PHOTOSYNTHETIC PATHWAY AND ONTOGENY AFFECT WATER RELATIONS AND THE IMPACT OF CO2 ON BOUTELOUA-GRACILIS (C-4) AND PASCOPYRUM-SMITHII (C-3)

The eastern Colorado shortgrass steppe is dominated by the C-4 grass, Bouteloua gracilis, but contains a mixture of C-3 grasses as well, inc luding Pascopyrum smithii. Although the ecology of this region has bee n extensively studied, there is little information on how increasing a tmospheric CO2 will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate pa rtitioning in B. gracilis and P. gracilis grown under present ambient and elevated CO2 concentrations of 350 mu l l(-1) and 700 mu l l(-1), respectively, and two deficit irrigation regimes. The experiment was c onducted in soil-packed columns planted to either species over a 2-mon th period under summer-like conditions and with no fertilizer addition s. Our objective was to better under stand how these species and the f unctional groups they represent will respond in future CO2-enriched en vironments. Leaf CO2 assimilation (A(n)), transpiration use efficiency (TUE, or A(n)/transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired) o f both species were greater at elevated CO2, although responses were m ore pronounced for P. smithii. Elevated CO2 enhanced photosynthesis, T UE, and growth in both species through higher soil water content (SWC) and leaf water potentials (Psi) and stimulation of photosynthesis. Co nsumptive water use was greater and TUE less for P. smithii than B. gr acilis during early growth when soil water was more available. Declini ng SWC with time was associated with a steadily increased sequestering of total non-structural carbohydrates (TNCs), storage carbohydrates ( primarily fructans for P. Smithii) and biomass in belowground organs o f P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO2 while the root:shoot ratio of B. gracil is was unresponsive to CO2. These partitioning responses may be the co nsequence of different ontogenetic strategies of a cool-season and war m-season grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for s ummer dormancy, while resource partitioning of the warm-season B. grac ilis remains unaltered. One consequence of greater partitioning of res ources into P. smithii belowground organs in the present study was mai ntenance of higher Psi and A(n) rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsive ness of P. smithii to CO2 enrichment compared to B. gracilis.