Growth and Partitioning in Pascopyrum smithii(C 3) and Bouteloua gracilis(C 4) as Influenced by Carbon Dioxide and Temperature

Abstract

This study investigated how CO 2and temperature affect dry weight (d.wt) accumulation, total nonstructural carbohydrate (TNC) concentration, and partitioning of C and N among organs of two important grasses of the shortgrass steppe, Pascopyrum smithiiRydb. (C 3) and Bouteloua gracilis(H.B.K.) Lag. ex Steud. (C 4). Treatment combinations comprised two temperatures (20 and 35°C) at two concentrations of CO 2(380 and 750 μmol mol -1), and two additional temperatures of 25 and 30°C at 750 μmol mol -1CO 2. Plants were maintained under favourable nutrient and soil moisture and harvested following 21, 35, and 49d of treatment. CO 2-induced growth enhancements were greatest at temperatures considered favourable for growth of these grasses. Compared to growth at 380 μmol mol -1CO 2, final d.wt of CO 2-enriched P. smithiiincreased 84% at 20°C, but only 4% at 35°C. Final d.wt of B. graciliswas unaffected by CO 2at 20°C, but was enhanced by 28% at 35°C. Root:shoot ratios remained relatively constant across CO 2levels, but increased in P. smithiiwith reduction in temperature. These partitioning results were adequately explained by the theory of balanced root and shoot activity. Favourable growth temperatures led to CO 2-induced accumulations of TNC in leaves of both species, and in stems of P. smithii, which generally reflected responses of above-ground d.wt partitioning to CO 2. However, CO 2-induced decreases in plant tissue N concentrations were more evident for P. smithii. Roots of CO 2-enriched P. smithiihad greater total N content at 20°C, an allocation of N below-ground that may be an especially important adaptation for C 3plants. Tissue N contents of B. graciliswere unaffected by CO 2. Results suggest CO 2enrichment may lead to reduced N requirements for growth in C 3plants and lower shoot N concentration, especially at favourable growth temperatures.