Effect of elevated carbon dioxide concentration at night on the growth and gas exchange of selected C4 species

Abstract

Biomass of certain C₄ species is increased when plants are grown at elevated CO₂ concentrations. Experiments using four C₄ species (Amaranthus retroflexus L., Amaranthus hypochondriacus L., Sorghum bicolor (L.) Moench and Zea mays L.) exposed both day and night from sowing to carbon dioxide concentrations of 370 (ambient) or 700 µmol mol^-1 (elevated) or to 370 µmol mol^-1 during the day and 700 µmol mol^-1 at night, determined whether any biomass increase at elevated CO₂ concentrations was related to a reduction in the night-time rate of CO₂ efflux at high night-time CO₂ concentrations. Of the four species tested, only A. retroflexus significantly increased both CO₂ assimilation (+13%) and plant biomass (+21%) at continuous elevated relative to continuous ambient concentrations of CO₂. This increase was not associated with improvement in leaf water potential during dark or light periods. In contrast, high CO₂ only during the night significantly reduced plant biomass compared to the 24 h ambient CO₂ treatment for both A. retroflexus and Z. mays. This indicates that the observed increase in biomass at elevated CO₂ for A. retroflexus was not caused by a reduction of carbon loss at night (i.e. increased carbon conservation), but rather a direct stimulation of daytime CO₂ assimilation, independent of any improvement in leaf water potential.