Accumulation of ammonium in Norway spruce (Picea ablies) seedlings measured by in vivo 14N-NMR

N-14-NMR and P-31-NMR have been used to monitor the in vivo pH in roots, stems, and needles from seedlings of Norway spruce, a typical ammonium-tolerant plant. The vacuolar and cytoplasmic pH measured by P-31-NMR was found to be c. pH 4.8 and 7.0, respectively, with no significant difference between plants growing with ammonium or nitrate as the N-source. The H-1-coupled (NH4+)-N-14 resonance is pH-sensitive: at alkaline pH it is a narrow singlet line and below pH 4 it is an increasing multiplet line with five signals. The pH values in ammonium-containing compartments measured by N-14-NMR ranged from 3.7 to 3.9, notably lower than the estimated pH values of the P-i pools. This suggests that, in seedlings of Norway spruce, ammonium is stored in vacuoles with low pH possibly to protect the seedlings against the toxic effects of ammonium (NH4+) or ammonia (NH3). It was also found that concentrations of malate were 3-6 times higher in stems than in roots and needles, with nitrate-grown plants containing more malate than plants grown with ammonium.