DISCUSSION
This study details the first-ever growth experiment on the Mn
hyperaccumulating tree, G. bidwillii , exposed here to a range of
Mn treatments and reports for new discovery of the Mn hyperaccumulative
trait in wild G. acmenoides , on the highly acidic Mn-rich soils
is intriguing given past analyses of samples sourced from a range of
other eastern Australian locations had not detected Mn hyperaccumulation
in this species (Fernando et al. 2009b). Foliar Mn concentrations in
experimental G. bidwillii to as high as ~3-fold
Mn hyperaccumulation threshold of 10 000 µg g-1 (van
der Ent et al. 2013) aligns with field data from earlier studies of this
species (Bidwell et al. 2002; Fernando et al. 2006b). A weak
relationship between substrate Mn supply and foliar Mn concentrations,
most notably, high Mn uptake by control plants is also consistent with
past field observations (Bidwell et al., 2002, Fernando et al., 2006b,
Fernando et al., 2007). The ability of Gossia bidwillii as shown
experimentally here and as previously noted in the field to vastly
over-accumulate Mn even in very low soil-supply is a common
characteristic hyperaccumulation, i.e., the ability to scavenge
from host substrates (Baker 1981; (Fernando et al., 2007).
Contrasting findings of previous field studies that have not foundG. acmenoides to hyperaccumulate Mn, and this present field study
describing strong Mn hyperaccumulation by this species may be
attributable to genetic differences, as also observed in Denhamia
founieri from New Caledonia by Fernando et al. (2008). The present
findings on G. acmenoides suggests heterogeneity across the
species, warranting further investigation of the genetic basis of Mn
hyperaccumulation in Gossia, for example, as interrogated by
Pollard et al (2002) for ubiquitous metal hyperaccumulating herbs of the
Northern Hemisphere. Variability of Mn hyperaccumulation trait inG. bidwillii has previously been reported, however there is
greater consistency across its broad natural range in comparison to the
emerging picture of G. acmenoides (Fernando et al., 2007).
Heterogeneity of metal accumulation has also been described in several
species of hyperaccumulators of metals other than Mn (Pollard et al.,
2002, Baker et al., 1994, Macnair, 2002). The extreme acidity of the
host soil (pH 4.20, Table 3) from which G. acmenoides was sampled
for this study warrants consideration in the context of Mn availability
at the root-soil interface. It is plausible that the apparently high
variation in Mn accumulation by G. acmenoides reflects specific
rhizosphere effects such as acidification and/or microbial associations
unique to a particular site location that renders soil-Mn highly
bioavailable. These findings yet again highlight the gaps in knowledge
around metal hyperaccumulation, with woody species such as Gossiaspp. poorly understood. At locations where G. acmenoideshas previously been found to accumulate\souts low levels of Mn, there
may also be ion competition among similar divalent cations such as
Ca2+, Mg2+ and
Mn2+ in addition to less acidic soil conditions
(Fernando et al., 2013, McLay et al., 2019, Bidwell et al., 2002). It is
notable that high concentrations of Ca and Mg relative to Mn were
reported in leaves of G. acmenoides (Bidwell et al. (2002).
In G. bidwillii , the behaviour of Mn in the oldest leaves
resembled that of Ca, Mg and Na which remained high at all treatment
levels but contrasted with that of K and P which decreased after
maturity. However, in G. acmenoides , K was high in young leaves
than in old leaves, whereas Ca, Mg, Na and Mn were higher in old leaves.
