To what aspect of environmental variability does photosynthesis acclimate?

Photosynthetic acclimation occurs when plants are exposed to a long-term environmental change and, in response, undergo a biochemical or morphological change. Sensitivity analyses have shown that photosynthetic acclimation can have large impact on simulations of the terrestrial carbon cycle, however, the mechanisms underlying these responses are not fully understood. This uncertainty hinders the inclusion of photosynthetic acclimation in models. Here, I addressed one aspect of this uncertainty by quantifying how photosynthetic acclimation responds to different levels of temperature and light variability. Specifically, I asked whether plants acclimate to midday or average daily conditions. I utilized Glycine max in a 7-week growth chamber experiment with 4 different variability treatments that all had the same average daily conditions but varied in low/high variability for temperature and light. I found that the treatments did not have a significant effect on measured traits related to photosynthetic biochemistry, morphology, or plant biomass, despite treatments that varied by 5°C and 400 µmol m-2 s-1 PAR during the middle of the day. The only treatment effect I detected was an increase in Narea with increased high light variability (HLV). This indicates that plants in HLV were increasing their nitrogen, not for photosynthesis but possibly for light dissipation. That said, the majority of my findings indicate that plants did not acclimate to aspects of environmental variability. These results, combined with previous studies, suggest that plants acclimate to average conditions because the only similarity between my treatments was the average daily condition. My findings suggest that models should use average conditions to simulate acclimation responses.