After exposure to a doubled CO, concentration of 750 pmol mol-' air for about 3 months, glucose and starch in the chlorenchyma of basal cladodes of Opuntia ficus-indica increased 175 and 57%, respectively, compared with the current CO, concentration of 370 pmol mol-', but sucrose content was virtually unaffected. Doubling the CO, concentration increased the nocturnal malate production in basal cladodes by 75%, inorganic phosphate (Pi) by 32%, soluble starch synthase activity by 30%, and sucrose-Pi synthase activity by 146%, but did not affect the activity of hexokinase. Doubling CO, accelerated phloem transport of sucrose out of the basal cladodes, resulting in a 73% higher dry weight for the daughter cladodes. Doubling CO, increased the glucose content in 14-dold daughter cladodes by 167%, increased nocturnal malate production by 22%, decreased total amino acid content by 61 %, and increased soluble starch synthase activity by 30% and sucrose synthase activity by 62%. No Plant Physiol 103: 51 9-524; P.S. J Exp Bot 45: 295-303), consistent with its higher source capacity and sink strength than under current CO,. These changes apparently do not result i n Pi limitation of photosynthesis or suppression of genes governing photosynthesis for this perennial Crassulacean acid metabolism species, as occur for some annual crops.Increases in the atmospheric CO, concentration generally enhance the rates of photosynthesis and growth for various C, plants, although the enhancement can diminish during long-term exposure to elevated CO, concentrations for annual plants with limited sinks (Sasek et al., 1985;Peet et al., 1986;Stitt, 1991). Such plants commonly respond to elevated CO, concentrations with a decrease of carboxylating enzyme activity and an increase of carbohydrate accumulation (Yelle et al., 1989a, 198913;Koner et al., 1995 vated CO, concentrations (Sage et al., 1989), the acclimation of photosynthesis may be a result of excess starch and sugar accumulation, which can cause feedback inhibition of photosynthesis (Sharkey and Vanderveer, 1989; Yelle et al., 1989a Yelle et al., , 1989bStitt, 1991). Based mainly on studies with annual crops and Arabidopsis, two hypotheses have been proposed to explain the feedback inhibition: (a) accumulation of starch and Suc can reduce the rates of their own synthesis, causing sugar phosphates to build up and deplete Pi pools in the chloroplasts and cytosol, thereby inhibiting photophosphorylation (Sharkey and Vanderveer, 1989;Stitt, 1991); and (b) buildup of sugars, especially Glc and SUC, can suppress (down-regulate) the expression of genes governing photosynthesis (Oosten et al., 1994;Sheen, 1994;Nie et al., 1995), resulting in a lowered photosynthetic rate.On the other hand, some C, tree species and perennial CAM plants do not show acclimation of photosynthesis during long-term exposure to elevated CO, concentrations (Idso and Kimball, 1994;Nobel and Israel, 1994;Ceulemans et al., 1995;Liu and Teskey, 1995). Doubling the atmospheric CO, concentration to 750 pmol mol-' a...