Differential tolerance to manganese in introduced and bred lines of Macroptilium atropurpureum

Abstract

A number of lines of Macroptilium atropurpureum, including cv. Siratro, were grown in two experiments at different manganese levels in sand culture. In one experiment 40 ppm manganese was used in conjunction with nitrogen either in inorganic form or as Rhizobium; in the other, two levels of excess manganese (30 and 45 ppm) were used with rhizobial nitrogen at a lower pH. Dry matter yields for tops, roots and nodules were recorded, and the plant material analysed for nitrogen, phosphorus, calcium and manganese. Dry matter yields of tops were reduced, and the ratio of root to total dry matter yield increased, by both Rhizobium and excess manganese. Tops, but not roots or nodules, showed considerable differences in manganese tolerance between lines. Excess manganese caused small but significant changes in concentrations of other minerals. Regressions of rhizobial on inorganic nitrogen showed that at the levels used excess manganese does not impair rhizobial efficiency. Numerical analyses suggested that manganese tolerance operates at three levels; more vigorous lines usually remain so in the presence of manganese, some lines appear to be able to resist the uptake of excess manganese, and for any given level of manganese concentration within the plant, some lines appear more sensitive than others. The results show that growth evaluation studies can be carried out equally well with rhizobial or inorganic nitrogen sources, and they suggest that it should be possible to improve the tolerance of M. atropurpureum lines by selection and/or breeding.