Cation and phosphate influences on the nodulating characteristics of indigenous serogroups of Rhizobium trifolii on soil grown Trifolium subterraneum L.

Abstract

The growth and nodulation responses of subclover ( Trifolium subterraneum L.) cultivar Mt Barker to different sources of phosphate [Ca(H 2PO 4) 2, Mg(H 2PO 4) 2 and KH 2PO 4] were evaluated in an acidic soil (pH 4.7 in 10 m m CaCl 2), low in extractable P (1–5 mg P kg − soil), in which two identifiable serogroups, 6 and 36, of Rhizobium trifolii were known to be indigenous. All P sources significantly increased ( P = 0.01) shoot dry weight of the plants. Although the P treatments had no influence on nodule number, or nodule mass, KH 2PO 4 increased significantly ( P = 0.05) the proportion of nodules occupied by serogroup 6, whereas the other two P sources had no effect. Nodule occupants of serogroup 36 were found mostly (87–97%) in bacteroid form regardless of treatment. No soil treatment resulted in the proportion of bacteroid dominated nodules of serogroup 6 reaching the levels achieved by serogroup 36. The morphology of serogroup 6 nodule occupants was influenced markedly by soil treatment. In all P sources the proportions of nodules occupied by serogroup 6 predominantly in bacteroid form were marginally in the majority (57–68%). In contrast, KCl was the only non-phosphate treatment where the proportion of nodules dominated by bacteroid forms of serogroup 6 was in the majority (77%). Immunofluorescence counts showed that the soil population of serogroup 36 outnumbered that of serogroup 6 by 4–11-fold either before planting, or at harvest and regardless of treatment or outcome of nodulation. Differences between the proportions of the soil populations of the two serogroups retained on, or passing through 0.4 μm pore size polycarbonate membrane filters were observed. Larger proportions of the preplant ( x = 48.9% ) or post-harvest ( x = 39.6% ) populations of serogroup 36 than of serogroup 6 ( x = 14.1 and 21.9%) passed through 0.4 μm pore size filters and were subsequently retained on 0.2 μm pore size filters. The ratios of 36 to 6 retained on the 0.4 μm filters ranged from 1.9:1 to 4.8:1 compared with 12.1:1 to 48.3:1 for the 0.2 μm filter-retained cells.