Nickel hyperaccumulation in Orthion and Mayanaea (Violaceae) from Mesoamerica

Abstract

Research on metal hyperaccumulating plants has concentrated on New Caledonia, Brazil, Cuba, the Mediterranean basin, and southeastern Asia, while other regions remain under studied. This work used a systematic approach in the targeted search for new hyperaccumulators in Mesoamerica, with a focus on characterizing nickel (Ni) hyperaccumulation in six species of Orthion and the monotypic genus Mayanaea (Violaceae), to complement earlier global studies on the related genus Hybanthus. We screened major herbarium collections, using x-ray fluorescence (XRF) analysis to measure metal concentrations in specimens of Orthion and Mayanaea ranging from Mexico to Nicaragua. In addition, fresh samples collected in Mexico were analyzed with associated rhizosphere soils. The results indicated that all species of Orthion and Mayanaea are able to hyperaccumulate Ni. Ni concentration ranges in leaves of herbarium specimens were (in mg kg⁻¹ dry weight): Orthion guatemalense (380–5100), O. malpighiifolium (24–5620), O. montanum (1620–5850), O. oblanceolatum (60–1440), O. subsessile (450–18,700), and O. veracruzense (22–3660). The available specimen of Mayanaea caudata had foliar Ni concentration of 5390 mg kg⁻¹. Field-collected samples of O. veracruzense and O. subsessile had similar Ni concentrations to herbarium specimens. Additionally, many Orthion species were facultative hyperaccumulators of cobalt. This is the first report of Ni hyperaccumulation in Violaceae from the Western Hemisphere. Ni hyperaccumulation in Orthion and Mayanaea is consistent with published phylogenies showing that they belong to a clade which also includes strong hyperaccumulators in the genus Hybanthus from Australia and New Caledonia. Orthion subsessile has suitable traits to become a potential crop for agromining of Ni.