Rising atmospheric carbon dioxide concentration ([CO2]) has generated considerable interest in the response of agricultural crops to [CO2]. The objectives of this study were to determine the effects of a wide range of daytime [CO2] on dark respiration of rice (Oryza sativa L. cv. IR-30). Rice plants were grown season-long in naturally sunlit plant growth chambers in subambient (160 and 250), ambient (330), or super-ambient (500, 660 and 900 μmol CO2 mol−1 air) [CO2] treatments. Canopy dark respiration, expressed on a ground area basis (Rd) increased with increasing [CO2] treatment from 160 to 500 μmol mol−1 treatments and was very similar among the superambient treatments. The trends in Rd over time and in response to increasing daytime [CO2] treatment were associated with and similar to trends previously described for photosynthesis. Specific respiration rate (Rdw) decreased with time during the growing season and was higher in the subambient than the ambient and superambient [CO2] treatments. This greater Rdw in the subambient [CO2] treatments was attributed to a higher specific maintenance respiration rate and was associated with higher plant tissue nitrogen concentration.
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