Fruit Size and Fruit Quality
Many aspects of an orchard operation affect its profitability . These include fruit orchard establishment and operations costs, debt, labor management, varieties, yield, fruit quality, and marketing. Of these, fruit quality has become increasingly important due to the competition from within the industry and the availability of different types of fruits through out the year. Of the factors that define fruit quality (such as color, size, finish, firmness, and flavor), size impacts greatly into the grower's pocket book because of the severe price penalties associated with not reaching a minimum size requirement. To improve fruit size, one must know understand the limitations to fruit size, and how some of these limitations may be overcome.
Genetics. The most important factor controlling fruit size is genetics. For example, crab apples are seldom larger than 1.5 inches in diameter, whereas the 'Wolf River' apple is often 5 inches. However, within these genetic limitations for each cultivar, the number and the size cells greatly influence final fruit size.
Cell size and number. Pome fruits such as the apple have relative constant growth during the season, with both cell division and cell enlargement determining the various final fruit sizes and storage characteristics. The period following anthesis (full bloom) is characterized by rapid cell division, whereas the growth of the fruit after 25-30 days after anthesis is due to cell expansion. In a study conducted in New York on 'Empire', it was determined that fruit growth rates 3-5 weeks after full bloom (AFB) were highly related to final fruit size. This period coincided with 'June drop'. The period of 6 to 10 weeks AFB also appear to be correlated to final size distribution, but much less than the earlier period. Interestingly, the growth rate in August and early September were shown to be important in final fruit size, but not related to the relative fruit size distribution. Studies have shown that early cell division is important for maintaining overall fruit quality. The best apple quality is achieved by growing relatively many cells of medium size rather than fewer cells of large size.
Moisture. The growth of the fruit after 25-30 days after anthesis is mostly due to cell expansion. This cell expansion is highly dependent upon an adequate supply of moisture which provides weight and diameter increases of the fruit, and one of the first responses to water deficit by an apple tree is that fruit growth slows down. Under water stress situations, water is taken from the fruits to supply the leaves. Since fruit growth occurs at an almost linear rate during the entire growing season, water stress anytime during the season should be avoided. It is difficult to determine how much water is needed to avoid water stress because many factors such as the amount of water in the soil profile, distance between the trees, and size of canopy, need to be considered.
Seed number. Generally, as seed number increases, the fruit weight increases, and since pollination has a direct effect on fruit set and seed number, ensuring that all conditions are right for adequate pollination is a prerequisite for achieving maximum fruit size.
Thinning. Thinning influences fruit density (the number of fruit per trunk or limb cross sectional area) which is a critical factor in determining fruit size. As fruit density decreases, the ratio of leaves to fruit increases, resulting in higher supply of photosynthates per fruit which results in larger fruit size (i.e., fewer fruits getting more nutrients). Early hand thinning or chemical thinning usually stimulates cell division and sometimes cell enlargement. The effect of thinning on fruit size declines with time. The largest response is obtained when thinning is done early, and little or no response is obtained when thinning is done late in the season. Although fruit position in a cluster affects fruit size (the king flower is the largest fruit at harvest), a study to determine the potential of fruit size by lateral fruits of 'Empire' demonstrated that single lateral fruits can be almost as large as single king fruits when thinning is done early (two weeks after bloom in this case).
Position and quality of fruiting spurs. Spur leaves are important in fruit production and fruit quality. As the leaf area per spur and spur efficiency increases, fruit set, fruit size, and the long term productivity of the tree increases. The most important factor influencing spur efficiency (total dry leaf weight/spur) is the amount of light penetrating and being intercepted by the canopy. A study on artificial shading of 'Golden Delicious' demonstrated that when light was reduced to below 30% of full sun during either the period from bud break through petal fall or from petal fall for the next four weeks, fruit set was reduced. The critical period for fruit size was from petal fall for the next 28 days. Spur quality is higher in the top third of the canopy and decreases as one moves into the interior of the canopy. A study conducted in New Zealand on 'Granny Smith' indicated that fruits located above the limb were the largest whereas fruits bellow the limbs were the smallest. Since these patterns of changes in fruit size are closely followed by light levels found at the various canopy positions, it must follow that any pruning and or training techniques and or practices which increase light penetration will have an impact on fruit size.
Nutrition. The supply of nutrients also controls positive fruit size. Proper nutrition helps to maintain spur quality as well as fruit, vegetative and root growth. Generally, high nitrogen fertilization results in larger fruit. However, this can in turn result in decreased fruit firmness. Therefore, nitrogen fertilization should be used cautiously.