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Low Root Temperature Effects on Soybean Nitrogen Metabolism and Photosynthesis ¹

^a Department of Agronomy, University of Wisconsin, Madison, Wisconsin 53706

The influences of low root temperature on soybeans (Glycine max [L.] Merr. cv. Wells) were studied by germinating and maintaining plants at root temperatures of 13 and 20 C through maturity. At 42 days from the beginning of imbibition, 13 and 20 C plants were switched to 20 and 13 C, respectively. Plants were harvested after 63 days. Control plants (13 C) did not nodulate, whereas those switched to 20 C did and at harvest had C₂H₂ reduction rates of 0.2 micromoles per minute per plant. Rates of C₂H₂ reduction decreased rapidly in plants switched from 20 to 13 C; however, after 2 days, rates recovered to original levels (0.8 micromoles per minute per plant) and then began a slow decline until harvest. Arrhenius plots of C₂H₂ reduction by whole plants indicated a large increase in the energy of activation below the inflection at 15 C. Highest C₂H₂ reduction rates (1.6 micromoles per minute per plant) were at 58 days for the 20 C control. Root respiration rates followed much the same pattern as C₂H₂ reduction in the 20 C control and transferred plants. At harvest, roots from 13 C-treated plants had the highest activities for malate dehydrogenase, glutamate oxaloacetate transaminase, and phosphoenolpyruvate carboxylase. Roots from transferred plants had intermediate activities and those from the 20 C treatment the lowest activities. Newly formed nodules from plants switched from 13 to 20 C had much higher glutamate dehydrogenase than glutamine synthetase activity.

Photosynthetic rates on a leaf area basis were about three times as high in the 20 C control as compared to 13 C control plants. Photosynthetic rates of plants switched from 20 to 13 C decreased to less than half the original rate within 2 days. Photosynthetic rates of plants switched from 13 to 20 C recovered to rates near those of the 20 C control plants within 2 weeks. All leaf enzymes assayed at harvest, with the exception of nitrate reductase, were highest in activity in the 20 C control plants.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, the National Soybean Crop Improvement Council, USDA-CSRS Grant 616-15-72, and the American Soybean Association Research Grant 75-ASARF-208-3.

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