Abstract
The relationship between dry matter production (W) and evapotranspiration (ET) was examined in the normal nodulating soybean cultivar Enrei, the supernodulating line Kanto 100, and the non-nodulating line ‘En1282’. In addition, using the normal nodulating soybean cultivar Fukuyutaka (F) and the non-nodulating line ‘T201’ (T) grafted soybeans, F/F (scion/stock) and F/T with (+) and without (-) flower removal and 2F/F were prepared. The W-ET relation was examined in F/F (+), F/F (-), F/T (-), F/T (-) and 2F/F (-). The difference in ET between nodulating and non-nodulating soybeans (ΔET) increased with the amount of symbiotic nitrogen (N) fixation in the plants. The difference in W between nodulating and non-nodulating lines (ΔW) corresponding to ΔET, which was calculated from ΔET (kg H2O)/transpiration coefficient (kg H2O g-1 dry weight) in non-nodulating soybeans, was closely correlated with the nodule weight (Ndw) and the amount of symbiotic N fixation. Although the supernodulating line Kanto 100 had a much larger Ndw/ΔW ratio than the normal nodulating soybean cultivars, no significant difference in N fixing cost was detected among the soybean cultivars (0.098±0.006 Ng g-1 dry matter). We concluded that ΔW between the nodulating and the non-nodulating soybean lines is a good indicator of the amount of symbiotic N fixation.
