Photosynthetic Responses of Thirteen Pasture Species to Elevated CO₂ and Temperature

Abstract

Thirteen common pasture species, (eleven C₃ and two C₄), were grown in controlled environments at 12/7, 18/13 and 28/23ºC and at 350 and 700 ppm CO₂ to evaluate the effects of elevated CO₂ on their photosynthetic responses. Photosynthesis was measured at the growth temperatures and at both 350 and 700 ppm CO₂. In C3 species, short-term (within minutes) increases in CO₂ had the greatest effect on photosynthesis, with an average of 50-60% higher rates in plants exposed to 700 ppm CO₂ at each temperature. However, there was a continuum of response between the C₃ species whereas C₄ species were unaffected by short-term changes in CO₂. There was also a long-term (4-8 weeks) response to high CO₂, with an average of about 40-50% higher rates of photosynthesis, with some response by C₄ species. Both short- and long-term responses were negatively correlated with the photosynthetic rate of each species at 350 ppm CO₂ and all species were less efficient at converting photosynthate to dry matter at elevated CO₂. These data show clearly that photosynthesis of these cool temperate pasture species can respond to elevated CO₂, especially at low temperatures. This will have consequences for predicting the potential effects of climate change, accompanied by rising CO₂, on pasture ecosystems.