Abstract
Foliar uptake of gaseous NO2 mainly occurs through the stomata and disrupts normal plant growth, but no detailed reports about the physiological responses of plants exposed to NO2 are available. In this study, to study leaf physicochemical responses, stomatal characteristics and chloroplast structure, we observed the leaves of Carpinus putoensis W.C.Cheng after exposure to NO2 (6 μL/L) for five time periods (0, 1, 6, 24, and 72 h) and after 30 days of recovery following NO2 exposure. Our results showed that short-duration exposure to a high concentration of NO2 had significant negative impacts (p < 0.05) on the chlorophyll content, photosynthesis and chloroplast-related physicochemical processes of C. putoensis leaves; with the exception of one hour of NO2 exposure, which was helpful for plant physiological responses. Moreover, NO2 exposure significantly increased the thickness of the palisade/spongy tissue and caused swelling of the thylakoids within the chloroplasts; this thylakoid swelling could be reversed by removing the pollutant from the air flow. Restoration of unpolluted air alleviated the toxic effects of NO2, as indicated by an increased chlorophyll content, net photosynthesis, and PSII maximum quantum yield. These results could support the development of a treatment for roadside trees that are exposed to NO2 as a major road pollutant.
Highlights
NO2 is a precursor of harmful secondary air pollutants such as ozone and particulate matter and it plays an important role in the phenomenon of acid rain [1,2]
We explored the changes in photosynthesis, stomatal behavior and chloroplast ultrastructure of C. putoensis under NO2 exposure for various durations
Our results showed that NO2 affected the Chl content in the C. putoensis leaf and its effects varied as a function of exposure time; 1 h NO2 exposure could lead to an increased Chl content
Summary
NO2 is a precursor of harmful secondary air pollutants such as ozone and particulate matter and it plays an important role in the phenomenon of acid rain [1,2]. Plants are influenced by the atmospheric pollutants that they absorb and suffer varying degrees of damage [3]. Liu treated Arabidopsis thaliana L. with NO2 at 0.48, 8.2, 17.4 and 38.5 μL/L to investigate variations in morphology, chlorophyll, superoxide free radical levels, protective enzyme systems, ascorbic acid levels, and glutathione levels after NO2 stress [6]. They found that A. thaliana initiated stress-resistant protective mechanisms under NO2 stress, during which superoxide free radicals acted as signals to Forests 2018, 9, 561; doi:10.3390/f9090561 www.mdpi.com/journal/forests
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