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The Effect of Elevated CO2 on the Optimum Temperature for Photosynthesis in Rice
Reference
Borjigidai, A., Hikosaka, K., Hirose, T., Hasegawa, T., Okada, M. and Kobayashi, K. 2006. Seasonal changes in temperature dependence of photosynthetic rate in rice under a free-air CO2 enrichment. Annals of Botany 97: 549-557.

What was done
Rice (Oryza sativa L. cv Akitakomachi) plants were grown from seed in greenhouses maintained at atmospheric CO2 concentrations of 370 and 570 ppm, after which they were transplanted to the field and grown in well-fertilized paddy culture in a FACE experiment conducted at the same CO2 concentrations under which the seeds were sprouted in both 2003 and 2004. At various times throughout the two growing seasons during the field portion of the study, photosynthetic measurements were made on the most recently fully-expanded leaves in full sunlight at a variety of different leaf temperatures.

What was learned
The authors report that "the optimal temperature of photosynthesis (Topt, the value where the photosynthetic rate was maximum) was significantly higher at elevated CO2: it ranged from 22 to 34.5C with an average value of 28.9C at ambient CO2, and from 29.5 to 37C with an average value of 33.5C at elevated CO2." As a result, since the increase in the air's CO2 concentration employed in this study was only 200 ppm, the 4.6C mean increase in Topt observed in this experiment would roughly translate to a 6.9C mean increase in Topt for a CO2 increase of 300 ppm, which is the concentration increase that is more commonly used in climate modeling studies of the effects of elevated CO2 on planetary temperature.

What it means
Since the mean increase in global temperature that is predicted to result from a 300-ppm increase in the air's CO2 concentration is considerably less than the 6.9C increase in the Topt of rice that is implied by this study to result from such a CO2 increase, and since the mean increase in global temperature predicted to result from an increase in the air's CO2 concentration is greater than that expected to occur in regions where rice is grown, it would appear that this particular variety of rice should have no problem adapting to any warming that might be produced by the ongoing rise in the air's CO2 concentration.

Reviewed 19 July 2006