Varietal Effects in Soybean Photosynthesis and Photorespiration1

The rate of photosynthesis for seedlings of 36 varieties of soybean [Glycine max (L.) Merrill] ranged from 12 mg CO2 assimilated dm−2 hr−1 in ‘Patterson’ to 24 mg CO2 dm−2 hr−1 in ‘Richland’ at saturating light intensity, determined to be 43 kilolux. Comparing the photosynthetic rate and average seed yield of several varieties indicates that varietal yield differences are not caused by differences in photosynthetic rate, although in some instances yield can be related to total photosynthate production. The photosynthetic rate of ‘Wayne’ seedlings was identical when grown in vermiculite, in sterilized soil, or in soils of medium and high productivity. The photosynthetic rates in air are 5 to 6 mg CO2 dm−2 hr−1 less than the rates obtained in a low oxygen atmosphere. Calculations based on photosynthetic rates and compensation points indicate that slightly more than onehalf of the decrease in photosynthesis in air is due to photorespiration, and that the remainder is due to an oxygen inhibition of photosynthesis. The compensation point at 43 kilolux was approximately 40 ppm CO2 for all varieties tested. Analysis of leaf extracts for glycolate oxidase activity indicated no correlation with photorespiratory activities, suggesting that although this enzyme may be required for photorespiration, it is not the limiting factor.