Kinetics of NO3- net fluxes in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association, as affected by the presence of NO3- and NH4+

The impact of mineral N supply, N-free or NO3- with or without NH4+, on the subsequent uptake of NO3- by maritime pine seedlings associated with the ectomycorrhizal fungus Rhizopogon roseolus was studied using ion-selective microelectrodes. NO3- net fluxes into N-starved non-mycorrhizal short roots (NMSRs) were low and measurable only over the NO3- concentration range of 0-70 mu M. The simple kinetics observed in those roots may reflect the dominant operation of a high-affinity NO3- transport system (HATS) which is constitutive. NO3- pretreatment increased the NO3- 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 NH4+ concentration. In contrast, NO3- uptake kinetics presented only one saturation phase in the fungus, whether associated with the plant or not. NO3- uptake was greater after a pretreatment in N-free or NO3- solution, but NH4+ pretreatment led to a threefold reduction in NO3- uptake. These results suggest that the regulation of NO3- 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.