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).