Canopy radiation- and water-use efficiencies as affected by elevated [CO2]

This study used an environmentally controlled plant growth facility, EcoCEL Ls, to measure canopy gas exchanges directly and to examine the effects of elevated [CO2] on canopy radiation- and water-use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 mu mol mol(-1)) and elevated [CO2] (746 mu mol mol(-1)) for 53 days in EcoCELLs. Whole cano py carbon- and water-fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO2] enhanced daily t otal canopy carbon- and water-fluxes by 53% and 11%, respectively, on a gro und-area basis, resulting in a 54% increase in radiation-use efficiency (RU E) based on intercepted photosynthetic active radiation and a 26% increase in water-use efficiency (WUE) by the end of the experiment. Canopy carbon- and water-fluxes at both CO2 treatments varied with canopy development. The y were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon- and water-fluxes were expressed on a leaf-area basis, no effect of CO2 was found for canopy water-flux while el evated [CO2] still enhanced canopy carbon-flux by 29%, on average. Night-ti me canopy carbon-flux was 32% higher at elevated than at ambient [CO2]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study p rovided direct evidence that plant canopy may consume more, instead of less , water but utilize both water and radiation more efficiently at elevated t han at ambient [CO2], at least during the exponential growth period as illu strated in this experiment.