| Spinach (Spinacia oleracea L.) is one of the vegetables prone to high accumulation of nitrate that is believed to give rise to harmful effect on human health and therefore causes much public concern. Much work has been done and a wide range of techniques have been developed and put in use to reduce nitrate concentration in the edible part of vegetables. Proper nutrient management is proven to be an effective and practical way to control nitrate level in plants. Therefore, hydroponical culture and soil pot experiments were conducted in greenhouse to investigate the effects of nitrogen and potassium levels and nitrogen forms on the growth and nitrate accumulation in a hybrid of spinach, New Century. The influence of dicyandiamide (DCD) and K application on the dynamics of NOs" and NH4+ in soil and its subsequent effect on nitrate accumulation in spinach were also examined. The results are summarized as follows:1. Biomass accumulation, activity of nitrate reductase, and crude protein and Vc contents in spinach increase with increasing N and K supply levels. N remarkably increases nitrate content both in leaves and in petioles, whereas K shows a different pattern of the effect on nitrate accumulation. Great increase in nitrate concentration was observed as K in cultural solution was enhanced from O.OOlmmol/L to 4mmole/L, but further addition of K (at 12mmol/L) significantly reduced nitrate content in comparison with K at 4mmol/L, suggesting the nitrate assimilation rate superior to the uptake rate at high K supply level. K also considerably depresses nitrite content. Results from this experiment shows that optimal N and K levels and ratio, 8mmol/L N and 4mmol/L K in this case, are essential, from a comprehensive view, for spinach to achieve high yield, crude protein and Vc content, and low nitrate and nitrite content.2. Hydroponical culture experiment demonstrates that the biomass, nitrate and nitrite content, crude protein content, and activity of nitrate reductase in spinach declines with the decreasing ratio of nitrate to ammonium, while Vc content is increased. The variations of biomass, the content of nitrate and the activity of nitrate reductase with nitrate/ammonium ratios have a similar pattern that is not affected by K levels. However, the content of nitrate<;sis not proportionally reduced with the decreasing ratio NOs" to NH4+; the greatest reduction of nitrate is shown when the ratio varies from 30/70 to 0/100. A positive correlation between the content of nitrate and the activity of nitrate reductase in spinach was observed. When the biomass accumulation, nitrate content and the quality indicated by Vc and crude protein are taken into account, the optimal ratio of NH/ to NC?~ is recommended to be 50/50 and 70/30 with K level at 4mmol/L.3. The treatments being deprived of N within seven days before harvest does not differ statistically from the treatment with continuous N supply in terms of biomass production of spinach. The contents of nitrate, nitrite, and crude protein decrease with the time being deprived of N in cultural solution before harvest, while Vc shows an increasing pattern. Most of the reduction of nitrate occurs in five days of N depriving before harvest. The content of nitrate in the petiole is negatively correlated with the days of N depriving before harvest. In consideration of biomass production, the content of nitrate, and other quality indices, depriving of N in the nutrient solution for five days before harvest is strongly suggested to minimize nitrate content in spinach.4. High concentration of NH4+-N with a corresponding low nitrate in soil added with DCD shows the strong inhibition of nitrification brought about by DCD. DCD does not show notable effect on growth of spinach but results in a substantial reduction in nitrate and nitrite contents, and a significant increase in Vc content in spinach. Our results suggests that application N with 10% DCD is an effective and practical approach to the problem of nitrate accumulation... |
