Polymetallic (zinc and cadmium) hyperaccumulation in the Australian legume Crotalaria novae-hollandiae compared to Crotalaria cunninghamii

Abstract

Abstract Purpose Hyperaccumulators are plants with the ability to tolerate and accumulate high concentrations of potentially phytotoxic metals. The Australian legume Crotalaria novae-hollandiae accumulates remarkably high concentrations of zinc (Zn), cadmium (Cd) and copper (Cu) in its shoots when growing on metalliferous (Zn-Cd ‘calamine’) soils. This study aimed to investigate zinc-cadmium tolerance in C. novae-hollandiae and to compare it with the closely related, but non-metalliferous, C. cunninghamii. Methods Crotalaria cunninghamii and C. novae-hollandiae were exposed to Zn (3–1000 μM) and Cd (0–60 μM) treatments in hydroponics culture. At the end of the experiment, harvested plants were segmented into roots, old and young stems, old and young leaves for elemental analysis with Inductively coupled plasma atomic emission spectroscopy (ICP-AES). Laboratory-based micro-X-ray fluorescence (μXRF) analysis was used to elucidate elemental distribution in a shoot and in leaflets. Results Crotalaria cunninghamii accumulated up to 1210 μg Zn g−1 and 47.6 μg Cd g−1 in its leaves, with a 75% reduction in biomass in the Zn treatment. Crotalaria novae-hollandiae accumulated up to 16,600 μg Zn g−1 and 1250 μg Cd g−1, with a 70% increase in biomass when exposed to Zn. The species both exhibited chlorosis and stunted growth in the Cd treatments, while only C. cunninghamii exhibited toxicity symptoms in Zn treatment. Conclusions Crotalaria novae-hollandiae has limited tolerance for Cd and based on the accumulation and distribution of foliar Zn and Cd it is suspected that C. novae-hollandiae has different uptake and tolerance mechanisms when compared to other widely studied Zn-Cd hyperaccumulators (such as Noccaea caerulescens and Arabidopsis halleri).