With increasing interest in the effects of elevated atmospheric CO2 on plant growth and the global carbon balance, there is a need for greater understanding of how plants respond to variations in atmospheric partial pressure of CO2. Our research shows that elevated CO 2 produces significant fine structural changes in major cellular organelles that appear to be an important component of the metabolic responses of plants to this global change. Nine species (representing seven plant families) in several experimental facilities with different CO2-dosing technologies were examined. Growth in elevated CO2 increased numbers of mitochondria per unit cell area by 1.3-2.4 times the number in control plants grown in lower CO2 and produced a statistically significant increase in the amount of chloroplast stroma (nonappressed) thylakoid membranes compared with those in lower CO2 treatments. There was no observable change in size of the mitochondria. However, in contrast to the CO2 effect on mitochondrial number, elevated CO2 promoted a decrease in the rate of mass-based dark respiration. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO 2 concentrations.C arbon dioxide enrichment of the global atmosphere is one of the most significant ecological changes produced by industrialization during the past two centuries, increasing the partial pressure of CO 2 in the atmosphere by 30% (1). Within the global carbon cycle, photosynthesis and respiration by plants are the primary biological processes linking inorganic carbon in the atmosphere (CO 2 ) with organic carbon in the biosphere. This knowledge has led to widespread interest in understanding more fully how these physiological responses are coupled to changes in atmospheric CO 2 partial pressures. Furthermore, the magnitude of carbon fluxes between the atmosphere and the biosphere resulting from both photosynthesis and autotrophic respiration is very large (approximately 120 and 60 billion metric tons per year, respectively) (2). The magnitudes of these carbon fluxes are so large that a change of only a few percent could account for the discrepancy between the known sources and sinks for carbon (2, 3). As atmospheric CO 2 partial pressures continue to rise as the result of human activities such as fossil fuel combustion and land clearing (3), it is critical to identify and quantify the biochemical, physiological, and ecological processes that are sensitive to atmospheric CO 2 . Clearly these processes can impact significantly the rate and͞or extent of changes in the CO 2 content of the Earth's atmosphere, as well as our conclusions concerning appropriate future policies regarding greenhouse gas emissions.An integrative understanding of the responses of plants to increased atmospheric CO 2 partial pressure is important because both short-and long-term photosynthetic and respiratory responses to elevated CO 2 have been demonstrated (recently reviewed in refs. 4-...