Green ‘blooded’ trees in New Caledonia: nickel hyperaccumulation in the tropics

Hyperaccumulator plants accumulate high concentrations of certain trace elements in their above-ground parts. This phenomenon is extremely rare, recorded in <0.2% of angiosperms but nickel (Ni) accumulation was found in 70% of hyperaccumulator species. Most hyperaccumulators are found in New Caledonia, Cuba and the Mediterranean Region.

Dr Antony van der Ent and his PhD student Adrian Paul from the University of Queensland with colleagues from France and New Caledonia investigated the occurrence of extremely nickel-rich green-coloured phloem tissues in the genus Hybanthus (Violaceae) in New Caledonia. The scientists found Hybanthus austrocaledonicus to be the only plant species outside of the Sabah/Palawan ultramafic rainforests or outside Central America which demonstrates this phenomenon. Synchrotron X-ray fluorescence microscopy (XFM) revealed that Ni is predominantly stored in the upper epidermis cell vacuoles of H. austrocaledonicus leaves and not in the mesophyll. 

In 1973 and 1980, two hyperaccumulating plants were found in New Caledonia, Hybanthus austrocaledonicus (accumulating up to 2.55 Wt% Ni) and H. caledonicus (accumulating up to 1.75 Wt% Ni). A herbarium specimen of H. austrocaledonicus from 1981 contained 6.80 Wt% Ni so the researchers went back to the original collection site to collect more samples to systematically assess the occurrence of Ni hyperaccumulation in H. austrocaledonicus and H. caledonicus populations. Dr van der Ent recently used micro-X-ray fluorescence (µ-XRF) to visualise nutrient uptake in hydrated plant tissues and investigated the frequency of hyperaccumulators of foliar elements in Malaysia. 

The researchers analysed 236 specimens at the Herbarium of New Caledonia and investigated the elemental concentrations of manganese (Mn), cobalt (Co) and Ni in three Hybanthus species endemic to New Caledonia using XRF scanning. Paul and colleagues  also collected samples of H. austrocaledonicus at Mont Dzumac for further analysis of macro-elements (Na, Mg, Al, P, S, K, Ca) and trace-elements (Cr, Mn, Fe, Co, Ni, Cu, Zn). They used cryo-scanning electron microscopy with energy dispersive spectroscopy (cryo-SEM-EDS) to visualise the distribution of Ni, C and O in frozen-hydrated leaf tissue  and XFM of leaf blades, bases, petioles and stems to visualise Ca, K, Ni and Zn in hydrated tissues. 

The Ni concentrations found in this study were much higher compared to previous findings from 1971 and 1983. The foliar concentrations for H. austrocaledonicus and H. caledonicus were 4.78 Wt% whilst the phloem concentration of H. austrocaledonicus  was 5.50 Wt% at Mont Dzumac. 

The XFM images revealed that Ni and Zn were mainly in the upper epidermis cell vacuoles of H. austrocaledonicus leaves but Ca was mainly concentrated adjacent to the epidermis and the sclerenchyma of vascular bundles. In the petiole, Ni was mainly concentrated at the cortex while Zn was located in the pith and around vascular tissues. In the stem, Ni and Zn distributions were alike and mainly concentrated in the bark.

“The same striking co-localisation patterns for Ni and Zn suggest similar storage mechanisms for these two metals in Hybanthus austrocaledonicus”, Paul and colleagues said. 

“To date, data remains scarce as highlighted by the low percentage [5%] of hyperaccumulators species investigated. Pursuing efforts by using newly developed technologies such as XFM (elemental distributions) and XRF (screening) coupled with transcriptomic investigations will enable a comprehensive understanding of the evolution of this fascinating phenomenon.”

Hyperaccumulator species not only hold many questions for scientists about plant physiology and adaptation but also can be used for phytomining on highly mineralised soils or on post-mine lands. The profit of a Ni phytomine in Australia is estimated to be ∼11,500 AU$/ha/harvest using the hyperaccumulator plant, Berkheya coddii. Find out more on the Global Hyperaccumulator Database.