Liu Yumei, Bai Longqiang, Sun Mintao, Wang Jun, Li Shuzhen, Miao Li, Yan Yan, He Chaoxing, Yu Xianchang, Li Yansu
The Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing, 100081, China.
College of Agricultural and Biological Engineering, Heze University, Heze, 274000, Shandong, China.
BMC Plant Biol. 2021 Apr 19;21(1):189. doi: 10.1186/s12870-021-02918-6.
Low temperature severely depresses the uptake, translocation from the root to the shoot, and metabolism of nitrate and ammonium in thermophilic plants such as cucumber (Cucumis sativus). Plant growth is inhibited accordingly. However, the availability of information on the effects of low temperature on nitrogen transport remains limited.
Using non-invasive micro-test technology, the net nitrate (NO) and ammonium (NH) fluxes in the root hair zone and vascular bundles of the primary root, stem, petiole, midrib, lateral vein, and shoot tip of cucumber seedlings under normal temperature (NT; 26 °C) and low temperature (LT; 8 °C) treatment were analyzed. Under LT treatment, the net NO flux rate in the root hair zone and vascular bundles of cucumber seedlings decreased, whereas the net NH flux rate in vascular bundles of the midrib, lateral vein, and shoot tip increased. Accordingly, the relative expression of CsNRT1.4a in the petiole and midrib was down-regulated, whereas the expression of CsAMT1.2a-1.2c in the midrib was up-regulated. The results of N isotope tracing showed that NO-N and NH-N uptake of the seedlings under LT treatment decreased significantly compared with that under NT treatment, and the concentration and proportion of both NO-N and NH-N distributed in the shoot decreased. Under LT treatment, the actual nitrate reductase activity (NRA) in the root did not change significantly, whereas NRA in the stem and petiole increased by 113.2 and 96.2%, respectively.
The higher net NH flux rate in leaves and young tissues may reflect the higher NRA in the stem and petiole, which may result in a higher proportion of NO being reduced to NH during the upward transportation of NO. The results contribute to an improved understanding of the mechanism of changes in nitrate transportation in plants in response to low-temperature stress.
低温严重抑制喜温植物如黄瓜（Cucumis sativus）对硝酸盐和铵盐的吸收、从根到地上部的转运以及代谢。植物生长因此受到抑制。然而，关于低温对氮素转运影响的信息仍然有限。
采用非损伤微测技术，分析了正常温度（NT；26°C）和低温（LT；8°C）处理下黄瓜幼苗初生根、茎、叶柄、叶脉、侧脉和茎尖的根毛区和维管束中硝酸盐（NO）和铵盐（NH）的净通量。在低温处理下，黄瓜幼苗根毛区和维管束中的净NO通量速率降低，而叶脉、侧脉和茎尖维管束中的净NH通量速率增加。相应地，叶柄和叶脉中CsNRT1.4a的相对表达下调，而叶脉中CsAMT1.2a - 1.2c的表达上调。氮同位素示踪结果表明，与正常温度处理相比，低温处理下幼苗对NO - N和NH - N的吸收显著降低，地上部中NO - N和NH - N的浓度和比例均下降。在低温处理下，根中的实际硝酸还原酶活性（NRA）没有显著变化，而茎和叶柄中的NRA分别增加了113.2%和96.2%。
叶片和幼嫩组织中较高的净NH通量速率可能反映了茎和叶柄中较高的NRA，这可能导致在NO向上运输过程中更多的NO被还原为NH。这些结果有助于更好地理解植物响应低温胁迫时硝酸盐转运变化的机制。
