Abstract
BackgroundLow 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.ResultsUsing non-invasive micro-test technology, the net nitrate (NO3−) and ammonium (NH4+) 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 NO3− flux rate in the root hair zone and vascular bundles of cucumber seedlings decreased, whereas the net NH4+ 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 15N isotope tracing showed that NO3−-N and NH4+-N uptake of the seedlings under LT treatment decreased significantly compared with that under NT treatment, and the concentration and proportion of both NO3−-N and NH4+-N distributed in the shoot decreased. Under LT treatment, the actual nitrate reductase activity (NRAact) in the root did not change significantly, whereas NRAact in the stem and petiole increased by 113.2 and 96.2%, respectively.ConclusionsThe higher net NH4+ flux rate in leaves and young tissues may reflect the higher NRAact in the stem and petiole, which may result in a higher proportion of NO3− being reduced to NH4+ during the upward transportation of NO3−. The results contribute to an improved understanding of the mechanism of changes in nitrate transportation in plants in response to low-temperature stress.
Highlights
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)
Net NO3− and NH4+ flux rates First, we observed that low temperature (LT) (8 °C) treatment significantly depressed the net NO3− flux rate in the root hair zone and in the vascular bundles of other organs of cucumber seedlings
The present results showed that CsNRT1.1 may not be the dominant gene involved in nitrate transportation in the petiole and midrib of cucumber
Summary
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). Results: Using non-invasive micro-test technology, the net nitrate (NO3−) and ammonium (NH4+) 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 NO3− flux rate in the root hair zone and vascular bundles of cucumber seedlings decreased, whereas the net NH4+ flux rate in vascular bundles of the midrib, lateral vein, and shoot tip increased. The NO3− absorbed by plants is first reduced to NH4+ before it can be metabolized This reduction is catalyzed by nitrate reductase (NR) and nitrite reductase (NiR) [9]. Activity of NR and NiR can be detected in many plant organs (e.g., the root, stem, cotyledon, inflorescence stalk, flower, petiole, and leaf) [11,12,13,14]
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