There are a few major bottlenecks, which limit the fruit production in different fruit-producing countries. The following are the main constraints in fruit production, which need attention.
Genetic Resource Conservation and Characterization
Main limiting factors of all the fruit-growing countries is lack of proper genetic resource conservation programme of fruit crops which is a backbone of the crop improvement programme. Urbanization, decline of old plant material, unchecked exploitation of wild resources is causing a great threat to survival of indigenous and rare species of fruit crops like mango, citrus, apple and papaya. Therefore, systematic efforts to conserve these materials on field condition or conservation in a conventional gene bank are needed. The documentation of germplasm is another key issue that must be addressed. Molecular characterization of important germplasm and DNA banking is the need of the hour.
Lack of Quality Seed and Genuine Planting Material
Because of long gestation period, high heterozygosity, lack of information on inheritance pattern, less number of seeds in fruits, and inadequate supply of genuine and
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certified planting material to the grower are some of the reasons for the low productivity of fruit crops. Papaya seeds are produced by controlled cross-pollination and by maintaining isolation distance (47) that causes non-availability of sufficient quantity of pure seed and quality planting material which in turn limit its commercial production. Ram and Majumder (46) have found that cv. Pusa Dwarf produced the highest seed yield (391.7 kg/ha) at lowest cost (Rs 61/kg), while Pusa Majesty gave the lowest yield (52.5 kg/ha) at the highest cost (Rs 416.60/kg). It is suggested that foundation seed production on commercial scale may be conducted on isolation fields (400 - 1,000 m distance) to meet the increasing demands for papaya seed in India. However, breeders seed should be produced under strictly controlled pollination to maintain genetic purity (45). Normally 2:1 ratio of bisexual and female plants has been recommended for seed production of papaya.
On the other hand in other perennial fruit crop like mango, plant multiplication being done by grafting techniques on descript rootstock resulted in inferior plants. Similarly, crops like guava, litchi and citrus are being multiplied through stooling, air- layering and budding which are sluggish and cumbersome and the plants are not multiplied through elite mother trees of superior quality. Therefore, multiplication should be done only from the mother plants of established superiority. It would be desirable to establish elite orchard of important fruit crops in the fruit growing state for the supply of authentic plant materials. Clonal selection would be an important aspect of this programme (58).
Lack of High-density Plantation
Most of the fruit orchards are at present planted at low density and such orchards provide low returns with long gestation period. Productivity of fruits is static and per capita land is decreasing. Lack of dwarfing rootstock and non-availability of precocious variety are the main reasons for low-density plantation. Therefore, transforming fruit industry through high-density planting is the dream of day for the horticulture. High- density planting increases productivity and fruit quality, shortens juvenility, gives high early returns and provides high land use and better use of natural resources like light, water and nutrient, besides easy harvesting. To have a full physiological control of the tree in high density, it would be essential to have dwarf tree. A shallow canopy (1.5 - 2.0 m depth) is needed in high density to achieve maximum efficiency for trapping sun energy through foliage and chanellizing metabolites for quality fruit production with maximum return. In India, high-density planting has been successfully demonstrated for high yield in case of banana, pineapple, papaya and mango but most of the orchards are still under the traditional low-density system, resulting in low average productivity
Fig. 1. Canopy management with application of paclobutrazol to maintain height control and for better harvest under high-density planting in Mango cv. Dashehari: (a) heading back from 1.5 m height (b) new shoot developing in response to heading cut; (c) profuse flowering in pruned and paclobutazol applied tree and (d) heavy fruiting in pruned coupled with paclobutazol treated tree.
b a
d c
(4, 19, 49). The high-density technology developed in mango utilizes vigorous seedlings rootstock of varying genotypes rather than dwarfing rootstock like in temperate fruits. The tree is managed dwarf through training and pruning and growth is restricted within
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planting distance provided between the trees. Closer the density higher the productivity has been the general guiding principle. The mango, being an evergreen tree, has been assumed earlier to be unresponsive to pruning unlike the grapes which are pruned regularly to induce and regulate growth, flowering and cropping. Pruning is particularly effective in trees which bear fruit on new shoots and thus it is done to induce healthy current season shoots from older wood. It was reported that pruning of mango trees may not be successful to regulate bearing as the new growth turns out to be purely vegetative (39). However, Iyer and Subramaniam (26) reported that pruning of one- year-old shoots at the base induced flowering. Pruning coupled with paclobutrazol has got remarkable success in high-density planting (Fig. 1a,b,c and d) of mango (60).
