The effect of solution pH and external calcium concentration on the early growth and nodulation of several annual Medicago species

Abstract

The study tested the hypothesis that annual medic species which nodulate well in acid soils in the field (M. murex Willd. and M. polymorpha L.) will nodulate better in acid solutions with low calcium concentrations than annual medics which nodulate poorly in acid soils (M, truncatula Gaertn.). Effects of pH (5.5 and 6.5) and calcium concentration (0.2, 0.5, 1 and 2 mM) on the early growth and nodulation of three annual medic species (M. truncatula, M. polymorpha and M. murex) were investigated. Increasing pH or calcium concentration did not increase plant growth for any of the species. However, nodulation was generally depressed by low pH for all species. Increasing calcium concentration in solution increased nodulation in all species. Effects of low pH and low calcium concentration in decreasing nodule number were much greater for M. truncatula than for M. polymorpha and M. murex. At pH 5.5, M. truncatula failed to nodulate at any calcium concentration, whereas a large proportion of M. murex plants nodulated at 1 mM calcium and some M. polymorpha plants nodulated at 2 mM calcium. At pH 6.5, M. polymorpha required 1 mM calcium in solution for maximum nodule number, and M. murex only 0.5 mM calcium, whereas nodule number for M. truncatula increased up to 2 mM calcium, the highest concentration used. The results provide the basis for a simple screening system to distinguish differences among annual medics in nodulation tolerance to acidity. The maintenance of ranking among species with respect to nodulation over a wide range of stresses induced by combinations of low pH and calcium concentration suggest that screening using a single stress combining these two components would be adequate. Nodulation differences between species can be simply and effectively assessed using a scoring system combining number size and location of nodules.