Water Availability

Water deficits often limit productivity in orchards. The effects of water stress on carbohydrate partitioning are primarily on growth of the top of a tree, while the dry matter partitioned to roots is generally more stable. This could be due to the roots experiencing less water stress than the shoots, which must draw water through the plant. It would also help establish a better functional balance of water-absorbing roots to transpiring shoots when the limiting resource is obtained by roots. This behavior has been utilized to develop an irrigation strategy suited for stone fruit in arid climates, in which irrigation is withheld in midseason if shoots are too actively growing. This inhibits shoot growth for easier management but will not affect fruit growth very much if it is done at the time of the midseason slow growth phase of stone fruit mentioned previously. This is a good example of the utilization of plant physiology studies for practical crop management.

Nutrient Availability

Variations in mineral nutrient availability are relatively common in spite of excellent work that has defined the requirements and fertilization for temperate fruit trees. In general, it appears that if nutrients are limiting, the growth of the top of the tree is reduced more than the growth of the roots (similar to the response to water deficits). In some studies, roots actually grow more with low nutrient availability, presumably to obtain more of the limiting nutrients. Conversely, with high nutrient availability, top growth is stimulated while root growth may be reduced. When nitrogen is too high, flower bud initiation is inhibited and a tree can become too vegetative.

Pruning and Canopy Management

Pruning is always a dwarfing process. The earlier the pruning, the greater the effect in the current season. There are many practices that growers use to regulate growth and sustained cropping of temperate fruit crops by directly or indirectly affecting carbon partitioning. The most direct is the removal of young fruitlets (called "thinning") to reduce competition so that the remaining fruit can grow larger and have better quality. The reduction in crop load of course also allows more carbohydrate partitioning to other organs and processes such as flower bud development.

Pruning and training techniques (bending or positioning of branches, etc.) also are used to structure trees and to maintain good sunlight distribution in tree canopies. Pruning reduces the number of growing points and therefore tends to increase vigor of remaining shoots. This tends to reduce the number of fruiting sites and lower crop loads, which will affect partitioning and make trees more vegetative.

The partitioning of carbohydrates produced by photosynthesis is a critical process in the growth and cropping of temperate fruit trees. It is a complex whole-plant process that is a function of the unique combination of competing organs of the tree and is greatly influenced by many internal and environmental factors. Many cultural practices are essentially designed to regulate carbohydrate partitioning for optimizing yields of quality fruit in a given season and in the following years.

Related Topics: FLOWER BUD FORMATION, POLLINATION, AND FRUIT SET; FRUIT GROWTH PATTERNS; LIGHT INTERCEPTION AND PHOTOSYNTHESIS; PLANT GROWTH REGULATION; PLANT NUTRITION; WATER RELATIONS

SELECTED BIBLIOGRAPHY

American Society of Horticultural Sciences Colloquium (1999). Carbohydrate economy of horticultural crops, Colloquium proceedings. HortScience 34:10131047.

Flore, J. A. and D. R. Layne (1996). "Prunus." In Zamski, E. and A. A. Schaffer (Eds.), Photoassimilate distribution in plants and crops (pp. 825-849). New York: Marcel Dekker. Lakso, A. N., J. N. Wunsche, J. W. Palmer, and L. Corelli-Grappadelli (1999). Measurement and modeling of carbon balance of the apple tree. HortScience 34: 1040-1047.

Palmer, J. W. (1992). Effects of varying crop load on photosynthesis, dry matter production and partitioning of Crispin/M.27 apple trees. Tree Physiol. 11:19-33.

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