Fertilization

Pollination is not, of course, the final goal but is one step in the process ending in fertilization. Following the deposition of pollen to the stigmatic surface of a flower, a complex chain of biochemical recognition factors determines whether the pollen grain will germinate and also whether the pollen tube will grow down the stylar tissue of the flower toward the ovary. Barriers to self-pollination may occur at both these sites. Fertilization does not occur until the pollen tube has reached the ovule, but because the ovule is receptive for a limited time, the speed at which the pollen tube grows down the style becomes important. The effective pollination period (EPP) is the difference between the duration that the ovule is receptive and the length of time taken for the pollen to reach the ovule. The main factor determining pollen tube growth, and therefore EPP, is temperature. For example, in pear, the rate of pollen tube growth is more than five times faster at 15°C than at 5°C. Cool weather during the bloom period is detrimental, as it discourages bee flight. In addition, pollen tubes grow slowly during cool weather and may not reach the ovule while it is receptive. Although temperature is the primary determinant of pollen tube growth rate, other factors also play a role, such as the nutrient status of flowers, wind, and probably the light environment of the flowers.

Fruit set refers to the stage in which flowers are retained on the tree and develop into fruit, or else abscise. Shedding of flowers and young fruitlets occurs in several waves. During the first wave, unpollinated flowers are shed, followed by flowers pollinated but not fertilized. A number of fertilized flowers are shed in subsequent waves, depending on fruit species. In the Northern Hemisphere, this is called "June drop" ("December drop" in the Southern Hemisphere). Required flower set to ensure a reasonable crop obviously depends on the intensity of flowering but generally is in the range of 5 percent for apple up to as high as 70 percent for cherry. The degree of abscission is seldom sufficient to regulate crop load to attain good fruit size and return bloom, so fruit thinning is also required.

For multiseeded fruit, the number of seeds in retained fruit is important. Higher seed counts result in larger fruit size, although there appears to be a closer relationship between seed number and fruit weight in fruit with a high seed count, such as kiwifruit, than with lesser-seeded fruit, such as apple or pear. Nevertheless, even in fruit such as apple, it holds true that larger fruit on average have more seeds than smaller fruit. Seeds are also important for uniform fruit shape. Fruit with uneven seed distribution are often flattened or lopsided, with the side having fewer seeds being less well developed (Figure F1.2).

FIGURE F1.2. Uneven fruit shape caused by incomplete pollination. Note well-developed seed in the top half of the fruit compared with poorly developed seed and poorer fruit development in the lower half.

Flowering of fruit trees, from flower initiation to fruit set, is a complex process, and the details are not well understood. With most temperate tree fruit, however, horticulturists have quite comprehensive knowledge of the requirements of the crop and appropriate management techniques. As growers recognize, the foundation for a successful crop is laid the previous year when flowers are initiated. From this early beginning through the stages of pollination and fertilization, the challenge is to maximize the potential for high-quality fruit production.

Related Topics: ANATOMY AND TAXONOMY; ORCHARD PLANNING AND SITE PREPARATION; PLANT GROWTH REGULATION

SELECTED BIBLIOGRAPHY

Chan, B. and J. C. Cain (1967). Effect of seed formation on subsequent flowering of apple. Proc. Amer. Soc. Hort. Sci. 91:63-68.

DeGrandi-Hoffman, G. (1987). The honey bee pollination component of horticultural crop production systems. Hort. Rev. 9:237-272.

Fallahi,E. (1999). The riddle of regular cropping: The case for hormones, nutrition, exogenous bioregulators, and environmental factors. HortTechnol. 9:315-331.

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