Differences in the kinetics and scale of signalling molecule production modulate the ozone sensitivity of hybrid poplar clones:: the roles of H2O2, ethylene and salicylic acid

Hydrogen peroxide (H2O2), ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC) and salicylic acid (SA) concentrations and ACC synthase (ACS) gene expression were measured to establish whether the high sensitivity of the Populus deltoides x maximowiczii clone Eridano to ozone (O-3) exposure, compared with the O-3-resistant Populus deltoides x euramericana clone I-214, is attributable to differences in the modulation of signal transduction pathways. In a time-course experiment, Populus deltoides (poplar) clones were exposed to acute fumigation with 150 nl l(-1) O-3 for 5 h. The two poplar clones showed differences in ethylene evolution, I-214 displaying earlier and less pronounced ethylene emission than Eridano. In both clones, ethylene evolution was accompanied by increased ACS transcript levels and enhanced emission of free ACC. I-214 exhibited a greater basal concentration of free SA and a lower concentration of the conjugated pool. However, a slight accumulation of free SA at the end of the 5-h exposure was found only in Eridano, together with an earlier minimal increase in the concentration of conjugated SA. The results show that both clones react to O-3 by producing H2O2, ethylene and SA, but the difference in sensitivity to the pollutant is probably attributable to differences in the kinetics and magnitude of this response.