As one of the most common pollutant gases and a major threat to ecosystems, NO2, which is a brown–red gas with a pungent odor, is an atmospheric pollutant that originates primarily from vehicle emissions, fuel burning in power plants, and industrial production of nitric acids (Chai et al., 2002). NO2 serves as a precursor of ozone and atmospheric particles, and it is an essential atmospheric pollutant for the formation of acid rain (Bermejo-Orduna et al., 2014; Rahmat et al., 2013). In recent decades, the emission of NO2 has increased in many countries worldwide (Frati et al., 2008; Munzi et al., 2009), particularly in China and other Asian countries (Hu et al., 2015; van der A et al., 2008). In China, the national concentration threshold for NO2 pollution is 0.0199 µL·L−1, and the atmospheric concentration of NO2 near highways reaches 0.0255–0.058 µL·L−1 (Nanjing Environmental Protection Bureau, 2016). Furthermore, with the progression of urban industrialization and automation, the NO2 concentration may continue to increase (Munzi et al., 2009). Therefore, it is critical to determine ecologically and environmentally friendly methods to reduce atmospheric NO2 concentrations.
Injury to plants is primarily due to the corrosive and high oxidative properties of NO2, which influence various physiological and biochemical processes after entering plants via the stomata (Lu et al., 2002). NO2 can cause reductions in growth, and the amount of damage experienced by a plant varies according to different factors, such as the concentration and length of exposure, plant age, edaphic factors, light, and humidity. Symptoms are often divided into “visible” and “invisible” (or hidden) injuries, which comprise an overall reduction in growth but no obvious signs of visible damage (Wellburn, 1990) and mainly affect the physiological metabolism of plants.
Chlorophyll (Chl) is the most important plant pigment because it has a crucial role in photosynthesis (Croft et al., 2017). Chen et al. (2010) performed NO2 fumigation of Cinnamomum camphora seedlings at low-volume fractions for 60 d and found that 4.0 µL·L−1 of NO2 significantly decreased the Chl content in the leaves of seedlings. Changes in the leaf color that resulted in yellow spots or yellow leaves were attributable to the change in Chl content. Liu et al. (2015) found that Arabidopsis thaliana initiated stress-resistant protective mechanisms triggered by superoxide free radicals under NO2 stress. Hu et al. (2015) reported that the net photosynthesis of Populus alba × P. berolinensis hybrid leaves significantly decreased after 14 and 48 h of exposure to 4 µL·L−1 of NO2, thus demonstrating that inhibition of photosynthesis may be responsible for the suppression of growth by NO2. Furukawa et al. (1984) found that exposure to 2 and 4 µL·L−1 of NO2 for 2 h decreased the net photosynthesis rate by 20% and 90%, respectively, but had no significant effect on reducing transpiration.
Ultrastructural changes associated with invisible injuries have often been linked to decreased transpiration and photosynthesis (Fink, 1988; Huttunen and Soikkeli, 1984), and some occurred in both mitochondria and chloroplasts. Four-year-old Norway spruce trees (Picea abies) were fumigated with 1 µL·L−1 of NO2 (low concentration) for 3 weeks, resulting in swollen thylakoids, decreased grana stacks, and enhanced starch contents compared with controls (Schiffgens-Gruber and Lütz, 1992). Mature bean (Phaseolus vulgaris) tissue was exposed to 1 and 2 µL·L−1 of NO2 for 1 and 2 h and compared with similar untreated tissues; the NO2-exposed bean tissue showed swelling of the thylakoids within the chloroplasts but no extrachloroplastic damage, and the thylakoid swelling could be reversed by removing the pollutant from the air flow or changing the rate of gas flow within the tissue (Wellburn et al., 1972). Chaparro-Suarez et al. (2011) found that NO2 was not absorbed by plants when the stomata were closed, and that species-dependent differences in uptake rates could be clearly related to stomatal behavior. Despite the results obtained by the aforementioned studies, considering that NO2 resulting from traffic pollution is a primary source of pollution in urban areas, and considering that landscape plants comprise an important part of the urban ecology, studies of the effects of NO2 on plants that occupy an important position in urban green spaces should be performed. However, most previous reports have focused on field crops and herbs rather than urban landscape plants, particularly trees.
The species Carpinus betulus originates from Turkey and Ukraine (Jia, 2002). It is a deciduous broad-leaf tree in the family Betulaceae. Carpinus betulus has an attractive shape, with dense, delicate leaves in summer and golden leaves and unique fruit in autumn (Jia, 2002). Various cultivars of this species can be used for landscaping, and it has been grown in many countries. Shi et al. (2015) investigated the photosynthetic characteristics of C. betulus from different regions and the influence of salt stress on the photosynthetic and fluorescent characteristics of C. betulus seedlings. However, studies of the effects of NO2 stress on the physiological response of C. betulus have not been reported (Taylor and MacLean, 1970).
Based on the aforementioned results, this study aimed to address the following hypotheses: 1) NO2 treatment has a negative influence on the photosynthesis, stomatal behavior, and chloroplast ultrastructure of C. betulus leaves; and 2) C. betulus returns to normal growth after the removal of NO2 stress. The results of this study may provide useful reference data for the selection of plant species for atmospheric absorption and a scientific reference for plant selection for urban gardening and the construction of ecological green space landscapes.
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