FRUITFULNESS & UNFRUITFULNESS

 

Fruit growing is a rewarding venture. It produces more yield of qualitative fruits which Warrants more return on unit basis of land provided it grows and bears properly. Sometimes, fruit trees fail to bear fruits untoward expectation. This is known as unfruitfulness.

Phylogénical, physiological, managemental, parasitical and climatological factors are attributed to the constraint of unfruitfulness.

Phylogenical factors

Sex Forms. Staminate and pistillate forms constitute basic sex forms in plants. The plants in which both the sexes fail to form zygote, unfruitfulness is noted. Unfruitfulness depends a lot upon distribution, structural peculiarity, differential maturity of sex forms ad viability of pollen to fertilize the ovary of flowers, genetic inheritance mechanism compatibility etc. In some plants male and female parts remain present within the flower itself. Such situation is termed as hermaphrodite. In hermaphrodite flowers, chances of unfruitfulness are restricted provided sex organs are compatible. There is the situation when male and female parts remain distributed in two different flowers on the same plants. This is termed as monoecy and the plants are termed as monoecious. In contrary to this, dioecious plants are also noted in which both the sex forms are found on two different plants separately. Further the chances of conjugation of sexes lower will be the chances of fruitfulness in plants and vice-versa. Monoecy favours easy conjugation whereas dioecy disfavours it. Some fruit trees are self-fruitful owing to compatibility of sex organs. They set fruits when pollinated by pollen from their own flowers or by pollen from another tree of the same variety. Most of the peaches, nectarine and sour cherry are self-fruitful. In contrast, many trees are self-unfruitful. Such trees fail to set fruits when pollinated by its own pollen or by the pollen of another tree of the same variety. Most of apple, pear, plum and cherry varieties are self unfruitful.

Many flowers have structural peculiarities. Their from and structure vary from each other. This is also referred to as heterostyly or dimorphism. In this situation, some flowers of the same species or varieties may have short style and long filament (thrum type heterostyly) while others may have long style and short filament (pin type heterostyly). Such types of peculiarities may prohibit self pollination and makes cross pollination obligatory. If cross pollination is constrained, unfruitfulness is encountered. In apple, mostly in Delicious group of varieties Golden Delicious, Royal Delicious stamens are quite upright and petals are spreading which permits bee visit even without touching to pollen or pistil. Heterostyly is of common occurrence in pomegranate, litchi, sapota, cashewnut, almond, carambola etc. In these crops, poor fruit setting is attributed to heterostyly.

The sexes of the flowers may have differential maturity status. It is termed as dichogamy. Under dichogamous condition either androecium matures first or gynoecium latter or vice versa. If androecium matures first, it is termed as protandry and such flowers are termed as protandrous. In Case, gynoecium matures first, it is termed as protogyny and such flowers are termed as protogynous. Protandry is observed in soursop (Annona muricata). sapota. macadamia nut, walnut, coconut and in passionfruit. Protogyny is noted in pomegranate, palm and in banana. In pistachio nut, avocado and in chestnut unusual type of dichogamy is found. In avocado Protogynous Diurnally Synchronous Dichogamy (PDSD) has been reported. The flowering synchronizes in such a way that each flower which appears on a tree is female at one time and male at another time. Diurnal synchronization is observed on the tree and the flowers behave like functional female during one part of the day and as functional male during another part of the day. The dichogamy is protogynous in nature and pistil matures prior to androecium. Thus, flower appears structurally bisexual but acts like unisexual. Such type of peculiarity contravenes self pollination and necessitates cross pollination. If cross pollination fail, plant remains unfruitful.

Duodichogamy is another sexual mechanism in plant which contravenes self pollination and may lead to unfruitfulness. It was discovered in Castanea (Fagaceae family) by Stout in 1928 who also coined this term. In this situation, each plant produces two batches of male flower that are tempórarily separated by a batch of female flowers in between. Such type of flower favours synchrony with-within individual and asynchrony among individual to ensure mating partners. It is an adaptive mating system in plants which results from competition of males to access small number of females. The plants flower in the order-male – Femalen - male. The resting period between flowering precludes selfing almost completely. Duodichogamy has been reported in Bridelia tomentosa (Phyllanthaceac), Acer, Dipteronia (Sapindaceae), Sawgrass (Cladium jamaicense).

It is very likely that the pollen produced by the flowers may not be always viable. On the basis of viability of pollen grains, a particular plant may be grouped either fertile or sterile. Sterility is very common in grape, J.H. Hale and Shanghai varieties of peach. Washington Navel orange and in Tahiti lime. The dropping of grape blossoms the partly developed berry of vinifera varieties (known as 'coulure") is ascribed to sterility. The degeneration occurring in generative or vegetative nucleus or in both leads to sterility of pollen grains and such plants remain unfruitful. Pollination from defective pollen leads to abortion of ovary. Growth and development of fruits are quite dependent upon the process of fertilization. Fertilization of ovary with defective pollen leads to its degeneration, it drop-off and the plant remains barren. Abortions of partially developed flowers, pistil, defective embryo and embryo sac are very common. Abortions of flowers due to degenerative pistil in plum and ornamental pomegranate, defective embryo in apple, defective embryo sac in oranges are very common.

