Lotus tenuis tolerates the interactive effects of salinity and waterlogging by ‘excluding’ Na+ and Cl- from the xylem

Salinity and waterlogging interact to reduce growth of poorly adapted species by, amongst other processes, increasing the rate of Na+ and Cl- transport to shoots. Xylem concentrations of these ions were measured in sap collected using xylem-feeding spittlebugs (Philaenus spumarius) from Lotus tenuis and Lotus corniculatus in saline (NaCl) and anoxic (stagnant) treatments. In aerated NaCl solution (200?mM), L. corniculatus had 50% higher Cl- concentrations in the xylem and shoot compared with L. tenuis, whereas concentrations of Na+ and K+ did not differ between the species. In stagnant-plus-NaCl solution, xylem Cl- and Na+ concentrations of L. corniculatus increased to twice those of L. tenuis. These differences in xylem ion concentrations, which were not caused by variation in transpiration between the two species, contributed to lower net accumulation of Na+ and Cl- in shoots of L. tenuis, indicating that ion transport mechanisms in roots of L. tenuis were contributing to better ‘exclusion’ of Cl- and Na+ from shoots, compared with L. corniculatus. Root porosity was also higher in L. tenuis, due to constitutive aerenchyma, than in L. corniculatus, suggesting that enhanced root aeration contributed to the maintenance of Na+ and Cl- ‘exclusion’ in L. tenuis exposed to stagnant-plus-NaCl treatment. Lotus tenuis also had greater dry mass than L. corniculatus after 56?d in NaCl or stagnant-plus-NaCl treatment. Thus, Cl- ‘exclusion’ is a key trait contributing to salt tolerance of L. tenuis, and ‘exclusion’ of both Cl- and Na+ from the xylem enables L. tenuis to tolerate, better than L. corniculatus, the interactive stresses of salinity and waterlogging.