Responses of seedling growth to daytime or continuous elevation of carbon dioxide

In some studies of ecosystem responses to elevated carbon dioxide, supplemental carbon dioxide is supplied only during the daytime, while in others it is supplied continuously. The purpose of my work was to determine for several species whether seedling growth differed depending on whether carbon dioxide was elevated only during the daytime or continuously. Acer rubrum, Amaranthus retroflexus, Glycine max, Medicago sativa, and Quercus prinus were grown in controlled environment chambers with 700 mumol mol(-1) [CO2] during the daytime and either 350 or 700 mumol mol(-1) [CO2] at night. For the herbaceous species A. retroflexus, G. max, and M. sativa, a comparison was made between responses to the [CO2] treatments under constant temperatures and responses with a day/night temperature difference. In all of these species and for all temperature regimes, plant growth differed significantly depending on whether [CO2] was elevated continuously or only in the daytime. With continuous elevation of [CO2], biomass at the final harvest ranged from 0.46 to 1.45 times as much as for daytime elevation of [CO2], depending on species. In each of the species examined, the response to the night [CO2] treatment was opposite for regimes with constant compared with diurnally varying temperatures. The results indicate that seedling growth often responds to the [CO2] at night when daytime [CO2] is elevated.