Scanning Microscopy


Root ultrastructure and elemental distributions in regions 3-4 mm behind the apex were examined by transmission electron microscopy and x-ray microanalysis in Plantago maritima ( grown ± 200 mM NaCl) and Plantago media (grown± 20 mM NaCl). In Plantago media vacuolation was incomplete in the cortical and xylem parenchyma cells. Epidermal transfer cells were observed in plants grown in the presence of NaCl. In Plantago maritima grown in the presence of NaCl, vacuolation was complete. In the xylem parenchyma cells of Plantago maritima controls the mitochondria were clustered adjacent to the vessels, rather than evenly distributed throughout the cytoplasm. X-ray microanalysis of root cortical cells from plants grown in the presence of NaCl and prepared by freeze-substitution showed that whereas the cytoplasm in the majority of cells in Plantago maritima contained relatively high concentrations of both Na (mean 81 mol m-3) and K (mean 107 mol m-3), that of Plantago media contained negligible Na and a much lower concentration of K (mean 29 mol m-3). Na was not accumulated in the vacuoles in either Plantago media or Plantago maritima. The ultrastructural and x-ray microanalytical observations are consistent with the hypotheses that differences in salinity tolerance between these species arise from both differences in the efficiency of ion transport to the shoot and in the compartmentation of ions in the root cortical cells.

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