Characteristics of Photosynthesis in Wheat Cultivars with Different Sensitivities to Ozone Under O3-Free Air Concentration Enrichment Conditions

With the help of the Chinese Ozone-Free Air Concentration Enrichment (O3FACE) platform, the responses of photosynthsis characteristics to elevated O3 concentration were investigated using winter wheat (Tritcium aestivum L.) cultivars Yannong 19 and Yangmai 16, which differ in sensitivities to O3. Under O3 treatment for 75 d, the net photosynthetic rate (Pn), stomtal conductance (Gs), and transpiration rate (Tr) decreased significantly in both cultivars, whereas the intercellular CO2 concentration (Ci) changed slightly. The O3-sensitive cultivar Yannong 19 had larger reductions in Pn (61.1%), Gs (68.0%), and Tr (57.4%) than Yangmai 16 (27.9%, 37.5%, and 27.9%, respectively). This indicated that the reduction of Pn mostly resulted from the nonstomatal factors in cooperation with stomal factors. In the chlorophyll fluorescence parameters, the maximal photochemical efficiency of PSII in the dark (Fv/Fm), the potential activity of PSII (Fv/Fo), the photochemical quenching (qP), and the rate of photochemical reaction (Prate) decreased in the O3 treatment, but the nonphotochemical quenching (NPQ) and the rate of thermal dissipation (Drate) showed a upward tendency. The change tendency of total soluble protein content and the amount of Rubisco was similar to that of chlorophyll fluorescence parameters and Pn. The results implied that the major nonstomal factors responsible for the Pn decrease under elevated O3 concentration were the Ribulose-1,5-bisphosphate (RuBP) carboxylation limitation and the damage of PSII. The change extents of all the parameters were larger in Yannong 19 than in Yangmai 16. The high Tr value and slow reduction of Rubisco amount in Yangmai 16 are probably crucial reasons for its high photosynthetic rate.