CONSEQUENCES OF GROWTH AT 2 CARBON-DIOXIDE CONCENTRATIONS AND 2 TEMPERATURES FOR LEAF GAS-EXCHANGE IN PASCOPYRUM-SMITHII (C-3) AND BOUTELOUA-GRACILIS (C-4)

Continually rising atmospheric CO2 concentrations and possible climati c change may cause significant changes in plant communities. This stud y was undertaken to investigate gas exchange in two important grass sp ecies of the short-grass steppe, Pascopyrum smithii (western wheatgras s), C-3, and Bouteloua gracilis (blue grama), C4, grown at different C O2 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 conce ntrations (350 and 700 mu mol mol(-1)) and two temperature regimes (fi eld average and elevated by 4 degrees C). Leaf gas exchange was measur ed during the second, third and fourth growth seasons. All plants exhi bited higher leaf CO2 assimilation rates (A) with increasing measureme nt CO2 concentration, with greater responses being observed in the coo l-season C-3 species P. smithii. Changes in the shape of intercellular CO2 response curves of A for both species indicated photosynthetic ac climation to the different growth environments, The photosynthetic cap acity of P. smithii leaves tended to be reduced in plants grown at hig h CO2 concentrations, although A for plants grown and measured at 700 mu mol mol(-1) CO2 was 41% greater than that in plants grown and measu red at 350 mu mol mol(-1) CO2. Low leaf N concentration may have contr ibuted to photosynthetic acclimation to CO2. A severe reduction in pho tosynthetic capacity was exhibited in P. smithii plants grown long-ter m at elevated temperatures. As a result, the potential response of pho tosynthesis to CO2 enrichment was reduced in P. smithii plants grown l ong-term at the higher temperature.