INFLUENCE OF NITROGEN RATES ON NITRATE ACCUMULATION IN VEGETABLES

Pot experiments with rape, chinese cabbbage and spinach to study the influence of nitrogen rates on nitrate accumulation in the vegetables were carrled out in greenhouse. Soil sampled from field of wheat. The results showed that application of certain amounts of nitrogen fertilizer increased these vegetable yields by 1.1 to 6.1 folds, but high nitrogen rates inhibited their growth. In contrast, nitrate contents in vegetables increased continuously with the increase of N rates, and significant positive correlation were found between the nitrogen rates and nitrate contents (r ranging from 0. 933 to 0. 957). The nitrate contents had significant difference with organs of vegetables. Without nitrogen fertilization, the nitrate contents were higher in roots than in shoot and higher in stems than in leaves. Application of nitrogen the nitrate contents were lower in the roots than in shoot and lower in stems than in leaves. However, the nitrate contents were always higher in petioles than in blades at any case. Comparing the vegetable growth, nitrate absorption and reduction found that nitrate reductase activities increased at high nitrogen level and nitrate reduction was less than absorption, so nitrate accumulation occured in the vegetables. Further, nitrate accumulation in plants increased more rapidly than hiomass growth with the increase of nitrogen rates. This might be the major cause for the rapid increase of nitrate contents in vegetables.