Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

Abstract

To evaluate leaf C balance during rapid pod-fill in soybean, measurements were made of CO2 assimilation at mid-day and changes in specific leaf weight, starch and sucrose concentrations over a 9 h interval. Assimilate export was estimated from CO2 assimilation and leaf dry matter accumulation. Chamber-grown ''Amsoy 71'' and ''Wells'' plants were subjected on the day of the measurements to 1 of 6 photosynthetic photon flux densities to vary CO2 assimilation rates. Rate of accumulation of leaf dry matter and rate of export both increased as CO2 assimilation rate increased in each cultivar. Starch concentrations were greater in ''Amsoy 71'' than in ''Wells'' at all CO2 assimilation rates. At low CO2 assimilation rates, export rates in ''Amsoy 71'' were maintained in excess of 1.0 m CH2O/dm2 leaf area per h at the expense of leaf reserves. In ''Wells'', export rate continued to decline with decreasing CO2 assimilation rate. The low leaf starch concentration in Wells at low CO2 assimilation rates may have limited export by limiting C from starch remobilization. Both cultivars exhibited positive correlations between CO2 assimilation rate and sucrose concentration, and between sucrose concentration and export rate. C fixation and C partitioning both influenced export rate via effects on sucrose concentration.