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).