Atmospheric carbon dioxide (CO2) concentrations are increasing and may exceed 800 ppm by 2100. This is increasing global mean temperatures and the frequency and severity of heatwaves. Recently, we showed for the first time that the combination of chronic warming and elevated carbon dioxide (eCO2) caused extreme upward bending during growth (i.e., hyponasty) of leaflets and leaf stems (petioles) in tomato (Solanum lycopersicum), which reduced growth. Other compound-leaved species (marigold [T. erecta] and soybean [G. max]) displayed increased leaf angle and cupping as well, while simple leaved (sunflower [H. annuus] and okra [A. esculentus]) did not. That study, however, examined only two levels of CO2 (400, 700 ppm) and temperature (30, 37°C) at the young-vegetative-plant stage. Further, we did not investigate underlying mechanisms for this warming+eCO2-induced hyponasty, which is likely to involve the plant hormones auxin and ethylene, based on their role in thermal hyponasty. In this study, warming+eCO2 hyponasty was evaluated in tomato across a range of temperatures and CO2 concentrations, and at multiple life stages. Ethylene and auxin tomato mutants were examined to explore the potential role of these hormones in warming+eCO2 hyponasty, and a light-filtering experiment was used to evaluate how light quality may affect plant hyponastic responses to warming+eCO2 treatments. Lastly, other species, especially Solanaceous species, were examined to determine if warming+eCO2 hyponasty is restricted to compound-leaved species.  At eCO2 (800 ppm), petiole angle increased ca. linearly as temperature increased from 30-38°C. Under heat stress (HS, 38°C), petiole angle increased at all eCO2 concentrations compared to ambient (600/800/1000 vs. 400 ppm). All life stages examined (juvenile, pre-reproductive, and flowering) had increased petiole angle in leaves developed during warming+eCO2, such that most leaves exhibited hyponasty in juveniles but only young leaves did so in adults (previously fully-developed leaves were unaffected). Ethylene-insensitive and -constitutively-producing, and auxin-constitutively-producing, mutants displayed similar changes in petiole angle with warming+eCO2 compared to the non-mutant reference. Auxin-insensitive mutants grown with warming+eCO2 displayed similar leaf angle to those grown at optimal temperatures and ambient CO2, indicating that auxin and not ethylene is involved in this hyponastic response. Blue light filters increased the proportion of blue light and decreased the proportion of red and far-red light, resulting in reduced leaf angle in plants grown at warming+eCO2 compared to standard chamber lights, indicating a role for light in this hyponasty. Only compound-leaved Solanaceous species exhibited increased warming+eCO2 hyponasty compared to control conditions, warming, and eCO2 treatments, while plants from other families had varying responses. Warming+eCO2 hyponasty may have negative consequences for the productivity of certain wild and domesticated plants in the coming decades, and understanding the underlying mechanisms of this response may prove useful for developing climate-change-resistant crops.