Carbon and Nitrogen Assimilation and Partitioning in Soybeans Exposed to Low Root Temperatures

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Abstract
Low root temperature effects on vegetative growth of soybean (Harosoy 63 × Rhizobium japonicum USDA 16) were examined in 35 day old plants exposed to temperatures of 15°C (shoots at 25°C) for an 11 day period. Duing this period various aspects of C and N assimilation and partitioning were monitored including shoot night and nodulated root respiration, C and N partitioning to six plant parts, C2H2 reduction, H2 evolution, leaf area, transpiration, net photosynthesis, and N2 fixation. The low temperature treatment resulted in a decrease in the net rate of N2 fixation but nitrogenase relative efficiency increased. In response, the plant retained N in the tissues of the nodulated root and decreased N partitioning to young shoot tissues, thereby inducing the remobilization of N from older leaves, and reducing leaf area development. The leaf area specific rate of net photosynthesis was not affected over the study period; however, shoot and nodulated root respiration declined. Consequently, C accumulated in mature leaves and stems, partly in the form of increased starch reserves. Three possibilities were considered for increasing low temperature tolerance in nodulated soybeans: (a) decrease in temperature optima for nitrogenase, (b) increased development of nodules and N2 fixation capacity at low temperature, and (c) alterations in the pattern of C and N partitioning in response to low temperature conditions.