Assimilate Utilization in the Leaf Canopy and Whole-Plant Growth of Soybean During Acclimation to Elevated CO2

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Abstract
Young vegetative soybeans (Glycine max ''Ransom'') were exposed to control (350 .mu.L L-1 CO2) or CO2-enriched (700 .mu.L L-1 CO2) environments for 22 days. Alterations in carbon acquisition, assimilate utilization by the leaf canopy, and whole-plant growth were followed to characterize plant acclimation at high CO2. Whole-plant dry weight (DW) progressively increased at high CO2 relative to the control throughout the experiment. The initial DW increases were associated with the accumulation of nonstructural assimilates in leaves and increased specific leaf weight (SLW). After 3 days, however, DW began to accumulate rapidly in stems and roots under high CO2, and SLW no longer increased relative to controls. Total leaf area did not increase significantly at high CO2 unitl 13 days after the start of treatments. Net assimilation rates declined under both CO2 conditions but remained higher at 700 .mu.L L-1 CO2 throughout the experiment. The increases in stem and root DW during week 1 at high CO2 were accompanied by (1) an early increase in the estimated rate of assimilate utilization in the canopy during the dark phase of the diurnal cycle and (2) a later increase in the estimated rate of assimilate utilization during the light phase. The results indicate that dark mobilization of assimilates from source leaves responded to variations in assimilate accumulation but that export of assimilates from source leaves in the light adjusted more slowly and appeared to be coordinated with large changes in sink activity in the whole plant.