Nitrate reductase (NR) activity in the presence of Mg2+ (NR act) representing the non-phosphorylated NR state and the activity in the presence of EDTA (NR max) representing maximum NR activity was measured in roots and shoots of 15 d grown aluminium and water stressed rice seedlings to examine changes in NR activation state due to these stresses. Seedlings subjected to a moderate water stress level of -0.5 MPa for 24 h or grown in presence of 80 microM Al3+showed decreased level of NR max but resulted in higher NR act and NR activation state. However, seedlings grown in presence of a higher level of 160 microM Al3+ showed a decline in NR act as well as NR max. With a higher water stress Level of -2.0MPa a marked decline in the levels of both NR act and NR max was observed, whereas NR activation state remained almost unaltered with severe water stress. NR activity appeared to be sensitive to H2O2, PEG-6000, NaCl and various metal salts. Incorporation of these components in the enzyme assay medium led to decreased affinity of enzyme towards its substrate with increase in Km and decrease in Vmax values. Addition of each of the osmolytes i.e. 1 mol/L proline, 1 mol/L glycine betaine or 1 mol/L sucrose in the enzyme assay medium caused a considerable protection to the enzyme against the damaging effects of stressful components. An enhanced level of proline and glycine betaine was observed in Al-stressed seedlings and sucrose in Al as well as water stressed seedlings. Results suggest that Al toxicity and water stress decrease total amount of functional NR in rice seedlings and the osmolytes proline, glycine betaine and sucrose appear to have a direct protective action on enzyme NR under stressful conditions

中文翻译：

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铝毒和水分胁迫下水稻幼苗硝酸还原酶活性的调节：渗透物作为酶保护剂的作用

在代表非磷酸化 NR 状态的 Mg2+ (NR act) 存在下的硝酸还原酶 (NR) 活性和代表最大 NR 活性的 EDTA 存在下的活性 (NR max) 在生长 15 天的铝和水胁迫的水稻幼苗，以检查由于这些胁迫引起的 NR 激活状态的变化。幼苗在-0.5 MPa 的中等水分胁迫水平下持续 24 小时或在 80 microM Al3+ 存在下生长，显示 NR 最大值水平下降，但导致更高的 NR 行为和 NR 激活状态。然而，在更高水平的 160 microM Al3+ 存在下生长的幼苗表现出 NR 行为以及 NR 最大值的下降。在 -2.0MPa 的较高水分压力水平下，观察到 NR act 和 NR max 水平显着下降，而 NR 活化状态几乎保持不变，严重的水分胁迫。NR 活性似乎对 H2O2、PEG-6000、NaCl 和各种金属盐敏感。在酶测定培养基中加入这些组分导致酶对其底物的亲和力随着 Km 的增加和 Vmax 值的降低而降低。在酶测定培养基中添加每种渗透剂，即 1 mol/L 脯氨酸、1 mol/L 甘氨酸甜菜碱或 1 mol/L 蔗糖，对酶产生了相当大的保护，使其免受应激成分的破坏作用。在铝胁迫的幼苗和铝中的蔗糖以及水分胁迫的幼苗中观察到脯氨酸和甘氨酸甜菜碱的水平提高。结果表明，Al 毒性和水分胁迫降低了水稻幼苗中功能性 NR 的总量和渗透物脯氨酸，