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. 1992 Aug;99(4):1582–1589. doi: 10.1104/pp.99.4.1582

Studies of the Uptake of Nitrate in Barley 1

V. Estimation of Root Cytoplasmic Nitrate Concentration Using Nitrate Reductase Activity—Implications for Nitrate Influx

Bryan J King 1, M Yaeesh Siddiqi 1, Anthony D M Glass 1
PMCID: PMC1080667  PMID: 16669077

Abstract

The cytoplasmic NO3− concentration ([NO3−]c) was estimated for roots of barley (Hordeum vulgare L. cv Klondike) using a technique based on measurement of in vivo nitrate reductase activity. At zero external NO3− concentration ([NO3−]o), [NO3−]c was estimated to be 0.66 mm for plants previously grown in 100 μm NO3−. It increased linearly with [NO3−]o between 2 and 20 mm, up to 3.9 mm at 20 mm [NO3−]o. The values obtained are much lower than previous estimates from compartmental analysis of barley roots. These observations support the suggestion (MY Siddiqi, ADM Glass, TJ Ruth [1991] J Exp Bot 42: 1455-1463) that the nitrate reductase-based technique and compartmental analysis determine [NO3−]c for two separate pools; an active, nitrate reductase-containing pool (possibly located in the epidermal cells) and a larger, slowly metabolized storage pool (possibly in the cortical cells), respectively. Given the values obtained for [NO3−]c and cell membrane potentials of −200 to −300 mV (ADM Glass, JE Schaff, LV Kochian [1992] Plant Physiol 99: 456-463), it is very unlikely that passive influx of NO3− is possible via the high-concentration, low-affinity transport system for NO3−. This conclusion is consistent with the suggestion by Glass et al. that this system is thermodynamically active and capable of transporting NO3− against its electrochemical potential gradient.

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Selected References

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