Growth, nutrient uptake and ectomycorrhizal function in Pinus sylvestris plants exposed to aluminium and heavy metals

The potential role of aluminium (Al) toxicity to trees has been of particular concern to forest owners and scientists since the early 1980's when Ulrich hypothesised that both Al and heavy metals were involved in forest dieback because of their increased concentrations in soil due to acidification. Since then, numerous studies have examined the effects of metals upon nutrient uptake by plants. However, most of these investigations have been carried out in the absence of mycorrhizal fungi, which, in most ecosystems, are crucial components in nutrient uptake by plants. The present work focused on the effects of elevated concentrations of Al and heavy metals on Scots pine (Pinus sylvestris L.) and the potential role of ectomycorrhiza in modifying these effects. Ectomycorrhizal colonisation enhanced the growth and nutrient uptake by seedlings. To some extent, colonisation also alleviated reduced nutrient uptake which was a feature of seedlings growing in the presence of the metals. This effect was particularly noticeable with respect to P uptake. In general, mycorrhizal seedlings grew better and had an improved P, K, Mg and S status compared with non-mycorrhizal seedlings. Significant differences were also found in nutrient uptake among seedlings colonised by different fungi. One fungus, Hebeloma cf. longicaudum, was more sensitive to the Al treatment than the pine seedlings. The use of the base cation / Al ratio as an indicator of the potential detrimental effects to trees to acidification and Al is discussed. The production of oxalic acid was found to increase when mycorrhizal and nonmycorrhizal seedlings were exposed to Al or Cu. Colonisation by Suillus variegatus or Rhizopogon roseolus, in particular, resulted in a marked increase. These results demonstrate that there is a capacity, especially by certain ectomycorrhizal fungi, for increased production of the metal-chelating oxalic acid when root systems are exposed to increased levels of metals. In a field experiment, spraying with solutions of Ni/Cu sulphate or acidified water did not affect the growth of small pine trees. As part of the same experiment, defoliation was carried out on the pine trees in order to reduce carbon supply below-ground. Defoliation altered the proportions of different mycorrhizal morphotypes: Tuberculate types decreased and smooth types increased. The treatment did not affect the level of mycorrhizal colonisation of short roots, which was nearly 100%