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Some Effects of Leaf Area, Solar Radiation, Air Temperature, and Variety on Net Photosynthesis in Field-Grown Soybeans¹

The effect of leaf area index, solar radiation, and air temperature on photosynthesis (net CO₂ assimilation) leaf canopies of three soybean (Glycine max L., Merr.) varieties was studied. Sections of field plots (1.16-m²) were enclosed in transparent chambers, and CO₂ assimilation rates were determined by infrared gas analysis. From a maximum of about 60 mg CO₂2/dm² (ground area) .hr on July 5, the assimilation rate declined to 53 mg CO₂/dm².hr by Sept. 9. Maximum variety differences were 8 mg CO₂/dm².hr. An interaction occurred between LAI and solar radiation in their effect of photosynthesis; the critic:d LAI was 5 to 6 at 0.2 ly/min solar radiation and greater than 8 (maximum LAI attained) at 1.2 ly/min. Below LAI of four canopies showed light saturation. Optimum air temperature was 25–30 C.

Key Words: Critical LAI • Glycine max

² Formerly Graduate Assistant (now Assistant Professor, Ohio Agr. Research and Development Center, Wooster, Ohio 44691) and Professor, Dept. of Agronomy, Iowa State University, Ames, Iowa 50010.

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