营养液中添加氯化钠降低硫浓度对韭菜硝酸盐累积的减控效应

  【目的】  韭菜易于吸收和累积硝酸盐，研究营养液中添加氯化钠 (NaCl) 和降低硫含量减少韭菜硝酸盐累积的效果，并从氮代谢途径初步探讨其减少硝酸盐累积的机理。  【方法】  采用新型韭菜专用营养液架床栽培系统进行了韭菜水培试验。在营养液中添加NaCl 12 mmol/L的同时，硫浓度设定为3、2、1和0 mmol/L 4个水平，分别用NaCl+S₃, NaCl+S₂, NaCl+S₁ 和 NaCl+S₀表示。韭菜生长30天后，取样分析了不同硫水平下韭菜中的硝酸盐含量及主要氮代谢途径中的氨基酸含量。  【结果】  在营养液硫供应水平3 mmol/L下，与营养液中不添加氯化钠处理 (CK) 相比，NaCl+S₃处理的韭菜硝酸盐累积降低了32.60%，地上部干重、可溶性糖和可溶性蛋白质含量、根系活力显著增加；NaCl+S₁处理的韭菜硝酸盐累积降低了53.30%，地上部干重、可溶性糖和可溶性蛋白质含量、根系活力显著增加。NaCl+S₁处理较NaCl+S₃处理更有利于降低硝酸盐含量。韭菜地上部的全氮含量无显著变化 (NaCl+S₃处理) 或略有增加 (NaCl+S₁处理)，表明添加低浓度氯化钠并未限制氮素吸收。NaCl+S₃和NaCl+S₁处理下，硝态氮还原活性 (硝酸还原酶NR) 和转氨活性 (谷氨酸草酰乙酸转氨酶GOT、谷氨酸丙氨酸转氨酶GPT) 增加，尤其是初级同化活性 (谷氨酰胺合成酶GS) 分别大幅增加了43.57%和71.43%。NaCl+S₃和NaCl+S₁处理下，丝氨酸途径的游离氨基酸代谢和天冬氨酸途径的蛋白质合成得到增强，韭菜的游离氨基酸总量基本保持不变或略有增加，而蛋白质水解氨基酸总量显著增加。  【结论】  韭菜专用营养液中添加NaCl 12 mmol/L并将硫浓度降低到1 mmol/L，可显著提高根系和氮代谢关键酶活性，在一定程度上改变游离氨基酸代谢途径和蛋白质合成途径，进而在显著增加韭菜干物质的同时，大大降低硝酸盐累积。

Decrement of nitrate accumulation in Chinese chives through adding NaCl and reducing SO42– concentration in the nutrient solution

  【Objectives】  Chinese chives are easy to accumulate NO₃?-N, so we tried to reduce the NO₃?-N accumulation through adding sodium chloride (NaCl) and reducing the SO₄2? concentration, and studied the mechanism of the nitrogen metabolic pathway.  【Methods】  Trellis growing system was used in a hydroponic experiment of Chinese chive. A specific nutrient solution was used as control (containing SO₄2? 3 mmol/L), and NaCl of 12 mmol/L was added into the solution, SO₄2? concentration of 3, 2, 1 and 0 mmol/L were prepared for treatments of NaCl+S₃, NaCl+S₂, NaCl+S₁ and NaCl+S₀. The Chinese chives were treated for 30 days before sampled for measurement of N, NO₃? and free amino acid contents, and some N-metabolic enzyme activities.  【Results】  Compared with CK, NaCl+S₃ remarkably decreased nitrate content by 32.60%, increased aboveground dry weight, nutrition quality and root activity of Chinese chives (P < 0.05). Reducing SO₄2? concentration to 1 mmol/L (NaCl+S₁) decreased nitrate by 53.30% and increased significantly NaCl+S₃ did in the aboveground dry weight, nutrition quality and root activity. The activities of nitrate reductase (NR) and transaminase (GOT, GPT) were significantly increased, and those of glutamine synthetase (GS) were increased by such an amazing range of 43.57% and 71.43% under NaCl+S₃ and NaCl+S₁ treatments. The NaCl+S₃ and NaCl+S₁ treatments led to the improved serine pathway for free amino acid metabolism and the aspartic pathway for protein synthesis. As a result, the total free amino acid content remained basically unchanged or slightly increased, while the total amino acid hydrolyzed by protein was significantly increased.  【Conclusions】  Adding 12 mmol/L of NaCl and reducing the SO₄2? concentration from 3 mmol/L to 1 mmol/L will stimulate greatly the root activity and the activities of key nitrogen metabolism enzymes, and lead to efficient free amino acid metabolic pathway and the protein synthetic pathway, as a result, increase the dry matter accumulation and nutrition quality of Chinese chives, and reduce nitrate accumulation in Chinese chive.