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Due to past limitations in experimental technology, canopy function has generally been inferred from leaf properties through scaling and/or indirect measurements. The development of a facility (EcoCELLs) at the Desert Research Institute has now made it possible to directly measure canopy gas exchange. In this experiment, sunflowers (Helianthus annus) were planted in the EcoCELLs and grown under ambient (399 mu mol mol(-1)) and elevated (746 mu mol mol(-1)) CO(2) concentrations. We continuously measured carbon flux during canopy development from which canopy quantum yield (phi(C)) was estimated. The results indicated that the total daily carbon flux was similar between elevated and ambient CO(2) treatments in the early stage of canopy development. After the canopy closed, carbon flux under elevated CO(2) averaged 53% higher than that under ambient CO(2). Assimilation/incident irradiance (A/I) curves of leaves at different canopy positions were used to estimate leaf quantum yields (phi(L)), and AII curves of canopies at late development stages were used to estimate phi(C). Elevated CO(2) enhanced phi(L) by 24%. There was little difference in phi(L) at different canopy positions, averaging 0.0542 at ambient CO(2) and 0.0671 at elevated CO(2). Canopy quantum yield (phi(C)) was higher by 32% at elevated than ambient CO(2). It increased with canopy development and was strongly correlated with leaf area index (LAI) by phi(C) = 0.0094 LAI/(0.0829 + 0.1137 LAI) at ambient CO(2) and phi(C) = 0.01382 LAI/(0.1129 + 0.1224 LAI) at elevated CO(2). In addition, the curvilinear relationship between radiation and canopy carbon fluxes suggests that canopy radiation use efficiency (CRUE) varied with radiation availability. The variability in phi(C) and CRUE with canopy development and light levels warrants further research on the notion drawn from earlier work that CRUE in non-stressed conditions is relatively constant. (C)2000 Elsevier Science B.V. All rights reserved.