Effects of elevated CO2 and O-3 on aspen clones varying in O-3 sensitivity: can CO2 ameliorate the harmful effects of O-3?

To determine whether elevated CO2 reduces or exacerbates the detrimental ef fects of O-3 on aspen (Populus tremuloides Michx.), aspen clones 216 and 27 1 (O-3 tolerant), and 259 (O-3 sensitive) were exposed to ambient levels of CO2 and O-3 or elevated levels of CO2, O-3, or CO2 + O-3 in the FACTS II ( Aspen FACE) experiment, and physiological and molecular responses were meas ured and compared. Clone 259, the most O-3-sensitive clone, showed the grea test amount of visible foliar symptoms as well as significant decreases in chlorophyll, carotenoid, starch, and ribulose-1, 5-bisphosphate carboxylase /oxygenase (Rubisco) concentrations and transcription levels for the Rubisc o small subunit, Generally, the constitutive (basic) transcript levels for phenylalanine ammonialyase (PAL) and chalcone synthase (CHS) and the averag e antioxidant activities were lower for the ozone sensitive clone 259 as co mpared to the more tolerant 216 and 271 clones. A significant decrease in c hlorophyll a, b and total (a + b) concentrations in CO2, O-3, and CO2+ O-3 plants was observed for all clones. Carotenoid concentrations were also sig nificantly lower in all clones, however, CHS transcript levels were not sig nificantly affected, suggesting a possible degradation of carotenoid pigmen ts in O-3-stressed plants. Antioxidant activities and PAL and 1-aminocyclop ropane-1-carboxylic acid (ACC)-oxidase transcript levels showed a general i ncrease in all O-3 treated clones, while remaining low in CO2 arid CO2+O3 p lants (although not all differences were significant). Our results suggest that the ascorbate-glutathione and phenylpropanoid pathways were activated under ozone stress and suppressed during exposure to elevated CO2. Although CO2+O3 treatment resulted in a slight reduction Of O-3-induced leaf injury , it did not appear to ameliorate all of the harmful affects of O-3 and, in fact, may have contributed to an increase in chloroplast damage in all thr ee aspen clones. (C) 2001 Published by Elsevier Science Ltd. All rights res erved.