Effects of Elevated Carbon Dioxide Levels and Air Temperature on Carbon Assimilation of Loblolly Pine

Abstract

Changes in the atmospheric carbon dioxide (CO₂) concentration directly affects photosynthesis, which, in turn, indirectly affects all other plant processes. Carbon dioxide is the substrate used in photosynthesis for the creation of sugars needed by the plant for all anabolic and catabolic activities. For trees, the present-day ambient CO₂ concentration (C_a) is not at saturation with respect to photosynthesis, therefore an increase in C_a can result, at least initially, in an increase in the rate of net carbon assimilation (A_n) (Ceulemans and Mousseau, 1994). For the most part, this effect is caused by a reduction in photorespiration. The higher CO₂ concentration causes an increase in the frequency of carboxylation reactions in relation to oxygenation reactions catalyzed by the enzyme rubisco in the initial step of the dark reactions, or Calvin cycle (Webber et al., 1994). An increased concentration of C₂ in the atmosphere also speeds the rate of diffusion of CO₂ into the mesophyll. But when conditions are less than optimum, a complex variety of interacting factors may significantly reduce the potential for carbon gain in elevated CO₂ concentrations. Nutrient limitations, drought, and excessive tempera- tures are among the common stresses in the field that may reduce the increase in A, expected under increased CO, concentrations (Conroy et al., 1990, Tschap- linski et al., 1993; El Kohen and Mousseau, 1994; Guehl et al., 1994; Lewis et al., 1994; Ziska and Bunce, 1994).