Critical Nitrate-N Concentration In Vegetables And Influencing Factors

Leafy vegetables are prone to accumulate nitrate N, and Nitrate N accumulated in vegetable can account 72-92% of the total nitrate taken up by human beings. Therefore nitrate accumulation and its regulation has always attracted serious attentions from scientists in different countries. Critical nitrate-N concentration (CNC) is the nitrate concentration in the plant when it just reaches its maximum growth. CNC reflect not only the nitrogen nutrition status in the plant, but also its potential ability of nitrate-N accumulation. Thereafter, N application based on CNC can meet with the nutrient requisition, and prevent nitrate N from over accumulating in the plant at the same time. However, research on determination methods of CNC, changes of CNC with the growing stages, and the relationships between CNC and plant growth and its construction, water metabolism, nitrate reductase activity (NRA) and nitrate distribution in metabolic and storage pool in the plant cell, have not been reported in China. So pot experiments with 3 different leafy vegetables and 30 different varieties of spinach as indicators were carried out to study these issues. The major results are as follows:1. Of the different organs and parts, the petiole is the optimum part for determining CNC in vegetables. The CNC of 3 vegetables had been determined and the results showed that the CNC of rape was the highest, then the Chinese cabbage, and that of spinach was the lowest. In addition, CNC decreased with the vegetable's growing to late stages. The further determination of CNC in 7 spinach varieties showed that CNC was different among varieties.2. CNC was closely related with plant growth and its construction. The CNC of the three different vegetables was negatively related with the ratios of their leaves blades to shoots. Spinach had the higher ratio and the lower CNC, but Chinese cabbage and rape' had the lower ratios, and their CNC were higher than that of spinach. Further research by different spinach varieties showed the increased degrees of the ratios with the increasing of N rates were also negatively correlated with their CNC.3. CNC cannot only reflect the nitrogen nutrition status in the vegetable, but also its critical water requirement and critical water use efficiency. The CNC of rape was higher than that of spinach, and so were its critical water requirement and critical water use efficiency. This indicated that the vegetable with higher CNC, its critical water requirement and critical water use efficiency were usually higher than others.4. No determined relationships between the CNC or nitrate concentrations and leafy nitrate reductase activities were found in either the three vegetables or 30 different spinach varieties. However, further determination showed that the ratios of in vivo NRA to in vitro NRA were negatively correlated with CNC between different spinach varieties. These results indicated that the higher expressing degrees of the potential NRA were one of the major reasons of lower nitrate accumulation in the vegetable.5. Compared to the Chinese cabbage at the same growing stage, spinach had the higher nitrate N concentration in metabolic pool and the higher ratio of nitrate N in metabolic pool to that in storage pool, and so did its leaf NRA, but its total Nitrate N accumulation and CNC were obviously lower. Furthermore, this kind of relationship was also found among different spinach varieties. Nitrate distribution in metabolic and storage pool in the plant cell was a key factor affecting nitrate N accumulation in vegetables.