[Effects of long-term ozone exposure on chlorophyll a fluorescence and gas exchange of winter-wheat leaves].

In order to provide basis for evaluating the effects of air pollutant such as O3 on crops yield and food security, the effects of O3 fumigation (ambient air, CK; 100 nL x L(-1), T1; 150 nL x L(-1), T2) on chlorophyll a fluorescence and gas exchange of a field-grown winter-wheat (Triticum aestivum L. Yang Mai 13) in different growing period were conducted via open-top chamber technique in conjunction with Diving-PAM fluorometer and LC pro + photosynthesis system. Results indicated that Fv/Fm caused by T1 was higher than 0.8, while the Pm, qP, (1-qP)/NPQ and Y(NO) were similar to those of CK, the NPQ and Y(NPQ) were increased by 13.5%-29.0% and 13.3%-22.7% respectively due to O3 stress. Under nature light (rapid light curve, RLC) and after dark adaptation (induction curve in steady-state, IC) the Yield of T1 was decreased by 4.6%-7.6% and 11.3%-19.3% respectively, with 8.0%-9.8% and 11.0%-23.1% reductions in Pn, and Gs compared to CK, respectively. In heading stage and blooming stage, the Ls of T, was greater than CK, but in filling stage and mature stage, it became lower compared to CK. The Fv/Fm was slightly lower than 0.8 under T2 treatment, with the Y(NO), (1-qP)/NPQ and c(i) were increased by 37.9%-75.6%, 157.1%-325.8% and 3.4%-18.1% relative to CK. Under RLC and IC condition, the Yield of T2 was respectively decreased by 10.2%-13.6% and 21.4%-29.1%, and the Pn, Ls, qP, Pm, NPQ and Y(NPQ) were decreased by 28.1%-39.9%, 5.2%-21.3%, 15.8%-30.4%, 27.6%-45.6%, 3.3%-52.9% and 5.7%-17.9% in comparison, respectively. Obviously the enhanced O3 causes a significant decrease in the capacity of photosynthesis of winter wheat, and the influence mechanism presents a series of dynamic changes according to growing seasons. The reduction of Fv/Fm under T1 treatment is a response of PS II reaction center to the increase of NPQ, and the decrease in Pn and Yield is a consequence of protective adjustment, by this approach, the antioxidant system and energy dissipation mechanism can thus prevent light damage to the PS II reaction center of winter wheat. Under T2 treatment, the CO2 assimilation and Q(A) re-oxidizing during actinic illumination are restricted, the energy dissipation mechanism was destroyed, and the reduction of photosynthesis was mainly due to damage in photosystem caused by O3 and excess light. The critical loads for O3 of PS II reaction center is between 100 nL x L(-1) and 150 nL x L(-1) close to 100 nL x L(-1). While the Fv/Fm value is not an effective index for assessing O3 influence on winter-wheat. Although the winter-wheat can have certain adapted ability to O3 stress, the growing enhancement of surface O3 is still a great threat to agricultural production in China.