Background Molecular clocks drive oscillations in leaf photosynthesis,
stomatal conductance, and other cell and leaf-level processes over ~24 h under
controlled laboratory conditions. The influence of such circadian regulation
over whole-canopy fluxes remains uncertain; diurnal CO2 and H2O vapor flux
dynamics in the field are currently interpreted as resulting almost
exclusively from direct physiological responses to variations in light,
temperature and other environmental factors. We tested whether circadian
regulation would affect plant and canopy gas exchange at the Montpellier
European Ecotron. Canopy and leaf-level fluxes were constantly monitored under
field-like environmental conditions, and under constant environmental
conditions (no variation in temperature, radiation, or other environmental
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 CO2 and H2O fluxes occurred under circadian entrainment in canopies
of an annual herb (bean) and of a perennial shrub (cotton). We also observed
that considering circadian regulation improved performance by 8–17 % in
commonly used stomatal conductance models. Conclusions Our results show that
circadian controls affect diurnal CO2 and H2O flux patterns in entire canopies
in field-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 plant
canopies