Uptake and Assimilation of Nitrate by Crop Plants and its Regulation

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The reduction of nitrate pool-size in plant tissues contributes greatly to the production of harmless vegetables and an increase of the utilization efficiency of fertilizer nitrogen. We proposed the practical technique for the reduction of nitrate pool-size in radish, chinese mustard and spinach, especially by using of cytokinin, B-vitamins and organic fertilizer.1. Nitrite and nitrate ions in water-extract obtained from each vegetable dry tissue could be distinctly separated by HPLC, at least within 10 min. However, no nitrite peak was found in all samples tested.2. Among the vegetable organs, the pool-size of nitrate was the biggest in leaf blade, followed by petiole and root. The ratio (%) of nitrate per total nitrogen was threefold higher in petiole and root than in leaf. blade. Therefore, the nitrate pool in the, former organs must be reduced.3. Kinetin decreased the content of nitrate and the ratio of nitrate per total nitrogen in the petiole of spinach, and this kinetin effect was marked in the case of foliar application. The soil kinetin-applied spinach showed relatively high values of nitrogen uptake and ^<15>N content. On the other hand, both the B-vitamins reduced the amount of nitrate nitrogen and the ratio of nitrate nitrogen per total nitrogen. A marked reductive effect was found in spinach treated with pantothenate (0.1 %). Pyridoxin decreased slightly the amounts of water-soluble and total oxalic acid.4. Organic fertilizer reduced the pool of nitrate in spinach tissue. This reduction was due to the delay of mineralization of organic matters.In conclusion, the author-wishes to emphasize that the stimulation of nitrate assimilation in vegetables by cytokinins derived from the natural plant resources or pantothenate is very important to achieving high utilization of fertilizer nitrogen, high yield and high quality of vegetable crops.