Consequences of growth at two carbon dioxide concentrations and two temperatures for leaf gas exchange in Pascopyrum smithii (C3) and Bouteloua gracilis (C4)

Continually rising atmospheric CO2 concentrations and possible climatic change may cause significant changes in plant communities. This study was undertaken to investigate gas exchange in two important grass species of the short-grass steppe, Pascopyrum smithii (western wheat-grass), C3, and Bouteloua gracilis (blue grama), C4, grown at different CO2 concentrations and temperatures. Intact soil cores containing each species were extracted from grasslands in north-eastern Colorado, USA, placed in growth chambers, and grown at combinations of two CO2 concentrations (350 and 700 μmol mol−1) and two temperature regimes (field average and elevated by 4°C). Leaf gas exchange was measured during the second, third and fourth growth seasons. All plants exhibited higher leaf CO2 assimilation rates (A) with increasing measurement CO2 concentration, with greater responses being observed in the cool-season C3 species P. smithii. Changes in the shape of intercellular CO2 response curves of A for both species indicated photosynthetic acclimation to the different growth environments. The photosynthetic capacity of P. smithii leaves tended to be reduced in plants grown at high CO2 concentrations, although A for plants grown and measured at 700μmol mol−1 CO2 was 41% greater than that in plants grown and measured at 350 μmol mol−1 CO2. Low leaf N concentration may have contributed to photosynthetic acclimation to CO2. A severe reduction in photosynthetic capacity was exhibited in P. smithii plants grown long-term at elevated temperatures. As a result, the potential response of photosynthesis to CO2 enrichment was reduced in P. smithii plants grown long-term at the higher temperature.