Photosynthate allocation in Pinustaeda. III. Photosynthate economy: its production, consumption and balance in shoots during the growing season

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Abstract
Using data on CO2 exchange, growth and biochemical composition, an approach was developed to calculate the seasonal production, consumption and surplus of photosynthate in shoots of loblolly pine (P. taeda L.). The amounts of CO2 released and of glucose substrate required in biosynthesis of shoot biomass over the growing season were also calculated. Individual growth flushes of different age-classes, as well as of an entire 1 yr old branch unit, of P. taeda were used as bases for calculations. Substrate requirements and CO2 production were relatively constant throughout the growing season. Within the whole 1 yr old branch unit, 63% of the photosynthate production was supplied by the two growth flushes of the previous year. Growth of the current year''s shoot represented the major consumption of photosynthate, and this made up 66% of the total photosynthate consumed in the branch unit. Surplus of photosynthate, which would be available for translocation from the branch, was about 38% of the total photosynthate produced. Most of the photosynthate surplus was derived from the growth flushes of the previous year. Source and sink relationships, with regard to photosynthate partitioning in the 1 yr old branch unit over the growing season, were clarified and described.