thus our present work has shown that it is possible to evolve a regular-bearing variety in mango by crossing regular bearing and biennial parents.
  • Another technique of hybridization has been standardized at the IARI, which altogether eliminates the necessity of bagging after pollination.
  • Besides being a time-consuming process, rebagging usually results in serious damage to the stigma and style.
  • On the basis of inheritance studies, 'Totapari Red Small' was utilized as a genetic marker in view of its dominant shoot colour and leaf characteristics compared with 'Dashehari'.
  • The final harvest from the panicles which were not bagged after pollination was almost twice that of the bagged panicles.
  • All the harvested fruits from the unbagged panicles after crossing turned out to be hybrids, as indicated by the dominant character of 'Totapari Red Small'.
  • Thus this technique has immense utility in obtaining large populations of hybrids more efficiently and more economically.
  • This is highly essential, because mango is a highly heterozygous crop and very large populations are needed for proper recombination.
  • Study of the hybrids in various cross-combinations has revealed that much desired characters like dwarf tree habit, precocity and regularity in bearing seem to be governed by recessive genes, as the number of hybrids having this combination in the Fl population is very low.
  • The regular-bearing parent must also have fruits of acceptable standard, otherwise it will be very difficult to get a hybrid which may be regular bearing and which may also have fruits of good quality.
  • By taking 'Neelum' as one of the regular-bearing parents, the improved regular-bearing hybrids 'Mallika', 'Amrapali' and 'Ratna' were evolved.
  • The problem is complicated by occurrence of incompatibility factors in the backcross of the hybrids with superior parents, resulting in negligible number of backcross progeny.
  • Very little progress is obtained in selfing of the hybrids also.
  • Therefore selection has to be confined to Fl progeny only.
  • Thus unless a very large progeny of F1 hybrids is raised, there is little likelihood of accomplishing the desired results.
  • Studies on evolution of improved types through physical and chemical mutagens are, in general, lacking in tree fruits, especially in mango. LD50 values ("Symbol" g - irradiation) for Neelum', 'Dashehari' and 'Malika' were 3.9, 2.9 and 2.4 krad respectively.
  • The effective dosages of EMS and NM4 for 'Dashehari' and 'Neelum' were 1.50 and 0.05% respectively. Primary effects of both physical and chemical mutagens were found to be more or less the same.
  • Some interesting changes in vegetative characters have also been established.
  • Out of these a few plants appeared promising for dwarfness, whereas in some others the fruit quality improved.

Yield

  • The yield of mango varies greatly, depending upon the variety and agro-climatic conditions prevailing in a region.
  • Grafted mango trees start bearing from the fifth year onward. However, seedling trees may take 8-10 years.
  • At the start of bearing the yield may be as low as 10-15 fruits (2-3 kg) per tree, rising to 50-75 fruits (10-15 kg) in the subsequent years, and to about 500 fruits (100 kg) in its tenth year.
  • In the age group-20- 40 years, a tree bears 1,000-3,000 fruits (200-600 kg) in an 'on' year.
  • The productive age of a grafted mango tree is usually 40-50 years, after which the yield declines.

 
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Ikisan - Fruit setting in Mango Orchards

