Pascopyrum smithii (C 3 ) and Bouteloua gracilis (C 4 ) are important forage grasses native to the Colorado shortgrass steppe. This study investigated photosynthetic responses of these grasses to long-term CO 2 enrichment and temperature in relation to leaf nonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlings were transferred to growth chambers and grown for 49 d at two CO 2 concentrations (380 and 750 μmol mol −1 ) at 20 and 35 °C, and two additional temperatures (25 and 30 °C) at 750 μmol mol −1 CO 2 . Leaf CO 2 exchange rate (CER) was measured at a plant's respective growth temperature and at two CO 2 concentrations of approx. 380 and 700 μmol mol −1 . Long-term CO 2 enrichment stimulated CER in both species, although the response was greater in the C 3 , P. smithii . Doubling the [CO 2 ] from 380 to 750 μmol mol −1 stimulated CER of P. smithii slightly more in plants grown and measured at 30 °C compared to plants grown at 20, 25 or 35 °C. CO 2 -enriched plants sometimes exhibited lower CER when compared to ambient-grown controls measured at the same [CO 2 ], indicating photosynthetic acclimation to CO 2 growth regime. In P. smithii , such reductions in CER were associated with increases in TNC and specific leaf mass, reductions in leaf [N] and, in one instance, a reduction in leaf conductance compared to controls. In B. gracilis , photosynthetic acclimation was observed more often, but significant changes in leaf metabolite levels from growth at different [CO 2 ] were generally less evident. Temperatures considered optimal for growth (C 3 : 20 °C; C 4 : 35 °C) sometimes led to CO 2 -induced accumulations of TNC in both species, with starch accumulating in the leaves of both species, and fructans accumulating only in P. smithii. Photosynthesis of both species is likely to be enhanced in future CO 2 -enriched and warmer environments, although responses will sometimes be attenuated by acclimation.
September 20, 1995 ; September 4, 1996