Interactive Effects of Elevated CO2 and Ozone on Leaf Thermotolerance in Field-Grown Glycine max

Humans are increasing atmospheric CO2, ground-level ozone (O-3), and mean and acute high temperatures. Laboratory studies show that elevated CO2 can increase thermotolerance of photosynthesis in C-3 plants. O-3-related oxidative stress may offset benefits of elevated CO2 during heat-waves. We determined effects of elevated CO2 and O-3 on leaf thermotolerance of field-grown Glycine max (soybean, C-3). Photosynthetic electron transport (Phi(et)) was measured in attached leaves heated in situ and detached leaves heated under ambient CO2 and O-3. Heating decreased Phi(et), which O-3 exacerbated. Elevated CO2 prevented O-3-related decreases during heating, but only increased Phi(et) under ambient O-3 in the field. Heating decreased chlorophyll and carotenoids, especially under elevated CO2. Neither CO2 nor O-3 affected heat-shock proteins. Heating increased catalase (except in high O-3) and Cu/Zn-superoxide dismutase (SOD), but not Mn-SOD; CO2 and O-3 decreased catalase but neither SOD. Soluble carbohydrates were unaffected by heating, but increased in elevated CO2. Thus, protection of photosynthesis during heat stress by elevated CO2 occurs in field-grown soybean under ambient O-3, as in the lab, and high CO2 limits heat damage under elevated O-3, but this protection is likely from decreased photorespiration and stomatal conductance rather than production of heat-stress adaptations.