Fruit Set

  • Mango inflorescence is primarily terminal but axillary and multiple panicles may also arise from axillary buds quite frequently.
  • The panicle consists of a main axis bearing many branched secondary axes.
  • The secondary branches may bear a cyme of 3 flowers, or tertiary branches may again arise on them which bear a cyme of 3 flowers, each flower borne on bracteate pedicel.
  • The flowers are closely clustered towards the apices of each branch or main axis and are either male or hermaphrodite.
  • The total number of flowers in a panicle may vary from 1,000 to 6,000, depending upon the variety.
  • The panicle of the mango also varies in length from a few centimetres to 60 cm.
  • It is the hermaphrodite flowers that after proper pollination and fertilization set fruit.
  • Therefore the initial fruit set will depend much on the number of hermaphrodite flowers in a panicle.
  • The percentage of hermaphrodite flowers in a panicle is subject to appreciable variation, depending upon the early or late emergence of the panicle and the variety.
  • Under north Indian conditions percentage of perfect flowers in the panicles of 'Dashehari' and'Langra' is 30.6 and 69.8 respectively.
  • In the south Indian mangoes it varies from 16.41 in 'Neelum' to 3.17 in 'Allampur Beneshan'.
  • The percentage of perfect flowers varies from 2 to 70 according to the variety.
  • The percentage of perfect flowers in the panicles of medium and late flushes in 'Dashehari' was respectively 2 and 7 times more than that in the panicles of early flush.
  • Panicles in the inner portion of the tree bore 1.5-2 times more perfect flowers than those located on the periphery.
  • The number of hermaphrodite flowers is the least in the upper pan of the panicle but the percentage is the highest.
  • Fruit set and ultimate retention per panicle are much higher in the medium and late emerged panicles than in the early ones.
  • There appears to be a close association between high temperature and an increased percentage of perfect flowers, and low temperature and a decreased percentage of perfect flowers.
  • The percentage of perfect flowers in some of the south Indian varieties has been observed to be much less under north lndian conditions.
  • This has been attributed to the lower maximum and minimum temperatures obtaining during the period of panicle development at Delhi compared with those obtaining at Kodur (:south India).
  • The percentage of perfect flowers in 'Janardhan Pasand' and 'Beneshan' is significantly increased with the aid of NAA (200 ppm) sprays.
  • The increase in the percentage of perfect flowers by NAA sprays results in much higher fruit set per panicle in 'Beneshan', which is shy when grown under north Indian conditions.
  • Mango is a cross-pollinated crop and pollination is essential for fruit set.
  • It 'is primarily accomplished by insects; housefly (Musca domestica L.) being the chief agent Pollen production in 'Dashehari', Langra' and 'Chausa' is significantly higher in the late flush than in the early flush (Sharma and Singh 1970).
  • It may be as high as 2,025 per anther in the late flush of panicles.
  • Pollen viability in mango is quite high (93%).
  • Pollen germination on the stigma is quite normal.
  • The stigma remains receptive even up to 5 days after anthesis.
  • Environmental factors, however, influence the receptivity of stigma appreciably.
  • For a better fruit set, need for adequate and efficient transfer of pollen has been emphasized by many workers.
  • However, unpollinated flowers of Dashehari' can continue on the panicles up to 13th day after anthesis.
  • At this stage the mean size of the ovary and the ovule may be about 2.5 times than at anthesis.
  • Therefore the initial fruit set should be recorded at 'marble' stage to have an accurate estimate.
  • Fruit set is a varietal character, depending upon several factors such as time of flowering, sex ratio, efficient cross-pollination and intensity of flower drop.
  • Varieties differ from one another in these respects and this leads to varying fruit set in different varieties.
  • Under Delhi conditions the panicles of the early flush of Dashehari' emerge rather early in the season and at 'that time the majority of the varieties are not found in flowering.
  • The weather is also much cooler at that time and the pollen transference is also less.
  • Consequently, the mean fruit set is negligible in the early flush of Dashehari'.
  • The panicles of the late flush of Dashehari', on the contrary, show high fruit set and ultimate retention per panicle.
  • Fruit drop an mango there is a heavy drop of hermaphrodite flowers and young fruits, amounting to 99% or more.
  • In the commercial varieties 'Bombai', 'Langra' and 'Fazli', only 13 to 28% of the bisexual flowers have been found to set fruits, out of which only 0.1 to 0.25% reached maturity.
  • This observation emphasizes the nature of heavy fruit drop in mango.
  • In general, in mango 0.1% or less hermaphrodite flowers develop fruits to maturity.
  • The maximum drop of fruits in 'Langra' and 'Dashehari' takes place in the fIrst 3 weeks of April and differs significantly from the drops in the following weeks.
  • Fruit drop is to some extent associated with the variety, as the variety 'Langra' is more prone to fruit drop than Dashehari'.
  • Among the external factors affecting mango fruit drop in the initial stages, mention may be made of mango hopper, mango mealy bug, powdery mildew and anthracnose.
  • Deficient nutrition of many developing embryos may be the most important internal factor leading to post-fertilization drop in mango.
  • This results due to competition among over-crowded fruitlets on a panicle.
  • Degeneration of the embryo in the initial stages of its development may yet be another cause of drop.
  • This occurs invariably if the flowers are self- pollinated.
  • So far no control measures have been evolved to check the fruit drop in mango.
  • Except in some varieties such as 'Langra' and 'Chausa', natural fruit drop does hardly pose a problem in normal bearing.
  • However, drop due to insect and disease attack must be checked in all the varieties by applying appropriate insecticides and fungicides.
  • Cross- pollination of the varieties ought to be assured by avoiding isolated planting of a single variety.
  • Check of natural fruit drop in mango will result in the intensification of alternate bearing habit.
  • Several growth-regulators are tried to control the fruit drop in 'Bombay Green', Dashehari', 'Langra' and 'Chausa'.
  • They found that 2,4-D gave better results in all varieties compared with NAA and 2,4,5-T.
  • Generally, 2,4-D produced better results at concentrations below 20 ppm, because at higher concentrations fruit and seed development was retarded.
  • About 30-50% more fruits were harvested than the control in 'Bombay green', Dashehari' and 'Chausa' when sprayed with 2,4-D, 6 weeks after fruit set (in the last week of April).
  • However, its toxic effect on the fruit development was clearly evident Spray of 2% urea on Dashehari' during the early stages of fruit development (7-21 April) also increased the final harvest compared with the control.
  • Spraying BA {1.5 x 103 m m) on panicles at anthesis followed by GA {7.2 x 102 m m) + NAAM {3.1 x 101 m m) application at the young fruit stage significantly reduced the fruit drop in mango.
  • Various plant growth-regulators were tried for the control of fruit drop in mango at the Central Institute of Horticultural Research for Northern Plains at Lucknow.
  • Single spray of NAA or 2,4-D each at 20 ppm or alar 100 ppm at pea stage of fruit gave promising results.

