THE MECHANISM OF NITRATE TRANSPORT ACROSS THE TONOPLAST OF BARLEY ROOT-CELLS

Nitrate-selective microelectrodes were used to measure not only nitrate activity in the cytoplasm and vacuole of barley (Hordeum vulgare L.) root cells, but also the tonoplast electrical membrane potential. For epidermal cells, the mean cytoplasmic and vacuolar pNO3 (-log10 [NO3]) values were 2.3 +/- 0.04 (n = 19) and 1.41 +/- 0.03 (n = 35), respectively, while for cortical cells, the mean cytoplasmic and vacuolar nitrate values were 2.58 +/- 0.18 (n = 4) and 1.17 +/- 0.06 (n = 13), respectively. These results indicate that the accumulation of nitrate in the vacuole must be an active process. Proton-selective microelectrodes were used to measure the proton gradient across the tonoplast to assess the possibility that nitrate transport into the vacuole is mediated by an H+/NO3- antiport mechanism. For epidermal cells, the mean cytoplasmic and vacuolar pH values were 7.12 +/- 0.06 (n = 10) and 4.93 +/- 0.11 (n = 22), respectively, while for cortical cells, the mean cytoplasmic and vacuolar pH values were 7.24 +/- 0.07 (n = 3) and 5.09 +/- 0.17 (n = 7), respectively. Calculations of the energetics for this mechanism indicate that the observed gradient of nitrate across the tonoplast of both epidermal and cortical cells could be achieved by an H+/NO3- antiport with a 1:1 stoichiometry.