No Significant Contribution of Arbuscular Mycorrhizal Fungi to Transfer of Radiocesium from Soil to Plants

Abstract

The diffuse pollution by fission and activation products following nuclear accidents and weapons testing is of major public concern. Among the nuclides that pose a serious risk if they enter the human food chain are the cesium isotopes 137 Cs and 134 Cs (with half-lives of 30 and 2 years, respectively). The biogeochemical cycling of these isotopes in forest ecosystems is strongly affected by their preferential absorption in a range of ectomycorrhiza-forming basidiomycetes. An even more widely distributed group of symbiotic fungi are the arbuscular mycorrhizal fungi, which colonize most herbaceous plants, including many agricultural crops. These fungi are known to be more efficient than ectomycorrhizas in transporting mineral elements from soil to plants. Their role in the biogeochemical cycling of Cs is poorly known, in spite of the consequences that fungal Cs transport may have for transfer of Cs into the human food chain. This report presents the first data on transport of Cs by these fungi by use of radiotracers and compartmented growth systems where uptake by roots and mycorrhizal hyphae is distinguished. Independent experiments in three laboratories that used different combinations of fungi and host plants all demonstrated that these fungi do not contribute significantly to plant uptake of Cs. The implications of these findings for the bioavailability of radiocesium in different terrestrial ecosystems are discussed.

Mycorrhizal association of maritime pine, Pinus pinaster, with Rhizopogon roseolus has contrasting effects on the uptake from soil and root-to-shoot transfer of 137Cs, 85Sr and 95mTc

“…This contrasts with the findings of Joner et al (2004) who recently concluded that arbuscular mycorrhizal fungi are unlikely to have any significant effect on Cs uptake by plants. However, other studies, including some on tree species, are less categorical.…”

Role and influence of mycorrhizal fungi on radiocesium accumulation by plants

“…Indeed, depending on the plant and AM fungal species and on the experimental conditions, the accumulation of radiocesium in mycorrhizal plants can be lower (Dighton and Terry, 1996;Berreck and Haselwandter, 2001), similar (Rogers and Williams, 1986;Rosén et al, 2005) or even higher (Mc Graw et al, 1979;Entry et al, 1999;Rosén et al, 2005) than in non-mycorrhizal plants. However, as noticed by Joner et al (2004), the experimental setups used in these studies did not assess fungal transport or immobilization, but rather the combined effect of AM fungi and root uptake on plant radiocesium accumulation. In addition, the participation of AM fungi in the immobilization or transport of radiocesium could not be evaluated by the sole observation of its accumulation in plants as plant-AM fungus associations and experimental conditions influenced plant growth and root-to-shoot biomass partition resulting in the well-known ''dilution effect'' (Timmer and Leyden, 1980;Smith and Read, 1997;Clark and Zeto, 2000).…”

“…However, neither the concentrations of K nor radiocesium in the soil solution were measured in these experiments. Joner et al (2004) concluded that arbuscular mycorrhizas are unlikely to play any significant role in plant radiocesium acquisition through the process of fungal transport, especially in agricultural ecosystems. However, Dupré de Boulois et al (2005b) suspected that the uptake of radiocesium by AM fungi in this study could have been strongly impaired by the sorption capacity of clay, the bio-availability of K in the soil solution and/or plant K nutritional status.…”

“…Within the plants, 90% of the 134 Cs was located in the roots, and thus only 10% in the shoot. Joner et al (2004) performed a new study using compartmented pot devices in order to quantify, in vivo, the role of AM fungi in the transport of radiocesium. To do so, three experiments were conducted in different laboratories using different protocols (i.e.…”

Use of mycorrhizal fungi for the phytostabilisation of radio-contaminated environment (European project MYRRH): Overview on the scientific achievements

“…Three separate experiments performed by three laboratories using different protocols and fungus/host plant combinations [19] to investigate whether AM fungi can transport Cs in soil-based systems. Here only the first experiment will be discussed.…”