Photosynthetic pathway and ontogeny affect water relations and the impact of CO on Bouteloua gracilis (C) and Pascopyrum smithii (C).

The eastern Colorado shortgrass steppe is dominated by the C grass, Bouteloua gracilis, but contains a mixture of C grasses as well, including Pascopyrum smithii. Although the ecology of this region has been extensively studied, there is little information on how increasing atmospheric CO will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate partitioning in B. gracilis and P. smithii grown under present ambient and elevated CO concentrations of 350 μl land 700 μl l, respectively, and two deficit irrigation regimes. The experiment was conducted in soil-packed columns planted to either species over a 2-month period under summer-like conditions and with no fertilizer additions. Our objective was to better understand how these species and the functional groups they represent will respond in future CO-enriched environments. Leaf CO assimilation (A ), transpiration use efficiency (TUE, or A /transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired) of both species were greater at elevated CO, although responses were more pronounced for P. smithii. Elevated CO enhanced photosynthesis, TUE, and growth in both species through higher soil water content (SWC) and leaf water potentials (Ψ) and stimulation of photosynthesis. Consumptive water use was greater and TUE less for P. smithii than B. gracilis during early growth when soil water was more available. Declining 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 of P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO, while the root:shoot ratio of B. gracilis was unresponsive to CO. These partitioning responses may be the consequence of different ontogenetic strategies of a cool-season and warm-season grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for summer dormancy, while resource partitioning of the warm-season B. gracilis remains unaltered. One consequence of greater partitioning of resources into P. smithii belowground organs in the present study was maintenance of higher Ψ and A rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsiveness of P. smithii to CO enrichment compared to B. gracilis.