Studies of the uptake of nitrate in barley : v. Estimation of root cytoplasmic nitrate concentration using nitrate reductase activity-implications for nitrate influx

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.