- Not all fruit trees in an orchard bear fruit equally or regularly and sometimes flowers and fruits do not bear fruit under similar conditions where a tree is bearing heavy.
- This failure to bear fruit can be called unfruitful.
- To understand the problem of unfruitful in orchards, it is necessary to be familiar with the following words.
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- It refers to the initial growth the ovary and its associated parts have taken after bloom to maturity.
- It is the stage of the plant when it is not only capable of flowering and fruiting but also taking these fruits to maturity and the inability to do so is unfruitfulness or barrenness.
- The inability of a plant not only to produce fruit but also to develop viable seeds is known as sterility or infertility. All fertile plants are fruitful but not all fruit plants are fertile (Seedless fruits).
- The ability of a plant to mature fruits after self-pollination.
- The ability of a plant to produce viable seeds after self-pollination.
Causes of Unfuitlessness
This unfruitfulness is one of the serious problems of many orchards and its causes need to be properly understood for effective control and achieving economically acceptable production levels.
There can be many reasons for this problem and they are broadly divided into two categories
- Internal factors
- External factors
1. Internal Factors
There are many internal factors that can be classified into the following major categories, they are:
- Evolutionary tendencies
- Genetic influence
- Physiological factors
i. Evolutionary Tendencies
In the process of growth, many conditions can lead to incomplete flower or different growth periods, unless appropriate measures are adopted.
Monoecious and Dioecious nature
- A plant with stamens and carpels in different flowers on the same plant is monoecious. E.g. Coconut, Areca, Pecan Nut, Capri Fig, and Hazelnut. Monoecious plants have little or no problems with pollination, fruit formation, and fruit set. However, pollinators need to be ensured.
- Plants that bear male and female flowers on different plants are called dioecious. eg. Papaya, dates, and strawberries. Similarly, some varieties of plum that are bisexual produce very little pollen.
- The abundant flowering of ornamental pomegranates without bearing fruit is the result of their being unisexual.
- Various sex forms have been reported in papaya by various scientists.
- There are two types of flower clusters in figs, namely staminate and pistillate flowers.
- To ensure a good fruit set, it is necessary to maintain some staminate trees (9:1) as pollinizers.
- A condition in a flower where the length of the style, relative to other parts of the flower, varies among flowers of different plants. In this case, the style of some flowers is short with long filaments and in flowers of some species or varieties, the style is long with short filaments.
- Thus the style and stigma at different heights prevent self-pollination.
- Brinjal has 4 types of flowers according to the length of the style i.e. long, medium, pseudo short, and truly short. Of these, pseudo short and true short are do not produce fruits.
- Similarly, in the delicious group of apples, the spreading petals with the extreme upright position of the stamens do not allow the bees to pollinate while collecting the nectar. When the pistils of heterostyled plants are pollinated with pollen from similar flowers or from other flowers with stamens of the same height, the combination may be fruitful but there is also the potential for varying levels of sterility. Here a system of pollination needs to be made.
- When the stigma receptivity period does not coincide with pollen viability in monoecious plants, it is known as Dichogamy.
- Self-pollination is prevented in perfect flower plants due to different maturation, as the two sex elements mature at different times.
- If the stamens ripen before the stigma becomes receptive, the flower is known as protandrous and if stigmas become receptive before the stamens produce viable pollens it is known as protogynous.
- It reduces the production of fruits. Protogynous condition is found in monoecious plants like walnut, hazelnut, etc. whereas protandry is present in many varieties of coconut.
- Most dioecious plants are also protogynous.
Abortive Flowers or aborted pistils or ovules
- It occurs in the pistil and stigma of many species of flowers and is responsible for failure to the fruit set.
- Abortion of partially developed flower buds is common.
- The establishment and maturation of two sexes depend on the degradation of two well-formed sex cells.
- Any interference with their development and functioning may result in sterility or unfruitfulness; Such conditions can be observed in some grape varieties and tomato varieties.
- Late flowering in strawberry clusters is always abortive.
- It is more common in indeterminate types of plants.
- Pistol degeneration takes the form of abortion and is more common in ornamental pomegranates.
- Some varieties of olives contain 10-60% aborted embryos. It is also common in some varieties of apples. Embryo sac abortion becomes more of a cause for seedlessness than it is fruitful in some cases.
Impotence of pollen
- Many grape varieties produce nonviable or impotent pollen, although they appear as perfect flowers.
