Characterizing nitrate uptake in lettuce using very-sensitive ion chromatography

The nitrate (NO3) uptake system (<1.0 mM) was characterized in lettuce (Lactuca sativa L.) grown under various levels of nitrogen (N) supply (Cn) by monitoring the time-course of depletion of NO3 from a complete nutrient solution. Nitrate absorption followed typical uptake kinetics, with saturation occurring at low (<15.0 mu M) solution NO3 concentrations. While Imax values for NO3 absorption were similar when depletion medium NO3 was measured by either ion chromatography (IC) or ultra-violet absorption (UV), Km and Co (the minimum concentration for net uptake) were approximately 2- and 6-fold lower, respectively, with IC. The results showed a significant loss of root absorption capacity in plants grown at high N levels (1.0 mM), however, neither Km or Co were appreciably affected by NO3 supply. Imax and Co values tended to decrease while Km remained relatively constant as plant age increased. A comparison of Km values measured by IC with their respective Co values indicated a broad concentration range, with Km values 10-20 times higher than Co concentrations. Overall, Km values for NO3 averaged 9.5 mu M, while Co concentrations ranged from 0.42 to 0.85 mu M In each case, plants depleted solution NO3 concentrations to submicromolar levels. Significant amounts of net efflux of NO3 were observed both before and after Co was reached. It is in the accurate determination of Co for NO3 that IC appears to have the greatest value in plant nutrition studies. The low detection limits for NO3 in solution by IC made possible the accurate determination of low Co concentrations, which in turn allowed for the reliable estimation of low Km values.