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Soybean Response to Nodulation by Wild-Type and an Isogenic Bradyrhizobium elkanii Mutant Lacking Rhizobitoxine Production

Delaware Agric. Exp. Stn, Dep. of Plant and Soil Sciences, College of Agric. Sci., Univ. of Delaware, Newark, DE 19717-1303

^* Corresponding author (kun{at}udel.edu).

Soybean [Glycine max (L.) Merr.] cultivars differ in their susceptibility to rhizobitoxine-producing (RT⁺) bradyrhizobia, but the physiological basis for this variation is uncertain. We conducted greenhouse experiments to determine the response of six cultivars, that differ in their RT susceptibilities, to nodulation by four strains of Bradyrhizobium: USDA 61 and 94 (RT⁺ wild-type B. elkanii strains), USDA 110 (RT wild-type B. japonicum strain), and RX 18E (a Tn5-induced mutant of USDA 61 lacking RT production in planta). At 45 d after planting, plants were rated for RT symptoms and analyzed for shoot N contents and shoot, root, and nodule biomass and RT concentrations. Foliar chlorosis was generally better correlated with shoot RT concentrations than with nodular concentrations. Strain USDA 94 generally produced more RT in planta than did USDA 61. When nodulated by USDA 61, cultivars rated as RT-resistant often had lower nodular RT concentrations than did RT-susceptible plants. Relative to RX 18E, nodulation by USDA 61 significantly reduced plant shoot weight for the RT-sensitive cultivar Hutcheson and tended to reduce shoot weight for all remaining cultivars except the highly resistant SS FFR 565; decreased shoot N content by 50.5 and 17.4% in cultivar Hutcheson and Williams, respectively; and had no significant effect on nodule number and nodule weight. These results indicate that mechanisms of RT resistance may differ among the cultivar-strain combinations examined and involve differences in RT production, RT export from the nodules, and RT detoxification during and following transport to the shoot.

Received for publication August 31, 1995.

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