Characterization of Sugar Transport in Storage Tissue of Carrot

Sugars are a major storage carbohydrate and a primary component of carrot root quality. The objective of this study was to determine the characteristics of sugar transport into the storage cells of carrot [ Daucus carota (L.)]. Tissue disks were incubated in a buffered solution (pH 6.5) containing various concentrations of sucrose, glucose, or fructose, 1 m m CaCl 2 , and 100 m m mannitol. Passive uptake was defined as uptake in the presence of 5 μ m carbonyl cyanide, - m chlorophenyl hydrazone (CCCP). Active uptake was the difference between total (–CCCP) and passive uptake. Characteristic, biphasic kinetics were observed for all sugars. At sugar concentrations below 10 m m , a saturating active component was operating. Above 10 m m , the influx was a nonsaturating, linear transport system. Active transport was pH dependent, showing high rates of uptake at low pH. Glucose and fructose did not inhibit sucrose influx and vice versa, but they did compete with each other. The kinetics of the hexose competition was noncompetitive inhibition. The competition studies suggested the presence of a separate carrier for each sugar. The evidence indicated that sugar transport across membranes of carrot storage cells is a combination of active and passive transport, consistent with transport kinetics observed in other crops. Active sugar uptake is a significant part of uptake, doubling influx at low apoplastic concentrations. At maturity, sucrose is the major transport and storage sugar. Glucose and fructose however, are transported at considerable rates in vitro , when they are present in the free space.