High-temperature responses of photosynthetic parameters

Terrestrial ecosystems currently absorb approximately one-third of anthropogenic CO₂ emissions, constituting a central component of the global carbon cycle. However, the persistence of terrestrial carbon uptake under future warming and extreme heat events remains highly uncertain, in part due to limited understanding of how photosynthetic processes respond to thermal stress. In current land surface models, the temperature sensitivity of photosynthesis is often represented using simplified empirical formulations that inadequately capture physiological failure under extreme conditions.

The widely applied Farquhar–von Caemmerer–Berry (FvCB) framework commonly employs Arrhenius-type temperature response functions with fixed parameters derived from empirical fitting, which perform reasonably well near moderate temperatures but struggle to represent rapid declines in photosynthetic capacity at high temperatures. Moreover, how these limitations differ between C₃ and C₄ plants, despite their contrasting photosynthetic pathways and thermal strategies, remains poorly constrained at the global scale.

Here, we integrate a global meta-analysis drawing on published and newly compiled datasets within a mechanistic framework to assess the thermal responses of photosynthetic processes across C₃ and C₄ species. Our results indicate that even C₄ plants, despite their comparatively high thermal tolerance, exhibit pronounced enzymatic constraints under extreme heat. We further identify a coherent pattern in which biochemical and photochemical processes respond over a similar temperature range; however, biochemical limitations consistently arise at lower temperatures than photochemical limitations, suggesting that heat stress leads to metabolic failure prior to photochemical impairment.

These findings suggest that current temperature response formulations in land surface models may systematically overestimate photosynthetic stability under extreme heat, underscoring the need for improved mechanistic representation of thermal sensitivity to better project terrestrial carbon uptake under future climate extremes.