 
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Ikisan - Irregular bearing of Mango Trees

Irregular Bearing

Introduction

  • Most of the commercial varieties of mango show the same pattern of bearing (biennial); but 'Baramasi' may exhibit erratic and off-season bearing; and 'Totapari Red Small', 'Neelum' and 'Bangalora', show distinct regularity; particularly the latter
  • Some varieties, though excellent in fruit quality, are shy-bearing such as 'Allampur Beneshan' and 'Himayuddin' of south India.
  • The off-season bearing in some of the south Indian varieties under Kanyakumari conditions appears to be much affected by the coastal climatic conditions.
  • However, behaviour of even such varieties is not consistent but by and large erratic.
  • Even the regular- bearing types, if they carry a heavy load of crop in 1 year, show a tendency towards reduced yield in the following year. Hence the basic tendency of bienniality exists even in the so-called regular-bearing varieties of mango.

Important Factors Governing Flowering in Mango

1. Growth pattern

  • A number of workers have studied the pattern of growth flushes in mango and have reported different periods of primary and extension growths, depending upon the variety and the environmental conditions under which they worked.
  • In fact, the growth in Mango takes place in different flushes, which vary in different parts of the country.
  • Most of the workers strongly feel that early initiation and cessation of growth, followed by a definite dormant period, will help the shoots to attain proper physiological maturity, which is essential for fruit-bud initiation in them.
  • However, there is now enough evidence to show that the growth of mango shoots is purely a varietal feature and that fruit-bud differentiation in regular- bearing varieties is an annual feature.
  • But in biennial-bearing varieties it is governed by 'on' and 'off' year phase of the trees rather than by the time of origin and cessation of growth of shoots.
  • Extension growth on primary shoots is a varietal feature.
  • The shoot depending upon the variety, may stop putting forth extension growth after May or continue till September or onwards.
  • The potentiality of these shoots to form flower buds will depend on the floriferous condition of the tree, which in turn will be determined by the amount of fruit load carried by the tree in the previous year.

