Regulation of assimilate partitioning in leaves

Concepts of the regulation of assimilate partitioning in leaves frequently consider only the allocation of carbon between sucrose and starch synthesis , storage and export. While carbohydrate metabolism accounts for a large pr oportion of assimilated carbon, such analyses provide only a restricted vie w of carbon metabolism and partitioning in leaf cells since photosynthetic carbon fixation provides precursors for all other biosynthetic pathways in the plant. Most of these precursors are required for biosynthesis of amino acids that form the building blocks for many compounds in plants. We have u sed leaf carbon : nitrogen ratios to calculate the allocation of photosynth etic electrons to the assimilation of nitrogen necessary for amino acid for mation, and conclude that this allocation is variable but may be higher tha n values often quoted in the literature. Respiration is a significant fate of fixed carbon. In addition to supplying biosynthetic precursors, respirat ion is required for energy production and may also act, in both light and d ark, to balance cellular energy budgets. We have used growth CO2 concentrat ion and irradiance to modify source activity in Lolium temulentum in order to explore the interactions between photosynthetic carbon and nitrogen assi milation, assimilate production, respiration and export. It is demonstrated that there is a robust correlation between source activity and foliar resp iration rates. Under some conditions concomitant increases in source activi ty and respiration may be necessary to support faster growth. In other cond itions, increases in respiration appear to result from internal homeostatic mechanisms that may be candidate targets for increasing yield.