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CROP PHYSIOLOGY & METABOLISM

Inhibition of Net Nitrate Uptake by Ammonium in Pima and Acala Cotton Roots

Dep. of Agronomy and Range Science, Univ. of California, One Shields Avenue, Davis, CA 95616-8515

* Corresponding author (rltravis{at}ucdavis.edu)

This study was conducted to determine whether the inhibition of net NO^-₃ uptake by NH⁺₄ in cotton roots is due to inhibition of NO^-₃ influx per se, and/or enhancement of efflux. Two cotton species, Pima (Gossypium barbadense L.) and Acala (G. hirsutum L.), which differ in NO^-₃ influx and efflux, were used. Seedlings were grown hydroponically for 10 to 11 d and then induced for NO^-₃ uptake with 0.01, 0.10, or 1.0 mM NO^-₃. Net uptake was determined by following NO^-₃ depletion from uptake solutions, containing 0.1 mM NO^-₃, in the presence or absence of 10 mM NH⁺₄. Roots induced with 0.01 mM NO^-₃ contained only 5 to 6 µmol NO^-₃ g^-1 fresh weight (FW) after the induction treatment. In these roots, net NO^-₃ uptake was equivalent to influx, and was not inhibited by NH⁺₄. Roots induced with 0.1 mM NO^-₃ contained 36 to 38 µmol NO^-₃ g^-1 FW, and NH⁺₄ inhibited net NO^-₃ uptake albeit after a lag of 12 min. In contrast, roots induced with 1.0 mM NO^-₃ contained 65 µmol NO^-₃ g^-1 FW, and NH⁺₄ immediately inhibited net uptake. Exposure to NH⁺₄ increased NO^-₃ efflux by both species. At similar root NO^-₃ concentrations, NH⁺₄ inhibited net NO^-₃ uptake and stimulated efflux more in Pima than in Acala cotton. These results indicate that the response of both cotton species to NH⁺₄ is dependent upon root NO^-₃ concentration, and supports the argument that NH⁺₄ has little effect on NO^-₃ influx, but inhibits net uptake by stimulating efflux.

Abbreviations: FW, fresh weight • HPLC, high-performance liquid chromatography

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Crop Science 2001 41: 991. [Full Text]