Abstract: Human activity is increasing atmospheric CO2 and both global mean and acute high temperatures (heat waves). Laboratory studies have shown that elevated CO2 can increase heat tolerance of photosynthesis in C3 plants. However, human-caused increases in ground-level ozone (O3), which causes oxidative stress, may offset benefits of elevated CO2 during heat waves. In this study, we determined the effects of elevated CO2 and O3 on the heat tolerance of leaves of field-grown Glycine max (soybean, C3). Photosynthetic electron transport (et) was measured in attached leaves heated in situ and in heated detached leaves under ambient CO2 and O3; biochemical assays were conducted on leaves of plants heated in the lab. Heat stress decreased et, and O3 exacerbated this decrease. Elevated CO2 prevented O3-related decreases during heat stress, but only increased et under ambient O3 in the field. CO2 and O3 effects on et during heat stress were light dependent. Heat stress decreased chlorophyll and carotenoid content, especially under elevated CO2. Neither CO2 nor O3 had any effect on production of heat-shock proteins. Heat stress increased catalase (except in high O3) and Cu/Zn-SOD (superoxide dismutase) content, but not Mn-SOD; CO2 and O3 decreased catalase and did not affect Mn- or Cu/Zn-SOD. Soluble carbohydrates were unaffected by heat stress, but increased in elevated CO2. Together, these results indicate that modest protection of photosynthetic metabolism during heat stress by elevated CO2 is observed in field-grown soybean under ambient O3, as in the lab, and high CO2 limits damage during heat stress under elevated O3, but this protection is likely related to decreased photorespiration and stomatal conductance rather than production of heat-stress adaptations.

Key words: anti-oxidants, global change, heat-shock proteins, photosynthesis.