Isoprene emission rates under elevated CO2 and O-3 in two field-grown aspen clones differing in their sensitivity to O-3

Isoprene is the most important nonmethane hydrocarbon emitted by plants. The role of isoprene in the plant is not entirely understood but there is evidence that it might have a protective role against different oxidative stresses originating from heat shock and/or exposure to ozone (O-3). Thus, plants under stress conditions might benefit by constitutively high or by higher stress-induced isoprene emission rates. In this study, measurements are presented of isoprene emission from aspen (Populus tremuloides) trees grown in the field for several years under elevated CO2 and O-3. Two aspen clones were investigated: the O-3-tolerant 271 and the O-3-sensitive 42E. Isoprene emission decreased significantly both under elevated CO2 and under elevated O-3 in the O-3-sensitive clone, but only slightly in the O-3-tolerant clone. This study demonstrates that long-term-adapted plants are not able to respond to O-3 stress by increasing their isoprene emission rates. However, O-3-tolerant clones have the capacity to maintain higher amounts of isoprene emission. It is suggested that tolerance to O-3 is explained by a combination of different factors; while the reduction of O-3 uptake is likely to be the most important, the capacity to maintain higher amounts of isoprene is an important factor in strengthening this character.