Protecting the photosynthetic performance of snap bean under free air ozone exposure

Tropospheric ozone (O₃) is a major air pollutant and causes serious injury to vegetation. To protect sensitive plants from O₃ damage, several agrochemicals have been assessed, including cytokinin (e.g., kinetin, KIN) and ethylenediurea (EDU) with cytokinin-like activity. In higher plant, leaves are primarily injured by O₃ and protective agrochemicals are often applied by leaf spraying. To our knowledge, the mitigating abilities of EDU and KIN have not been compared directly in a realistic setup. In the present research, impacts of elevated O₃ (2× ambient O₃, 24hr per day, for 8days) on an O₃ sensitive line (S156) of snap bean (Phaseolus vulgaris), which is often used for biomonitoring O₃ pollution, were studied in a free air controlled exposure system. The day before starting the O₃ exposure, plants were sprayed with a solution of EDU (300ppm), KIN (1mmol/L) or distilled water, to compare their protective abilities. The results demonstrated that 2× ambient O₃ inhibited net photosynthetic rate and stomatal conductance, increased the minimal fluorescence yield of the dark-adapted state, decreased the maximal quantum yield of PSII photochemistry, and led to visible injury. KIN and EDU alleviated the reduction of the photosynthetic performance, and visible injury under O₃ fumigation. The plants sprayed with EDU showed greater ability to mitigate the O₃ damage than those sprayed with KIN. Chlorophyll fluorescence imaging may have detected more precisely the differences in O₃ response across the leaf than the conventional fluorometer.