4 | DISCUSSION
The tropical tree species Tabebuia rosea showed clear signs of
photosynthetic acclimation to elevated temperature and
CO2: treatment plants had moderately higher thermal
optima for photosynthetic parameters VCMax and
JMax, and downregulated respiration rates at high
temperature, thereby improving photosynthetic performance at treatment
conditions and supporting our hypothesis of acclimation. Higher thermal
optima of net photosynthesis were observed regardless of measurement
CO2 concentration, indicating that the shift in the
optimum temperature was not solely due to suppression of
photorespiration by elevated CO2 in the treatment dome.
The observed reduction in VCMax in treatment plants is
consistent with acclimation to elevated CO2. However,
reduced VCMax has also been observed as a result of
warming alone—e.g. in (sub-) tropical Eucalyptus provenances
(Crous et al. 2018)—suggesting that the observed decrease in
VCMax can be related to increased CO2,
increased temperature, or a combination of the two. Indeed, in the
tropical tree species A. glandulosa neither warming nor elevated
CO2 significantly reduced VCMax, but the
combination of the two did (Fauset et al. 2018)., In contrast to common
observations in studies in which only CO2 is increased
(e.g., Ainsworth & Rogers 2007; Klein & Ramon 2019), stomatal
conductance and stomatal density were not reduced in our combined
warming and CO2 enrichment experiment, suggesting that
the CO2 effect may be temperature dependent.
As hypothesized, net photosynthesis of leaves developed under control
conditions and transferred to treatment conditions for 1–2 weeks, did
not acclimate to the same extent as leaves that had developed under
treatment conditions, as indicated by smaller shifts in temperature
optima, and greater overlap in the credible intervals of parameter
estimates. We did not find support for our hypothesis that stomatal
conductance would exert stronger control over net photosynthesis under
treatment conditions; plants were not more inhibited by low stomatal
conductance under treatment conditions than under control conditions.
Accounting for stomatal conductance by examining temperature responses
at a fixed Ci showed that stomatal conductance had only
a small influence on TOpt, as the shifts in
TOpt were comparable for P270 and
P505 as they were for P400 and
P800. The activation of Rubisco was not limiting net
photosynthesis under growth conditions, neither in control nor treatment
plants, but transferred plants exhibited signs of Rubsico activase
limitation. Collectively, these results indicate that T. roseaplants exhibited strong acclimation to the combined effects of 4°C
warming and doubling of CO2. Acclimation appeared to be
underpinned by changes in the biochemical parameters, particularly in
shifts in TOpt of VCMax associated with
adjustment of ΔS.
Our Bayesian approach for fitting temperature response curves was
motivated by complications associated with fitting these nonlinear
models with ordinary least square regression.
With five saplings per treatment
the plant-level replication in our study was modest, and small sample
size may constrain the ability to accurately identify treatment effects.
The Bayesian approach enabled us to more reliably estimate how
treatments affected the key parameters, and to more thoroughly probe the
uncertainty around the estimated treatment effects. This is valuable for
difficult-to-estimate model parameters such as TOpt:
Uncertainty of TOpt can be asymmetric, especially when
constrained by the lower or upper limits of temperature control of leaf
cuvettes of commercially available portable photosynthesis systems. For
example, the posterior distributions of TOpt of
P400 were left skewed, because of the small number of
observations below TOpt, and those of
VCMax tended to be right skewed (Fig. 2). Even with few
data points above TOpt of VCMax and
JMax, its estimates were relatively well constrained
with the help of informed priors, with the 10% and 90% percentiles
separated by on average only 2.3°C.