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

Accurate estimation of the photorespiratory CO2 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 CO2 release in the light (DL) are commonly estimated using the Laisk method, based on measurements of net CO2 assimilation rate (Anet) at low chloroplastic CO2 concentrations (cc), 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 Anet-cc response predicted by the photosynthetic theory. Here, we revisited the temperature dependence of Γ* and DL using the improved Laisk-FvCB framework that simultaneously constrains the nonlinear Anet-cc 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 DL, including some physiologically unrealistic negative DL 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 C3 species.