Terrestrial plants often experience drastic temporal changes in photosynthetic photon flux density (PPFD) under natural light regime. Photosynthetic process in response to changing PPFD, hereafter referred to as dynamic photosynthesis, is expected to be affected by environmental temperature. In contrast to our knowledge on photosynthesis under constant PPFD, we know little about dynamic photosynthetic response to temperature, particularly in terms of its temperature dependence in plants acclimated to different temperature environments. Addressing this question will improve our understanding of dynamic photosynthesis in natural environment, where temperature elevation and increased temperature variability both occur under climate change.
In this study, we examined leaf photosynthetic CO2 exchange and fluorescence in Populus koreana × trichocarpa cv. Peace at all four temperatures (15°C, 20°C, 25°C, 30°C) for plants grown at the same four temperatures (15°C, 20°C, 25°C, 30°C), respectively. Using the hybrid species with little stomatal response to varying PPFD, we were able to address the following questions:
- How instantaneous changes in temperature (measurement temperature) affects photosynthetic response to sudden changes in PPFD?
- How growth temperature affects the above instantaneous effects, i.e., how plants acclimated to various growth temperature differ in their response to instantaneous changes in temperature?
Results/Conclusions
- Under in situ circumstances, i.e., when photosynthetic rate in response to sudden increase of PPFD were measured at the same temperature as the growth temperature, the time required to reach 50% of the steady-state photosynthetic rate (t50) decreased, and 1/τ, indicating acceleration of induction response, increased with temperature, until an optimum temperature at around 26°C.
- We then separated the effect of growth and measurement temperatures on dynamic photosynthesis. We found that steady-state photosynthetic characteristics such as Vcmax, Jmax and light-saturated photosynthetic rate changed pronouncedly with growth temperature, indicating a clear photosynthetic acclimation to growth temperature. However, the dynamic photosynthetic characteristics including t50 and 1/τ exhibited a much clearer trend along measurement rather than growth temperature gradient, though they also showed higher temperature optimum and less sensitivity in relation to measurement temperature when plants were grown at higher temperature.
These results suggest that, compared to steady-state photosynthesis, dynamic photosynthesis is less influenced by growth temperature and more dominated by measurement temperature; but temperature acclimation of dynamic photosynthesis is still critical in its determination of temperature optimum and sensitivity.