STUDIES OF THE UPTAKE OF NITRATE IN BARLEY .5. 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-mu-M 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 [19921 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.