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CROP PHYSIOLOGY & METABOLISM

Sensitivity of N₂ Fixation Traits in Soybean Cultivar Jackson to Manganese

USDA-ARS, Agronomy Dep., Agronomy Physiology Lab., IFAS Building 350, 2005 SW 23rd Street, Univ. of Florida, P.O. Box110965, Gainesville, FL 32611-0965 USA

* Corresponding author (trsincl{at}gnv.ifas.ufl.edu)

There are large increases in leaf ureides, allantoin, and allantoic acid upon water deficit in N₂ fixing soybean [Glycine max (L.) Merr.], which are likely to trigger a feedback inhibition of nodule activity. The degradation of ureides in the leaves appears to be a major factor associated with N₂ fixation tolerance to water deficit. Since one of the possible enzymes responsible for allantoic acid degradation depends on Mn as a cofactor, we investigated the possibility that the previously demonstrated N₂ fixation tolerance to water deficit of the cultivar Jackson may result from a superior ability to accumulate Mn. Indeed, Jackson was found in field and greenhouse experiments to have higher leaf Mn concentrations than other genotypes over a range of Mn availability. Acetylene reduction activity after treating Jackson plants grown on hydroponic solutions with ureide did not vary, however, with leaf Mn concentration. This was in contrast to a soybean line with N₂ fixation sensitive to water deficit in which acetylene reduction activity following ureide treatment was greater as the leaf Mn concentration increased. In addition, ureide degradation rates in Jackson leaf samples with differing Mn concentrations were insensitive to Mn concentration. These results indicate that ureide degradation likely did not involve Mn as a cofactor and that ureide degradation in Jackson is catalyzed by an enzyme not requiring Mn as a cofactor. It was concluded that the capacity to maintain a high leaf Mn concentration in Jackson under sufficient Mn availability was a trait with no causal relationship to N₂ fixation tolerance of water deficit.

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