High-density orcharding having closer planting (3.0m x 3.0 m) in mango for regular crop is practised through training and annual pruning after crop harvest and induction of flowering through paclobutrazol in alternate bearing varieties like Dashehari (49). However, no paclobutrazol is to be applied in regular varieties like Rumani, Amrapali, Sindhu, Tomy Atkins and Sensation (48). The training helps to develop proper frame of trees in early stages of growth, while pruning helps to curtail growth and maintain tree vigour on sustainable basis for regular fruiting year after year along with control of disease and pest. The high-density orcharding provides 8-9 time higher yield than the traditional orcharding (49).
In Israel, productivity of mango has been doubled by adopting high-density planting technology. Mango tree training technique for high density for the hot tropics has been developed (11). The plant density in papaya plays a vital role in productivity per unit area. The yield per unit area can be enhanced by increasing plant density. It is generally grown at planting density of 1,400-1,700 plants/ha. A plant population of 2,500/ha is recommended for high-density planting (4). With the development of dwarf cultivars like Pusha Nanha and Ranchi Dwarf, it is possible to plant papaya still closer. Ranchi Dwarf planted @ 2,922 plants/yield 98.05 tonnes/ha fruits and Pusha Nanha planted at 1.25 m x 1.25 m (6,400 plant/ha) yielded 60 - 65 tonnes/ha as compared to traditional yield of 15 - 20 tonnes/ha (44). Growth control is primary requirement of high density orcharding which can be achieved by dwarfing rootstock, pruning, dwarf scion varieties, use of chemical etc. Recent advancement in tree physiology has shown that growth retardant has tremendous potential to control the growth of tree with or without dwarfing rootstock and scion cultivars which is prerequisite of high-density orcharding. In mango, dwarfing rootstock or scion cultivar is not available, chemical like paclobutrazol was found effective to control the tree growth by reducing the xylem : phloem ratio with higher yield.
The ability to influence the development and productivity of tree fruits rests in genetic or cultural techniques. Breeding to improve fruit production has so far had limited success. Among several agro-techniques, such as high-density planting, control of tree size and canopy management is some of the important technology to achieve high productivity per unit area both in short duration and perennial crops. In India, high-density planting is being recommended in fruits like pineapple, banana, papaya, citrus and mango. Neverthless, high-density planting systems were recommended long ago, but the growers have not started adopting the technique. Delay in acceptance of the high density planting system can be attributed to the lack of a reliable and universally acceptable method to control tree vigor and higher initial capital investment. In mango, dwarf and compact trees have been identified (Amrapali in India and other selections in Thailand and Pakistan) but to develop an ideal plant type, the terminal buds of shoots need to be pinched off in atleast three successive flushes of growth. The polyembryonic, Sabre has reportedly been effective as a dwarfing rootstock in South Africa, although in Israel it was unsuccessful. Therefore, a hi-tech strategy for high-density planting in horticultural crops in India would call for the introduction of dwarfing gene for the management of tree size and canopy shape.
Tree Flowering and Erratic Bearing
The flowering process is of vital importance to fruit crop productivity as yield is directly dependent upon its success or failure. The erratic and irregular flowering in most of fruit crops cause low orchard efficiency. Each fruit tree in commercial groves does not bear equal crops year after year. Climatic variations in particular year, environmental factors such as photoperiod, temperature, plant water stress and genetical nature of variety as well as physiological changes occurring in trees during floral induction period are the main reason for erratic bearing and accounting for low productivity of fruits. Alternation in cropping habit of mango is used as a synonym for poor yields. In mango, growth flushes are very erratic and occur up to 3-4 times per year on individual stems, depending upon cultivar and growth condition and they tend to flower only after 9-10 months as to attain proper physiological maturity (42). In north Indian commercial varieties, only March flush is the major one that accounts for over 80 per cent of annual growth and its shoots has the maximum potential for becoming new fruiting shoot (21). The other flushes are minor ones and are not appreciably related to flowering. Therefore, bienniality / irregularity in flowering would ensue because of inability of one shoot to bear vegetative growth and flower in the same year.
