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Effects of Elevated CO2 Concentration and High Temperature on Growth and Yield of Rice : II. The effect on yield and its components of Akihikari rice

Yield and its component organs of rice (cv. Akihikari) were examined for populations grown under two different CO2 concentrations (350 and 690μLL-1)×four temperature regimes in temperature gradient chambers (TGCs) in two cropping seasons of 1991 and 1992. The temperature treatments ranged 27.2∼31.1°C in 1991 and 26.0∼29.3°C in 1992 on average over the entire growth period. The relative yield increases by nearly doubling the CO2 concentration under the lowest temperature conditions were 40% and 22% in 1991 and 1992, respectively. These yield increases were mainly attributable to the increased spikelet number per unit area by elevated CO2, whereas the CO2 effects on ripening percentage and weight of single grain mass were relatively small. The difference in the CO2 enhancement rate in the spikelet number and hence in the yield between the two years was considered to reflect the difference in the nitrogen (N) application rate, as total amounts of N applied were 24 g m-2 in 1991 and 12 g m-2 in 1992. With the increase in temperature, yields at ambient and elevated CO2 concentrations decreased drastically with a more pronounced reduction with elevated CO2, resulting in no CO2 enrichment effect on rice yield at higher temperatures. The yield decline at higher temperatures was primarily due to an increase in the number of sterile spikelets and slightly due to the increase in imperfectly ripened grains. The spikelet sterility was most closely related to the daily maximum temperature averaged over the flowering period.