Abstract
Background : Molecular clocks drive oscillations in leaf photosynthesis, stomatal conductance, and other cell andleaf-level processes over ~24 h under controlled laboratory conditions. The influence of such circadian regulation overwhole-canopy fluxes remains uncertain; diurnal CO2and H2O vapor flux dynamics in the field are currently interpretedas resulting almost exclusively from direct physiological responses to variations in light, temperature and otherenvironmental factors. We tested whether circadian regulation would affect plant and canopy gas exchange at theMontpellier European Ecotron. Canopy and leaf-level fluxes were constantly monitored under field-like environmentalconditions, and under constant environmental conditions (no variation in temperature, radiation, or otherenvironmental cues). Results : We show direct experimental evidence at canopy scales of the circadian regulation of daytime gas exchange:20–79 % of the daily variation range in CO2and H2O fluxes occurred under circadian entrainment in canopies of anannual herb (bean) and of a perennial shrub (cotton). We also observed that considering circadian regulation improvedperformance by 8–17 % in commonly used stomatal conductance models. Conclusions : Our results show that circadian controls affect diurnal CO2and H2O flux patterns in entire canopies infield-like conditions, and its consideration significantly improves model performance. Circadian controls act as a‘memory’of the past conditions experienced by the plant, which synchronizes metabolism across entire plantcanopies.