Uptake of Acidic and Basic Sugar Derivatives in Lemna gibba G1.

The uptake of acidic and basic sugar derivatives in Lemna gibba L. was studied. Uronic acids applied to the experimental solution (50 millimolar) induced a small decrease of the membrane potential (10 +/- 1 millivolt galacturonic acid, and 20 +/- 4 millivolt glucuronic acid). After incubation of the plants in a 0.1 millimolar solution of these substrates, no decrease in the concentration of reducing groups in the external solution was detected. Respiration increased by 31% with 50 millimolar galacturonic acid, whereas no effect was found with the same concentration of glucuronic acid. Glucosamine caused a considerable concentration-dependent membrane depolarization. ((14)C)glucosamine uptake followed Michaelis-Menten kinetics together with a linear component. Influx of this substrate was inhibited by glucose but the type of competition could not be clearly distinguished. Glucosamine, 50 millimolar, inhibited the respiration rate by 30%. The glucosamine uptake was pH-dependent, with maximum uptake at around pH 7. Lack of enhancement of uptake by low pH as well as the permanent membrane depolarization suggest a uniport mechanism for the charged species of the substrate and an electroneutral diffusion of the uncharged species.