| Summary: | We previously showed that chronic warming plus elevated carbon dioxide (eCO<sub>2</sub>) causes extreme upward bending of leaflets and petioles (i.e., hyponasty) in tomato (<i>Solanum lycopersicum</i>), which reduces growth. In that study, only two levels of CO<sub>2</sub> (400, 700 ppm) and temperature (30, 37 °C) were tested in young vegetative plants, and the underlying mechanism for warming + eCO<sub>2</sub> hyponasty was not investigated. In this study, warming + eCO<sub>2</sub> hyponasty was evaluated in tomato across a range of temperatures and CO<sub>2</sub> concentrations, and at multiple life stages. Based on their roles in thermal hyponasty, ethylene and auxin tomato mutants were examined, and light quality manipulated, to explore the mechanism for warming + eCO<sub>2</sub> hyponasty. At eCO<sub>2</sub> (800 ppm), the petiole angle increased roughly linearly with temperature from 30 to 38 °C. Under high temperature stress (38 °C), the petiole angle increased similarly at all eCO<sub>2</sub> concentrations (600/800/1000 vs. 400 ppm). All life stages examined had an increased petiole angle in leaves developed during warming + eCO<sub>2</sub>, such that most leaves in juvenile plants exhibited hyponasty but only young growing leaves did so in adults. Auxin-insensitive mutants displayed a reduced petiole angle compared to auxin-sensitive, ethylene-sensitive, ethylene-insensitive, and non-mutant genotypes, indicating that auxin, but not ethylene, is likely a main component of this hyponastic response. Reduced far-red-to-red light plus increased blue light reduced petiole hyponasty compared to non-filtered white light during warming + eCO<sub>2</sub>. These results indicate that eCO<sub>2</sub> affects the well-characterized thermal hyponastic response of leaves, which has implications for future plant responses to climate change.
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