Rhizospheric NO affects N uptake and metabolism in Scots pine (Pinus sylvestrisL.) seedlings depending on soil N availability and N source

Abstract

Signals controlling the competitive interactions for nitrogen between plants and soil microorganisms in the rhizosphere are to date mainly unknown, thus, we investigated the effects of rhizospheric NO concentration with varying soil N availability on N uptake and metabolism in fine roots of Scots pine seedlings. Our results show for the first time that NO affected N uptake by seedlings dependent on N source and soil N availability. The commonly accepted suppression of nitrate uptake in the presence of ammonium and glutamine was overruled at high NO. This study also provides first evidence for the effects of NO on nitrate and arginine uptake in a tree species and that rhizospheric NO does not only affect deciduous trees (i.e., beech), but also coniferous tree species (i.e., pine). We investigated the interaction of rhizospheric nitric oxide (NO) concentration (i.e. low, ambient or high) and soil nitrogen (N) availability (i.e. low or high) with organic and inorganic N uptake by fine roots of Pinus sylvestrisL. seedlings by 15N feeding experiments under controlled conditions. N metabolites in fine roots were analysed to link N uptake to N nutrition. NO affected N uptake depending on N source and soil N availability. The suppression of nitrate uptake in the presence of ammonium and glutamine was overruled by high NO. The effects of NO on N uptake with increasing N availability showed different patterns: (1) increasing N uptake regardless of NO concentration (i.e. ammonium); (2) increasing N uptake only with high NO concentration (i.e. nitrate and arginine); and (3) decreasing N uptake (i.e. glutamine). At low N availability and high NO nitrate accumulated in the roots indicating insufficient substrates for nitrate reduction or its storage in root vacuoles. Individual amino acid concentrations were negatively affected with increasing NO (i.e. asparagine and glutamine with low N availability, serine and proline with high N availability). In conclusion, this study provides first evidence that NO affects N uptake and metabolism in a conifer. © 2012 Blackwell Publishing Ltd.