The intra-plant distribution of
15N in common bean, cowpea and soybean having different levels of responses to shading and N
2-fixing ability were analyzed under shaded and non-shaded conditions. Maize was used as a reference (non N
2-fixing) plant. Seedlings were grown in pot soils for 3 weeks then transferred to shaded (55% of control) and non-shaded (control) conditions in a greenhouse, and sampled at 13 days and 24 days after shading. The proportion of plant N derived from N
2-fixation (%Ndfa) estimated by the natural
15N abundance method was higher in cowpea and soybean (74−91%) than in common bean (37−38%). Shade treatment reduced %Ndfa significantly in cowpea and soybean. The difference in δ
15N between shoot and root (Δδ
15N
s-r) was the highest in maize followed by common bean, cowpea and soybean. Shading increased Δδ
15N
s-r in each legume species, particularly in cowpea and soybean. A significant negative correlation was found between Δδ
15N
s-r and %Ndfa in all legumes at both sampling dates (R
2 = 0.67−0.96, P<0.1). The slope and Y-intercept of the regression line was similar at the sampling dates, but varied with the species. The slope was −0.05 in cowpea, −0.06 in common bean, and −0.11 in soybean. The Δδ
15N
s-r value estimated by extrapolation of the regression line was 2.9, 2.5 and 8.6‰ at 0 %Ndfa, and −3.2, −2.8 and −2.6‰ at 100 %Ndfa, in common bean, cowpea and soybean, respectively. The consistent relationships between Δδ
15N
s-r and %Ndfa found among legume species suggest that Δδ
15N
s-r could be used as a parameter for estimating %Ndfa without using a reference plant, although the component of regression line was characteristically different among legume species.
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