Fig. 1. Effect of different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP), a NO donor, on the biomass of ryegrass seedlings under benzoapyrene (B[a]P) stress. (A) Aboveground dry weight, (B) Belowground dry weight, (C) Aboveground fresh weight, (D) Belowground fresh weight, (E) Aboveground plant height, (F) Belowground root length. Data are mean ± standard deviation, n = 3. Different letters indicate significant differences at p < 0.05.

Fig. 2. Effect of different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP), a NO donor, on the photosynthetic pigment content of ryegrass seedlings under benzoapyrene (B[a]P) stress. (A) Chlorophyll a content, (B) Chlorophyll b content, (C) Carotenoid content, (D) Total chlorophyll content. FW – fresh weight. Data are mean ± standard deviation, n = 3. Different letters indicate significant differences at p < 0.05.

Fig. 3. Effect of different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP), a NO donor, on photosynthetic gas exchange parameters of ryegrass seedlings under benzoapyrene (B[a]P) stress. (A) Net photosynthetic rate (Pn), (B) Stomatal conductance (Gs), (C) Intercellular carbon dioxide concentration (Ci), (D) Transpiration rate (Tr), (E) Water use efficiency (WUE). Data are mean ± standard deviation, n = 3. Different letters indicate significant differences at p < 0.05.

Fig. 4. Effect of different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP), a NO donor, on chlorophyll fluorescence parameters of ryegrass seedlings under benzoapyrene (B[a]P) stress. (A) PSII potential activity (F_v/F₀), (B) PSII maximum light energy conversion efficiency (F_v/F_m), (C) Actual quantum yield of PSII photochemistry (ФPSII), (D) Steady-state fluorescence (F_s), (E) Photochemical quenching coefficient (qP), (f) Non-photochemical quenching coefficient (qN). Data are mean ± standard deviation, n = 3. Different letters indicate significant differences at p < 0.05.

Fig. 5. Effect of different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP), a NO donor, on antioxidant enzyme activity of ryegrass seedlings under benzoapyrene (B[a]P) stress. (A) Superoxide dismutase (SOD) activity. (B) Catalase (CAT) activity. (C) Peroxidase (POD) activity. Data are mean ± standard deviation, n = 3. U – unit of enzyme activity, FW – fresh weight. Different letters indicate significant differences at p < 0.05.

Fig. 6. Principal component analysis (PCA) plots of growth, photosynthetic characteristics and chlorophyll fluorescence parameters and antioxidant enzyme activity of ryegrass for different concentrations (100, 200, 300 and 400 μmol L^-1) of sodium nitroprusside (SNP) in combination with benzoapyrene (B[a]P) as well as control.