Effects of Light Intensity and Carbon Dioxide Concentration on Photosynthetic Rate of Soybean1

Photosynthetic response curves to CO2 concentration and light intensity were determined for the soybean varieties ‘Hark’ and ‘Chippewa‐64’. At light intensities above 5,380 lux, the photosynthetic rates of both varieties were limited by the CO2 concentration of the surrounding atmosphere within the range investigated (270 to 1670 ppm). At normal atmospheric CO2 concentration (300 ppm), photosynthesis of both varieties was light saturated at about 21,530 lux. At a CO2 concentration of 1670 ppm, photosynthesis was not light saturated in either variety at the highest light intensity tested (75,350 lux). The photosynthetic rate of the variety Hark was higher than that of the variety Chippewa‐64 at 45 of the 48 combinations of light intensity and CO2 concentration tested.