Revisiting the Temperature Dependence of the Photorespiratory CO ₂ Compensation Point (Γ*)

ABSTRACT

Accurate estimation of the photorespiratory CO ₂ compensation point (Γ*) is essential for describing the balance between Rubisco carboxylation and oxygenation and for parameterising biochemical models of photosynthesis. Γ* and the rate of CO ₂ release in the light ( D _L ) are commonly estimated using the Laisk method, based on measurements of net CO ₂ assimilation rate ( A _net ) at low chloroplastic CO ₂ concentrations (c _c ), under several sub‐saturating irradiance levels. However, many widely used temperature dependence relationships for Γ* (Γ*( T )) were derived using conventional linear implementations of the Laisk method, despite the intrinsically nonlinear behaviour of the A _net –c _c response predicted by the photosynthetic theory. Here, we revisited the temperature dependence of Γ* and D _L using the improved Laisk–FvCB framework that simultaneously constrains the nonlinear A _net –c _c response across multiple irradiance levels. Gas exchange of sunflower leaves was measured across a wide temperature range from 3.9°C to 42.0°C. The conventional linear implementation generated highly dispersed pairwise intersections and unstable estimates of both Γ* and D _L , including some physiologically unrealistic negative D _L values at low temperatures. In contrast, the mechanistically constrained Laisk–FvCB framework produced physiologically meaningful temperature responses and substantially reduced methodological artefacts associated with linear extrapolation. Using this framework, we derived a revised in vivo Γ*( T ) relationship described by an Arrhenius‐type function with Γ*(25) = 43.4 μmol mol ⁻¹ and an apparent activation energy of 27.7 kJ mol ⁻¹ , such that Γ*( T ) = 43.4 exp[11.176 (( T  − 25)/( T  + 273.15))], where T is leaf temperature in °C. Comparison with other widely used Γ*( T ) formulations showed substantial divergence at temperature extremes, often exceeding the variability expected from realistic interspecific differences in Rubisco specificity among C ₃ species.