Revisiting the Temperature Dependence of the Photorespiratory CO₂ Compensation Point (Γ*)

Area of Science:

• Plant Physiology
• Photosynthesis Research
• Biochemical Modeling

Background:

• The photorespiratory CO2 compensation point (Γ*) is vital for understanding Rubisco activity and photosynthesis. Current estimation methods, like the Laisk method, often rely on linear assumptions that conflict with theoretical photosynthetic responses.
• Existing temperature dependence relationships for Γ* (Γ*(T)) are frequently derived from linear implementations of the Laisk method, potentially introducing significant errors, especially at temperature extremes.
• These linear approaches can lead to unstable estimates and physiologically unrealistic results, such as negative CO2 release rates (D_L) at low temperatures.

Purpose of the Study:

• To re-evaluate the temperature dependence of Γ* and D_L using a more robust, nonlinear framework.
• To compare the performance of the improved Laisk-FvCB framework against conventional linear methods for estimating Γ* and D_L.
• To derive a revised, mechanistically constrained Γ*(T) relationship for C3 plants.

Main Methods:

• Utilized an improved Laisk-FvCB framework to analyze the nonlinear net CO2 assimilation rate (A_net) versus chloroplastic CO2 concentration (c_c) response across multiple irradiance levels.
• Conducted gas exchange measurements on sunflower leaves across a broad temperature range (3.9°C to 42.0°C).
• Constrained the nonlinear A_net-c_c response simultaneously across different irradiances to improve parameter estimation.

Main Results:

• The conventional linear Laisk method produced highly dispersed estimates and physiologically unrealistic negative D_L values at low temperatures.
• The Laisk-FvCB framework yielded physiologically meaningful temperature responses for Γ* and D_L, significantly reducing methodological artifacts.
• A revised Arrhenius-type function for in vivo Γ*(T) was derived: Γ*(T) = 43.4 exp[11.176 ((T - 25)/(T + 273.15))], with Γ*(25) = 43.4 μmol mol⁻¹ and an activation energy of 27.7 kJ mol⁻¹.

Conclusions:

• The Laisk-FvCB framework provides a more mechanistically sound and accurate approach for estimating Γ* and D_L temperature dependencies compared to linear methods.
• The derived Γ*(T) relationship offers a more reliable representation of photorespiration across a wider temperature range for C3 plants.
• Discrepancies between the revised Γ*(T) and existing formulations highlight the limitations of previous linear approaches, particularly at temperature extremes.