Diurnal changes in leaf chemical constituents and (14)C partitioning in cottonwood

Diurnal changes in concentrations of leaf chemical fractions and partitioning of photosynthetically fixed (14)C within the plant and among chemical fractions were studied in rapidly growing cottonwood (Populus deltoides Bartr. ex Marsh.) seedlings. During the light period, leaf weight (mg cm(-2)) increased by about 25% primarily as a result of the accumulation of starch and sucrose, and to a lesser extent because of an increase in the content of amino acids and the chloroform fraction (pigments plus lipids). In contrast, reducing sugars and organic acids decreased in concentration. The partitioning of (14)C within the plant also changed during the light period. Acropetal transport to developing leaves and stem decreased from 81 to 55% of the total (14)C translocated from a source leaf in 4 hours, whereas basipetal transport to stem and roots increased from 13 to 37%. Although assimilation rate ((14)C fixed in 0.5 h) remained constant during the light period, the percentage of fixed (14)C translocated out of the source leaf in 4 h decreased from 27 to 9%. This change in transport rate of recently fixed (14)C was caused by a shift in (14)C partitioning from transport sucrose to storage starch. During the light period, the incorporation ratio ((14)C-sugar/(14)C-starch) decreased from 40 at 0700 h to 2 at 1900 h. The partitioning of carbon to different chemical fractions within the source leaf and the interactions or feedback between different sinks and the source leaf have a major influence on plant growth and development. Control of this carbon partitioning is located in both source and sink leaves.