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Limited evidence indicates that moderate leaf hyponasty can be induced by high temperatures or unnaturally high CO2. Here, we report that the combination of warming plus elevated CO2 (eCO(2)) induces severe leaf hyponasty in tomato (Solanum lycopersicum L.). To characterize this phenomenon, tomato plants were grown at two levels of CO2 (400 vs. 700 ppm) and two temperature regimes (30 vs. 37 degrees C) for 16-18 days. Leaf hyponasty increased dramatically with warming plus eCO(2) but increased only slightly with either factor alone and was slowly reversible upon transfer to control treatments. Increases in leaf angle were not correlated with leaf temperature, leaf water stress, or heat-related damage to photosynthesis. However, steeper leaf angles were correlated with decreases in leaf area and biomass, which could be explained by decreased light interception and thus in situ photosynthesis, as leaves became more vertical. Petiole hyponasty and leaf-blade cupping were also observed with warming + eCO(2) in marigold and soybean, respectively, which are compound-leaved species like tomato, but no such hyponasty was observed in sunflower and okra, which have simple leaves. If severe leaf hyponasty is common under eCO(2) and warming, then this may have serious consequences for food production in the future.