Adaptation of cucumber seedlings to low temperature stress by reducing nitrate to ammonium during it's transportation.

Background: 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.

Results: 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.

Conclusions: 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.