2.Nitrogen and carbohydrate reserves

  • The seasonal changes in the carbohydrate reserves and nitrogen content of mango shoots and their relationship with flower-bud initiation have been studied by a number of workers.
  • In almost all the varieties studied excepting in 'Baramasia', it was found that higher starch reserve, total carbohydrates and C:N ratio in the shoots favoured flower initiation in mango.
  • A recent study on the nitrogenous constituents in the mango stem and leaves showed that the total nitrogen content was higher in the stem and leaves of trees which were expected to initiate flower buds irrespective of the varieties they belonged to.
  • The available evidence indicates that nitrogen and carbohydrate reserves play an important role-if not the primary role-in flower-bud initiation.
  • Perhaps the accumulation of these compounds may create a favourable condition for the synthesis and action of the substances actually responsible for flower induction in these plants.
  • A comparative study of carbohydrates, proteins and amino acids in the bark of juvenile and mature mango plants at the time of flower initiation was made.
  • There was an increased accumulation and metabolism of these constituents in the mature plants compared with the juvenile plants.
  • The inability of juvenile plants to form flower buds under natural condition could hence be attributed to lack of sufficient reserves and their possible hormone- directed redistribution and mobilization.
  • The maximum leaves available are 30 in number on a shoot in the 'on' year, could not support the growth of a single fruit to normal size.
  • For the normal development of a mango fruit it requires 60-90 fully expanded leaves for development, if the assimilates are to be met from current photosynthesis.
  • As a consequence the developing fruit withdraws large amount of reserve metabolites from vegetative organs during the 'on' year.
  • This might contribute to biennial and erratic bearing in mango.

3. Hormonal control of flower formation

  • Evidences will be considered here in regard to growth hormones such as auxins, gibberellins, and inhibitors claimed to have flower-inducing properties in fruit plants.

(a) Auxins

  • The chromatograms of shoot extracts are taken from 'on' and 'off' trees of 'Dashehari' and regular- bearing trees of 'Totapari Red Small' contained 2 growth-active zones at RfO.3-0.5 and 0.8-1.0.
  • The eluates of both the zones contained growth- promoting substances, as shown by the first internode and coleoptile section tests in Avena. However, the major zone of growth-promoting activity was located at Rf 0.4-0.5 in the chromatograms of shoot extracts of both the varieties.
  • The growth substances present at Rf 0.4- 0.5 exhibited the same Rf value as IAA, in isopropanol/ammonia/water but none of the chromogenic tests confirmed its identity with lAA.
  • The shoots from 'Dashehari' of 'on' year and from 'Totapari Red Small' trees, which initiated flower buds during experimental period, contained higher level of the above growth-promoting substances during the period of flower-bud initiation than those from 'Dashehari' of 'off. year tree which remained vegetative.
  • The chromatograms 0f the shoot extracts taken from the 'on' year trees of 'Dashehari' and 'Totapari Red Small' trees sampled during November-December when sprayed with Ehrlich Reagent, showed a pink colour at Rf 0.4-0.5.
  • This pink colour did not develop in the chromatogram of 'Dashehari' of 'off year.
  • While the appearance of this pink band and the increase in the auxin level is associated with the onset of flower-bud initiation in mango, existence of any causal relationship between them is not known.
  • Available evidence indicates that the growth active zone at Rf 0.4-0.5 contains a complex group of unidentified substances.
  • It is possible that the auxin is only active in combination with a receptor complex, which cannot be measured in the usual ways.
  • The shoot tips of 'Dashehari' contain 2 acidic (a1 and a2) and 1 non-acidic (a3) auxin.
  • Chromatographic studies and bioassay of these auxins led to the tentative identification of a2 as indole-3-acetic acid (IAA) and a3 as 3- indole-acetonitrile (IAN).
  • Auxin could not be identified chemically.
  • In December, when the flower-bud different, shoot tips of 'Dashehari' in 'on' year contained about 35 times more auxin a3 than those in 'off year.
  • Auxin a3 appeared to play major role in the induction of flowering in mango.
  • However if endogenous auxin is the only factor favouring flowering in mango, external application of synthetic auxins would have initiated flowering in the 'off year.
  • The results so far obtained do not support it.
  • This indicates that factors other than auxins are also involved in the initiation of flowering in mango.

