Partitioning To Tree Organs

To grow and produce a crop, all the critical organs of a fruit tree must receive carbohydrates for growth and maintenance. A unique feature inherent to perennial crops is that flower buds for the next year's crop are developing on a tree during the growth of the current year's crop. So although growers and consumers are interested in the fruit component, the perennial nature requires that vegetative organs (shoots, roots, and structure) and developing flower buds receive an adequate share of carbohydrates to sustain cropping in following years. There have been many studies of the actual partitioning of dry matter (not precisely carbohydrates, but a close approximation) in temperate tree fruit.

The individual organs on a fruit tree have genetically programmed growth patterns and carbohydrate requirements. However, it appears that overall partitioning of carbohydrates within a tree is not a genetically programmed process, but a result of the unique combination of competing organs and their relative abilities to vie for limited carbohydrates. The degree of competition among organs is determined by organ activity and distance from carbohydrate source.

In a young tree that is not yet cropping, shoots and roots receive substantial amounts of carbohydrates, as the development of both the top and bottom of the tree is needed. The relative amounts that roots receive, however, tend to decline with tree age. Once a tree begins to reach its final size (either natural or contained by grower pruning) and begins to produce fruit, the patterns of partitioning change (Figure C1.1).

The timing of growth and thus requirement for carbohydrates varies among tree organs. Shoot and leaf area growth is generally strongest in early season, with varying levels of decline in midseason as canopy development is completed. Shoot growth in apple generally declines markedly in midseason, while in stone fruit, shoot growth can continue quite strongly even after harvest. Shoots and the trunk continue to increase in diameter after terminal bud set until leaf fall.

FIGURE C1.1. Growth in peach tree dry weight and the partitioning into tree organs as affected by tree development over many years (Source: Flore and Layne, 1996; Figure 3, from original data of D. Chalmers and B. v. d. Ende, 1975, Annals of Botany 39:423-432. Reprinted from Eli Zamski and Arthur Schaffer, eds., Photoassimilate Distribution in Plants and Crops: Source-Sink Relationships, p. 830, by courtesy of Marcel Dekker, Inc.)

FIGURE C1.1. Growth in peach tree dry weight and the partitioning into tree organs as affected by tree development over many years (Source: Flore and Layne, 1996; Figure 3, from original data of D. Chalmers and B. v. d. Ende, 1975, Annals of Botany 39:423-432. Reprinted from Eli Zamski and Arthur Schaffer, eds., Photoassimilate Distribution in Plants and Crops: Source-Sink Relationships, p. 830, by courtesy of Marcel Dekker, Inc.)

Patterns of fruit growth differ quite markedly between pome and stone fruit (see FRUIT GROWTH PATTERNS for details). Pome fruit growth rate (weight gain per day) increases rapidly in the first third of the season, then levels off and is fairly stable until harvest. Stone fruit growth rate increases initially similar to pome fruit, but then shows a decline in midseason, followed by another peak before harvest. The final stage of growth is very accelerated and many fruit will increase 40 to 60 percent in a matter of two to three weeks. Growth of wood structures has not been examined in many cases. Also, patterns of growth of new roots in fruit trees are not consistent, even from year to year under the same trees. It appears that root production only occurs when the environment (temperature and water) and internal competi tion allow. This suggests that roots are very weak competitors in spite of their obvious importance. In stone fruit, a flush of root growth is often noted after fruit harvest near the end of the season.

Partitioning to Fruit

A common expression in crop physiology for the percentage of total dry matter partitioned to the harvested portion (the fruit in this case) is the "harvest index" (HI). In temperate tree fruit, HI will be zero in the early years before cropping, but it can become very high in mature trees. The HI values from our studies and those reported in the literature for pome and stone fruit trees have been summarized. In general, the HI increases over orchard development from planting to maturity. For apples, values of 30 to 50 percent seem common, but can be as high as 65 to 80 percent of dry matter produced each year (Figure C1.2). Although fewer studies have been done with stone fruit, peaches also are reported to reach HI values of up to 70 percent. Comparable studies have not been done with cherries, but their yields are typically about half those of peaches due in part to very short fruiting seasons. We would expect cherries to have somewhat lower maximum HI values.

