Response of Soybean Canopy Photosynthesis to CO2Concentration, Light, and Temperature

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Abstract
Photosynthetic rates of outdoor-grown soybean (Glycine max L. Merr. cv. Bragg) canopies increased with increasing CO2 concentration during growth, before and after canopy closure (complete light interception), when measured over a wide range of solar irradiance values. Total canopy leaf area was greater as the CO2 concentration during growth was increased from 160 to 990 mm3 dm−3. Photosynthetic rates of canopies grown at 330 and 660 mm3 CO2 dm−3 were similar when measured at the same CO2 concentrations and high irradiance. There was no difference in ribulose bisphosphate carboxylase/oxygenase (rubisco) activity or ribulose 1,5-bisphosphate (RuBP) concentration between plants grown at the two CO2 concentrations. However, photosynthetic rates averaged 87% greater for the canopies grown and measured at 660 mm3 CO2 dm−3. A 10°C difference in air temperature during growth resulted in only a 4°C leaf temperature difference, which was insufficient to change the photosynthetic rate or rubisco activity in canopies grown and measured at either 330 or 660 mm3 CO2 dm−3. RuBP concentrations decreased as air temperature during growth was increased at both CO2 concentrations. These data indicate that the increased photosynthetic rates of soybean canopies at elevated CO2 are due to several factors, including: more rapid development of the leaf area index; a reduction in substrate CO2 limitation; and no downward acclimation in photosynthetic capacity, as occur in some other species.