Leaf ureide degradation and N2 fixation tolerance to water deficit in soybean1

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Abstract
Accumulation of ureides in leaves is associated with the sensitivity of N2 fixation in soybean to soil water deficit. Consequently, ureide degradation in leaves may be a key to increasing soybean tolerance to dry soils. Previous research indicated that allantoic acid degradation is catalysed by different enzymes in cultivars Maple Arrow and Williams. The enzyme found in Williams requires manganese as a cofactor. The first objective of this study was to determine if the two degradation pathways were associated with differences in N2 sensitivity to soil water deficits. N2 fixation of Williams grown on low‐Mn soil was sensitive to stress, but it was relatively tolerant when grown on soil amended with Mn. N2 fixation in Maple Arrow was relatively tolerant of soil drying regardless of the Mn treatment. The second objective of this study was to expand the study of the degradation pathway to nine additional genotypes. Based on ureide degradation in the presence and absence of Mn, these genotypes also segregated for the two degradation pathways. Those genotypes with the Mn‐dependent pathway tended to have drought‐sensitive N2 fixation, but there was one exception. The genotypes not requiring Mn for ureide degradation were drought‐tolerant except for one genotype. These results demonstrated the possibility for increasing N2 fixation tolerance to soil water deficits in soybean by selection of lines with high ureide degradation rates, which were commonly associated with the Mn‐independent pathway.