GAS-EXCHANGE AND CARBOHYDRATE AND NITROGEN CONCENTRATIONS IN LEAVES OF PASCOPYRUM-SMITHII (C-3) AND BONTELOUA-GRACILIS (C-4) AT DIFFERENT CARBON-DIOXIDE CONCENTRATIONS AND TEMPERATURES

Pascopyrum smithii (C-3) and Bouteloua gracilis (C-4) are important fo rage grasses native to the Colorado shortgrass steppe. This study inve stigated photosynthetic responses of these grasses to long-term CO2 en richment and temperature in relation to leaf nonstructural carbohydrat e (TNC) and [N]. Glasshouse-grown seedlings were transferred to growth chambers and grown for 49 d at two CO2 concentrations (380 and 750 mu mol mol(-1)) al 20 and 35 degrees C, and two additional temperatures (25 and 30 degrees C) at 750 mu mol mol(-1) CO2. Leaf CO2 exchange rat e (CER) was measured at a plant's respective growth temperature and at two CO2 concentrations of approx. 380 and 700 mu mol mol(-1). Long-te rm CO2 enrichment stimulated CER in both species, although the respons e was greater in the CER P. smithii. Doubling the [CO2] from 380 to 75 0 mu mol mol(-1) stimulated CER of P. smithii slightly more in plants grown and measured al 30 degrees C compared to plants grown at 20, 25 or 35 degrees C. CO2-enriched plants sometimes exhibited lower CER whe n compared to ambient-grown controls measured at the same [CO2], indic ating photosynthetic acclimation to CO2 growth regime. In P. smithii, such reductions in CER were associated with increases in TNC and speci fic leaf mass, reductions in leaf [N] and, in one instance, a reductio n in leaf conductance compared to controls. In B. gracilis, photosynth etic acclimation was observed more often, but significant changes in l eaf metabolite levels from growth at different [CO2] were generally le ss evident. Temperatures considered optimal for growth (C-3: 20 degree s C; C-4: 35 degrees C) sometimes led to CO2-induced accumulations of TNC in both species, with starch accumulating in the leaves of both sp ecies, and fructans accumulating only in P. smithii. Photosynthesis of both species is likely to be enhanced in future CO2-enriched and warm er environments, although responses will sometimes be attenuated by ac climation. (C) 1997 Annals of Botany Company.