Different O3 response of sensitive and resistant snap bean genotypes (Phaseolus vulgaris L.): The key role of growth stage, stomatal conductance, and PSI activity

The ozone (O-3) 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 O-3 levels often found in Mediterranean sites; (ii) contributing to identify possible ecophysiological mechanisms determining the different O-3 sensitivity of S156 and R123, along different plant growth stages. In contrast with what reported by other authors, differences in stomatal O-3 uptake were evident between sensitive and resistant plants. In particular, R123 plants showed an O-3-induced stomatal closure (-38.1% than the control) during flowering and the onset of visible injury, a behaviour that can be regarded as an O-3 avoidance mechanism. At the end of the fumigation period (AOT40 = 4.66 ppm h), 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 O-3 stress. Increased energy demand for maintenance and repair processes also determined increased dark respiration rates (Rd) in all fumigated plants; nocturnal stomatal conductance (g(sN)) was also enhanced, differently in the two genotypes, with possible implication for higher nocturnal stomatal uptake of S156 in field conditions. No clear genotype x O-3 effect was instead detected on pod biomass, although a significant O-3-induced reduction of yield was evident in both genotypes.