Nodulation and growth of nodules in the common bean (Phaseolus vulgaris) under salt stress.

In order to determine the respective parts of the nodulation and the growth of nodules in the decrease of the nodule dry matter observed in common beans exposed to salinity, the following experiment was carried out. After germination, plants inoculated with an efficient strain of Rhizobium CIAT899 were grown in salt free medium or one containing 25 or 50 mM NaCl. In another treatment, plants were submitted later (after nodulation, 28 DAS) to 25 mM NaCl. Salt decreases the different parameters of growth. However, its effects were more pronounced in plants with early exposure to salt. The nodule DW and, to a lesser degree, growth of roots were the most affected. The mean number of nodules per plant was systematically less affected by salt than nodule DW. Also, the number of nodules per unit of root DW was generally the same or even larger in plants submitted to salt than controls. These results showed that the decrease of the nodule DW in the presence of salt is owed to an inhibition of the infection resulting more from a limitation of root growth than from a low predisposition of these organs to the infection and to a reduction of the nodule growth resulting from a restriction of the feeding of these organs with photosynthetates. This last parameter seems especially sensitive to salt.