Influence of carbohydrates on photosynthesis in single, rooted soybean leaves used as a source-sink model. Plant Cell Physiol

The single rooted leaf of soybean (Glycine max L. Merr.) was used to study source-sink relationships in photosynthesis. When the leaves were kept under a regime of 10 h light (410-480 ^mol photons m~2 s"1, 400-700 nm)-14 h dark, they did not expand, the increase in leaf dry weight almost stopped, and photosynthetic activity remained at a high and constant level for.8 d while the dry weight of the roots increased at a constant rate throughout the period. Thus, under this condition the leaf and the root system served as the only source and sink organs, respectively. When leaves grown for 7 d under this condition were placed under continuous light to alter the source/sink balance in photosynthate, the root dry weight increased at a constant rate equal to that found under the 10 h light—14 h dark condition. The leaf dry weight markedly increased and by day 5 of continuous light had increased 1.6-fold, mainly as a consequence of accumu-lation of starch and sucrose, which were not translocated for root growth. The continuous light caused an abrupt decrease in the photosynthetic activity (40 % of initial value by day 5). However, the activity recovered almost completely after a 32-h transfer to darkness. Significant negative correlations existed between photosyntheti