- Sterility in grape varieties is the result of impotent pollen.
- Sterile pollen in the grape results from degeneration processes in the generative nucleus or arrested development prior to mitosis in the microspore nucleus. It’s ‘J.H. Hale’ peaches, ‘Washington Naval’ oranges, and ‘Tahiti’ limes are also commonly found.
ii. Genetic Influences
- Self-sterility is a condition that is determined by genetic inheritance but can develop in a favorable environment.
- Self-sterility affects its offspring as well as hybrids.
Sterility and Unfruitfulness Due to Hybridity
- Sterility and failure due to hybridization
- Generally wider the cross, the higher the level of sterility.
- The cross between peach and plum has an abundance of flowers but without the pistil with distorted stamens.
- The signs of the flowers were constantly sterile.
- A hybrid between a pear and a quince was seedless.
- Most citranges (a cross between sweet orange and Citrus trifoliate) do not produce any fertile female gametes.
- Seedlessness in most varieties of banana and pineapple is due to the hybrid nature of their ancestors.
- Most triploid varieties of apple produce null pollen.
- Many hybrids between Vitis rotundifolia and Euvitis are completely sterile.
- Similar was the case with the hybridization of Vitis vinifera and Vitis rotundifolia.
- Self-unfruitfulness and self-sterility due to incompatibility between pollen and ovule of the same plant or same variety are one of the common causes.
- Pollen and ovule are fertile but they fail to affect conjugation.
- Self-incompatible varieties in apple, pear, plum, and aonla require other pollinating varieties for fruit set.
- Self-incompatibility is found in some mango varieties like ‘Langra’, ‘Dashehri’ and ‘Chausa’.
- In addition, self-unfruitfulness and self-sterility are found in apples, pears, plums, almonds, apricots.
iii. Physiological factors
Slow Pollen Tube Growth
- The slow growth of pollen tube results in unfruitfulness.
- There is a difference in the growth rate of self and cross-pollinated apples, pears, cherries, and some citrus fruits. This can be considered a type of incompatibility due to chemotropic or hormone effects.
- Furthermore, fertilization must occur within a short period of time, failing which abscission will take place at the base of the style or ovary pedicel, and the fruit is not set.
Premature or delayed Pollination
- Premature or delayed pollination does not produce fruit.
- Tobacco flowers are susceptible to damage from premature pollination.
- When mature pollen grains are attached to the immature pistil, they germinate, enter the style, enter the ovule, and flower drop if the ovule is not ready for fertilization.
- However, in the case of oranges, there was no harmful effect of premature pollination, while some damage was observed in tomatoes.
- Persimmons, pears, plums, and peaches showed low fruit set due to premature pollination.
- Similarly, delay in pollination causes the flowers to drop without stopping. Delaying pollination for 1 or 2 days did not affect the fruit set. However, some species may produce polyembryonic seeds with further delay.
Plant nutritional status
The nutrient status of a plant just before or shortly after flowering is an important factor that determines the percentage of flowers being set and carried to maturity. This can affect pollen viability or the fertility of pistils.
- Effect on Pollen Viability: There is a significant difference in the germination percentage of pollen collected from older apple trees and stronger young trees of the same variety.
- Effect on pistil: Due to excessive fruiting of trees, dryness, or lack of nutrients in the soil, the defective pistil is produced. Excessive fruiting weakens the fruit tree and adversely affects the production in the coming season. A close association has been reported between defective pistil and unfruitfulness in American plums.
A lack of carbohydrates in Vitis vinifera is a common cause of flower drop. The lack of carbohydrates in greenhouse-grown tomatoes results in an abort of flowering and ultimately the plant does not bear fruit.
Fruit setting in flowers under different conditions
- In many fruits, there is more competition among fruits produced at the terminal growth under normal nutritional conditions which are fruit set and mature but fruit set is low.
- This positional competition takes place between fruits and branches as well as between different fruits which influences unfuitfulness.
- Strong and weak spurs – The nutritional status of spurs is positively correlated with fruit set in apples. Spurs on vigorous limbs/branches with large leaves produce more fruit than weak branches. More flowers eventually lead to more fruit sets and more flowers are usually produced on stronger branches. Similarly, only single flowers bear fruit and mature into fruit and most of the flowers produced in clusters dropdown.
