Chlorate as a Transport Analog for Nitrate Absorption by Roots of Tomato

Several studies have indicated that chlorate (ClO3-) and nitrate (NO3-) may share a common transport system in higher plants. Here, we compared the interactions between ClO3- and NO3-uptake by roots of intact tomato (Lycopersicon esculentum cv T5) plants. Exposure to ClO3- for more than 2 h inhibited both net ClO3- and K+ uptake, presumably because of ClO3- toxicity; consequently, subsequent measurements were conducted after short exposures to ClO3-. The apparent affinity and apparent maximum rate of absorption for net ClO3- and NO3- uptake were very similar. Interactions between ClO3- and NO3- transport were complex; 50 [mu]M NO3- acted as a mixed inhibitor of net ClO3- uptake, but 50 [mu]M ClO3- had no significant effect on net NO3- uptake, and 500 [mu]M ClO3- had no significant effect on 15NO3- influx. If the two ions share a single common high-affinity transport system, it is much more selective for NO3- than would be suggested by the similarity of net NO3- and ClO3- uptake kinetics. Our results indicate that, although NO3- may interfere with root ClO3- uptake, ClO3- is not a useful analog for the root high-affinity NO3- transport system.