Abstract
Plants and other organisms in the ecosystem compete for the limited nitrogen (N) in the soil. Formation of a symbiotic relationship with arbuscular mycorrhizal fungi (AMF) may influence plant competitiveness for N. However, the effects of AMF on plant nitrate (NO(3) (-)) uptake capacity remain unknown. In this study, a pot experiment was conducted to investigate the effects of N application and Rhizophagus irregularis inoculation on the root absorbing area, uptake kinetics of NO(3) (-), and the expression of NO(3) (-) transporter (NRT) genes in Populus × canadensis 'Neva'. The results showed that R. irregularis colonized more than 70% of the roots of the poplar and increased root active absorbing area/total absorbing area. The uptake kinetics of NO(3) (-) by poplar fitted the Michaelis-Menten equation. Mycorrhizal plants had a higher maximum uptake rate (V (max)) value than non-mycorrhizal plants, indicating that R. irregularis enhanced the NO(3) (-) uptake capacity of poplar. The expression of NRTs in roots, namely, NRT1;2, NRT2;4B, NRT2;4C, NRT3;1A, NRT3;1B, and NRT3;1C, was decreased by R. irregularis under conditions of 0 and 1 mM NH(4)NO(3). This study demonstrated that the improved NO(3) (-) uptake capacity by R. irregularis was not achieved by up-regulating the expression of NRTs in roots. The mycorrhizal pathway might repress root direct pathway in the NO(3) (-) uptake by mycorrhizal plants.