In wild type and in long– and short–circadian period mutants of Arabidopsis thaliana, plants with a clock period matched to the environment contain more chlorophyll, fix more carbon, grow faster, and survive better than plants with circadian periods differing from their environment. This explains why plants gain advantage from circadian control.

Circadian clocks produce an internal estimate of time that synchronizes biological events with external day-night cycles (1). Clocks with similar properties and regulatory architecture have evolved at least four times, indicating that circadian rhythms confer a selective advantage (2). In plants, circadian rhythms control gene expression, stomatal opening, and the timing component of photoperiodism, which regulates seasonal reproduction, but the basis for their contribution to fitness during vegetative growth remains undetermined

We demonstrate that when correctly tuned, the Arabidopsis circadian system enhances chlorophyll content, photosynthetic carbon fixation, and growth. We also show that circadian enhancement of photosynthesis leads to improved survival and competitive advantage.

We hypothesized that matching the endogenous clock period (τ) with the period of exogenous light-dark cycles (T) [so called “circadian resonance” (7)] provides an advantage by optimizing the phase relation between clock-controlled biology and exogenous day-night cycles.

To test this hypothesis, we compared the performance of wild-type plants with lines having mutations that alter clock period length, in a range of environmental period lengths (“T cycles”) that were either matched or unmatched to the endogenous clock period.

Last, the effect of circadian arrhythmia on growth and physiology was investigated in well-characterized arrhythmic plants overexpressing the molecular oscillator component CCA1 (CCA1-ox), and compared with rhythmic wild types

We assessed the contribution of circadian resonance to carbon fixation, biomass, and leaf chlorophyll.