Effect of elevated calcium concentration in infected cells of soybean (Glycine max (L.) Merr.) nodules on nitrogenase activity and N input to the plant
1998
Streeter, J.G.
Recent evidence from other workers indicates that NH4+ transport through the symbiosome membrane of soybean (Glycine max (L.) Merr.) nodules may occur via an ion channel; the transport channel was reported to be inhibited by very low concentrations of Ca. Two approaches were used to assess the Ca-sensitivity of N input from nodules; first, by providing up to 12-fold excess Ca in nutrient solutions. Second, strains of Bradyrhizobium japonicum that synthesize or do not synthesize an anionic polysaccharide in the peribacteroid space were compared; the rationale here was that the polysaccharide may bind Ca in quantities sufficient to avoid Ca inhibition of N transport. After 7-9 weeks of plant growth under the nutrient treatments, plant growth, mineral content, and N variables were measured; results from two similar experiments are reported. The concentration of Ca (and other elements) was estimated in situ in infected cells of nodules by using Energy Dispersive X-ray Analysis. The 12-fold increase in Ca supply led to increases in [Ca] in the infected cells of 2- to 3-fold. Increases in [Ca] were correlated with a decline in [K] in nodules containing polysaccharide. Elevation of [Ca] in infected cells depressed N content of shoots by 17-30% depending on the rhizobial strain and experiment. Effects of Ca on acetylene reduction activity was not always statistically significant, but the estimates of nitrogenase activity were consistent with the results for N content of shoots. The overall results support the idea that N transport from bacteroids is sensitive to Ca concentration in the peribacteroid space. There was no significant difference between bacterial strains, indicating that the polysaccharide deposited in the peribacteroid space does not enhance N transport by binding Ca when Ca concentration is high.
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