2.5 | Temperature responses of photosynthesis
parameters
The temperature responses of VCMax and
JMax were fitted according to Medlyn et al .
(2002) as:
\(\text{f\ }\left(T_{k}\right)=k_{\text{Opt}}\times\ \frac{H_{d}\ \times\ e^{\left(\frac{H_{a}\ \times\ \left(T_{k}\ -\ T_{\text{Opt}}\right)}{T_{k}\ \times\ R\ \times\ T_{\text{Opt}}}\right)}}{H_{d}-\text{\ H}_{a}\ \times\ \left(1-e^{\left(\frac{H_{d}\ \times\ \left(T_{k}\ -\text{\ T}_{\text{Opt}}\right)}{T_{k}\ \times\ R\ \times\ T_{\text{Opt}}}\right)}\right)}\)Eqn. 3
where temperatures are in Kelvin, kOpt is
VCMax or JMax at TOpt,
Ha represents the activation energy—it describes the
exponential rise of the curve before TOpt—
Hd is the ‘de-activation energy’, reflecting the rate of
decrease above TOpt, and R is the universal gas
constant (8.314 J K–1 mol–1). We
also calculated the entropy parameter ΔS (sensu Medlyn et al. 2002),
which is related to TOpt as:
\(T_{\text{Opt}}=\frac{H_{d}}{S-\text{R\ ln}\left(\frac{H_{a}}{H_{d}-H_{a}}\right)}\)Eqn. 4
and thus:
\(S\ =\frac{H_{d}}{T_{\text{Opt}}}+R\ \ln\left(\frac{H_{a}}{H_{d}-H_{a}}\right)\)Eqn. 5
To improve our ability to obtain
robust parameter estimates we pooled measurements made on different
leaves within each treatment—the experimental design and the number of
leaf-level replicates was not amenable to a random effects model. Fig.
S1 shows leaf-level curves in comparison to pooled-data curves.
Net photosynthesis at the ambient conditions of control and treatment
domes, that is, at about 400 ppm (P400) and at 800 ppm
(P800), respectively, were extracted from each A-Ci
curve, and P400 and P800 were fitted
with a parabolic function following Gunderson, O’Hara, Campion, Walker
& Edwards (2010) as:
\(P_{400}\text{\ or\ }P_{800\ }=\ P_{\text{Opt}}\ -b\ \times\ \left(T_{k}\ -\ T_{\text{Opt}}\right)^{2}\)Eqn. 6
where POpt is the rate of net photosynthesis at 400 or
800 ppm CO2 at optimum temperature TOpt,
and b is a shape parameter that is inversely proportional to the width
of the parabolic curve. The full equation used for estimating parameters
for all four treatment categories is shown in Table S1. Net
photosynthesis was also fitted with Eqn 3, for which the results are
presented in Table S3.
In addition, for each curve the net photosynthesis rate at
Ci of 270 (P270) and 505 ppm
(P505) were calculated. These Ci values
correspond with the mean Ci associated with measurement
CO2 concentrations of 400 and 800 ppm, respectively. By
analyzing the temperature response of photosynthesis at a given
Ci, the temperature and associated VPD effects on
stomatal conductance are accounted for, and hence, comparison of the
parameters obtained from the P400 and
P270 (and P800 and P505)
can reveal the role of stomatal conductance in determining the
temperature optimum of photosynthesis (Kumarathunge et al. 2019).