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

The effect of sodium chloride (NaCl), sodium sulfate (Na 2 SO 4 ), 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 Na 2 SO 4 , 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 Na 2 SO 4 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 Mg 2+ application.