High concentrations of Na+ and Cl- ions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress

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.