Effect of elevated carbon dioxide concentration at night on the growth andgas exchange of selected C-4 species

Biomass of certain C-4 species is increased when plants are grown at elevat ed CO2 concentrations. Experiments using four C-4 species (Amaranthus retro flexus L., Amaranthus hypochondriacus L., Sorghum bicolor (L.) Moench and Z ea mays L.) exposed both day and night from sowing to carbon dioxide concen trations of 370 (ambient) or 700 mu mol mol(-1) (elevated) or to 370 mu mol mol(-1) during the day and 700 mu mol mol(-1) at night, determined whether any biomass increase at elevated CO2 concentrations was related to a reduc tion in the night-time rate of CO2 efflux at high night-time CO2 concentrat ions. Of the four species tested, only A. retroflexus significantly increas ed both CO2 assimilation (+13%) and plant biomass (+21%) at continuous elev ated relative to continuous ambient concentrations of CO2. This increase wa s not associated with improvement in leaf water potential during dark or li ght periods. In contrast, high CO2 only during the night significantly redu ced plant biomass compared to the 24 h ambient CO2 treatment for both A. re troflexus 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 st imulation of daytime CO2 assimilation, independent of any improvement in le af water potential.