Juvenile nitrogen uptake capacities and root architecture of two open-pollinated families of Picea abies. Effects of nitrogen source and ectomycorrhizal symbiosis.

This study was carried out to find early physiological differences occurring in young seedlings between two contrasting Picea abies open-pollinated families (OPF), one with high- and one with low-growth performance in the field by, determining their N uptake capacities and their root architecture. We used three potential N-sources in forest soil solution, NO3-, NH4+ and amino acids, to establish N uptake rates by the plants, whether or not associated with a fungus isolated from the field and identified as Paxillus involutus. NO3- fluxes were determined locally at the root surface using NO3(-)-selective microelectrodes whereas NH4+ and amino acid (L-glutamate and L-aspartate) uptake rates were calculated from their depletion of the incubation solution by the whole root system. Root systems were digitised in order to determine the number and the length of different root types. In non-mycorrhizal plants, the results showed that the most distinguishing parameters between OPF were NO3- uptake rates measured in the white tip of the secondary roots and the root architecture, with higher values determined in high-growth than in low-growth field performance OPF. The presence of the mycorrhizal fungus decreased NO3- uptake rates in both OPF and had an opposite effect on root architecture by increasing it in low-growth and decreasing it in high-growth field performance OPF, respectively. In non-mycorrhizal plants, NH4+ and amino-acid uptake rates were not different between OPF. Mycorrhizal symbiosis did not change NH4+ uptake rates whereas it increased that of amino acids, specifically that of L-aspartate in the low-growth field performance OPF. Taken together these results suggest that the measurement of local fluxes in roots of young plants could be a good potential tool for the early evaluating of growth capacity of Picea abies OPF.