Direct effects of atmospheric carbon dioxide concentration on whole canopydark respiration of rice

The purpose of this study was to test for direct inhibition of rice canopy apparent respiration by elevated atmospheric carbon dioxide concentration ( [CO2]) across a range of short-term air temperature treatments. Rice (cv. I R-72) was grown in eight naturally sunlit, semiclosed, plant growth chamber s at daytime [CO2] treatments of 350 and 700 mu mol mol(-1.) Short-term nig ht-time air temperature treatments ranged from 21 to 40 degrees C. Whole ca nopy respiration, expressed on a ground area basis (R-d), was measured at n ight by periodically venting the chambers with ambient air. This night-time chamber venting and resealing procedure produced a range of increasing cha mber [CO2] which we used to test for potential inhibitory effects of rising [CO2] on R-d. A nitrous oxide leak detection system was used to correct R- d measurements for chamber leakage rate (L) and also to determine if appare nt reductions in night-time R-d with rising [CO2] could be completely accou nted for by L. The L was affected by both CO2 concentration gradient betwee n the chamber and ambient air and the inherent leakiness of each individual chamber. Nevertheless, after correcting R-d for L, we detected a rapid and reversible, direct inhibition of R-d with rising chamber [CO2] for air tem peratures above 21 degrees C. This effect was larger for the 350 compared w ith the 700 mu mol mol(-1) daytime [CO2] treatment and was also increased w ith increasing short-term air temperature treatments. However, little diffe rence in R-d was found between the two daytime [CO2] treatments when night- time [CO2] was at the respective daytime [CO2]. These results suggest that naturally occurring diurnal changes in both ambient [CO2] and air temperatu re can affect R-d. Because naturally occurring diurnal changes in both [CO2 ] and air temperature can be expected in a future higher CO2 world, short-t erm direct effects of these environmental variables on rice R-d can also be expected.