Kinetics of NO(3)(-) net fluxes in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association, as affected by the presence of NO(3)(-) and NH(4)(+)

摘要

The impact of mineral N supply, N-free or NO(3)(-) with or without NH(4)(+), on the subsequent uptake of NO(3)(-) by maritime pine seedlings associated with the ectomycorrhizal fungus Rhizopogon roseolus was studied using ion-selective microelectrodes. NO(3)(-) net fluxes into N-starved non-mycorrhizal short roots (NMSRs) were low and measurable only over the NO(3)(-) concentration range of 0-70 microM. The simple kinetics observed in those roots may reflect the dominant operation of a high-affinity NO(3)(-) transport system (HATS) which is constitutive. NO(3)(-) pretreatment increased the NO(3)(-) net fluxes and led to a complex kinetics that may reflect the operation of other HATS. A simple kinetics was observed in plants pre-incubated at high NH(4)(+) concentration. In contrast, NO(3)(-) uptake kinetics presented only one saturation phase in the fungus, whether associated with the plant or not. NO(3)(-) uptake was greater after a pretreatment in N-free or NO(3) (-) solution, but NH(4)(+) pretreatment led to a threefold reduction in NO(3) (-) uptake. These results suggest that the regulation of NO(3)(-) transport systems varies between the host and the fungal partner. This variation is likely to contribute to the positive effect of mycorrhizal association on N uptake in plants when the N supply is low and fluctuating.