Iron‐deficiency specifically limits nodule development in peanut inoculated with Bradyrhizobium sp.

summary Severely iron‐deficient peanuts ( Arachis hypogaaea L.) grown on calcareous soils in central Thailand failed to nodulate until given foliar iron applications. Glasshouse experiments were conducted on two cultivars (Tainan 9 and Robut 33–1) to identify which stage of the nodule symbiosis was most sensitive to iron‐deficiency. Iron‐deficiency did not limit growth of soil or rhizosphere populations of peanut liradyrhizobium. Similar numbers of root nodule initials formed in the roots of both control and iron‐sprayed plants, showing that iron‐deficiency did not directly affect root infection and nodule initiation. Plants sprayed with iron produced greater numbers of excisable nodules and carried a greater nodule mass than untreated plants. Five days after iron application, nodules on sprayed plants of CV. Tainan 9 contained 200–fold higher bacteroid numbers per unit weight and 14–fold higher concentrations of leghaemoglobain. The onset of nitrogenase activity was also delayed by iron deficiency in both cultivars. Tainan 9 appeared more sensitive to iron‐deficiency than Robut 33‐1 in terms of nodule mass produced, but both cultivars showed the same effect of iron‐deficiency on nitrogenase activity per plant. It is concluded that the failure of the infecting rhizobia to obtain adequate amounts of iron from the plant results in arrested nodule development and a failure of nitrogen fixation.