Host plant specificity of a native variety of Stylosanthes gracilis

Abstract

In three greenhouse experiments nodulation difficulties of the variety IRI-1022 of Stylosanthes gracilis (H.B.K.) were studied. This variety is one of the most promising in the winter-dry regions of Central Brazil. In the first experiment six Rhizobium strains were compared on three varieties of S. gracilis. The varieties FAO 13381 and Deodoro II showed effective symbiosis with two local strains and with the Australian CB-756, but variety IRI-1022 showed poor N-fixation with the two local strains and did not form one nodule with strain CB-756. In a second experiment 24 strains of the "cowpea" group isolated from 8 different species were tested on the variety IRI-1022. Only the three homologous strains and one strain from Centrosema pubescens and one from Calopogonium mucunoides were able to produce nodules on this variety but none was highly effective. In the third experiment six homologous local strains were compared with soil inoculant obtained in five soil series. Two soils did not produce nodules but the three other soils produced more effective symbiosis than the pure strains. Nodule efficiency was evaluated by the regression of total plant nitrogen on nodule weight. In S. gracilis it varied with variety and with the inoculant. With pure strains nodule efficiency was four times higher with the variety Deodoro II than with the variety IRI-1022. Nodule efficiency in variety IRI-1022 was three times greater with soil inoculants.