(b) Gibberellins

  • Exogenous application of gibberellic acid reduces flower-bud initiation in a wide range of fruit crops such as apple, pear, citrus and mango.
  • This inhibitory action is surprising because many of these fruit crops initiate flower buds during long days and gibberellins are active as flower promoter in long-day plants.
  • The physiological response of apple to N-dimethyl amino-succinamic acid have indicated the possibility that the flower-inducing properties of 'B 995' in apple may be mediated through the blockage of gibberellin action.
  • Thus the flower-promotion properties of growth retardants in fruit trees such as apple and citrus may be owing to their antagonistic effect on the endogenous gibberellins.
  • The extracts of 'Dashehari' mango shoots contained substances wit11 gibberellin-like activity , which promoted the leaf sheath elongation in Ds dwarf maize mutant plants.
  • The amount of such gibberellin-like substances was higher in the shoot extract of 'Dashehari' of 'off year compared with that of 'on' year (which were differentiating flower buds).
  • Gibberellic acid (GA3) at concentration of 10-1 and 10-2M is applied on the buds of 'on' year tree just before flower-bud differentiation inhibited flowering by 95 and 75% respectively.
  • Concentrations of10-3 and 10-4M inhibited flowering to a much lesser extent, but delayed the emergence of panicles by nearly 2 weeks.
  • These results show that gibberellins are antagonistic to flowering in mango.
  • Factors X2, X3, X4, X5(2), X6(1), K7 and K8 were tentatively identified as gibberellins A1, A7, A9, A4, A3, A5 and A6. respectively. on the basis of their behaviour in column, paper and thin layer chromatography. Factor X 1 could not be identified.
  • The endogenous level of gibberellins X1, X2, X3, X4. X5 and X6 was higher in shoot tips of 'off year than in those of 'on' year.
  • This suggests that the failure of flowering in an 'off year may be accompanied by a higher level of gibberellins in the shoot tips.
  • They further reported in 1979 that the seed was the major source of gibberellin in the fruit, the epicarp containing only traces.

(c) Cytokinins

  • Qualitative and quantitative analyses of endogenous cytokinins in mango ('Dashehari') shoot tips were made.
  • The endogenous levels of cytokinins were higher in the shoot tips of 'on'-year trees than of 'off -year ones at the time of flower-bud differentiation.
  • Eleven cytokinin-like substances could be isolated from the shoot tips of 'on'-year trees/employing ion- exchange and paper chromatography.
  • These cytokinins were numbered M1 to M11 on the basis of their Rf values and spectral properties.
  • Only 2 of these. M9 and M10, could be tentatively identified as zeatin riboside and zeatin respectively.
  • The Rf values and UV spectra of other cytokinins were different from those reported for known cytokinins in the literature.
  • These results suggest that flowering in mango shoot tips is associated with a high level of endogenous cytokinins.
  • Zeatin, zeatin riboside, N6 adenine riboside and an unidentified compound (which appeared to be a cytokinin glucoside) were isolated from the extracts of immature mango ('Irwin') seeds.
  • Cytokinin concentration in panicle and pulp of mango was highest 5-10 days after full bloom and decreased rapidly thereafter.
  • The amount of cytokinins/seed increased from 10 days after full bloom, reaching a peak at 40 days after full bloom.

(d) Inhibitors

  • The presence of certain inhibitors. Similar to abscisic acid, in the shoots of mango trees.
  • The findings that the shoots of 'Dashehari' from 'on'- year trees and of Totapari Red Small' trees contain relatively higher level of this inhibitor during flower-bud initiation than those of 'Dashehari' from 'off year trees indicate that it may also be involved in the initiation of flowering in mango.
  • Since the inhibitor is antagonistic to both GA3 and auxin effects on cell elongation, it may perhaps check vegetative growths of mango, thereby providing conditions suitable for flower-bud initiation.
  • However, the exact role of the inhibitor in the initiation of flower buds in mango cannot be assessed until more information is gathered about its chemical and biological properties.

4. Climatological factors

  • Climatic factors are associated with the biennial bearing of mango trees in 2 ways: (i) by damaging the crop directly by destroying the fruit bud, blossoms and fruits, or (ii) by creating conditions that indirectly affect the production of flower or fruit on the tree adversely.
  • Frost, high temperature accompanied by low humidity and hailstorm fall under the first category , as they directly damage the fruit bud and developing fruits respectively and thereby reduce the crop considerably.
  • Cloudy weather and rains during blossoming period reduce the crop indirectly by creating favourable conditions for the spread of mango hoppers and of diseases such as powdery mildew and anthracnose.
  • Besides, mango trees in extreme humid places and under milder climatic conditions may not fruit at allowing to their increased tendency toward vegetative growth.
  • Even if they flower, owing to excessive humidity the pollen is never in a suitable condition to be transferred by insects for pollination purpose.
  • Thus the above factors are contributory in nature and bring about biennial bearing only indirectly.
  • They are partly responsible for the marked fluctuatin in crop yield from year to year.
  • At times, severe attack of mango hopper, blossom blight or early spring frost destroys completely the fruit blossoms and this converts an 'on' year into an 'off year condition.
  • The following year, which would have been normally an 'off year becomes an 'on' year, in which an excessive amount of fruit bud is laid down, resulting in heavy bearing.
  • Consequently, the following year is turned into a completely 'off year and thus the biennial rhythm is initiated.

