Summary
Acidity reduced nodule numbers at pH below 5.5, and virtually prevented nodulation at pH 4.5. In this range (pH 5.5 to 4.5) it did not affect root growth or the number of root hairs.
In lightly inoculated solution cultures, acidity inhibited the extensive multiplication of Rhizobium which was necessary to establish a rhizosphere population sufficiently large to induce nodulation.
In heavily inoculated solutions, containing 108 to 109 Rhizobium cells per litre, Rhizobium rapidly accumulated in the rhizosphere by a pH-independent process which seemed to be largely mechanical and to involve little multiplication. Thus large rhizosphere populations accumulated within a few hours even at pH 4.4. Nevertheless, acidity still prevented root hairs from curling and becoming infected. Raising the pH from 4.4 to 5.4 at this stage allowed curling and subsequent steps to proceed without delay.
Shortly after curling was completed, the pH could be lowered to 4.4 without hindering the development of infection and the normal completion of nodulation. Hence the prevention of nodulation by acidity, in heavily inoculated solution cultures, can be attributed to prevention of a step which approximately coincides with the curling of root hairs. This step occupied less than 12 hours of the 4 to 7 days required for visible nodules to appear.
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Munns, D.N. Nodulation ofMedicago sativa in solution culture. Plant Soil 28, 129–146 (1968). https://doi.org/10.1007/BF01349181
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DOI: https://doi.org/10.1007/BF01349181