Salt Tolerance of Trifolium alexandrinum L. IV. Ion Measurements by X-Ray Microanalysis in Unfixed, Frozen Hydrated Leaf Cells at Various Stages of Salt Treatment

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Abstract
Bulk-frozen leaf tissue of salt-treated (50 mM NaCI) T. alexandrinum plants harvested at various stages of the salt treatment was fractured in a scanning electron microscope. By means of X-ray microanalysis, Na+ and Cl\- was measured in single cells of the veins and of the surrounding tissue. From the corrected peak values Na+ : K+ ratios were calculated for each cell. For each sampling interval the average percentage of cells falling into various categories of Na+ :K+ and CI- :K+ was determined in all tissues. In the phloem parenchyma the percentage of cells having a very high Na+ :K+ ratio (>4) rose between days 10 and 16 of the salt treatment, whereas the percentage of xylem parenchyma cells falling into this category declined. It is concluded that the increasing Na+ :K+ ratios in the phloem are the result of both an enhanced inflow of Na+ into the veins (measured in a previous experiment) and the retranslocation of Na+ from the xylem to the phloem. The results support the hypothesis of intraveinal recycling of Na+ in salt-stressed young leaves of T. alexandrinum and may explain the gradual damage to the phloem transfer cells observed with progressive salt treatment.