We examined the effects of moderately elevated ozone at an open-air field on protein expression patterns in the
leaves of two Finnish hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) clones with different O3 sensitivities.
The soil-grown plants were exposed for three growing seasons to O3 1.3 to 1.4 times the ambient concentration (approximately
35 ppb), mirroring increases expected in the near future. Contrary to our expectations, O3 treatment had no
major effects on the protein patterns of the two hybrid aspen clones. Altogether, 447 protein spots were reproducibly detected
in the SYPRO Ruby -stained gels. The intensity of nine of these spots was significantly affected by O3; six and
three were up- and down-regulated, respectively. In both clones, chronic long-term O3 stress up-regulated WRKY transcription
factor and a Myb protein and transposon protein. The clones showed contrasting responses in RuBisCO large
subunit fragment, thioredoxin (TRX) intermediate and glyceraldehyde phosphate dehydrogenase (GPDH). In the more
tolerant clone 55, RuBisCO large subunit fragment and TRX were down-regulated and GPDH up-regulated, while in the
more sensitive clone RuBisCO large subunit fragment and TRX were up-regulated and GPDH down-regulated. Although
our study found rather small changes in hybrid aspen proteome, it showed the significance of long-term O3 exposure on
growing trees. This study would prove useful to scale up O3 responses on mature trees at the ecosystem level.