The potential of the vegetation to sequester C is determined by the balance between assimilation and respiration. Respiration is under environmental and substrate-driven control, but the circadian clock might also contribute. To assess circadian control on night-time dark respiration (RD) and on light enhanced dark respiration (LEDR) - the latter providing information on the metabolic reorganization in the leaf during light-dark transitions - we performed experiments in macrocosms hosting canopies of bean and cotton. Under constant darkness (plus constant air temperature and air humidity), we tested whether circadian regulation of RD scaled from leaf to canopy respiration. Under constant light (plus constant air temperature and air humidity), we assessed the potential for leaf-level circadian regulation of LEDR. There was a clear circadian oscillation of leaf-level RD in both species and circadian patterns scaled to the canopy. LEDR was under circadian control in cotton, but not in bean indicating species-specific controls. The circadian rhythm of LEDR in cotton might indicate variable suppression of the normal cyclic function of the tricarboxylic-acid-cycle in the light. Since circadian regulation is assumed to act as an adaptive memory to adjust plant metabolism based on environmental conditions from previous days, circadian control of RD may help to explain temporal variability of ecosystem respiration.This study benefited from the CNRS human and technical resources allocated to the ECOTRONS Research Infrastructures as well as from the state allocation ‘Investissement d'Avenir’ AnaEE-France ANR-11-INBS-0001, ExpeER Transnational Access program, Ramón y Cajal fellowships (RYC-2012-10970 to VRD and RYC-2008-02050 to JPF), the Erasmus Mundus Master Course MEDfOR, internal grants from UWS-HIE to VRD and ZALF to AG and Juan de la Cierva-fellowships (IJCI-2014-21393 to JGA). We remain indebted to E. Gerardeau, D. Dessauw, J. Jean, P. Prudent (Aïda CIRAD), J.-J. Drevon, C. Pernot (Eco&Sol INRA), B. Buatois, A. Rocheteau (CEFE CNRS), A. Pra, A. Mokhtar and the full Ecotron team, in particular C. Escape, for outstanding technical assistance
