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

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 m M NO − 3 Net uptake was determined by following NO − 3 depletion from uptake solutions, containing 0.1 m M NO − 3 , in the presence or absence of 10 m M NH + 4 Roots induced with 0.01 m M 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 m M 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 m M 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.