Effects of heavy metals on nitrogen uptake by Paxillus involutus and mycorrhizal birch seedlings

Abstract

The effects of the heavy metals Cu, Cd, Ni, Pb and Zn on [ 14C]methylamine and [ 14C]aminoisobutyric acid uptake were studied in the free-living fungus Paxillus involutus and in mycorrhizal and non-mycorrhizal birch roots. The uptake of both N sources by P. involutus was inhibited by the five metals tested. However, Cu 2+ and Pb 2+ had a greater inhibitory effect. Non-competitive inhibitions were determined between heavy metals and [ 14C]methylamine uptake. [ 14C]Methylamine uptake was reduced by one third by 2 μM Cd 2+ and Cu 2+ in non-mycorrhizal roots, whereas that of mycorrhizal roots was not affected. However, it was reduced by 30 to 80% by 200 μM Cd 2+ and Cu 2+ irrespective of the mycorrhizal status. [ 14C]Aminoisobutyric acid uptake in mycorrhizal roots was not significantly affected by Cd 2+ and Cu 2+, whereas that of non-mycorrhizal roots was decreased by 77% at 200 μM Cu 2+. [ 14C]Aminoisobutyric acid uptake was 4.5 to 6 fold higher in mycorrhizal roots, compared with non-mycorrhizal roots, even under metal exposure. The high efficiency of N acquisition by mycorrhizal birch seedlings under metal exposure might be regarded as a mechanism of stress avoidance.