Effect of NaCl on nitrate reduction in first leaves of barley

One-week-old barley seedlings grown on complete nutrient solution with 12 mM NO3- were used. Preincubation of the first leaf segments in 30 mM phosphate buffer with NaCl (50 mM) was carried out in vivo under light. This induced a dramatic decrease in subsequent dark nitrate reduction and resulted in a diminution of nitrite yield in the same solution during the subsequent dark incubation. The extent of the decrease in nitrite was correlated with the duration of light exposure. Addition of nitrate (up to 100 mM) to the incubation medium in order to increase and prolong nitrite production had no beneficial effect when NaCl was present in the same solution. Experiments were carried out with wild-type and NR-mutant plants characterized by the absence of NADH-dependent nitrate reductase activity due to a mutation inducing molybdenum cofactor deficiency. At sodium concentrations in the leaves within the range of 100-200 mM on a fresh weight basis, nitrate assimilation was blocked, although nitrate reductase activity was still present. These results can be explained by the influence of Na+ on the cell membrane. Cessation of nitrate reduction under such conditions was most probably due to the restriction of nitrate flow into the substrate pool caused by sodium ions. The severity of salt stress in plants can be predicted by evaluating the substrate pool of nitrate with an in vivo nitrate reductase test similar to that employed by the authors for diagnostics of nitrate nutrition in the held. A model for nitrate reduction and nitrate pool sequestration is proposed.