The high concentrations of Mn, Ca, Mg and Na in old leaves of G.
bidwillii and G. acmenoides could be attributed to phloem
immobility of the aforementioned elements (Marschner, 2002, Graham et
al. 1988) and vice versa for K in G . acmenoides andG. bidwillii . Moreover, this behaviour could be due to the
similarities in divalent cations of Mn, Ca and Mg. Similar observation
of high Mn in old leaves has been reported in G. bidwillii(Bidwell et al., 2002) and in other Mn hyperaccumulators includingPhytolacca americana (Xu et al., 2006), Macadamia
integrifolia (Fernando et al., 2009a), G. fragrantissima(Abubakari et al. 2021) and in crop plants (Millikan, 1951). In
contrast, G. grayi and G. shepherdii were reported to
accumulate higher Mn concentrations in young leaves than in older leaves
(Fernando et al., 2018). A previous report by Bidwell et al. (2002) of
decreased Ca and Mg with an increase in Mn concentration in old leaves
of G. bidwillii contradicts the findings of this study. The
nutritional dynamics of plants as unusual as metal hyperaccumulators are
yet to be fully understood, and clearly cannot be assumed to align with
broader understanding of plant nutrition largely drawn from crop models
(Marschner 2002).
The phytoavailability of Al and Mn are known to occur in soils of low pH
(<5), however the pH of soils on which G. bidwillii was
cultivated in this present study shows that the solubility of Al was low
(>5), yet G. bidwillii was able to take up high
amounts of Al in old leaves which qualifies it as an Al hyperaccumulator
with concentration 6-fold higher than the Al hyperaccumulation threshold
set at 3000 µg g-1 (Jansen et al., 2003, Jansen et
al., 2001). Exceptionally high Al concentrations in old G.
bidwillii leaves, even under Mn treatment, could be facilitated by
organic anions involved in Mn transport (Bidwell et al., 2002). This
suggests that G. bidwillii may be able to take up Al via
an anion channel, a mechanism that appears to be a peculiar trait among
Al tolerant species (Zhang et al., 2001, Ryan et al., 1997, Piñeros and
Kochian, 2001, Kollmeier et al., 2001). It should be cautioned here that
while these observations of Al over accumulation by G. bidwilliihave been made under experimental conditions, it has not been observed
in the field, even on lateritic soils rich in Al (Fernando, Bidwell
etc). The notable limit in uptake of Al in G. acmenoides even
though its soil pH was suitably low for Al mobilisation for plant
uptake, was most likely due to the very low soil Al concentrations.
Furthermore, it has also been suggested that Al inhibits uptake of Ca
and Mg in non- Al accumulators (Kochian et al., 2005, Ryan and Kochian,
1993, Rengel and Zhang, 2003), and this was found in old leaves ofG. acmenoides in this present study. Species within the Myrtaceae
family have been listed to contain Al hyperaccumulators (Jansen et al.,
2003, Jansen et al., 2001) and previous studies by Fernando et al.
(2009b) have shown that other Gossia spp., includingG. hillii , G. inophloia , G. lewensis and G.
macilwraithensis, can be Al hyperaccumulators after they were exposed
to Al treatments.
The distribution patterns of Mn in leaves in wild G. acmenoidesand Mn-dosed G. bidwillii have been shown by XFM to be distinctly
different (Figs. 3 and 4). Strong Mn distribution throughout young and
old leaves in wild G. acmenoides , while highly concentrated at
the apex and lamina of treated G. bidwillii suggest that Mn
movement with the transpiration stream rather than against it. This
indicates that, increasing transpiration rate throughout the leaves ofG. acmenoides , and towards the apex and lamina of G.
bidwillii led to higher Mn accumulation in those parts of the studied
species. Similar observations to that of G. bidwillii have been
reported for the Mn hyperaccumulators Acanthopanax
sciadophylloides (Memon et al., 1980) and G. fragrantissima(Abubakari et al., 2021a).
This study newly revealed the strong and facultative Mn
hyperaccumulative trait in G. acmenoides and confirmed it inG. bidwillii , demonstrating both species to be strong Mn
hyperaccumulators with concentrations in both young and old leaves well
exceeding the Mn hyperaccumulation threshold. Laboratory based-XFM
revealed distinct Mn distribution patterns in the leaves of G.
acmenoides and G. bidwillii . Further work should be undertaken
using synchrotron X-ray Florescence Microscopy with more precision and
higher resolution to investigate Mn distribution at the cellular and
subcellular levels in order to elaborate hypotheses for its metabolic
pathways to be elucidated with genetic studies.