The endogenous circadian (～24 h) system allows plants to anticipate and adapt to daily environmental changes. Stomatal aperture is one of the many processes under circadian control; stomatal opening and closing occurs under constant conditions， even in the absence of environmental cues. To understand the significance of circadian-mediated anticipation in stomatal opening， we have generated (specifically guard cell) Arabidopsis () plants in which the oscillator gene () was overexpressed under the control of the guard-cell-specific promoter， The plants showed a loss of ability to open stomata in anticipation of daily dark-to-light changes and of circadian-mediated stomatal opening in constant light. We observed that under fully watered and mild drought conditions， plants outperform wild type with larger leaf area and biomass. To investigate the molecular basis for circadian control of guard cell aperture， we used large-scale qRT-PCR to compare circadian oscillator gene expression in guard cells compared with the "average" whole-leaf oscillator and examined gene expression and stomatal aperture in several lines of plants with misexpressed Our results show that the guard cell oscillator is different from the average plant oscillator. Moreover， the differences in guard cell oscillator function may be important for the correct regulation of photoperiod pathway genes that have previously been reported to control stomatal aperture. We conclude by showing that CONSTANS and FLOWERING LOCUS T， components of the photoperiod pathway that regulate flowering time， also control stomatal aperture in a daylength-dependent manner.