Effect of pH on nitrogenase activity and mineral composition of Medicago truncatula cv. Jemalong and M. polymorpha cv. Serena inoculated with Rhizobium meliloti

Abstract

The further nodulation, nodule development and nitrogen fixation by nodulated Medicago polymorpha and M. truncatula plants transferred from pH 7.0 to three lower pH conditions (4.5, 5.0 and 5.5) in hydroponic culture was examined. Under these conditions, further nodule initiation by strains CC169 and WSM540 was severely retarded. Nodule development (mg nodule dry weight plant^-1) was similar on plants at pH 5.0 and 5.5 to that on plants kept at pH 7.0, but at pH 4.5, it was less. Under the lower pH conditions, the nodules were 2–3 times the mass of those found at pH 7.0. Nitrogen fixation, expressed as µg N₂ mg^-1 (nodule dry weight) d^-1, was largely unaffected by pH, and the apparently lower levels of nitrogen fixation at pH 4.5 could be attributed to an effect on nodule development. The determination of nitrogenase activity by acetylene reduction assay showed a strong influence of pH on the observed rates (per plant, or on a mg nodule dry weight basis). No explanation can be offered for this anomoly, but it does indicate the need for caution in applying the assay in this type of experimentation. The four host x strain combinations showed a similar response to the lower pH conditions, despite the fact that M. polymorha fixed higher levels of nitrogen, at a higher rate per mg nodule dry weight, than M. truncatula.