The ozone (O3) biomonitoring system based on sensitive (S156) and resistant (R123) genotypes of snap bean (Phaseolus vulgaris L.), was tested in a fumigation experiment, with the aim of: (i) assessing the performance of the S156/R123 system under simulated climatic conditions and O3 levels often found in Mediterranean sites; (ii) contributing to identify possible ecophysiological mechanisms determining the different O3 sensitivity of S156 and R123, along different plant growth stages. In contrast with what reported by other authors, differences in stomatal O3 uptake were evident between sensitive and resistant plants. In particular, R123 plants showed an O3-induced stomatal closure (-38.1% than the control) during flowering and the onset of visible injury, a behaviour that can be regarded as an O3 avoidance mechanism. At the end of the fumigation period (AOT40=4.66ppmh), despite the reduction of assimilation and the photoprotective down regulation of PSII photochemistry, the capacity to reduce the final electron acceptors beyond PSI was up-regulated, particularly in S156, while the PSI activity remained constant in both genotypes, an unusual response to O3 stress. Increased energy demand for maintenance and repair processes also determined increased dark respiration rates (Rd) in all fumigated plants; nocturnal stomatal conductance (gsN) was also enhanced, differently in the two genotypes, with possible implication for higher nocturnal stomatal uptake of S156 in field conditions. No clear genotype×O3 effect was instead detected on pod biomass, although a significant O3-induced reduction of yield was evident in both genotypes. © 2012 Elsevier B.V.

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