Is salt stress of faba bean (Vicia faba) caused by Na+ or Cl- toxicity?

The effect of sodium chloride (NaCl), sodium sulfate (Na2SO4), and potassium chloride (KCl) on growth and ion concentrations of faba bean (Vicia faba L. cv. Troy) was studied. After 14 or 15 d of isoosmotic treatment with 100 mM NaCl or 75 mM Na2SO4, respectively, plants developed toxicity symptoms. These symptoms were characterized by local and nonchlorotic wilting spots, which later turned to black, necrotic spots. In contrast to NaCl or Na2SO4 treatment, plants treated with 100 mM KCl did not show these symptoms. The symptoms occurred on those leaves that accumulated highest concentrations of Na+ and showed highest Na+ : K+ ratios. Our results indicate that Na+ toxicity inducing K+ deficiency is responsible for the spot necrosis of faba bean. Additionally, chlorotic symptoms occurred. The concentrations of Na+ and Cl- were determined in chlorotic leaves and in isolated chloroplasts. The reduction of chlorophyll in leaves after NaCl exposure may be explained in terms of high Cl- concentrations in the chloroplasts and appears to depend on high Na+ concentrations. Chlorotic toxicity symptoms can be avoided by additional Mg2+ application.