Chlorophyll <i>a</i> Fluorescence Kinetics Reveal That Ozone Stress Redistributes Electron Transport Limitations Between PSII and PSI in Wheat

ABSTRACT Tropospheric ozone (O 3 ) reduces crop productivity, but the sites at which ozone alters photosynthetic electron transport remain unclear. We tested whether acute exposure to O 3 shifts the dominant site of electron transport limitation in a cultivar‐specific manner, rather than causing a uniform decline in PSII function. The induction kinetics of chlorophyll a fluorescence in two wheat cultivars, PAN 3434 and PAN 3161, were analysed after controlled exposure to 100 ppb O 3 as an acute mechanistic perturbation, using charcoal‐filtered air as a low‐ozone reference treatment. O 3 induced different cultivar‐dependent changes in fluorescence rise phases. In PAN 3434, suppression of the J–I and I–P phases were consistent with a restricted reduction of the plastoquinone pool and a reduced acceptor‐side capacity of PSI, accompanied by declines in the density of the active PSII reaction centre and the probabilities of electron transport. In contrast, PAN 3161 showed smaller changes in these parameters, consistent with partial preservation of reaction centre function and intersystem electron transport. These contrasting photochemical signatures were associated with cultivar‐specific yield reductions. The findings indicate that acute O 3 exposure alters distinct segments of the PSII–PSI electron transport chain in a cultivar‐specific manner, providing mechanistic information relevant to physiological phenotyping and cultivar screening for ozone tolerance in wheat.