INTRODUCTION
Hyperaccumulators are unique plants able to concentrate extraordinarily high concentrations of specific trace elements in their foliage and other aerial parts (Baker & Brooks, 1989; Reeves, 2003; van der Entet al. , 2013). They achieve such extreme levels of accumulationvia enhanced uptake and translocation mechanisms that are yet to be fully understood (Baker, 1981; Baker, 1987). Manganese (Mn) hyperaccumulation is recognised at the notional threshold concentration of 10 000 µg g-1 Mn in dry weight shoot tissue (van der Ent et al., 2013). The hyperaccumulation of Mn is a rare trait documented primarily within the genera Alyxia , Denhamia (Maytenus) , Gossia, Grevillea, Macadamia andVirotia distributed over eastern Australia and New Caledonia (Losfeld et al., 2015, Fernando et al., 2008, Jaffré, 1977, Jaffré, 1980, Fernando et al., 2009b), Malaysia (Nkrumah et al., 2018) and recently from Papua New Guinea (Do et al., 2019).
There are 20 Australian Gossia species with a wide latitudinal distribution, ranging from northern New South Wales (320S) to the northern tip of the Cape York Peninsula (100S) in Queensland (Snow et al., 2003). Gossia bidwillii and G. acmenoides (Myrtaceae) have a very smooth bark which is irregularly covered with relatively large, coloured patches. These species, as well as G. lucida and G. grayi, are called “python bark” Gossia’s due to the resemblance of their bark to the skin colouring of the python snake (Snow et al., 2003).Gossia bidwillii , is the only Australian Gossia to thrive on ultramafic soils (McLay et al., 2019), and also the first Mn hyperaccumulator described in Australia (Bidwell et al. , 2002). That discovery instigated subsequent research on Mn hyperaccumulation in several other Australian Gossia species (Fernando et al. , 2007; Fernando et al. , 2008a; Fernando et al. , 2009b; Fernando et al. , 2013, McLay 2018). All these published studies have been based on freshly collected field samples or preserved material obtained from herbaria. There have been certain consistent observations throughout, for example, the Mn hyperaccumulative trait in Gossia bidwillii , and the Mn non-hyperaccumulation by G. acmenoides, as captured in a recent phylogenetic study of Gossia (McLay et al 2018). While these two species are known to be sympatric on Mn-rich soils, they appear taxonomically partitioned into separate clades (McClay et al (2018).
Studies on fresh field material have reported foliar Mn concentrations of 19 200 µg g-1 in G. bidwillii (Bidwell et al., 2002). Recent growth experiment on G. fragrantissima has shown it can take up to 545 µg g-1 Co, 17 400 µg g-1 Mn and up to 13 000 µg g-1 Zn (Abubakari et al. 2021a) whereas freshly collected field samples ofG. grayi and G. shepherdii were observed to contain up to 13 700 µg g-1 and 11 000 µg g-1foliar Mn respectively (Fernando et al., 2018). In vivocryo-scanning electron microscopy (SEM)/energy dispersive X-ray analysis (EDS) showed Mn localization inG. bidwillii to be different from other hyperaccumulating species. Foliage hyperaccumulated metals are usually known to accumulate in non-photosynthetic tissues such as the epidermal cells and associated dermal structures including trichomes and leaf hairs (Vázquez et al., 1992, Küpper et al., 2000, Küpper et al., 2001, Krämer et al., 1997, Mesjasz-Przybylowicz et al., 2001, Bhatia et al., 2003, Bidwell et al., 2004, Broadhurst et al., 2004), whereas in G. bidwillii Mn was found to be primarily localised in photosynthetic cells (Fernando et al., 2006b, Fernando et al., 2006a, Fernando et al., 2007). Laboratory and synchrotron X-ray Florescence Microscopy (XFM) have revealed marginal accumulation of Co, Mn and Zn in leaves, with localization of Co, Mn and Zn in epidermal cells of G. fragrantissima (Abubakari et al. 2021a).
To date, no attempt has been made to examine the effects of Mn dosing treatments on G . bidwillii under controlled experimental conditions, mainly due to the relatively slow growth rate of woody species such as this. Neither have there been any studies to examine Mn accumulation of G. bidwillii to Mn accumulation by a closely related species. Gossia acmenoides was selected here for a field investigation into Mn accumulation in its natural habitat because it is commonly sympatric to G. bidwillii . This study aims to: i) measure the response of propagated G. bidwillii plants to Mn treatment under controlled conditions, and ii) assess Mn uptake and accumulation in the aforementioned G. bidwillii plants with that of Gossia acmenoides on naturally Mn enriched soils. By employing laboratory XFM to determine in situ distributions of Mn and other elements in G. bidwillii and G.acmenoides, this study also investigates Mn distribution patterns in the leaf tissues of these two species, as well as test within-species age-related distributional differences, i.e ., between their young and old leaves.