Elevated CO2 Increases Severity of Thermal Hyponasty in Leaves of Tomato

We previously showed that chronic warming plus elevated carbon dioxide (eCO2) causes extreme upward bending of leaflets and petioles (i.e., hyponasty) in tomato (Solanum lycopersicum), which reduces growth. In that study, only two levels of CO2 (400, 700 ppm) and temperature (30, 37 °C) were tested in young vegetative plants, and the underlying mechanism for warming + eCO2 hyponasty was not investigated. In this study, warming + eCO2 hyponasty was evaluated in tomato across a range of temperatures and CO2 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 + eCO2 hyponasty. At eCO2 (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 eCO2 concentrations (600/800/1000 vs. 400 ppm). All life stages examined had an increased petiole angle in leaves developed during warming + eCO2, 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 + eCO2. These results indicate that eCO2 affects the well-characterized thermal hyponastic response of leaves, which has implications for future plant responses to climate change.