Two modern cultivars [Yangmai16 (Y16) and Yangfumai 2 (Y2)] of winter wheat (Triticum aestivum L.) with almost identical phenology were investigated to determine the impacts of elevated ozone concentration (E-O 3 ) on physiological characters related to photosynthesis under fully open-air field conditions in China. The plants were exposed from the initiation of tillering to final harvest, with E-O 3 of 127% of the ambient ozone concentration (A-O 3 ). Measurements of pigments, gas exchange rates, chlorophyll a fluorescence and lipid oxidation were made in three replicated plots throughout flag leaf development. In cultivar Y2, E-O 3 significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to nonstomatal factors, e.g. lower maximum carboxylation capacity, electron transport rates and light energy distribution. In cultivar Y16, by contrast, the effects of E-O 3 were observed only at the very last stage of flag leaf ageing. Since the two cultivars had almost identical phenology and very similar leaf stomatal conductance before senescence, the greater impacts of E-O 3 on cultivars Y2 than Y16 cannot be explained by differential ozone uptake. Our findings will be useful for scientists to select O 3 -tolerant wheat cultivars against the rising surface [O 3 ] in East and South Asia. Nomenclature:%D 5 the fraction of photons dissipated in the antenna %P 5 the fraction of photons utilized in PSII photochemistry %X 5 the fraction of absorbed photons by PSII neither used in photochemistry nor dissipated in the PSII A 5 photosynthetic rate AOT40 5 the cumulative O 3 exposure over a threshold of the 1 h average [O 3 ] of 40 ppb during daytime A sat 5 light-saturated photosynthesis Car 5 Carotenoid Chl 5 Chlorophyll C i 5 intercellular CO 2 concentration AQY 5 apparent quantum efficiency DAFE 5 days after full expansion of flag leaves Fv 0 /Fm 0 5 actual photochemical efficiency of PSII in the saturated light gs 5 stomatal conductance J max 5 the maximum rate of electron transport l 5 stomatal limitation to photosynthesis M7 5 the mean of the daily 7 h average [O 3 ] MDA 5 malondialdehyde O 3 5 ozone OTC 5 open-top chambers Correspondence: K. Kobayashi, tel. 1 81 3 5841 1292, fax 1 81 3 5841 5186, PPFD 5 photosynthetic photon flux density PhiPSII 5 light-adapted apparent quantum efficiency of PSII qP 5 quenching of photochemical efficiency of PSII SUM06 5 sum of hourly average [O 3 ] ! 0.06 ppm during daytime Vc max 5 the maximum carboxylation efficiency Y16 5 wheat variety Yangmai 16 Y2 5 wheat variety Yangfumai 2.
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