Despite the fact that most plants accumulate both sodium (Na⁺) and chloride (Cl^–) ions to high concentration in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na⁺ accumulation. There have also been some recent concerns about the ability of hydroponic systems to predict the responses of plants to salinity in soil. To address these two issues, an experiment was conducted to compare the responses to Na⁺ and to Cl^– separately in comparison with the response to NaCl in a soil-based system using two varieties of faba bean (Vicia faba), that differed in salinity tolerance. The variety Nura is a salt-sensitive variety that accumulates Na⁺ and Cl^– to high concentrations while the line 1487/7 is salt tolerant which accumulates lower concentrations of Na⁺ and Cl^–. Soils were prepared which were treated with Na⁺ or Cl^– by using a combination of different Na⁺ salts and Cl^– salts, respectively, or with NaCl. While this method produced Na⁺-dominant and Cl^–-dominant soils, it unavoidably led to changes in the availability of other anions and cations, but tissue analysis of the plants did not indicate any nutritional deficiencies or toxicities other than those targeted by the salt treatments. The growth, water use, ionic composition, photosynthesis, and chlorophyll fluorescence were measured. Both high Na⁺ and high Cl^– reduced growth of faba bean but plants were more sensitive to Cl^– than to Na⁺. The reductions in growth and photosynthesis were greater under NaCl stress and the effect was mainly additive. An important difference to previous hydroponic studies was that increasing the concentrations of NaCl in the soil increased the concentration of Cl^– more than the concentration of Na⁺. The data showed that salinity caused by high concentrations of NaCl can reduce growth by the accumulation of high concentrations of both Na⁺ and Cl^– simultaneously, but the effects of the two ions may differ. High Cl^– concentration reduces the photosynthetic capacity and quantum yield due to chlorophyll degradation which may result from a structural impact of high Cl^– concentration on PSII. High Na⁺ interferes with K⁺ and Ca²⁺ nutrition and disturbs efficient stomatal regulation which results in a depression of photosynthesis and growth. These results suggest that the importance of Cl^– toxicity as a cause of reductions in growth and yield under salinity stress may have been underestimated.

Soil salinity affects large areas of the world's cultivated land, causing significant reductions in crop yield. Despite the fact that most plants accumulate both sodium (Na⁺) and chloride (Cl^–) ions in high concentrations in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na⁺ accumulation. It has previously been suggested that Cl^– toxicity may also be an important cause of growth reduction in barley plants. Here, the extent to which specific ion toxicities of Na⁺ and Cl^– reduce the growth of barley grown in saline soils is shown under varying salinity treatments using four barley genotypes differing in their salt tolerance in solution and soil-based systems. High Na⁺, Cl^–, and NaCl separately reduced the growth of barley, however, the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na⁺ and Cl^– stress. The results demonstrated that Na⁺ and Cl^– exclusion among barley genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. High concentrations of Na⁺ reduced K⁺ and Ca²⁺ uptake and reduced photosynthesis mainly by reducing stomatal conductance. By comparison, high Cl^– concentration reduced photosynthetic capacity due to non-stomatal effects: there was chlorophyll degradation, and a reduction in the actual quantum yield of PSII electron transport which was associated with both photochemical quenching and the efficiency of excitation energy capture. The results also showed that there are fundamental differences in salinity responses between soil and solution culture, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.