Canopy quantum yield in a mesocosm study

Due to past limitations in experimental technology, canopy function has gen erally been inferred from leaf properties through scaling and/or indirect m easurements. The development of a facility (EcoCELLs) at the Desert Researc h Institute has now made it possible to directly measure canopy gas exchang e. In this experiment, sunflowers (Helianthus annus) were planted in the Ec oCELLs and grown under ambient (399 mu mol mol(-1)) and elevated (746 mu mo l mol(-1)) CO2 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 CO2 treatments in the early stage of canopy developmen t. After the canopy closed, carbon flux under elevated CO2 averaged 53% hig her than that under ambient CO2. Assimilation/incident irradiance (A/I) cur ves of leaves at different canopy positions were used to estimate leaf quan tum yields (phi(L)), and AII curves of canopies at late development stages were used to estimate phi(C). Elevated CO2 enhanced phi(L) by 24%. There wa s little difference in phi(L) at different canopy positions, averaging 0.05 42 at ambient CO2 and 0.0671 at elevated CO2. Canopy quantum yield (phi(C)) was higher by 32% at elevated than ambient CO2. It increased with canopy d evelopment and was strongly correlated with leaf area index (LAI) by phi(C) = 0.0094 LAI/(0.0829 + 0.1137 LAI) at ambient CO2 and phi(C) = 0.01382 LAI /(0.1129 + 0.1224 LAI) at elevated CO2. In addition, the curvilinear relati onship between radiation and canopy carbon fluxes suggests that canopy radi ation use efficiency (CRUE) varied with radiation availability. The variabi lity in phi(C) and CRUE with canopy development and light levels warrants f urther research on the notion drawn from earlier work that CRUE in non-stre ssed conditions is relatively constant. (C)2000 Elsevier Science B.V. All r ights reserved.