Implications for measuring Vcmax
In terms of measuring Vcmax , the required timing
of Vcmax measurements depends on the aims of the
measurements. For comparing maximum apparentVcmax , our data suggest that for most species,
photosynthetic CO2 responses should be measured in the
morning, prior to midday. If the aim is to obtain data useful for model
parameterization, there are two avenues: daily average or diurnal
photosynthetic CO2 response measurements. For daily
average measurements, care is needed to make sure that the measurements
occur across the entire light period of the photoperiod, with the
recognition that there may be biases in conclusions based on averageVcmax . For diurnal measurements, there are again
two approaches: RACiR, and the one-point method to estimatingVcmax . We have demonstrated this diurnal
application of RACiR, however RACiR may not be feasible in all cases due
to equipment or instrument precision. However, De Kauwe et al .
(2016) showed that the one-point method for estimating apparentVcmax can produce similar estimates to those from
full photosynthetic CO2 responses, providing sufficient
time is allowed for leaf gas exchange to reach steady state in the
chamber (Burnett et al. 2019). Given that diurnal dynamics inVcmax can incur large changes on timescales of
< 2 hrs, diurnal dynamics would affect temperature response
measurements of Vcmax , with the effect dependent
on which temperatures are measured earlier or later. It may be possible
to circumvent these effects using RACiR-based approaches to temperature
responses, although the timescales for chamber equilibration times may
limit such an approach. We call for more diurnal measurements ofVcmax as current measurement approaches may be
introducing biases into conclusions based on the assumption theVcmax is diurnally static.