Nodulation ofMedicago sativa in solution culture

Summary Calcium and hydrogen ions interacted on nodulation. Increasing acidity from pH 5.6 to pH 4.8 increased the calcium concentration required to nodulate 50% of the plants, from 0.1 mM to 6 mM. Calcium concentration below 0.2 mM or pH below 4.8 inhibited nodulation at all tested levels of the other variable. Root extension and root-hair production were insufficiently affected by calcium or pH to explain reductions in nodule numbers. Initiation of infection, the most acid-sensitive stage of the nodulation process, was also the most calcium-demanding stage at pH 5.2. Once infections were initiated, infection threads still developed and nodules still grew despite transfer of the plants to solutions too low in calcium to have permitted infection to begin. Pretreatments at 0.5 mM and 8 mM calcium at pH 5.2 before inoculation had no significantly different effects on nodulation. Observations on root-hair distribution suggest that developing nodules can suppress further infection by suppressing the emergence of root hairs on newly developing roots.