Abstract: Studies on the distribution and remobilization of nitrate in vegetable plants are important theoretical bases for decreasing nitrate content in vegetables and increasing nitrate use efficiency by plants. Hydroponic experiments and plant electro-physiological methods were used to determine tissue nitrate content and vacuolar nitrate activity of different organs of two pakchoi cultivars, which were significantly different in nitrate accumulation. Results obtained were as follows: Vacuolar nitrate activities of leaf blades, petioles and roots were 4.6, 2.0 and 7.4 times of tissue nitrate content respectively, inferring that nitrate activity in vacuoles was much higher than in cytosol. Nitrate contents in leaf blades, petioles and roots of Liangbaiye 1 were 2.4, 1.9 and 3.6 times of those of Shanghaiqing. Tissue nitrate contents in different organs of pakchoi were in the decreasing order of petiole > leaf blade > root, but no significant difference was found among these three organs. After pakchoi plants were transferred to the solution without nitrate, both the tissue nitrate contents and vacuolar nitrate activity significantly decreased, while cytosolic nitrate activity and NRA in leaf blades maintained relatively unchanged. Nitrate was predominantly stored in vacuole and could be reused by plants. Tissue nitrate stored in roots and petioles could be remobilized by leaf blades, and vacuolar nitrate could be released into cytosol to maintain a steady cytosolic nitrate concentration, and thus the nitrate reductase activity was kept in normal level. Liangbaiye l, a high nitrate accumulating cultivar, had a high vacuolar nitrate activity, and its tissue nitrate contents decreased more slowly than Shanghaiqing after transferring to the solution without nitrogen, which probably was caused by its low NRA.

Key words: Pakchoi (Brassica chinensis L.), Vacuole, Cytosol, Nitrate activity