Effect of zinc and harvest times on decrease nitrate accumulation and nitrate reductase enzyme activity in lettuce and spinach grown hydroponically

Leafy vegetables play a crucial role in human dietary. Excessive application of nitrogen fertilizer leads to accumulation of high levels of nitrate in the tissues and causes further consequence. In order to evaluate the impact of zinc and harvest times on the reduction of nitrate accumulation and nitrate reductase activity، an experiment was conducted as completely randomized design with factorial arrangement and three replicates in Hoagland and Arnon nutrient solutions. Treatments were two levels of zinc (7 and 50 µg. L-1)، two harvest times (29 and 46 days) and two plants of lettuce and spinach. The results indicated that with increasing zinc concentration، nitrate concentration in lettuce shoots in both times decreased and nitrate concentration in roots decreased during the second harvest، whereas no reduction was found in roots and shoots of spinach plants. Nitrate reductase activity was increased in lettuce and spinach plants with application of high zinc levels. Reduced enzyme activity، and increased nitrate accumulation was observed over time in both experimental plants. The results of this study indicated that nitrate metabolism in plants affected by activity of nitrate reductase enzyme، the species of plant، harvest time and plant nutrients such as the zinc.