| Abstract: | Compartmental analyses of intact roots of barley (Hordeum vulgareL. cv. Klondike) plants, grown with different levels of NO3–(up to 1·0 mol m–3) in the external media, wereundertaken using 13NO3–. Two additional treatments, namelysodium dodecyl sulphate (SDS) or brief exposure to high temperature,designed to investigate the identity of the three NO3–compartments revealed by compartmental analyses, provided supportfor the identification of the latter as corresponding to superficialsolution, apoplasm, and cytoplasm. Half-lives for exchange ofthese compartments, 3 s, 30 s, and 7 mm, were unaffected bythe level of NO3– provided during growth. Independentestimates of 13NO3– fluxes obtained by direct methodsagreed well with values of fluxes calculated from the compartmentalanalyses. Cytoplasmic NO3–], estimated from the compartmental analyses,were in the range from 1–37 mol m–3, and increasedwith increasing NO3–] of the medium. Such values forcytoplasmic 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 concentrationrange above 10 mol m–3. A model, based upon localizeddistribution of nitrate reductase activity in epidermal cells,is proposed in which the proposed passive low affinity NO uptakeat high external NO3–] is restricted to epidermal cells. During loading periods with 13NO3–, significant amountsof 13N were translocated to the shoot. Two pools of 13N, onebeing the root symplasm, appear to participate in the transferof labelled N to the shoot. Key words: Barley, compartmentation, nitrate, nitrate reductase, 13N |
