The kinetics of nitrate uptake from flowing solutions by rice: Influences of pretreatment nitrate concentration and irradiance

The kinetics of nitrate (NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}){dollar} uptake by intact rice plants was studied with a system which was designed and built to measure the depletion of NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} in solutions flowing over the plant roots. The system was characterized through a comprehensive uncertainty propagation calculation, and through experiments demonstrating that the fluid in the plant root chamber was mixed well. A Michaelis-Menten kinetic model was applied, allowing the uptake kinetics to be characterized by two parameters, K{dollar}sb{lcub}rm m{rcub}{dollar} (the apparent half-velocity constant), and V{dollar}sb{lcub}rm max{rcub}{dollar} (the apparent maximum uptake rate). The uncertainty propagation calculation revealed that the measurement system was capable of determining the kinetic parameters with a high degree of accuracy.; Preliminary experiments suggested that pretreatment NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} concentration (50 to 800 {dollar}mu{dollar}M), affected the NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} uptake rate of intact rice plants from solutions containing 200 {dollar}mu{dollar}M NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}.{dollar} The preliminary experiments also validated experimental protocols which were used in the kinetic experiments.; Kinetic experiments revealed that both K{dollar}sb{lcub}rm m{rcub}{dollar} and V{dollar}sb{lcub}rm max{rcub}{dollar} varied with the pretreatment NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} concentration (50 to 800 {dollar}mu{dollar}M), although the changes in V{dollar}sb{lcub}rm max{rcub}{dollar} were more consistent than those in K{dollar}sb{lcub}rm m{rcub}.{dollar} Plants pretreated in high concentrations of NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} had lower V{dollar}sb{lcub}rm max{rcub}{dollar} and higher K{dollar}sb{lcub}rm m{rcub}{dollar} than plants pretreated in lower concentrations of NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}.{dollar} The changes in the parameters compensate for the pretreatment NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} concentration to maintain a fairly constant uptake rate. Changes in the kinetic parameters occur within 4 h of a change in the background NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} concentration, and continue for more than 24 h.; Kinetic experiments also revealed that a 24 h pretreatment without illumination stopped NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} uptake. Four to eight hours of irradiation was required after the 24 h light deprivation before NO{dollar}sbsp{lcub}3{rcub}{lcub}-{rcub}{dollar} uptake resumed. The uptake rates after 8 h of illumination were still well below those of plants which had been irradiated throughout pretreatment.; The implications of the results to nitrogen removal by artificial wetland wastewater treatment system are discussed.