Photosynthate allocation in Pinustaeda. II. Seasonal aspects of photosynthate allocation to different biochemical fractions in shoots

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Abstract
Research was conducted to study (1) seasonal changes in tissue composition and (2) dynamic allocation of photosynthate to different biochemical fractions in growing shoots of the current and previous years of 15-year-old loblolly pine (Pinustaeda L.). As the growing season progressed, the shoots increased rapidly in total biomass; however, old needles formed in the previous year lost dry weight. The most striking biochemical change was a rapid decrease in starch content of old needles early in the growing season. Soluble sugars and organic acids in old needles also decreased slightly. Contents of biochemical constituents generally increased in current axes and needles as the organs grew and increased in weight, although there were decreases in organic acid contents of current axes early in the growing season. Formation of nonlabile major constituents (cellulose, hemicelluloses, lignin, and protein) in current-year shoots proceeded at different rates. These rates were interpreted in terms of aspects of shoot growth and development. Seasonal allocation of photosynthates into biochemical constituent fractions in current year growth flushes was calculated. Results suggested that photosynthates were allocated to constituents of different functional categories (metabolism, storage, structure, and protection) at different times of the growing season in such a way that structural growth was of first priority. Thus, allocation of photosynthates to structural growth early in the season enables branches to compete for favorable growing conditions.