5. Cultural practices

  • The earlier notion- of some workers that biennial bearing problem is for most part a nutritional one appears to be wrong in view of the observations made by subsequent workers.
  • The failure of cultural practices or deblossoming in inducing regular cropping in well-grown mango trees goes further to prove that the habit is not an acquired one.
  • It appears to be inherent in the tree from the very beginning and becomes more and more operative as the tree grows in age and in that it may be aided indirectly by environmental factors.
  • The regularity of bearing in mango is a cultural problem which can be corrected by influencing nutritive conditions of the tree.
  • Observations at the Horticultural Research Institute, Saharanpur, had also shown that liberal manuring, irrigation and ploughing given to the 20-year-old biennial-bearing trees failed to induce regular cropping in them.
  • The investigations pertaining to manuring of the mango trees from the very beginning of bearing phase have not yielded any consistent results so far (ICAR Scheme at Saharanpur, unpublished).
  • Thus it is evident that while nutritional requirements of the fruit trees must be met for optimum performance of the trees, biennial bearing cannot be overcome by addition of nutrients.

6. Crop load

  • Generally, moderate blossoming is one of the chief conditions of annual fruit bearing in fruit trees.
  • The biennial habit can be minimized by undertaking some measures that reduce the number of fruit buds setting into fruits.
  • Investigations carried out in some of the mango varieties have shown interesting relationship between the effect of the total number of fruits harvested on the emergence of new shoots and their subsequent fruit-bud differentiation.
  • In 'Dashehari' when the number of fruits harvested was optimum, fewer shoots were produced, which in the following year failed to differentiate fruit buds.
  • In the 'off year when there were no fruits, large number of new shoots were produced which successfully differentiated fruit buds in the following year.
  • Thus the fruit load on the tree appears to be the main factor governing production of shoots, their fruit-bud differentiation and ultimately the biennial bearing.
  • Increased crop load seems to have a systematic depressing effect on many manifestations of tree growth.
  • Apparently, in mango the fruiting is an exhausting process.
  • The number of fruits retained till harvest is a varietal feature.
  • However, the total number of fruits that are harvested is important because of their deleterious influence on the production of new shoots and their subsequent fruit-bud differentiation.
  • Therefore the load of fruits appears to be the main conditioning factor for 'on' or 'off year in mango.
  • It thus appears that the limiting factor to the initiation of flower buds is the presence of a crop.
  • The maximum available (30 leaves) on a single shoot could not support the growth of a single fruit to normal size in the 'on' year.
  • Therefore the fruit development depended not only on current assimilates but also to a great extent on the reserve.
  • The utilization of reserve metabolites from vegetative organs during the 'on' year could contribute to biennial or erratic bearing.
  • The load of crop in its very initial stages of fruit growth (within a fortnight of full bloom) inhibits the flower-bud formation for the crop of the following year.
  • Non-bearing and bearing units on the same tree are growing under the same environmental conditions with their alternating potentials for flower-bud formation.
  • In the bearing units of a tree even when new shoots are initiated which attain physiological maturity quite in time, these fail to differentiate flower buds if such a unit carries an optimum load of crop.
  • No definite information is available whether there is a single factor or a complex of factors deficient in such shoots.
  • Besides, the fact that it is an inherent characteristic of a variety (because there are varieties that do flower in the succeeding season despite crop load), biochemical studies indicate that non-differentiating shoots do differ in certain chemical constituents from the differentiating ones.
  • Perhaps a high level of starch, some auxin- like regulators and inhibitors and a low level of gibberellins may be deemed favourable for flowering in shoots.
  • The manner of bringing about this right physiological status of shoots is a matter for future research.

