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

摘要

This study was conducted to determine whether the inhibition of net NOa?’3 uptake by NH+4 in cotton roots is due to inhibition of NOa?’3 influx per se, and/or enhancement of efflux. Two cotton species, Pima (Gossypium barbadense L.) and Acala (G. hirsutum L.), which differ in NOa?’3 influx and efflux, were used. Seedlings were grown hydroponically for 10 to 11 d and then induced for NOa?’3 uptake with 0.01, 0.10, or 1.0 mM NOa?’3 Net uptake was determined by following NOa?’3 depletion from uptake solutions, containing 0.1 mM NOa?’3, in the presence or absence of 10 mM NH+4 Roots induced with 0.01 mM NOa?’3 contained only 5 to 6 ??mol NOa?’3 ga?’1 fresh weight (FW) after the induction treatment. In these roots, net NOa?’3 uptake was equivalent to influx, and was not inhibited by NH+4 Roots induced with 0.1 mM NOa?’3 contained 36 to 38 ??mol NOa?’3 ga?’1 FW, and NH+4 inhibited net NOa?’3 uptake albeit after a lag of a‰?12 min. In contrast, roots induced with 1.0 mM NOa?’3 contained 65 ??mol NOa?’3 ga?’1 FW, and NH+4 immediately inhibited net uptake. Exposure to NH+4 increased NOa?’3 efflux by both species. At similar root NOa?’3 concentrations, NH+4 inhibited net NOa?’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 NOa?’3 concentration, and supports the argument that NH+4 has little effect on NOa?’3 influx, but inhibits net uptake by stimulating efflux.