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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.

Key Words: varietal differences in photo synthetic rate

² Associate Professor of Agronomy and formerly Research Associate, (now Agronomist-in-charge, U. S. Regional Soybean Laboratory, Urbana, Ill., Crops Research Division, ARS, USDA.)

Received for publication February 6, 1967.