THE EFFECT OF SALT STRESS ON THE CATABOLISM OF SUGARS IN LEAVES AND ROOTS OF A MANGROVE PLANT, AVICENNIA-MARINA

Respiration and related aspects of metabolism were investigated in the roots and leaves of 2-year-old trees of the mangrove plant, Avicennin marina in the presence of 100, 250 and 500 mM NaCl. The rate of respi ration of leaves increased with increasing concentrations of NaCl in t he incubation medium, but respiration of roots was not similarly affec ted. In order to examine the relative rates of catabolism of glucose b y the glycolysis-tricarboxylic acid (TCA) cycle and the oxidative pent ose phosphate pathway (PP pathway), we determined the rates of release of (CO2)-C-14 from [1-C-14]glucose and from [6-C-14]glucose in segmen ts of roots and leaves. The ratios of rates (C-6/C-1) in roots varied from 0.30 to 0.44, while ratios of 0.85 to 0.99 were obtained when lea ves were incubated in the presence of various concentrations of NaCl. It appeared that the PP pathway was more involved in sugar catabolism in the roots than in the leaves of A. marina. Uniformaly C-14-labelled sucrose, incubated with segments of roots and leaves for 18 h, was co nverted to CO2, amino acids (mainly glutamine), organic acids (mainly malic acid), sugars and ethanol-insoluble macromolecules. The incorpor ation of radioactivity into most of these components was not significa ntly affected by NaCl. However, in leaves (but not in roots) the relea se of (CO2)-C-14 from [U-C-14]sucrose was enhanced by NaCl at 250 mM a nd 500 mM, while the rate of incorporation of radioactivity into macro molecules was reduced by high concentrations of NaCl. Incorporation of radioactivity from [U-C-14]sucrose into malic acid was enhanced in bo th roots and leaves by an increase in the concentration of NaCl from 1 00 mM to 500 mM (this concentrations is similar to that in sea water). Independent of the concentration of NaCl, more than half of the radio activity in the neutral fraction from leaves was incorporated into an unidentified sugar, while in the same fraction from roots, the radioac tivity was associated with glucose, fructose and sucrose. On the basis of these results, a discussion is presented of the characteristics of catabolism of sugars in A, marina in relation to salt resistance.