Medicago sativa and Medicago murex differ in the nodulation response to soil acidity

Nodulation of Medicago sativa by Sinorhizobium meliloti is challenged by acidity, but the ability of M. murex to nodulate in acid soils provided the opportunity to compare the symbiotic development between the two species in an acid sandy soil. Soil acidity had different effects on the nodulation of Medicago spp. In soil of pH 4.3, M. murex produced fewer nodules than plants grown in soil of pH 7.0, but these nodules developed at a similar rate to those on plants grown in soil of neutral pH. The uppermost nodule on M. murex formed lower down the tap-root of plants grown in soil of pH 4.3. In identical soils, M. sativa produced fewer nodules when grown in the acidic soil and nodules appeared later compared to those on plants grown in soil of pH 7.0. However, the location of the uppermost nodule was the same in plants grown in soils of pH 4.3 and 7.0. It is suggested that M. murex formed the first nodules near the actively growing root tip, while M. sativa formed nodules later at the more mature region of the root, above the root tip.