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.