Ozone tolerance in Phaseolus vulgaris depends on more than one mechanism

Two bean cultivars with different ozone-sensitivity, i.e. the O3-sensitive Cannellino and the O3-tolerant Top Crop, were exposed to acute O3-stress with the aim of evaluating physiological and biochemical traits responsible for O3-tolerance. The cultivars differed in their constitutive ability to prevent and counter O3-induced oxidative damage. The ability to dissipate excess-energy, via regulated and unregulated nonphotochemical quenching mechanisms (NPQ, NO) was constitutively greater in Top Crop than in Cannellino, and allowed the O3-tolerant cultivar to compensate light-induced declines in PSII and preserve photosystem II from excitation-energy to a superior degree than the O3-sensitive one. The inevitable generation of reactive oxygen species was countered to a significantly smaller extent in the O3-sensitive cultivar, because of constitutively lower activities of superoxide dismutase and catalase than the O3-tolerant cultivar. Indeed, Cannellino underwent an early accumulation of H2O2 and an almost complete disruption of cell structure and irreversible damages in the photosynthetic apparatus