Salt resistance in two cashew species is associated with accumulation of organic and inorganic solutes

This study establishes relationships between salt resistance and solute accumulation in roots and leaves of two contrasting cashew species. The sensitive (Anacardium microcarpum) and resistant (A. occidentale) species showed maximum root LD50 values (the external NaCl concentration required for a 50% reduction in dry weight) of 63 and 128 mM NaCl, whereas the shoot LD50 values were 90 and 132 mM, respectively. The salt sensitivity was directly associated with Na+ accumulation and especially with the Cl− content in leaves and to a minor extent in roots. The accumulation of saline ions was associated with higher net uptake rates by roots and transport rates from root to shoot in the sensitive cashew species. The K+/Na+ ratios were not associated with salt resistance either in roots or leaves. Proline and free amino acid concentrations were strongly increased by salinity, especially in the leaves of the resistant species. The soluble sugar concentrations were not influenced by NaCl treatments in leaves of both species. In contrast, the root soluble sugar content was significantly decreased by salinity in the sensitive species only. In conclusion, the higher salt sensitivity of A. microcarpum is associated to an inefficient salt exclusion system of the leaves, especially for Cl−. On the other hand, the resistant species displays higher concentrations of organic solutes especially a salt-induced accumulation of proline and free amino acids in leaves.