Research values for apples and peaches are extremely high compared to most crops and may in fact be too high for sustained cropping. Nonetheless, the data indicate the great ability of these crops to produce fruit. The factors mentioned earlier of preexisting structure, long leaf area duration, and low respiration are likely key to high maximum HI values in temperate fruit trees. Most annual field crops have HI values of 20 to 50 percent, excluding root systems (whereas the fruit tree values quoted previously included roots).

Partitioning to Vegetative Organs

As the HI increases with onset of cropping or different levels of crop at any stage, there are concurrent declines in partitioning to other organs. All vegetative organs receive fewer carbohydrates, but the greatest relative reductions are in root systems (Figures C1.1 and C1.2), followed by shoots, stems, and leaves. Leaf areas are reduced somewhat, as increases in crop load tend to reduce shoot growth, although amounts vary among studies. Partitioning to wood structures tends to decline rapidly with initial low yields, but then levels out to

FIGURE C1.2. Effects of increasing crop load on partitioning of dry weight in dwarf apple trees (Source: Reproduced with permission from Palmer, 1992, Tree Physiol. 11:19-33.)

more constant amounts with higher crop loads. Apparent baseline amounts of leaf and structure partitioning are likely due to early season vegetative growth that occurs before fruit become strong competitors. Also, minimum quantities of leaf area and shoot structure are required for setting large crops of fruit.

In the case of roots, several studies in apples and peaches indicate that net seasonal dry weight gain may be near zero with the heaviest crop loads. This maybe deceiving, however, since some of the carbohydrates partitioned to root systems are lost due to death of fine roots after only a few weeks or months. Trees can have fairly high root turnover rates, and new roots may be functional for a period of only a few weeks. Ignoring these roots is analogous to not including the leaves that fall each autumn in the partitioning to the top of the tree.

Current studies indicate that only a small percentage of the new roots produced each year will survive to become larger structural roots that we can easily find and measure. Nonetheless, cropping dramatically reduces the partitioning of carbohydrates to permanent roots in temperate fruit crops.

Partitioning to and from Carbohydrate Reserves

Another important process that requires carbohydrates is the accumulation of reserves. These are not as easily measured or interpreted. The nonstructural carbohydrate reserves are usually stored as poly-saccharides such as starch or hemicellulose. They provide the critical carbohydrate supply for early season flower and shoot development before leaves can begin new photosynthesis. The general seasonal pattern of carbohydrate reserves is a maximum in early winter, followed by some decline with early bud development before visible growth and a rapid decline to a minimum shortly after bloom. Then, there is a gradual increase in reserves as the season progresses, reaching a maximum again as leaves fall.

For pome fruit, such as apple, the bloom minimum in reserves occurs about one month after budbreak when trees have about 20 percent of their leaf area. About 30 to 50 percent of extractable reserves are utilized before recovery begins. A tree can then support fruit growth with current photosynthesis. In stone fruit, however, bloom and budbreak occur at about the same time, the pattern changing with latitude and year. Therefore, reserve carbohydrates are needed for a longer period to support fruit growth until the leaf canopy can develop. In cherries, four to six leaves per shoot must be formed before shoots begin to contribute carbohydrates back to a tree. Also, it appears that at the minimum, stone fruit carbohydrate reserves are almost fully utilized before recovery. The partitioning of carbohydrate reserves in the spring is primarily to growing shoot tips and flowers, and to new root tips (area of first growth). Many of the carbohydrates are used for respiration, as early growth is intensive and requires a lot of energy.

Partitioning to Flower Bud Development

Flower buds are developing for subsequent years at the same time fruit and other organs are growing. Although partitioning of carbohy drates to these tiny buds is necessary, the amount is extremely small, and it is not clear if the carbohydrate supply is ever limiting. However, fruit load and effective leaf area seem to affect the process of flower bud initiation. Early thinning will promote flower bud initiation, while early leaf fall will decrease it. There is evidence that flower bud initiation or triggering is hormonally controlled, but there is also evidence that general carbohydrate relations may be involved as well. Some research suggests the initial triggering of flower bud development may be hormonally controlled, but the subsequent bud development that determines flower quality the next year may be carbohydrate related. The relative importance of the two factors remains to be clarified.

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