Several chemicals and plant growth regulators like Chlormequat Chloride (35), Ethephon, KNO3, Salicylic acid and triazoles particularly paclobutrazol were found
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effective to regulate the flowering and bearing in mango, apple and citrus by inhibiting vegetative growth of shoots and promoting flowering (7, 16, 29, 33, 55, 62, 63 and 64). Amongst them paclobutrazol is being widely used to increase flowering, enhance yield and control the alternate bearing habit in commercial monoembryonic mango orchards of India (9 and 61), China (68) Australia (51) and South Africa (21 and 70). Triazole having anti-gibberellin activity induce flowering even in the 'off' year of bearing by regulating the synthesis of gibberellins (61). There have been numerous studies on the inhibitory effect of GA3 on flowering of fruit crops (1, 14, 38, 41 and 56). Paclobutrazol is also being commercially used to advance the harvesting of the mango varieties by about a month (61). However, its application through judicious nutrient management also needs to be integrated for continued and sustainable production. On the other hand KNO3 which is commercially used in the Philippines for regular flowering and fruiting in mango did not give consistent result in commercial mangoes cultivated in India. It was believed that these inconsistent results are due to the fact that in the Philippines and in some other regions where the growth is continuous and the cultivars are polyembryonic whereas in India the cultivars are monoembryonic and period of plant growth is well defined and not continuous. Ataide and Jose (3) reported that application of ethephon stimulate the floral differentiation of flowering buds, whereas KNO3 stimulates the break of dormancy of already differentiated buds.
Papaya having uncertainty in flowering and fruiting is a large perennial herb. It is highly problematic, complicated and interesting fruit crop from botanical, genetical, cytogenetical and horticultural point of view. It is one of the most important crops grown in tropics and subtropics. The important papaya-growing countries are Zaire, Mexico, Brazil, India and Indonesia. United States of America is the main importing country. Papaya fruits are valued for fresh consumption and papain production. Its varied use in brewing, meat, fish and food industry, has made it a crop of commerce. The major bottleneck in papaya (Carica papaya L.) cultivation is its inherent heterozygosity, dioceous nature and susceptibility to a number of viral diseases. Sex reversal under varied environmental condition is also one of the reasons for low productivity of papaya in per unit area (8). It is a polygamous plant and has many sex forms. There are three basic sex forms: hermaphrodite or bisexual, pistillate or female, and staminate or male (65). Amongst these flowers, only female is stable whereas flowers of hermaphrodite and male vary in sex expression under different environment conditions. Storey (67) classified papaya flowers into eight categories: (i) staminate, (ii) teratological staminate, (iii) reduced elongata, (iv) elongata, (v) carpelloid elongata, (vi) pentandria, (vii) carpelloid pentandria and (viii) pistillate.
Staminate flowers are produced by male plants whereas teratoiogical staminate flowers by sex reversing male. Type third to seventh are normally produced by hermaphrodite plants and type eigth are produced by female plants. Planting period is also found to determine the sex expression as high percentage of female (66.0 per cent) produced during November planting closely followed by September (65.0 per cent) in cv. Pusa Delicious. Sex in papaya is determined by three homologous genes complexes on sex chromosomes (24 and 66). The genes are so tightly linked that no crossing-over occurs among them, thus the complexes are transmitted to offspring with pleiotropic effect on phenotypic expression. Sex in papaya cannot be identified unless they flower but the ratio can be predicted provided it is pollinated under controlled condition. However, the sex can be identified at seedling stage through chemical analysis (15). The leaves of male plant were found richer in carbohydrate, phosphorus and chlorophyll content than those of female plants, which are richer in nitrogen and potash.
Thus through this analysis the male plant can be removed at early stage and could be replaced by female seedling which in turn increase the yield. It was estimated that the average yield of papaya could vary from 37.23 tonnes/ha with a maximum of 6 per cent male plants present to 19.96 tonnes/ha with 50 per cent male plants. The male plants serve only as pollenizer and hence, it would be adequate to leave one male plant for every 20 female plants. Thus, removal of male plants allowing the robust growth of female plants is a potential strategy to increase the production of papaya. The profitable productive life of papaya is two-and-a-half years under northern Indian conditions provided the crop is well managed, therefore, they should be replaced by the new plantation for getting profitable yield.
