Abstract
To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO2 assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour 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 one of six photosynthetic photon flux densities in order 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 milligram CH2O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, 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 carbon from starch remobilization.
Both cultivars exhibited positive correlations between CO2 assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.
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Selected References
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