Measures to Overcome Biennial Bearing

Deblossoming and Thinning of Fruits

  • Deblossoming is a more severe form of fruit thinning, employed to conserve the reserves of the shoots which could otherwise be depleted later on in the development of fruits.
  • Thus the deblossomed tree, instead of developing panicles and producing fruits, puts on new vegetative growth in mango, which flowers and fruits the next year.
  • Deblossomed shoots in mango are tend to bloom the following year and partial defloration is important in regulating the crop, since excessive fruiting during 1 year brings about biennial bearing in the mature mango trees.
  • The partial or complete removal of flowers or young fruits in the 'on' year increases flowering the next year, but only slightly.
  • Investigations conducted at Saharanpur indicate that the effect of deblossoming on subsequent fruiting habit of the tree seems to be a varietal feature.
  • While the response of 'Dashehari' to deblossoming is a precise one, 'Langra' fails to show the same response.
  • 'Dashehari' tree deblossomed either half or branch-wise yields a fair crop every year, but no such tendency is noticeable in 'Langra'.
  • Perhaps the response to deblossoming is less marked in over-vigorous or vigorous trees.
  • For fruit-bud formation deblossoming of a few individual shoots is of no consequence in mango.
  • Even a small deblossomed branch fails to register any difference in its behaviour the following year.
  • However, branch-wise, regulation of crop in 'Dashehari' is quite possible by resorting to deblossoming only once in 10 years.
  • Thus, instead of taking full load of crop in 1 year, it can be distributed over 2 years for better profit.
  • While thinning of fruits during a month of full bloom has proved beneficial in reducing alternate bearing in a number of fruits (apple, olive etc.), it is hardly possible to practice thinning in mango due to quite a heavy fruit drop in the initial stages of fruit growth.
  • Moreover, fruits may grow up to 'pea' stage even without pollination and fertilization, but will drop down afterwards.
  • Since thinning, if at all to be considered, must be done before the fruitlets attain pea-size, it would be difficult to resort to judicious thinning in mango either by hand or by chemical sprays.
  • Fruit drop continues till the end of April and panicles thinned earlier might not carry any crop at all.
  • The defruiting experiments carried out at this Division revealed that the emergence of shoots early in the season does not ensure flowering in such shoots in the following year.
  • The exhaustion is caused by the fruitlets when these are still in pea stage of growth and the tree takes 1 year to recoup, thus causing biennial bearing in the mango.

Pruning

  • Pruning is primarily adopted to maintain a proper physiological balance between growth and fruiting.
  • It is a thinning process as well for the number of potential fruit buds.
  • As pruning reduces the fruit load in temperate fruits, it minimizes the tendency towards biennial bearing.
  • In evergreen fruits like mango, pruning is never practised.
  • At harvesting, if the fruits are plucked along with the panicle, light pruning is effected automatically and the tree could send forth, from the distal lateral buds, new growths if it has the inherent capacity to do so.
  • Generally the biennial-bearing varieties of mango do not do so, but regular varieties like 'Neelum' send forth new growths immediately after harvest, which differentiate flower buds in the following season.
  • In the biennial varieties even if some new growth is produced, these are incapable of differentiating fruit buds in the following season.
  • The mango responds to pruning.
  • Two mature healthy 'Mulgoa' trees and 1 of 'Banganpalli' characterised by poor and erratic cropping were drastically pruned by the removal of large branches to open out the trees and thinning of terminal shoots.
  • A year later the shoots were thinned again. It resulted in very high yield of excellent quality.
  • An interesting feature was the production of fruit on old branches as well as on shoots.
  • This may perhaps be a case of rejuvenation of old mango trees, as pruning in normal trees is likely to adversely affect the process of fruit- bud initiation.

