Effect of Al Concentration in Culture Solution on the Growth and N₂-Fixation of Some Tropical Pasture Legumes

Four tropical pasture legumes and a temperate pasture legume were grown at graded levels of Al concentration in a culture solution in order to examine the effect of AI on their growth and N2-fixing activities. In addition two tropical pasture legumes dependent upon combined-N or symbiotically fixed-N were exposed to graded levels of AI concentration and data on their weights, N2-fixing activities, root length were collected. The results obtained were as follows. 1) The tolerance to Al in terms of the relative weight of the whole plant in the Al treatments to that of plants which did not receive Al was in the order of: Macroptilium atropurpureum ≑ Stylosanthes humilis ≑ Centrosema pubescens> Desmodium intortum> Trifolium repens. 2) The critical concentration of Al with respect to N2-fixing activities was about 20 ppm in some tropical pasture legumes except for M. atropurpureum, in which AI concentration up to 100 ppm in the culture solution had no adverse effect on N2-fixing activity. 3) In both M. atropurpureum and C. pubescens, the whole plant weight decreased more in the plants dependent upon symbioticaIly fixed-N than combined-N. 4) In M. atropurpureum, 20 ppm AI treatment decreased the total root length and increased the root thickness whereas in C. pubescens, above that level of AI, the total root length decreased and the root thickness increased to a smaller extent.