- Ringing or girdling also accumulates an additional stock of food material, which leads to fruit set and development parthenocarpically.
- In the process of fruiting, the embryo is more important for development i.e. if the nutritional conditions are favorable, it is accompanied by the development of the seed coat and fruit wall, if not, only the later part of the development is higher degree retardation.
- Inadequate nutrient supply reduces the number of seed-forming ovules, and excessive nutritional deficiency results in both reduced fruit wall and the large number of ovules, rendering seeds incapable of production.
- In greenhouse cucumbers, nutrient deficiencies stunt the growth of growing fruits depending on the condition of the fruit and the timing of pollination. If some of the cucumbers are plucked, the remaining fruits begin to develop again.
- Strawberry produces bisexual flowers and if nutritionally deficient, pistillate flowers can be produced. However, nutrient status has an indirect effect on compatibility.
- External Factors
i. Nutrient supply: In some families such as Graminae, Cruciferae, and Leguminaceae, sterility is usually due to overnutrition or overfeeding. ‘Jonathan’ apple becomes self-sterile in rich soil, self-fertile in nutrient-poor soil. High fertility levels are usually associated with good pistil development and low levels with weak pistils and good stamens in grapes. Low fertility in olives leads to partial or complete degeneration of the pistil.
ii. Pruning and training: Pruning produces more true hermaphrodite conditions in the grape variety ‘Hope’. If pruning is not done, the variety remains sterile and produces aborted pistils.
iii. Location: A Jonathan apple that is sterile at one location may be self-fertile at another.
iv. Season: The hybrid grape ‘Adarsh’ is self-impotent at the beginning of the season but becomes self-potent later.
v. Temperature: High temperature dries up the stigma secretion at flowering and inhibits pollination. Tomato varieties grown at high temperatures do not produce any fruit.
vi. Light: Exposure of strawberry plants to long light periods causes the development of stamens and pistils in the flowers.
vii. Pests and Diseases: Mango hoppers, powdery mildew, etc. adversely affect fruit set and development in mango and grapes.
Spraying when the trees are in bloom i.e. at the time of flowering reduces the setting of fruits. Some fungicides have an inhibitory effect on pollen grains, i.e. at 200 to 10000 ppm, copper fungicides inhibit germination of pollen grains on the stigma.
Steps to Overcome the Problems of Unfruitfulness
Knowing that there can be many reasons for unfruitfulness, it is necessary to take necessary corrective measures which should start from the planning stage and extend to an established orchard.
- Crop and variety should be selected on the basis of climatic and soil conditions of the horticulture site.
- There should be provision for windbreaks and shelterbelts for areas prone to wind damage.
- Before planting an orchard, the soil should be optimized by adding organic matter, modifiers/amendments, and nutrients based on the soil analysis.
- In order to avoid pollination problems due to heterosexuality, dichogamy incompatibility, sterility, embryo abortion, hybridity, etc., pollinators (bees) should be used with effective pollinating varieties.
- Timely and appropriate amounts of chemical plant regulators can affect pollen tube slow growth, premature and delayed pollination.
- The problem caused by aging can be overcome by replanting or rejuvenating old trees.
- Problems caused by excessive fruiting or overbearing can be controlled by thinning at the appropriate level.
- Irrigation management will play an important role in the event of drought and waterlogging.
- The problem caused by the uneven distribution of flowers on the tree should be managed through thinning and crop regulation.
- Maintaining critical nutrient status in tree leaves for maximum crop production by adopting correct nutrition programs based on plant and soil analysis.
- Crops requiring regular pruning will have to adopt standard practices depending on the crop, variety, and climate.
- Management of diseases caused by pathogens should be done through effective plant protection measures following an integrated approach.
- The problem of fruit set due to the tendency of the alternate bearing should be addressed through the replacement of regular fruiting varieties and through crop regulation.
It is important to analyze the problem and then suggest corrective measures. Basically, planning should be done in such a way that the future is problem-free and then the right package of practices should be adopted.
1.Chadha, K.L. Handbook of Horticulture (2002) ICAR, NewDelhi
2.Jitendra Singh Basic Horticulture (2011) Kalyani Publications, New Delhi
3.K.V.Peter Basics Horticulture (2009) New India Publishing Agency
4. Jitendra Singh Fundamentals of Horticulture, Kalyani Publications, New Delhi