Carbon and nitrogen assimilation and partitioning in soybeans exposed to low root temperatures.

Low root temperature effects on vegetative growth of soybean (Harosoy 63 x Rhizobium japonicum USDA 16) were examined in 35 day old plants exposed to temperatures of 15 degrees C (shoots at 25 degrees 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, C(2)H(2) reduction, H(2) evolution, leaf area, transpiration, net photosynthesis, and N(2) fixation. The low temperature treatment resulted in a decrease in the net rate of N(2) 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 N(2) fixation capacity at low temperature, and (c) alterations in the pattern of C and N partitioning in response to low temperature conditions.