Journal of Experimental Botany, Vol. 51, No. 344, pp. 559-566,
March 2000
© 2000 Oxford University Press
Weather and nodule mediated variations in
13C and
15N values in field-grown soybean (Glycine max L.) with special interest in the analyses of xylem fluids
1 Plant Nutrition Diagnosis Laboratory, National Agriculture Research Center, Kannondai 3-1-1, Tsukuba, Ibaraki 305-8666, Japan
2 Institute of Applied Biochemistry, University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305-8572 Japan
The nodulating soybean (Enrei) and its non-nodulating mutant (EN 1282) were grown in outdoor plots for 2 years (1994: extraordinary dry, high temperature, 1995: ordinary year). Carbon and nitrogen accumulation,
13C and
15N values in plant parts and xylem fluids and
15N values in the water-extractable soil N were analysed throughout the growing period. Plant growth in 1994 was rapid during the early growth stages, but no pods were produced. In 1995, plant growth was normal and pods were formed. The
13C values of the leaves were less negative in 1994 than in 1995 and the nodulated plants showed less negative
13C values than non-nodulated plants in both years. The
13C values of the leaves during the vegetative phase were positively correlated to the leaf N concentrations. Leaf N concentrations in their turn were influenced by nodulation and weather conditions and/or soil available N. The
15N values in the plants and xylem fluids were lower in the nodulated soybean than in non-nodulated soybean in both years, and estimates of the contribution of N2 fixation in nodulated plants based on plant top
15N values were 714% in 1994 and 3763% in 1995. The
13C values of xylem fluids did not differ between nodulated and non-nodulated plants. Thus, the expected contribution by phosphopenolpyruvate carboxylase-mediated CO2 fixation in the root nodules to plant C-incorporation could not have been significant.
Key words:
13C,
15N, nodule CO2 fixation, N2 fixation, soybean.