PHOTOSYNTHATE SUPPLY AND UTILIZATION IN ALFALFA - A DEVELOPMENTAL SHIFT FROM A SOURCE TO A SINK LIMITATION OF PHOTOSYNTHESIS

Long-term CO2 enrichment, 14CO2 feeding and partial defoliation were employed as probes to investigate source/sink limitations of photosynthesis during the development of symbiotically [with Rhizobium meliloti] grown alfalfa. In the mature crop, long-term CO2 enrichment does not affect the rates of net photosynthesis, relative growth, 14C export to nonphotosynthetic organs or the rates of 14C label incorporation into leaf sucrose, starch or malate. The rate of glycolate labeling is 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 seeding crop, long-term CO2 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. A source limitation of seedling photosynthesis and a sink limitation of photosynthesis in more mature plants is inplied. Both the supply and the utilization of photosynthate may limit seasonal photosynthesis in alfalfa.