Enhancement of Glutamate Dehydrogenase Activity in Excised Pea Roots by Exogenously Supplied Acids

Abstract

Summary The addition of acids to the nutrient medium and the increase in the concentration of hydrogen ions in the external medium and within the roots therein obviously the determining factor causes an increase in GDH activity in excised pea roots. This increase is dependent on a proteosynthesis indicated by the complete inhibition of the increase by actidione; it is not, however, the result of generally enhanced proteosynthesis. Nitrate reductase activity was not induced by the increase in the concentration of H + ions; the activity of aspartate aminotransferase was slightly decreased. The increase in GDH activity caused by the addition of the acids was abolished by the addition of bases (incl. NH 4 OH), if the pH of the medium was shifted by this treatment nearer to neutral region (pH from 5 to 7). The shifting of pH into the alkaline region resulted in the inhibition of that increase in GDH level caused by the omission of sugar from the medium. The GDH level was not influenced significantly by this treatment in roots cultured with sugar. Among ammonium salts, NH 4 Cl (10 meq NH 4 + ) caused a statistically significant increase in GDH activity and simultaneously a statistically significant increase in the concentration of H + ions in roots cultured with sugar, whereas (NH 4 ) 2 HPO 4 (10 meq NH 4 + ) tendend to bring about an opposite, even though statistically insignificant, effect. These results indicate that the effect of higher concentrations of ammonium salts on GDH activity observed in the roots of intact plants by other authors does not reside in the direct influence of NH 4 + ions on GDH synthesis, but in the combination of (1) indirect influence of physiologically acid ammonium salts mediated by the increase in the concentration of H + ions inside the roots and of (2) another indirect influence mediated by the mechanism controlled by sugars (which was described and discussed in our previous paper - Sahulka et al., 1975). The GDH level could be increased up to four times during a 22 h incubation even in excised pea roots by the combined effects of the addition of acids and of the omission of sugar from the medium.