The interaction of calcium and hydrogen ions in the nodulation of subterranean clover

Abstract

A study was made of the effects of calcium and hydrogen ions on the nodulation of subterranean clover (Trifolium subterraneum L.) in water culture. A compound interaction was found. At pH 4.0 or less no nodules were formed at any calcium concentration. At a 0.01mM calcium concentration no nodules were formed at any pH used. Above these critical values, almost maximum nodulation could be obtained by an increase in either calcium concentration or pH, so that each factor was to a large degree replaceable by the other. Calcium and hydrogen ions in the range of concentrations which produced these marked interactions on nodulation had no measurable effect on plant growth (nitrate was supplied in solution). The effects of calcium and hydrogen ions on the growth of the Rhizobium strain used were also markedly different from their effects on nodulation. Hydrogen ions had a dominating effect on Rhizobium growth, while calcium ions had no effect at any pH and were required at most in trace amounts. Hydrogen ions depressed calcium uptake by the plants. It is suggested that, in the range of concentrations where they were replaceable, the effects of calcium and hydrogen ions on nodulation were through their influence on the level of calcium in the plants. It is concluded that the calcium requirement for nodulation of subterranean clover is higher than for growth of the host plant or for growth of Rhizobium.