Increase in the levels of air pollution due to the increase in industrial and agricultural technology has prompted investigation of mechanisms that contribute to air pollution tolerance in plants. Pollen grains of Thuja orientalis were collected from controlled (less polluted) and polluted areas (mainly SO 2 , NO 2 , CO, HC and APM). Thuja pollen is considered inaperturate and granulate. The exine is shed during rehydration, leaving the male gametophyte naked. The pollen grains collected from polluted areas are smaller and more fragile compared to control ones. The exine splitting is faster and higher in polluted pollen grains. SDS-PAGE (sodium dodecyl sulphate-polyacrylamide gel electrophoresis) pattern do not show significant differences in polluted pollen than those in the control group. HPLC (high pressure liquid chromatography) analysis demonstrates that air pollution induces flavonoids accumulation to significantly higher levels in polluted pollen than in controlled ones. These observations suggest that plants try to respond suitably by adjusting their metabolism so that minimum damage is done due to air pollutants. Their protective responses may include an increase in antioxidant enzymes and metabolites and induction of protection-related secondary metabolite genes especially flavonoids.

Major antioxidants were studied in three Citrus (C. reticulata, C. sinensis, and C. paradisi) under lowtemperature stress (1, −1, −3°C). Regulatory effects of exogenous proline (0, 15, 20 mM) were studied as well. The results indicated that treatment of exogenous proline reduced both hydrogen peroxide and lipid peroxidation in three Citrus species. The high-performance liquid chromatography chromatograms at −3°C showed that exogenous proline increases chlorogenic, gallic, p-coumaric, and ferulic acid from phenolic acids as well as naringin, quercetin, and rutin from flavonoids. The amount of oxalic, citric, and ascorbic acid increased with increasing proline concentration but tartaric acid decreased. Exogenous proline increased Gamma-aminobutyric acid although its content showed no significant difference between 15 and 20 mM. Both lowtemperature stress and treatment of proline induced the activity of ascorbate peroxidase (APX) and catalase. In contrast, lipoxygenase activity reduced with temperature decrease as well as increasing exogenous proline levels. Also, treatments of low temperature and proline showed that the levels of endogenous proline, catalase and APX in C. reticulata are higher compared with two other species and their amount increases with the reduction of temperature.

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Cadmium selenide nanoparticles (CdSe NPs) were synthesized by an easy and simple method and their properties were assessed by XRD, TEM and SEM techniques. The effects of CdSe NPs as well as Cd2+ ions on Lemna minor plants were investigated. The absorption of CdSe NPs by the plants had some adverse consequences that were assessed by a range of biological analyses. The results revealed that both CdSe NPs and the ionic form of cadmium noticeably caused toxicity in L. minor. Morphological parameters as well as peroxidase (POD) activity were deteriorated. In contrast, the activities of some other antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)) as well as the contents of total phenol and flavonoids went up. Taken all together, it could be implied that CdSe NPs as well as Cd2+ were highly toxic to plants and stimulated the plant defense system in order to scavenge produced reactive oxygen species (ROS).