Inhibition of net nitrate uptake by ammonium in pima and acala cotton roots

This study was conducted to determine whether the inhibition of net NO3- up take by NH4+ in cotton roots is due to inhibition of NO3- influx per se, an d/or enhancement of efflux. Two cotton species, Pima (Gossypium barbadense L.) and Acala (G. hirsutum L.), which differ in NO3- influx and efflux, wer e used. Seedlings were grown hydroponically for 10 to 11 d and then induced for NO3- uptake with 0.01, 0.10, or 1.0 mM NO3-. Net uptake was determined by following NO; depletion from uptake solutions, containing 0.1 mM NO3-, in the presence or absence of 10 mM NW. Roots induced with 0.01 mM NO3- con tained only 5 to 6 mu mol NO3- g(-1) fresh weight (FW) after the induction treatment. In these roots, net NO3- uptake was equivalent to influx, and wa s not inhibited by NH4+. Roots induced with 0.1 mM NO3- contained 36 to 38 mu mol NO3- g(-1) FW, and NH4+ inhibited net NO3- uptake albeit after a lag of approximate to 12 min. In contrast, roots induced with 1.0 mM NO3- cont ained 65 mu mol NO3- FW, and NW immediately inhibited net uptake. Exposure to NH4+ increased NO3- efflux by both species. At similar root NO3- concent rations, NW inhibited net NO3- uptake and stimulated efflux more in Pima th an in Acala cotton. These results indicate that the response of both cotton species to NH4+ is dependent upon root NO3- concentration, and supports th e argument that NH4+ has little effect on NO3- influx, but inhibits net upt ake by stimulating efflux.