Mycorrhizal infection and resistance to heavy metal toxicity in Calluna vulgaris

Abstract

Although studies of-the biology of mycorrhizas have repeatedly demonstrated their capacity to enhance nutrient capture in circumstances where essential elements are present in growth limiting quantities^1–3, knowledge of their function in environments containing potentially toxic concentrations of metallic elements is lacking. It is known that mycorrhizal infection can increase the uptake of copper and zinc from soil solutions containing low concentrations of these metals⁴, but any such increase in circumstances of high metal concentration would clearly be disadvantageous. Because of the large number of situations, both natural and man-made, in which heavy metal toxicity can restrict plant growth, it is of great ecological and applied interest to increase our understanding of those factors which permit growth of plants on heavily contaminated soils. One of the most successful colonists of such soils in northern Europe is the strongly mycorrhizal ericaceous plant Calluna vulgaris⁵. Calluna has a characteristic ‘ericoid’ mycorrhizal infection which, in contrast to the situation found in vesicular-arbuscular (VA) mycorrhizas, has been shown primarily to enhance nitrogen rather than phosphorus uptake^6,7. We have examined the possibility that mycorrhizal infection may influence the resistance of Calluna to high levels of heavy metals. The growth, survival and heavy metal content of two races of Calluna, one from a metal-polluted site and one from an unpolluted natural heathland, have been compared when plants were grown in the mycorrhizal (M) and non-mycorrhizal (NM) condition in sand cultures supplemented with different levels of copper and zinc. We report here that whereas NM plants show no tolerance of these metals at high concentrations, mycorrhizal infection provides a major degree of resistance to the toxicity and that infection leads to significant reduction of the heavy metal content of the shoot.