Limited impact of arbuscular mycorrhizal fungi on clones of Agrostis capillaris with different heavy metal tolerance

Abstract

The present study addresses the question whether heavy metal (HM) tolerance of host plants determines how arbuscular mycorrhizal (AM) symbiosis affects their response to increasing HM stress. Two clones of the pseudometallophyte Agrostis capillaris, one originating from a HM-contaminated site (PH) and one from an uncontaminated site (NC), were grown along a gradient of polymetallic contamination and either inoculated or not with an HM-tolerant AM fungal strain of Rhizophagus irregularis.Plant growth and chlorophyll concentrations decreased with rising substrate contamination much more in the NC clone. This HM-sensitive clone also accumulated significantly more Pb, Cd, Zn and Cu in its shoots when grown in the contaminated substrates. Contrary to our hypothesis, these marked inter-clonal differences little affected the outcome of the interaction of A. capillaris with AM fungi along the contamination gradient. Inoculated plants showed generally higher chlorophyll concentrations and a lower carotenoids/chlorophyll ratio. The rise in the chlorophyll a/b ratio due to the AM inoculation in the most contaminated substrate indicated a role of AM fungi in the alleviation of intense HM-induced oxidative stress. The AM inoculation also increased plant P concentrations, especially in the roots, but this influence diminished with rising contamination. These promotional effects of the AM inoculation were, however, not reflected in plant growth, and inoculated plants showed unchanged or even reduced shoot dry weight. The impact of AM fungi on HM uptake and root-to-shoot translocation depended on the contamination level, clone and particular metal, without any significant effect recorded in the contaminated substrates for Pb, which was the main contaminant. In spite of evidencing certain positive effects of AM fungi on the physiological state of their hosts, our data provide little support for AM inoculation as an economically feasible strategy for A. capillaris-based revegetation of HM-contaminated sites.