Alteration in intra-plant distribution of δ¹⁵N in response to shading in legumes

The intra-plant distribution of ¹⁵N in common bean, cowpea and soybean having different levels of responses to shading and N₂-fixing ability were analyzed under shaded and non-shaded conditions. Maize was used as a reference (non N₂-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 N₂-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 δ¹⁵N between shoot and root (Δδ¹⁵N_s-r) was the highest in maize followed by common bean, cowpea and soybean. Shading increased Δδ¹⁵N_s-r in each legume species, particularly in cowpea and soybean. A significant negative correlation was found between Δδ¹⁵N_s-r and %Ndfa in all legumes at both sampling dates (R² = 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 Δδ¹⁵N_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 Δδ¹⁵N_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.