DISCUSSION
Foliar total phosphorus and nitrogen
concentrations
Our first hypothesis was that with decreasing soil P availability, the
mass-based foliar total P
concentrations of both B. attenuata and B. sessilis would
decrease in response to decreased P availability, and this was
supported. An important finding was that the area-based foliar P
concentrations was higher in B. attenuata than in B.
sessilis in all three substrates, but there was no significant
difference in mass-based foliar P concentration in any of the three
substrates. We found the same patterns of mass-based foliar total P
concentration for both B. attenuata and B. sessilis grown
in all three substrates; the foliar total P concentrations of both
species were greatest in sand, and about 35% lower in SLIM. The
greatest carboxylate-extractable P concentration was in sand, and the
lowest was in limestone gravel (Fig. S1).
In contrast to the similar mass-based foliar total
P concentrations in the two
species in each substrate, the mass-based foliar total N concentration
of B. sessilis was almost twice that of B. attenuata grown
in the same substrate. Leaf N
concentrations in B. attenuata and B. sessilis grown on
SLIM were approx. 20% lower than those in plants grown in sand and
SLAT. Our result differ results on Hakea prostrata (Proteaceae)
showing that P availability did not influence leaf N concentration
(Prodhan et al. , 2016). The foliar total N concentrations of both
species were very low compared with those of plants from other
environments (Reich et al. , 1991), which reflects the low foliar
rRNA concentration in B. attenuata (Sulpice et al. , 2014),
and, presumably, in B. sessilis , based on the similar size of
their nucleic acid P pools. In our study, the relatively low foliar N
concentrations in B. attenuata and B. sessilis indicate
that protein concentrations were very low, which implies a low demand
for rRNA and, thus,
P.
Whilst the leaf N concentrations in both species were low compared with
the global average (Reich et al. , 1991), they were distinctly
higher inB.
sessilis than in B. attenuata . The higher N concentration
correlated with greater allocation of P to the nucleic acid fraction inB. sessilis . However, rates of photosynthesis and leaf N
concentrations expressed on an area basis were similar for the two
species, and hence so was the photosynthetic N-use efficiency (PNUE).
Therefore, the ‘extra’ N in B. sessilis on a mass basis was a
reflection of a lower investment in sclerenchymatic tissue, as evidenced
by its lower LMA). A low ribosome abundance can be expected to decrease
the rate of protein synthesis, and hence the protein and N
concentrations. Therefore, lower leaf N concentrations in B.
attenuata compared with B. sessilis on the three substrates
tested is consistent with lower rRNA concentrations and lower rates of
protein synthesis. However, since we studied mature non-growing leaves,
which do not rapidly change in protein concentration (Kuppusamy et
al. , 2014), the faster rate of protein synthesis must have been
balanced by faster rate of protein breakdown, and hence protein
turnover.
We found that RGR was strongly correlated with leaf N and nucleic acid P
concentrations in both species, and that RGR and N concentration inB. sessilis were significantly greater than those in B.
attenuata . This supports our second hypothesis that B. sessilis ,
which exhibits a more opportunistic growth strategy than B.
attenuata (Shi et al. , 2020), will have a higher foliar
NTotal : PTotal ratio than B.
attenuata and invest more P in nucleic acid P to support the higher N
concentration. The higher leaf N concentration found here and higher
capacity to acquire P (Shi et al. , 2020) in B. sessilisthan in B. attenuata when grown in the more P-limiting SLIM may
explain the different distribution patterns of the two species in the
environment. This higher capacity to acquire P presumably allows it to
colonise and become established on different P-impoverished soils (sand
over laterite or over limestone), compared with B. attenuata,which is restricted to deep sand (FloraBase, http://florabase.
dpaw.wa.gov.au/).
Foliar traits and P
fractions
The different foliar P-allocation patterns combined with differences in
LMA between the two species reflects differences in their life history
strategies and resource requirements. Plants like B. sessiliswith an r selection life history typically grow fast (Clarkeet al. , 2013) and produce seeds before the next catastrophe,i.e. fire or drought (Bowen & Pate, 2017, Knox & Clarke, 2005,
Pate et al. , 1990). This strategy
may require relatively greater
investment in P-rich rRNA and, thus, ribosomes, to support rapid protein
synthesis and turnover, including replacement of damaged proteins
(Raven, 2012). A high protein synthesis capacity may provide flexibility
to acclimate to variable and changing environments (i.e. shallow
sand over laterite or limestone, where water availability may fluctuate)
and complete the life cycle quickly. Unlike B. sessilis , B.
attenuata with larger seeds (Shi et al. , 2020) and higher LMA,
has the ability to resprout from epicormic buds or lignotubers (Groom &
Lamont, 2011, Pate et al. , 1991), a strategy associated with a
slower RGR (Bowen & Pate, 2017, Knox & Clarke, 2005, Pate et
al. , 1990). Thus, selection inB. attenuata was based on a
lower investment in nucleic acid P, as well as the ability to allocate
more biomass to deep roots compared with B. sessilis (Shiet al. , 2020). Thus, it does not need to grow fast and complete
its life cycle quickly (Bowen & Pate, 2017, Knox & Clarke, 2005, Pateet al. , 1990).
The Pi concentration in slow-growing B. attenuata was higher than
that in the faster-growing B. sessilis when grown in sand and
SLAT, slightly higher than when grown in SLIM. Cell vacuoles serve as a
reservoir for excess Pi in most plants, which can then be drawn upon as
P availability decreases (Mimura, 1995). Changes in total foliar P
concentration with Pi supply generally reflect the accumulation of Pi in
vacuoles, which is typically greater in slow-growing species than in
fast-growing ones (Güsewell, 2004). Thus, fast-growing species convert
Pi into growth-sustaining organic P, rather than accumulate Pi, as in
slow-growing species.
The metabolite P concentrations
and the proportions of total P in metabolites for B. sessiliswere significantly higher than those for B. attenuata for
plants grown in SLIM. Moreover, the PPUE of B. sessilis was
greater than that of B. attenuata grown on all substrates. Hidaka
& Kitayama (2009) suggested that high PPUE is sustained by the
allocation of a greater proportion of P to metabolic P (metabolite P +
Pi) than to structural P, as we have showed here. In addition, B.
sessilis had a lower LMA than B. attenuata on all substrates
tested; however, B. attenuata had higher lipid P concentrations
when grown in SLAT and SLIM in response to the lower P availability
compared with sand alone. This finding was partially in line
with a study that showed that the
concentration of structural P is greater in slow-growing plants with
high LMA than in fast-growing plants with low LMA (Villar et al. ,
2006). In other words, a greater
proportion of nucleic acid P, a lower proportion of lipid P and a lower
LMA in B. sessilis than in B. attenuata are all traits
associated with a higher RGR and shorter leaf life-span (Veneklaaset al. , 2012). Overall, the
unique foliar traits of the two species revealed different patterns of P
allocation in response to soil P availability and associated with growth
strategy that may define the ecological niches in which they are found
(Figure 6).