Before considering the partitioning of carbohydrates, seasonal production of carbohydrates must be understood. Temperate tree fruit are perennial plants with permanent structures that provide physical frameworks of canopies and root systems that do not need to be reproduced each season, as in annuals. Additionally, these structures contain reserves of carbohydrates and mineral nutrients that can immediately be used for early growth. Temperate tree fruit can develop full canopies and intercept sunlight to produce carbohydrates very early in the season compared to annual crops because they have thousands of preformed buds. Indeed, orchards may almost reach full light interception at the time many annual crops are planted. This leads to a rapid increase in total carbohydrate production early in the season.
In midseason, once tree canopies are established, carbohydrate production tends to be relatively stable. Compared to annuals that are planted at very close spacings and develop very high canopy densities, temperate tree fruit tend to have lower midseason carbohydrate production rates. This is due to the necessary alleyways that lower maximum sunlight interception to values typically about 40 to 70 percent for most mature orchards. Late in the season, however, temperate tree fruit tend to maintain carbohydrate production due to the ability of their leaves to sustain good photosynthesis rates for several months, while the leaves of annual crops typically senesce. Even with slower decline of leaf function, the shorter, cooler days combined with leaf aging gradually reduce carbohydrate production in the fall. This extended life span of green, active leaf area is referred to as a long "leaf area duration" and has been well correlated to high dry matter production in crops. Consequently, fruit trees often do well in marginal climates.
Photosynthesis produces carbohydrates for growth and energy. The energy needed to drive growth and maintenance of a tree is generated by using carbohydrates. The percentage of fixed carbohydrates used for respiration over a season will vary with the activity of growth, but it is reported to be as much as 40 to 60 percent in some annual plants. Unfortunately, we do not have good estimates of the respiration of root systems due to the difficulty of measurement, especially in field soils. Nor do we have good estimates of the amounts of carbohydrates lost because of leaching. However, a mature temperate fruit tree tends to have a relatively low respiration rate since (1) the structure of the tree is already built, (2) the woody structure has a high percentage of dead cells (wood vessels, fibers, etc.) that do not require much energy for maintenance, (3) the leaves tend to live long so the tree does not need to constantly produce energy-expensive new leaves to be productive, and (4) the fruit primarily accumulate carbohydrates directly so there are relatively few costs related to synthesizing proteins, lipids, etc. Combined, these characteristics make carbohydrate production and utilization quite efficient in temperate fruit trees.
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