Growth and partitioning in Pascopyrum smithii (C-3) and Bouteloua gracilis (C-4) as influenced by carbon dioxide and temperature

This study investigated how CO2 and 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 smithii Rydb. (C-3) and Bouteloua gracilis (H.B.K.) Lag. ex Steud. (C-4). Treatment combinations comprised two temperatures (20 and 35 degrees C) at two concentrations of CO2 (380 and 750 mu mol mol(-1)), and two additional temperatures of 25 and 30 degrees C at 750 mu mol mol(-1) CO2. Plants were maintained under favourable nutrient and soil moisture and harvested following 21, 35, and 49 d of treatment. CO2-induced growth enhancements were greatest at temperatures considered favourable for growth of these grasses. Compared to growth at 380 mu mol mol(-1) CO2, final d.wt of CO2-enriched P. smithii increased 84% at 20 degrees C but only 4% at 35 degrees C. Final d.wt of B. gracilis was unaffected by CO2 at 20 degrees C, but was enhanced by 28% at 35 degrees C. Root:shoot ratios remained relatively constant across CO2 levels, but increased in P. smithii with reduction in temperature. These partitioning results were adequately explained by the theory of balanced root and shoot activity. Favourable growth temperatures led to CO2-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 CO2. However, CO2-induced decreases in plant tissue N concentrations were more evident for P. smithii. Roots of CO2-enriched P. smithii had greater total N content at 20 degrees C, an allocation of N below-ground that may be an especially important adaptation for C-3 plants. Tissue N contents of B. gracilis were unaffected by CO2. Results suggest CO2 enrichment may lead to reduced N requirements for growth in C-3 plants and lower shoot N concentration, especially at favourable growth temperatures. (C) 1996 Annals of Botany Company