EFFECTS OF NITRITE, CHLORATE, AND CHLORITE ON NITRATE UPTAKE AND NITRATE REDUCTASE-ACTIVITY

Effects of NO2-, ClO3-, and ClO2- on the induction of nitrate transport and nitrate reductase activity (NRA) as well as their effects on NO3- influx into roots of intact barley (Hordeum vulgare cv Klondike) seedlings were investigated. A 24-h pretreatment with 0.1 mol m-3 NO2- fully induced NO3- transport but failed to induce NRA. Similar pretreatments with ClO3- and ClO2- induced neither NO3- transport nor NRA. Net ClO3- uptake was induced by NO3- but not by ClO3- itself, indicating that NO3- and ClO3- transport occur via the NO3- carrier. At the uptake step, NO2- and ClO2- strongly inhibited NO3- influx; the former exhibited classical competitive kinetics, whereas the latter exhibited complex mixed-type kinetics. ClO3- proved to be a weak inhibitor of NO3- influx (K(i) = 16 mol m-3) in a noncompetitive manner. The implications of these findings are discussed in the context of the suitability of these NO3- analogs as screening agents for the isolation of mutants defective in NO3- transport.