Abstract
Broad bean, hairy vetch, cowpea and groundnut were grown under low nitrogen (N) and high N fertilizer application conditions. The nodule weight of plants was heavier under low N than under high N. Evapotranspiration (ET) increased linearly with the increase in total dry weight (W), but the value of ET at the same W was larger under low N conditions. The difference in the W- ET relationship was characterized by the intercept (ET when W was zero, ETW=0) in broad bean, hairy vetch and cowpea and by the slope of the regression line (transpiration coefficient) in groundnut. Supply of N from symbiotic N fixation was detected under low N conditions, whereas the loss of N exceeded the N supplied from symbiotic N fixation under high N conditions except for hairy vetch. The difference in W between low and high N conditions (ΔW) corresponding to the difference in ET was correlated with the difference in the nitrogen balance. Taken together ΔW is a good indicator of the differences in symbiotic N fixation.