Genetic inheritance mechanism determines fruiting status of the plant. Wider will be the crosses, lower will be the chance of fertility. There may be unequal distribution of chromosome during hybridization and it leads to sterility. Peach and plum hybrid known as Blackman or mule; Kamdesa a hybrid between peach and sour cherry, both are completely sterile and bear barren flowers. Pyronia-a hybrid between pear and quince, flowers and sets fruits, but the fruits are seedless: Duke cherry-a hybrid between sweet and sour cherry though fruits, but there is only a few viable seeds: Citrange-a hybrid between Poncirus trifoliata and sweet orange is fruitful but fail to produce fertile female gametes. Walnut cultivars--the Royal (Persian x California) and Paradox (Persian x Eastem Black), raspberry hybrids, grape hybrids between Vitis rotundifolia and Euvitis group, banana and pineapple all are sterile. Hybridity is attributed to sterility in all these cases.

Compatibility in the sense of reproduction may be defined as the ability of pollen grains to germinate in the gynoecium of the flower. In contrast, under incompatible situation. though both pollen and ovary are fertile but they fail to conjugate. Self incompatibility has been reported in apple, pear, plum, cherry, almond, blackberry, dewberry, citrus, loquat etc. Self incompatibility has been reported in commercial cultivar of loquat like Late Yellow, Golden Yellow, Tanaka and California Advance. All commercial cultivars of mango in North India--Dashehari, Langra, Chausa etc. and lemon cultivars--Pant Lemon, Kagzi Kalan etc. don't bear crop if planted solely as single cultivar due to self incompatibility.

Physiological factors

Poor germination of pollen, pre-mature or delayed pollination, slow growth of pollen tube, nutritional status of plants etc. are important physiological attributes which govern fruitfulness.

In compatible and fertile combinations of flowers, pollen tube growth is proper. In otherwise situation, pollen tube growth is slow. It has been observed that when pistils of heterostyled plants were pollinated with stamens of proportionate height (legitimate pollination), the degree of fertilization of ovary was high. In self-incompatible combinations poor growth of pollen tube has been recorded. Heterostyly also favours poor growth of pollen tube.

If mature pollen grains are applied to receptive pistils, they germinate, penetrate the style and enter the ovule and bring fruitfulness. In contrast, if pollination is done when pistil is not receiptive, fruit setting fails and unfruitfulness is encountered. In self incompatible and also in dioecious plants, failure in pollination at right stage leads to unfruitfulness.

Nutritional status of tree has important bearing on fruitfulness. Poor nutritive condition results in improper development of flowers and also poor production of pollen. In many fruit plants, a year of heavy crop load is followed by a year of unfruitfulness. This may be considered in the light of depletion of carbohydrate reserve, mineral deficiency, hormonal imbalance etc, When the plant repairs and recovers itself from deficient state, it comes into flowering and fruiting naturally provided other factors are favourable.

Managemental factors

It includes all the operations carried out in the orchard for better fruitfulness. Deciduous fruit plants are invariably pruned during dormancy Evergreen plants being unable to sustain pruning shock are not pruned in general. However, commonly accepting, pruning practices in bearing orchard, by virtue of maintaining a balance between vegetative and reproductive growth, allowing recouperation of tree after crop load, favouring interception of solar radiation and thus by invigorating the tree, favour fruitfulness.

Proper supply of nutrients is desirable for fruitfulness in orchards. It has been observed that plants with proper nitrogen supply and good carbohydrate assimilation, bear fruits successfully. However, heavy application of nitrogen, produces much vegetative growth which utilizes more of carbohydrate reserve rather to supply carbohydrate to the plants and thus renders the tree unfruitful.

Use of clonal rootstock brings precocity in bearing owing to attained physiological maturity. In kinnow mandarin, following Troyer citrange rootstock, early bearing is achieved, whereas, Sohsarkar delays it. Use of seedlings for raising orchard, brings delayed fruiting.

A plant remains fruitful only if water supply is proper. Moisture deficit may lead to the formation of abscission layer which forces the dropping of leaves, flowers and fruits and makes the tree barren. Water affect synthesis and translocation of carbohydrate and regulates fruiting situation. In general, during flowering, irrigation is avoided in the orchard. Citrus is very sensitive to application of irrigation water and irrigating citrus orchard while flowering makes the tree unfruitful.

Parasitical factors

Insect-pests and diseases are major parasites of plants. Severe attack of these parasites renders nil cropping. Attack of hopper, catterpillar, butterfly, thrips, aphids, mealy bug affect fruitfulness badly.

Diseases like powdery mildew, downy mildew, fire blight, scab, etc, cause considerable damage to flowers and fruits and depending upon severity, there may be total crop failure in the orchard.

Climatological factor

Temperature, humidity, rainfall, hail, frost, cloud etc. affect state of fruitfulness in plants, Temperature influences flowering, pollination, fruit set, growth and over all development of fruits. In temperate fruits, flowering takes place only when there is fulfilment of chilling temperature requirement. In citrus flowering appears satisfactorily well in response to cessation of growth during winter. Due to excessive high temperature, sapota remains fruitful in arid areas owing to drying of stigmatic fluid. Pollination is a major event in tearfulness. Temperature around 15-40°C is considered congenial for bee visit. Rainfall affect bee activity, Rain at the time of flowering, washes away the pollen and poses unfruitfulness in plants. Wind serves as an important source of pollen transference. Heavy wind velocity hinders by flight checks pollination. Frost and hailstorm injure plant or plant part. Spring frost severely damage blossom in temperate fruit crops. Cloudy weather is considered congenial for the spread of powdery mildew. In this disease, white powdery mass of fungus appears over entire aerial portion of the plant. In case of attack during flowering, complete crop failure is noticed.