Photosynthate supply and utilization in alfalfa : a developmental shift from a source to a sink limitation of photosynthesis.

Long-term carbon dioxide enrichment, (14)CO(2) feeding, and partial defoliation were employed as probes to investigate source/sink limitations of photosynthesis during the development of symbiotically grown alfalfa. In the mature crop, long-term CO(2) enrichment does not affect the rates of net photosynthesis, relative growth, (14)C export to nonphotosynthetic organs, or the rates of (14)C label incorporation into leaf sucrose, starch, or malate. The rate of glycolate labeling is, however, substantially reduced under these conditions. When the mature crop was partially defoliated, a considerable increase in net photosynthesis occurred in the remaining leaves. In the seedling crop, long-term CO(2) enrichment increased dry matter accumulation, primarily as a result of increases in leaf starch content. Although the higher rates of starch synthesis are not maintained, the growth enhancement of the enriched plants persisted throughout the experimental period. These results imply a source limitation of seedling photosynthesis and a sink limitation of photosynthesis in more mature plants. Consequently, both the supply and the utilization of photosynthate may limit seasonal photosynthesis in alfalfa.