Plant tissues were extracted in distilled water for 2 h at
90 ◦ C, as described previously
). Chloride was determined with a chloride electrode
Sodium and potassium in the extracts were measured by atomic absorption spectroscopy (Unicam 919).
PTS fluorescence was measured at excitation= 403 nm and emission = 510 nm with a fluorescence spec- trometer (Perkin-Elmer LS-3B).