Lead uptake by Picea abies seedlings:: Effects of nitrogen source and mycorrhizas

Heavy metals, and particularly lead (Pb), have accumulated in the humic layer of forest soils in Central Europe and North America. Among other environmental factors, rhizosphere pH and mycorrhizas have the potential of modifying heavy metal uptake of forest trees and, thus, heavy metal stress. To investigate the influence of rhizosphere pH and mycorrhizas on Pb uptake, we grew Norway spruce seedlings, nonmycorrhizal or ectomycorrhizal with Laccaria bicolor or Paxillus involutus in root boxes in a quartz sand/nutrient solution culture system. The seedlings were supplied with NH4NO3-N or NO3-N for 6 weeks. During the last 3 weeks, 5 mu M lead (Pb) was added to the nutrient solution. The pH at the rhizoplane of short roots measured with antimony microelectrodes was higher with NO3- nutrition than with NH4NO3 nutrition. In shoots, no Pb was detected. Total Pb in roots was reduced by mycorrhizal infection but was not affected by the N source. The portion of Pb that could be desorbed from roots by washes in 5 mM CaCl2 or 0.4 mM HCl was 47 or 10%, respectively. This portion comprised mainly apoplastic Pb and was neither affected by the N source nor by the presence of mycorrhiza. Mycorrhizal infection had no effect on the pH at the rhizoplane. The element content of semi-thin sections of mycorrhizal or nonmycorrhizal short roots was examined with X-ray microanalysis. The Ca and Mg contents in the cortex cell walls of both mycorrhizal and non-mycorrhizal seedlings were strongly increased with NO3- nutrition, indicating a higher cation exchange capacity of the cortex apoplast with NO3- nutrition. Despite of the higher binding capacity of the cortex cell walls, the Pb content was lower with NO3- nutrition than with NH4NO3 nutrition. In addition, the Pb content in the stele was also reduced with NO3- nutrition. Mycorrhizal infection had no effect on the Pb content of cell walls in the cortex and stele. In an accompanying solution culture experiment conducted under similar conditions, increased solution pH (pH 5 vs. pH 3.2) had a similar effect on Pb contents in root cortex cell walls as had NO3- nutrition in the sand culture experiment. We conclude that in forest ecosystems where NO3- is the main N source, increased rhizosphere pH may reduce Pb uptake into the root cortex.