硝态氮难以在菠菜叶柄中还原的原因初探

【目的】蔬菜硝态氮过量累积危害人类健康，叶柄是蔬菜硝态氮累积的主要器官，揭示其累积硝态氮的原因是解决这一问题的关键。【方法】以3个菠菜品种为供试材料，设置不同氮水平进行盆栽试验，在不同生长期采样，测定叶柄硝态氮含量、内外源硝酸还原酶活性、细胞的硝态氮代谢库与贮存库大小，以及加入叶片硝酸还原酶后叶柄组织的亚硝态氮生成速率。【结果】叶柄硝态氮含量与其硝酸还原酶活性、代谢库大小无明显关系，但内外源硝酸还原酶活性的比值高、贮存库小，加入叶片硝酸还原酶后叶柄组织的亚硝态氮生成速率高的品种，其叶柄硝态氮含量低。【结论】叶柄潜在硝酸还原酶活性的实际表达程度、叶柄细胞液泡的大小、硝态氮由贮存库（液泡）进入代谢库（细胞质）的难易程度是造成硝态氮难以在叶柄中还原及品种间叶柄硝态氮含量差异的重要原因。

A Preliminary Study on Why It Is Difficult to Reduce Nitrate Spinach Petiole

Nitrate accumulation in vegetables has proposed detrimental impact on human heath, and understanding how nitrate can accumulate in plant is the key to solve the problem. Pot experiment was carried out, using three spinach (Spinacia oleracea L.) cultivars obviously different in nitrate concentration as test plants and at two N rates of 0.1 and 0.3 g N kg-1 on the basal application of 0.3 kg P2O5 kg-1 soil, to study the nitrate accumulation, distribution and allocation, and its reduction in the vegetable petioles. The petiole nitrate concentration, in vivo and the in vitro nitrate reductase activity (NRA) in the tissue of petiole, the nitrate metabolic pool size (NMPS) and storage pool size (NSPS) of the petiole cell, and the formation rate of nitrite by petiole tissue when addition of blade nitrate reductase were determined. The obtained results showed that the nitrate concentration of the petiole had no direct relationship with its NRA over different cultivars, but was negatively correlated with the ratio of in vivo to in vitro NRA. The NMPS of the petiole cell showed no certain relation with the nitrate concentration, but the NSPS was positively correlated with the nitrate concentration in petiole of different cultivars. When the nitrate reductase extracted from leaf blades was added to the culture solution of petiole tissue, the formation rate of nitrite by petiole tissue was all increased for the three spinach cultivars, and the formation rate was obviously higher for the cultivars with relatively lower petiole nitrate concentration. This indicated that the expressing degrees of the potential NRA in the petiole tissue, the NSPS of the petiole cell, and the capacity for the cell transporting nitrate from its storage pool (the vacuole) to metabolic pool (the plasma) was the main reason why it is difficult for nitrate to be reduced in petiole and nitrate concentrations in the petiole were different over spinach cultivars.