Murphy A T, Lewisf O A M
Botany Department, University of Cape Town, Rondebosch 7700, South Africa.
New Phytol. 1987 Oct;107(2):327-333. doi: 10.1111/j.1469-8137.1987.tb00184.x.
Maize plants (Zea mays L. cv. R201) were grown to 21 d in pH-controlled gravel culture with 2 mM inorganic N supplied as nitrate alone, ammonium alone or 1:1 nitrate + ammonium. At 21 d, the N feeding solutions were replaced with N solutions, and xylem sap collections were made 4 and 8 h after the commencement of feeding. Leaf and root material was harvested also for in vitro nitrate reductase and glutamine synthetase activity assays. Xylem sap analyses showed that in nitrate-only fed plants the major supply of nitrogen from root to shoot was in the nitrate form (60%) with 35 % carried as amino compounds. However, 93% of N was transported to the shoot as nitrate and only 6% in amino compounds, indicating the more direct routing of newly absorbed nitrogen to the shoot via the former. Leaf NRA was seven-fold greater than that of the root, confirming the shoot as the major site of nitrogen assimilation in plants fed only nitrate. In ammonium-only fed plants, 84% of xylem N was found in organic form (66% N), the remainder translocating as ammonium, identifying the root as the major site of ammonium N assimilation. In ammonium + nitrate fed plants, 64% of xylem N was present as organic N (55% N), 34% as nitrate (43 % N), indicating shared N assimilation between shoot and root, with root assimilation predominating. In plants receiving nitrate, glutamine was the major N compound translocated, in plants receiving only ammonium, asparagine predominated. GS activity was approximately the same in root and shoot and showed no response to N source. The significance of these results is discussed with respect to the reported increased productivity of maize fed a mixed nitrate-ammonium N source.
将玉米植株（玉米品种R201）种植在pH值可控的砾石培养体系中，培养21天，提供2 mM无机氮，分别为单独的硝酸盐、单独的铵盐或1:1的硝酸盐+铵盐。在第21天，将供氮溶液换成无氮溶液，并在供氮开始后4小时和8小时收集木质部汁液。同时收获叶片和根系材料，用于体外硝酸还原酶和谷氨酰胺合成酶活性测定。木质部汁液分析表明，仅供应硝酸盐的植株中，从根到地上部的主要氮供应形式是硝酸盐（60%），35%以氨基化合物形式运输。然而，93%的氮以硝酸盐形式运输到地上部，只有6%以氨基化合物形式运输，这表明新吸收的氮通过前者更直接地运输到地上部。叶片硝酸还原酶活性比根高7倍，证实地上部是仅供应硝酸盐植株中氮同化的主要部位。在仅供应铵盐的植株中，84%的木质部氮以有机形式存在（66%的氮），其余以铵盐形式转运，这表明根是铵态氮同化的主要部位。在供应硝酸盐+铵盐的植株中，64%的木质部氮以有机氮形式存在（55%的氮），34%以硝酸盐形式存在（43%的氮），这表明地上部和根之间存在氮同化共享，且以根同化为主。在接受硝酸盐的植株中，谷氨酰胺是主要运输氮化合物，在仅接受铵盐的植株中，天冬酰胺占主导。谷氨酰胺合成酶活性在根和地上部大致相同，且对氮源无响应。结合已报道的混合硝酸盐-铵盐氮源饲喂玉米提高生产力的情况，对这些结果的意义进行了讨论。
