Specificity and Reversibility of the Bicarbonate Effect on Hexose Uptake by Maize Root Tips

The inhibition of hexose uptake by bicarbonate ions was investigatedin detail in order to test the specificity and reversibilityof the effect and to compare it with those of other electrolytes.The degree of inhibition was similar at pH 7.0 and pH 8.0. AtpH 4.5 no influence of a high concentration of CO₂ on 3-O-methylglucoseuptake was found. Therefore, the inhibition of hexose uptakeby bicarbonate cannot be explained by consequences of CO₂ influx.The inhibition of sugar absorption by calcium and potassiumions was similar to that exerted by bicarbonate in so far asit was observed at higher pH only. The inhibition exerted bysodium salts of different monovalent weak acids was limitedto lower pH and needed some time to become established or reversed.The bicarbonate effect was independent of time and reversiblewithout a lag phase. Sodium salts of strong mineral monovalentacids did not differ significantly in their effect on sugaruptake. Bicarbonate inhibited phosphate uptake in a similarmanner to hexose uptake but strongly stimulated the absorptionof potassium. The bicarbonate effect is assumed to result froma change in the degree of coupling of secondary active transportto the proton pump. Key words: Inhibition, Transport coupling, pH, Proton pump