2.7 | Activation state of Rubisco 
We evaluated whether photosynthesis at high temperature was limited by the activation state of Rubisco using the method described in Sage, Way & Kubien (2008). In this approach, measurements of the initial slopes of A-Ci curves are compared with estimates of these slopes that are based on the FvCB model that implicitly assumes that Rubisco is fully-activated in Eqn 9 below; lower values for the measured than the modeled slopes indicate Rubisco inactivation. By comparing the model against observations, the assumption in the model of fully-activated Rubisco is tested. Rubisco activase requires sufficient ATP to sustain high activity, and if the electron transport rate (i.e., JMax) is reduced at high temperature, the resulting reduction in ATP supply could inhibit Rubisco activase. Therefore, the temperature effect on Rubisco activase is assessed at low [CO2], where electron transport is not limiting.
The measured initial slopes for each A-Ci curve were determined with linear regressions using all data points for which Ci< 200 µmol mol CO2. The initial slopes were modeled as:
\(\text{Initial\ slope}\ =\ \frac{V_{\text{CMax}}}{\left(\Gamma^{*}\ +\ K_{\text{c\ }}\times\ \left(1\ +\ \ \frac{O}{K_{o}}\right)\right)}\)Eqn.
Kc and Ko are the Michaelis–Menten constants of Rubisco activity for CO2 and O2, respectively, and O is the oxygen concentration in the chloroplast stroma. O2 was assumed to equal 210 mbar, and Γ*, Kc and Ko were taken from Bernacchi et al . (2001).