Net NH4+ and NO3− flux, and expression of NH4+ and NO3− transporters in roots of Picea glauca

Insights into the physiology of nitrogen (N) uptake help us to understand the adaption of boreal coniferous forests to their environment. We compared fluxes of nitrate and ammonium in white spruce [Picea glauca (Moench) Voss] roots, measured using a non-invasive microelectrode ion flux measurement system (MIFE), and transcript abundance of ammonium and nitrate transporter genes in roots, determined by real time PCR. Seedlings were pretreated with water, or 50 μM or 1,500 μM NH4NO3 + 200 μM CaSO4 + 25 μM KH2PO4. Measurements were made on seedling roots 0–5, 5–10, 10–20 and 20–30 mm from the root tip. As ammonium and nitrate transporter family members in spruce are still uncharacterized, primers for real time PCR were designed to cover one family with each set of primers (AMT1, AMT2, NRT1, NRT2). The expression patterns obtained by real time PCR differed significantly among transporter family, treatments and root segments. Expression of AMT1 did not show a relationship with distance from the root tip, but the expression of AMT2 was generally greater 0–5 mm from the root tip than in segments farther from the tip. Expression of NRT1 was greatest 10–30 mm from the root tip, while expression of NRT2 was greatest 5–10 mm from the tip in all treatments, except the 1,500 μM NH4NO3 treatment. MIFE measurements showed the highest N uptake and proton efflux near the root tip and declining fluxes with increased distance from the root tip in the 50 μM N treatment. Significant net ammonium efflux was observed from some root segments in the 1,500 μM N treatment. Transporter gene expression and ion fluxes were not correlated. Though the measured net fluxes of ammonium were greater than the measured net fluxes of nitrate, the nitrate transporters were, in general, more highly expressed than the ammonium transporters.