ION DISTRIBUTION ACROSS THE CORTEX OF SOYBEAN NODULES - POSSIBLE INVOLVEMENT IN CONTROL OF OXYGEN DIFFUSION

X-ray microanalysis was used to study ion distribution in nodules of s oybean [Glycine max (L.)] cv. Clarke formed with two strains of Bradyr hizobium japonicum; RCR3407 and RCR3442. Previous studies have shown t hat the oxygen diffusion barrier in nodules of the RCR3442 symbiosis h as slower responses to acetylene and increased external oxygen than th ose of the RCR3407 symbiosis and this variation is correlated with dif ferences in glycoprotein content. X-ray maps and cross-cortical cell c ounts of nodules formed by either strain show very similar zonal distr ibutions of Mg, K, S and Ca across the cortex. Levels of K appear to b e similar but levels of Mg, S and Ca seem to be lower in nodules of th e RCR3442 symbiosis. These results suggest that the content and distri bution of Mg, S and Ca reflect an involvement in the operation of the cortical diffusion barrier.