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Summary 1. Urban-rural transects can be utilized as natural gradients of temperature and also as a tool to predict how plant ecology and physiology might respond to expected global change variables such as elevated temperatures, CO2 and inorganic nitrogen deposition. 2. We investigated differences in respiration ( R) and the balance of electron partitioning through the cytochrome (CP) and alternative (AP) pathways in leaves of mature Quercus rubra L. trees along a transect from New York City to the Catskill Mountains over the course of one growing season. In addition, we investigated the effects of elevated temperature on Q. rubra seedlings in a controlled environment study. 3. In the field study, we found that urban-grown leaves often respired at greater rates than leaves grown at other sites and that this was likely due to higher leaf nitrogen. At each site, R at the prevailing growth temperature declined steadily throughout the growing season despite higher temperatures at the end of the summer. Differences in R were associated with changes in the relative abundances of cytochrome and alternative oxidase proteins. Oxygen isotope discrimination ( D), which reflects relative changes in AP and CP partitioning, was negatively correlated with daily minimum temperature in trees grown at the colder rural sites, but not at the warmer urban sites. 4. In the growth cabinet study, we found that R acclimated to elevated temperatures and that this was accompanied by a steady increase in D. 5. These findings that AP partitioning increases with both high and low temperatures show that the AP may play an important role in plant responses to environmental conditions that elicit stress, and not simply to specific conditions such as low temperature. [ABSTRACT FROM AUTHOR]