Photosynthesis and Water Relations of a C4 and a C3 Grass Under Doubled Carbon Dioxide

The objective of this project was to determine during an entire season the photosynthetic rate and water potential of a C3 grass (Kentucky bluegrass, Poa pratensis L.) and a C4 grass (big bluestem, Andropogon gerardii Vitman) grown with 2 levels of CO2 (ambient or twice ambient) and 2 water levels (field capacity or no water added). Elevated CO2 increased the photosynthetic rate of Kentucky bluegrass by an average of 76%. During mid-season (11 June-16 July), photosynthetic rate of high-water big bluestem with ambient CO2 was about the same as that of low-water big bluestem with doubled CO2• Under both water treatments, water potential of each grass was elevated by CO2 enrichment. The results suggest that the photosynthetic rate of both C3 and C4 grasses grown in dry regions may increase as atmospheric CO2 concentration increases.