Reach for the sky: Elevated CO2 plus warming cause hyponastic growth in tomatoes

Background/Question/Methods

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. However, that study examined only two levels of CO2 (400, 700 ppm) and temperature (30, 37oC) 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. Lastly, other Solanaceous species were examined to test the hypothesis that warming + eCO2 hyponasty is restricted to compound-leaved species in this family.

Results/Conclusions

At eCO2 (800 ppm), petiole angle increased ca. linearly as temperature increased from 30-38oC. 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 juvenile plants but only young leaves did so in adults (previously fully-developed leaves were unaffected). Ethylene-insensitive (nr) and -constitutive (epi) mutants displayed similar changes in petiole angle with warming + eCO2 compared to the non-mutant reference (cv. Celebrity), indicating that ethylene is not a main component of the mechanism of this hyponastic response. Auxin-insensitive (dgt) mutants had no hyponastic response to warming plus eCO2 treatments and auxin-constitutive (e) mutants displayed similar leaf angle changes to non-mutant reference (cv. Celebrity), indicating auxin may be a main component of warming and eCO2 induced hyponasty. Only compound-leaved Solanaceous species exhibited warming + eCO2 hyponasty. Warming + eCO2 hyponasty may have negative consequences for the productivity of certain wild and domesticated agronomic species in the coming decades, and understanding the underlying mechanisms of this response may prove useful for developing climate-change resistant crops.