Leaf Gas Exchange As a Factor in the Differential Ozone Sensitivity of Two Soybean Genotypes.

Ozone is a phytotoxic air pollutant that limits crop productivity. Plants exhibit a wide range of responses to ozone exposure. Understanding the mechanisms of ozone tolerance will contribute to breeding efforts to improve crops in the future. Ozone entry through leaf stomata initiates the stress response. Thus, stomatal conductance plays a critical role in controlling ozone uptake. To investigate stomatal conductance as a potential mechanism to explain ozone tolerance, experiments were conducted that compared leaf gas exchange characteristics of two soybean [Glycine max (L.) Merr] genotypes, Mandarin Ottawa (ozone-sensitive) and Fiskeby III (ozone-tolerant). Three week-old plants were exposed for 1 week to either charcoal filtered air (< 10 ppb ozone) or 70ppb ozone in Continuously Stirred Tank Reactors (CSTRs) in a greenhouse. Measurements of photosynthetic rates, stomatal conductance, and foliar injury were recorded. In the controls, we consistently found stomatal conductance to be approximately 30% lower for Fiskeby III relative to Mandarin Ottawa while the two genotypes exhibited similar high levels of photosynthesis. Ozone exposure induced significant foliar injury on leaves of Mandarin Ottawa, and the injury was associated with declines in both stomatal conductance (by 50%) and photosynthesis (by 32%). In contrast, ozone exposure resulted in minimal foliar injury on leaves of Fiskeby III with only a small decline in photosynthesis (by 9%) and a further reduction in stomatal conductance (by 25%) observed. The results confirm Fiskeby III to be an ozone-tolerant genotype and suggest that reduced stomatal conductance contributes to the observed ozone-tolerance through limiting ozone uptake by the plant. The capacity of Fiskeby III to maintain high rates of photosynthesis at lowered stomatal conductance is an unusual trait that could be used to enhance soybean ozone tolerance.