Photoperiods and temperature play an important role in sex expression (23). The appearance of a large number of modified forms occurs in progenies from appropriate hybridization when grown under a temperature regime of 13-32°C. Temperature from 22 to 26°C are said to be best for flowering and fruit production (52). It was observed that stamen carpellody is expressed under cool temperature (40). The fruit develops from the carpellody are misshapen and unmarketable. The incidence of carpellody also declines with increasing plant age and may be related to internodal length. Female sterility occurs at warm temperature (40). Excessive nitrogen and moisture also favours stamen carpellody, while plant stress influences female sterility (32).
Several chemical and plant growth regulators have been used to improve fruit production via producing more normal and female flower. Hermaphrodite trees sprayed with 2,3-dichloroisobutyrate (DCIB, 2-6 g/litre) and 2,3-dichloropropionate (Dalapon, 2 to 6 g/litre) produced more normal and female like (Carpelloid) flowers than untreated
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trees (32). TIBA, NAA or IAA was also found to induce the flower significantly (17). The papain yield, which is considered as one of the important products of papaya, could be increased four-fold compared with control by the application of 200 ppm ethrel (12).
Model for Management of Erratic Flowering in Mango
There are three main processes, which determine the fate of mango flowering, i.e. competence, induction and determination. Competence is exhibited, if a cell / tissue / organ is exposed to a signal and it responds in the expected manner only when they first attained readiness to flower stage. When they are ready to flower, then they are said to have attained competence. Induction occurs when a signal gives a unique developmental response from competent tissue and determination is shown, if, a cell or groups of cells exhibits the same development fate. For the flowering gibberellic acid levels must first fall below threshold level for its competence to flower to be expressed. Adequate assimilates (carbohydrates) must be on hand to support flowering and fruit growth. In an environment where GA levels are high, no starch accumulation can take place. Therefore, GA concentration needs to fall below a certain threshold level, so that starch can accumulate within the tree. Fortunately paclobutrazol in an inductive agent, which slow, down the synthesis of GA and provided the stimulus, which bring the change from vegetative phase to reproductive state.
From competence tissue, flower initiation can proceed. In this model nitrogen is also crucial for flowering. Presumably, there is also a threshold for nitrogen content that, if, exceeded will allow the plant to flower. Most probably, thiourea, application triggers flowering by exceeding this threshold level. However, thiourea was also found to be useful for the vegetative flush if sprayed after harvesting of fruits. It was estimated that less than 0.1 per cent of the hermaphrodite flowers develop into mature fruits, the rest fall to the ground (57). Assuming there are 1,00,000 flowers and each flower contains to 10 µg of nitrogen, then each time a tree flowers, it loses 1 kg of nitrogen. The tree will, therefore, need to have adequate nitrogen reserves for flower and subsequent fruit formation. Threshold level of nitrogen also reported to exist in citrus for proper fruiting (34). However, more research is needed in this area for validating the model.
Occurrence of Post-bloom Vegetative Flush
Heavy flower and fruit drop is a serious problem in mango. This is attributed to several causes, such as genetic, hormonal, insect pests, diseases, degeneration of embryo, lack of pollination and competition between fruitlets. However some mango
cultivars like Alphonso, Langra etc. produce heavy vegetative flush during flowering and fruit set. Yields from such trees were invariably very poor inspite of profuse flowering. Flushing tendency was observed more pronounced in young orchards (30). The possible reason which affected vegetative growth and fruit retention could be the competition between the sinks. Although, seeds in growing fruits are generally considered as powerful sinks for mobilisation of the photosynthates, in this case, the post-bloom vegetative flush may become a powerful sink than the seed and fruits. Such type of phenomenon is common in apple and pear (43 and 69). Removal of post-bloom vegetative flush was found the best remedy for remarkable increase in fruit retention and yield.
Domination of Low-Efficient Orchard
Low photosynthetic efficiency in fruit crops is one of the important factors responsible for low yield and inferior orchard efficiency (18). It is because the first enzyme for photosynthesis system, ribulose biphosphate carbolase (Rubisco), which fixes CO2 also catalyses an alternative reaction involving oxygenation of sugar biphosphate. Both carboxylase and oxygenase reactions occur at the same site and compete with each