Terrestrial biosphere models (TBMs) are extremely sensitive to the parameterization of the Farquhar, von Caemmerer & Berry model of photosynthesis, particularly the apparent maximum carboxylation capacity (Vcmax) of Rubisco. New instrumentation and approaches have enabled the rapid measurement of apparent Vcmax that paves the way for the investigation of diurnal variation in Vcmax and improved understanding of the potential impact on representation of photosynthesis in TBMs. Here we show that reductions in Vcmax over the course of a photoperiod can be as great as 50% and, when incorporated into a model of daily CO2 assimilation, show that net carbon gain can change between -19 and 215% when compared to the current TBM assumption of a constant Vcmax. Given the obvious impact on TBM representation of photosynthesis, we recommend a renewed focus on the measurement of diurnal responses in photosynthetic capacity across biomes to advance understanding and enable model representation of this important phenomenon.