4.1 | Acclimation of photosynthesis to extreme
atmospheric and climate change conditions
Experimental research on photosynthetic responses to elevated
CO2 has focused on economically important species and on
temperate and boreal ecosystems, leaving a knowledge gap in the tropics
(Leakey, Bishop & Ainsworth 2012) where high temperatures are
increasingly common (Tiwari et al. 2020), especially in association with
El Niño events (Rifai, Li & Malhi 2019). Physiological processes in
tropical trees are obviously the same as those for temperate vegetation,
but temperature regimes in the tropics have long been relatively stable,
and current increases of the already high ambient temperatures in the
tropics may result in different thermal sensitivities than in
non-tropical species (Corlett 2011).
The observed shifts in the short-term temperature response of net
photosynthesis towards a higher optimum temperature under treatment
conditions is consistent with acclimation of photosynthesis to higher
temperatures (Yamori, Hikosaka & Way 2014; Way & Yamori 2014; Slot &
Winter 2017a). Furthermore, TOpt of
VCMax and JMax were moderately higher
under treatment than control conditions, consistent with observed
warming effects on other ecosystems (Kattge & Knorr 2007). However, at
400 ppm TOpt was almost 5°C higher in treatment plants
that experienced ~5°C higher daytime temperatures than
in controls, while TOpt commonly only increases by
~0.4°C per degree increase in growth temperature at
ambient CO2 (Yamori et al. 2014; Slot & Winter 2017a).
TOpt of net photosynthesis also increases when
temperature responses are measured at elevated CO2because the beneficial effect that elevated CO2 has on
reducing photorespiration is greater at higher than at lower leaf
temperatures (Brooks & Farquhar 1985). In the current study
TOpt increased regardless of measurement
CO2 (Fig. 1), but higher Ci in the
treatment plants can account for reduced photorespiration and higher
TOpt. The difference in TOpt between
control and treatment plants thus reflects a response to the combination
of elevated temperature and elevated CO2, consistent
with observations of additive effects of warming and elevated
CO2 on TOpt of two boreal tree species
(Dusenge et al. 2020).
Prolonged exposure to elevated CO2 can result in
feedback inhibition of photosynthesis if more carbohydrates are
synthesized than are exported to sink organs (e.g. Neales & Incoll,
1968). Accumulation of carbohydrates can then lead to reduced levels of
Rubisco protein, and a reduction in VCMax (Moore, Cheng,
Sims & Seemann
1999). VCMaxindeed tended to be lower in treatment than control plants across much
of the ecologically relevant temperature range (Fig. 3), as were net
photosynthesis and leaf nitrogen content. Furthermore, higher LMA in
treatment plants may indicate accumulation of carbohydrates.
Photosynthesis measured at 800 ppm—outside the carboxylation limited
range of Ci—was reduced as much as at 400 ppm in
treatment plants, so while there may have been feedback inhibition,
lower VCMax alone cannot account for reduced net
photosynthesis.
JMax was, however,
also reduced, as is commonly observed with plants grown at elevated
CO2 (Ainsworth & Rogers 2007), probably related to the
reduced nitrogen content in leaves of treatment plants (Table 1).
The mechanisms underlying thermal acclimation are not fully understood,
but a relationship between TOpt and the
JMax/VCMax ratio has long been known.
Here we found weak relationships, both for
JMax/VCMax at 30°C and
JMax/VCMax at their respective
TOpt (Fig. S5). Changes in the activation energy
(Ha, in Eqn 3) have been found to play an important role
in acclimation in some studies (e.g. Hikosaka et al. 2006; Kumarathunge
et al. 2019; Dusenge et al. 2020), whereas Kattge & Knorr (2007) found
that ΔS, the entropy term of the instantaneous temperature response, was
significantly impacted by plant growth temperature . In the current
study TOpt of P400 and
P800 scaled with ΔS of VCMax and with
TOpt of VCMax (Fig. S5). In contrast,
TOpt of P400 and P800did not correlate significantly with Ha of either of the
biochemical parameters. An issue with the use of ΔS is that it is
strongly influenced by the value of Hd (Stinziano et al.
2018), and Hd can generally not be reliably estimated
due to the lack of data points above TOpt of
VCMax. Even when using informed priors we were not able
to estimate Hd reliably and had to use a fixed value of
200 kJ mol–1. The resulting ΔS estimates, although
not very well-constrained, correlated much better with
TOpt of P400 and P800than the JMax / VCMax ratio, lending
support to Kattge & Knorr (2007).