The increases in atmospheric carbon dioxide (CO(2)) concentrations can enhance plant growth and change their nutrient demands. We report that when tomato (Lycopersicon esculentum 'Zheza 809') plants were grown in iron (Fe)-limited medium (with hydrous ferric iron oxide) and elevated CO(2) (800 microL L(-1)), their biomass and root-to-shoot ratio were greater than plants grown in ambient CO(2) (350 microL L(-1)). Furthermore, the associated increase in Fe concentrations in the shoots and roots alleviated Fe-deficiency-induced chlorosis. Despite the improved nutrient status of plants grown in Fe-limited medium under elevated CO(2), the Fe-deficiency-induced responses in roots, including ferric chelate reductase activity, proton secretion, subapical root hair development, and the expression of FER, FRO1, and IRT genes, were all greater than plants grown in the ambient CO(2). The biomass of plants grown in Fe-sufficient medium was also increased by the elevated CO(2) treatment, but changes in tissue Fe concentrations and Fe deficiency responses were not observed. These results suggest that the improved Fe nutrition and induction of Fe-deficient-induced responses in plants grown in Fe-limited medium under elevated CO(2) are caused by interactions between elevated CO(2) and Fe deprivation. Elevated CO(2) also increased the nitric oxide (NO) levels in roots, but treatment with the NO scavenger cPTIO inhibited ferric chelate reductase activity and prevented the accumulation of LeFRO1, LeIRT1, and FER transcripts in roots of the Fe-limited plants. These results implicate some involvement of NO in enhancing Fe-deficiency-induced responses when Fe limitation and elevated CO(2) occur together. We propose that the combination of elevated CO(2) and Fe limitation induces morphological, physiological, and molecular responses that enhance the capacity for plants to access and utilize Fe from sparingly soluble sources, such as Fe(III)-oxide.