The Effect of Nitrate Levels and Harvest Times on Fe, Zn, Cu, and K, Concentrations and Nitrate Reductase Activity in Lettuce and Spinach

Leafy vegetables are considered as the main sources of nitrate in the human diet. In order to investigate the effect of nitrate levels and harvest times on nitrate accumulation، nitrate reductase activity، concentrations of Fe، Zn، Cu and K in Lettuce and Spinach and their relation to nitrate accumulation in these leafy vegetables، two harvest times (29 and 46 days after transplanting)، two vegetable species of lettuce and spinach and two concentrations of nitrate (10 and 20 mM) were used in a hydroponics greenhouse experiment with a completely randomized design and 3 replications. Modified Hoagland and Arnon nutrient solutions were used for the experiment. The results indicated that by increasing nitrate concentration of solution، nitrate accumulation in roots and shoots of lettuce and spinach increased significantly (P ≤ 0. 05)، and the same trend was observed for the nitrate reductase activity in the shoots of the two species. Increasing the nitrate concentrations of solution، reduced the shoot dry weight and the concentration of Fe and Cu in both species، where as it increased the K and Zn concentrations in the shoots of the two species in each both harvest times، the nitrate accumulation increased، but the nitrate reductase activity decreased in the shoots of the two species over the course of the growth. The Concentration of Fe، Cu and K decreased in the shoots of lettuce and the spinach with the time، despite the increase in Zn concentration in the shoots. The results also indicated that increasing nitrate concentrations of solution to the levels greater than the plant capacity for reduction and net uptake of nitrate، leads to the nitrate accumulation in the plants. Nitrate accumulation in plant tissue led to decreases in fresh shoot yield and Fe and Cu concentrations and nitrate reductase activities in both lettuce and spinach.