Differential gene expression in senescing leaves of two silver birch genotypes in response to elevated CO2 and tropospheric ozone

Long-term effects of elevated CO2 and O-3 concentrations on gene expression in silver birch (Betula pendula Roth) leaves were studied during the end of the growing season. Two birch genotypes, clones 4 and 80, with different ozone growth responses, were exposed to 2x ambient CO2 and/or O-3 in open-top chambers (OTCs). Microarray analyses were performed after 2 years of exposure, and the transcriptional profiles were compared to key physiological characteristics during leaf senescence. There were genotypic differences in the responses to CO2 and O-3. Clone 80 exhibited greater transcriptional response and capacity to alter metabolism, resulting in better stress tolerance. The gene expression patterns of birch leaves indicated contrasting responses of senescence-related genes to elevated CO2 and O-3. Elevated CO2 delayed leaf senescence and reduced associated transcriptional changes, whereas elevated O-3 advanced leaf senescence because of increased oxidative stress. The combined treatment demonstrated that elevated CO2 only temporarily alleviated the negative effects of O-3. Gene expression data alone were insufficient to explain the O-3 response in birch, and additional physiological and biochemical data were required to understand the true O-3 sensitivity of these clones.