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Shading and Temperature as Environmental Factors Affecting Growth, Nodulation, and Symbiotic N₂ Fixation by Soybeans¹

Soybeans [Glycine max (L.) Merr.] are subjected to varying amounts of light and temperature changes when grown in the field. A study was made on the effect of shading and different temperatures of the root medium upon growth and N₂ fixation of soybeans. Four shading treatments (0, 18, 40, and 62%) were imposed by screen cloths and two fertilizer N levels were used. Soybeans were also grown with four root temperatures (15, 20, 25, and 30 C) and four fertilizer N rates (0, 15, 30, and 45 ppm N). Growth, total N, nonstructural carbohydrate, nodule count, and nodule mass were determined on all experiments and, in addition, acetylene reduction was done on the shading experiment.

Plants with no shade produced more dry matter and had higher N content, higher nonstructural carbohydrate, higher nodule mass and number than when shaded. Total nodule activity (acetylene reduction) was greatest at 18% shading, however. Fertilizer N increased nonstructural carbohydrate and specific nodule activity but decreased nodule number and mass, and total nodule activity.

Temperature increase from 15 to 30 C resulted in an increase in relative growth rate and N content of plant tops, whereas total N and nonstructural carbohydrate in roots was decreased. There was no significant effect of increasing temperature upon growth of roots or nodule number or nodule mass. The N content of plant tops was increased while nodule number and nodule mass were decreased by fertilizer N application.

The most significant finding was that 18% shading produced the highest acetylene reducing activity of soybeans.

Key Words: Acetylene reduction • Light intensity • Carbohydrate reserve

² Former graduate assistant in agronomy, now at NAPB Inculant Laboratories, P. O. Box 404, Princeton, IL 61356, and professor of agronomy, respectively.

Received for publication December 6, 1978.