Cultural Practices

  • To overcome biennial bearing in mango, a number of cultural practices are recommended.
  • These include open planting with wind-break, regular ploughing, liberal manuring at appropriate intervals, ringing and adequate irrigation.
  • The importance of manuring to overcome biennial bearing in mango.
  • By good annual cultural treatments, trees can be kept reasonably healthy, and total failure of crops which usually occurs between 2 heavy croppings can be averted.
  • Although mango trees can be kept reasonably healthy and in better cropping condition by adequate manuring, irrigation and other cultural operations, experimental evidence is still lacking that they can be made to put on optimum crop every year on the entire trees.
  • The practice of smudging to induce flowers in the mango is reported from the Philippines.
  • Smudging should be done after the monsoon.
  • The heat causes lesser effect than CO2 on 'Carabao' mango, and the use of CO2 results in the maximum development of buds.
  • It is the heat and not the smoke that causes flowering, and the best results are obtained when the last growth is well matured and the terminal buds are well formed. It is not the heat from smoke but a gaseous combustion product from the smudge fire that stimulates flowering.
  • He believes that potentially ethrel could be used commercially to induce off-season or regular and heavy seasonal flowering.
  • He suggests that weekly application of 150-250 ppm ethrel for 4-6 weeks should result in heavy and uniform flowering of healthy mature trees.
  • Spray of 1,000 mg/litre ethephon 5 times at weekly intervals during November and early December induced heavy flowering in ringed and non-ringed juvenile mango seedlings.
  • The treated plants produced flower buds that opened by the end of December, whereas the unsprayed ringed seedlings produced only a new flush of vegetative growth.
  • This shows the potentiality of ethephon (2-chloroethyl phosphonic acid) in the induction of flowering in mango.
  • Smudging cannot always be depended upon to stimulate flowering.
  • Since it is a cumbersome process, its practical utility in India seems to be limited.
  • Although ringing as a means to obtain flowering in mango has been suggested by a number of workers.
  • The effect of ringing on flower-bud formation is immediate during the next flowering, and no residual effect remains.
  • However, we have observed here at the Division of Horticulture that ringing cannot induce an 'off' year branch to flower in mango.

Role of Plant Growth-Regulators and Chemicals

  • It appears that external application of various growth-regulators plays a very little role in the correction of biennial-bearing phenomenon in mango.
  • The role of endogenous growth substances has already been discussed.
  • It has also been stated that a high level of auxin-like substance is necessary for flower-bud initiation in mango.
  • There was no any flower-inhibiting effect of auxin such as NAA and 2,4,5- T even at very high concentrations, when sprayed at the time of flower-bud initiation.
  • They also failed to induce flowering in 'off year trees by either spraying auxins or MH (an anti-auxin). 4-5 sprays of 200 ppm ethrel (2-chloroethyl phosphonic acid) at an interval of 15-20 days starting from September, on 'off year 'Langra' trees, produced normal flower panicles in place of growth flush produced by the untreated branches.
  • They further observed that consecutive applications of 200 ppm ethephon for 3 years did not show any decline in the vigour and yielding ability of the treated 'Langra' trees.
  • High concentrations induced moderate to heavy leaf abscission in the experimental trees.
  • However, spray of ethrel under north Indian conditions has not given consistent results for the control of biennial bearing in mango.
  • The application of chlorflurenol (morphactin IT 3456) at 250 and 500 ppm overcame apical dominance and induced lateral branching in nucellar mango seedlings of 'Mylepalium'.
  • The foliar sprays with cycocel and alar each at 5,000 ppm at monthly intervals induced early and intensive flowering.
  • An increased endogenous level of ascorbic. acid associated with flower-bud formation indicated a positive correlation between ascorbic acid level and flower formation.
  • The 'Mulgoa' trees when treated with cycocel (chlormequat) or alar (daminozide) each at 5,000 ppm at monthly intervals during May-January, produced greater number of flowering shoots from 25% in the control to 85-87% respectively.
  • Both retardants significantly lowered the respiration rate at all growth stages.
  • The levels of chlorophyll and carotenoids were consistently higher in the treated leaves.
  • In Philippines that 'Carabao' shoots 8.5 months old required only 7 days from spraying 10- 160 g/litre KNO3 to attain 100% flowering.
  • 'Pahautan' shoots 4-8 months old showed 60-80% flowering in 7 days and 100% in 14 days with 10-80 g/litre.
  • Eleven out of 12 'Picot trees flowered 8 days after spraying 10 g/litre KNO on the dark green shoots of undetermined age.
  • Flower induction was uniform.
  • All the control shoots and unsprayed portions of the canopy remained vegetative long after termination of the experiments.
  • The results may show early induction of flowering rather than initiation of flower buds in 'off-year phase of these polyembryonic varieties under the Philippines conditions.
  • Application of KNO3 to 'off. year trees has not given any consistent result in the Indian varieties.
  • To overcome the problem of biennial bearing in mango, experiments were conducted for the last 8 years with different treatments like ethephon, cycocel, alar and different nitrates at the Central Institute of Horticultural Research for Northern Plains at Lucknow.
  • However none of the plant growth-regulators and other chemicals were found promising in inducing flowering in the 'off' year (personal communication).

 
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