STUDIES OF THE UPTAKE OF NITRATE IN BARLEY .3. COMPARTMENTATION OF NO3-

Compartmental analyses of intact roots of barley (Hordeum vulgare L. cv. Klondike) plants, grown with different levels of NO3- (up to 1.0 mol m-3) in the external media, were undertaken using (NO3-)-N-13. Two additional treatments, namely sodium dodecyl sulphate (SDS) or brief exposure to high temperature, designed to investigate the identity of the three NO3- compartments revealed by compartmental analyses, provided support for the identification of the latter as corresponding to superficial solution, apoplasm, and cytoplasm. Half-lives for exchange of these compartments, 3 s, 30 s, and 7 min, were unaffected by the level of NO3- provided during growth. Independent estimates of (NO3-)-N-13 fluxes obtained by direct methods agreed well with values of fluxes calculated from the compartmental analyses. Cytoplasmic [NO3-], estimated from the compartmental analyses, were in the range from approximately 1-37 mol m-3, and increased with increasing [NO3-] of the medium. Such values for cytoplasmic [NO3-] are inconsistent with an earlier proposal (Siddiqi, Glass, Ruth, and Rufty, 1990; Glass, Siddiqi, Ruth, and Rufty, 1990) of passive NO3- uptake in the concentration range above 1.0 mol m-3. A model, based upon localized distribution of nitrate reductase activity in epidermal cells, is proposed in which the proposed passive low affinity NO3- uptake at high external [NO3-] is restricted to epidermal cells. During loading periods with (NO3-)-N-13, significant amounts of N-13 were translocated to the shoot. Two pools of N-13, one being the root symplasm, appear to participate in the transfer of labelled N to the shoot.