Calcium and magnesium effects on symbiotic nitrogen fixation in the alfalfa (M. sativa) –Rhizobium meliloti system

An investigation of the roles of calcium and magnesium ions in symbiotic nitrogen fixation by legumes has shown that alfalfa plants (Medicago sativa L. cv. Saranac and Apollo) deficient in either cation were poorly nodulated and retarded in growth on nitrogen‐free media. This effect was reversed by supplementation with normal levels of these cations. After recovery, the calcium deficient seedlings showed continuing effects of early mineral deficiencies but recovered to 75% of the nitrogenase activity and had nearly the same yield as control plants. Magnesium deficient plants recovered nitrogenase activity to the same degree but grew to only about 50% of the weight of controls. Supplementation of non‐deficient seedlings grown on N‐free media with varying amounts of Ca2+ and Mg2+ resulted in the identification of an optimal ratio of calcium and magnesium near 2 when neither cation was growth limiting. A highly significant positive correlation was obtained between yield of dry matter and the fraction of total expressed nitrogenase activity that was actually available for dinitrogen reduction (nitrogen reducing equivalent). Bacteroids isolated from root nodules and freed of plant cytoplasmic components required high magnesium levels for maximal utilization of externally supplied ATP and dithionite. Ca+ was antagonistic to this activity but complemented Mg2+ in stimulating the respiration‐supported nitrogenase activity of intact bacteroids which had been treated with a chelating agent. The effects of calcium and magnesium on the nitrogenase system of intact bacteroids may be due to binding of the Ca2+ ions to the bacteroid membrane.