Uptake of Acidic and Basic Sugar Derivatives in Lemna gibba G1

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Abstract
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. (14C)glucosamine uptake followed Michaelis-Menten kinetics together with a linear component. Influx of this sustrate was inhbited 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.