Manganese accumulation and tissue-level distribution in Australian Macadamia (Proteaceae) species

Background and aims: Australian Macadamia species are known for their edible nuts as well as their ability to take up high concentrations of manganese (Mn) into their leaves. However, little is known about the ecophysiology of Mn in species of Macadamia. Methods: This study investigated (i) Mn accumulation in wild Macadamia using handheld X-ray fluorescence spectroscopy (XRF) of herbarium collections, and (ii) Mn uptake in three out of four known Macadamia species (M. tetraphylla, M. integrifolia and M. ternifolia) in a pot experiment that delivered Mn treatments in both solid (as natural Mn-minerals mixed with quartz sand) and liquid form (as MnSO4 solution added to soil). Tissue samples were analysed for bulk elemental concentrations and for in vivo distribution of Mn and other elements using laboratory μXRF analysis. Key results: The results revealed that wild M. ternifolia and M. integrifolia are strong Mn accumulators with up to 9600 μg Mn g−1 and 8500 μg Mn g-1, respectively, followed by M. jansenii (6400 μg Mn g-1) and M. tetraphylla (5100 μg Mn g-1). Under the controlled conditions, high Mn accumulation was demonstrated for M. integrifolia and M. tetraphylla with greater than 7500 μg Mn g−1 in mature leaves in the highest Mn treatment level. Our results further showed that Mn was efficiently translocated from the roots to the shoots with translocation factor >1 in the studied species. The laboratory μXRF analysis revealed that Mn was absent from the vascular bundles and concentrated in the areas between veins. Conclusions: All the four Macadamia species have high foliar Mn concentrations qualifying them as strong Mn accumulators in nature as determined by handheld XRF analysis of herbarium specimens. Under experimental conditions, M. integrifolia and M. tetraphylla were confirmed to be strong Mn accumulators. Manganese was shown by laboratory μXRF analysis to be concentrated in the inter-veinal areas of the leaf lamina