Abstract
Clethra barbinervis Sieb. et Zucc. is a tree species that grows naturally at several mine sites and seems to be tolerant of high concentrations of heavy metals, such as Cu, Zn, and Pb. The purpose of this study is to clarify the mechanism(s) underlying this species’ ability to tolerate the sites’ severe heavy-metal pollution by considering C. barbinervis interaction with root fungal endophytes. We measured the heavy metal concentrations of root-zone soil, leaves, branches, and fine roots collected from mature C. barbinervis at Hitachi mine. We isolated fungal endophytes from surface-sterilized root segments, and we examined the growth, and heavy metal and nutrient absorption of C. barbinervis seedlings growing in sterilized mine soil with or without root fungal endophytes. Field analyses showed that C. barbinervis contained considerably high amounts of Cu, Zn, and Pb in fine roots and Zn in leaves. The fungi, Phialocephala fortinii, Rhizodermea veluwensis, and Rhizoscyphus sp. were frequently isolated as dominant fungal endophyte species. Inoculation of these root fungal endophytes to C. barbinervis seedlings growing in sterilized mine soil indicated that these fungi significantly enhanced the growth of C. barbinervis seedlings, increased K uptake in shoots and reduced the concentrations of Cu, Ni, Zn, Cd, and Pb in roots. Without root fungal endophytes, C. barbinervis could hardly grow under the heavy-metal contaminated condition, showing chlorosis, a symptom of heavy-metal toxicity. Our results indicate that the tree C. barbinervis can tolerate high heavy-metal concentrations due to the support of root fungal endophytes including P. fortinii, R. veluwensis, and Rhizoscyphus sp. via growth enhancement, K uptake promotion and decrease of heavy metal concentrations.
Highlights
Natural vegetation has been observed on the deposits containing high concentrations of heavy metals
The root fungal endophytes P. fortinii, R. veluwensis, and Rhizoscyphus sp. were separately grown on 1% malt extract agar (1% MA) and five mycelial disks (8-mm i.d.) on the edge of each mycelium were inoculated into a 500-ml Erlenmeyer flask containing 400 ml of 1% malt extract liquid medium
Heavy metal concentrations of root-zone soil, leaves, branches, and fine roots of C. barbinervis at our study site
Summary
Natural vegetation has been observed on the deposits containing high concentrations of heavy metals. These plant species must have evolved to adapt to the heavy-metal environment [1], because only plants showing heavy-metal tolerance can grow there. Et Zucc.) can be observed throughout the Hitachi mine forest, one of the three main copper mines of Japan in old days, C. barbinervis was not among the species used for tree planting in the early 1900s [2]. C. barbinervis is a deciduous, broad-leaved, pioneer tree species, which grows naturally at mine sites [5]. The heavy-metal tolerance mechanism(s) of C. barbinervis has yet to be clarified
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