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Winter wheat is a grass species widely planted in northern and central China, where the increase of aerosols, air pollutants and population density are causing significant reduction in solar irradiance. In order to investigate the adaptation of winter wheat (Triticum aestivum L., cv. Yangmai 13) to low irradiance conditions occurring in the downstream plain of the Yangtze River (China), plants were subjected to four solar irradiance treatments (100%, 60%, 40%, and 20% of environmental incident solar irradiance). Significant increases in chlorophyll (Chl) and xanthophyll (Xan) pigments, and decreases in Chl a/b and Xan/Chl ratios were observed in plants under low light. Light-response curves showed higher net photosynthetic rates (P N ) in fully irradiated plants, that also showed a higher lightcompensation point. Shaded plants maintained high values of minimal fluorescence of dark-adapted state (F o ) and maximum quantum efficiency of PSII photochemistry (F v /F m ) that assess a lower degree of photoinhibition under low light. Reduced irradiance caused decreases in effective quantum yield of PSII photochemistry (Φ PSII ), electron transport rate (ETR), and nonphotochemical quenching coefficient (q N ), and the promotion of excitation pressure of PSII (1 -q P ). The activities of the antioxidant enzymes superoxide dismutase and peroxidase were high under reduced light whereas no light-dependent changes in catalase activity were observed. Thiobarbituric acid reactive species content and electrolyte leakage decreased under shaded plants that showed a lower photooxidative damage. The results suggest that winter wheat cv. Yangmai 13 is able to maintain a high photosynthetic efficiency under reduced solar irradiance and acclimates well to shading tolerance. The photosynthetic and antioxidant responses of winter wheat to low light levels could be important for winter wheat cultivation and productivity.

Mole fractions of atmospheric CO 2 (XCO 2) have been continuously measured from October 2014 to March 2016 at the Guangzhou Panyu Atmospheric Composition Site (23.00°N, 113.21°E; 140 m MSL) in the Pearl River Delta (PRD) region using a cavity ring-down spectrometer. Approximately 66.63%, 19.28%, and 14.09% of the observed values were filtered as background, pollutant source, and sink due to biospheric uptake, respectively, by applying a robust local regression procedure. Their corresponding mean values were 424.12 ± 10.12 ppm (×10 −6 mol mol −1), 447.83 ± 13.63 ppm, and 408.83 ± 7.75 ppm. The background XCO 2 levels were highest in spring and winter, moderate in autumn, and lowest in summer. The diurnal XCO 2 was at a minimum from 1400-1600 LST (Local Standard Time) and a maximum at 0500 LST the next day. The increase of XCO 2 in spring and summer was mainly associated with polluted air masses from south coastal Vietnam, the South China Sea, and the southeast Pearl River Estuary. With the exception of summer, airflow primarily from marine regions southeast of Taiwan that passed over the Pearl River Estuary had a greater impact on XCO 2 , suggesting an important potential source region.

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