In order to select for improved photosynthesis, it is first necessary to identify genetic variation. The photosynthetic rate of
an unstressed young leaf under high light and ambient CO2 reflects, firstly, the underlying content and characteristics of
photosynthetic enzymes and, secondly, the ease with which CO2 can reach the sites of carboxylation from the atmosphere.
The first constraint is related to the nitrogen content per unit leaf area, its allocation to photosynthetic enzymes such as
Rubisco and the kinetic properties of the enzymes. The second constraint is associated with stomatal and mesophyll
conductances. By analyzing gas exchange data with the standard Farquhar, von Caemmerer and Berry C3 photosynthesis
model (Von Caemmerer and Farquhar 1981), it is possible to identify the underlying components that are associated with
variation in the measured photosynthetic rate. However, to measure a sufficiently detailed CO2 response curve in the field
requires about 30 minutes limiting how many genotypes can be screened. We have been developing a higher throughput
method for assessing photosynthetic properties of wheat leaves using reflectance spectra. This was calibrated in the field
against CO2 response curves measured with conventional gas exchange. While the LI6400 is able to control leaf
temperature to some extent, it was not always possible to achieve a common temperature of 25 C in the field (especially
when ambient temperature was above 30 C combined with high irradiance). In fact, leaf temperatures for the experiments
conducted in Australia and Mexico ranged between 20 and 34 C. Checks were made by repeated measurement of several
leaves through a day at different temperatures. When the standard Farquhar, von Caemmerer and Berry C3 photosynthesis
model was applied, we found that the derived Rubisco activity corrected to 25 C was not constant. This prompted us to
conduct an experiment where CO2 response curves were measured repeatedly with the same leaf over a range of
temperatures. From these measurements, we have derived a set of activation energies and values for Rubisco kinetic
parameters at 25 C.
