Alteration in Intra-plant Distribution of δ¹⁵N in Response to Shading in Legumes

Abstract

The intra-plant distribution of ¹⁵N in common bean, cowpea and soybean having different levels of responses to shading and N₂-fi xing 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 signifi cantly in cowpea and soybean. The difference in δ¹⁵N 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 signifi cant negative correlation was found between Δδ ¹⁵N_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 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.

Key words:

• Common bean
• Cowpea
• δ ¹⁵N
• Natural ¹⁵N abundance method
• Nitrogen fixation
• %Ndfa
• Soybean