Light-stimulated biphasic amino acid uptake by xylem parenchyma cells

Abstract

The secondary xylem of tomato (Lycopersion esculentum cv Money-maker) internodes was isolated and was shaken in solutions of 14C-labelled amino acids. The uptake showed biphasic Michaelis-Menten kinetics. Light stimulated both uptake systems. During the uptake of amino acids the pH of the medium rose, more in the light than in the dark. Addition of 30 mmol 1−1 K+ to the medium retarded the pH rise. High K+ concentrations (>5 mmol l−1) decreased the uptake, whereas low K+ concentrations (< 2 mmol l−1) stimulated it. The observations indicated amino acid/proton co-transport into xylem parenchyma cells.