Exogenous sodium sulfide improves morphological and physiological responses of a hybrid Populus species to nitrogen dioxide

Gaseous nitrogen dioxide (NO₂) can disturb normal plant growth and trigger complex physiological responses. NO₂-induced responses are influenced by biotic or abiotic factors. In this study, we investigated the effects of exogenous sodium sulfide (Na₂S, 5 mmol L^?1) on epidermis and stomata related physico-chemical responses of hybrid poplar cuttings (Pouplus alba × P. berolinensis) to gaseous NO₂ (4 μl 1^?1) for three time periods (0, 14 and 48 h). We also investigated hydrogen sulfide (H₂S), nitrate-nitrogen and nitrate reductase activity (NR) in control and Na₂S treated plants. Our results showed that NO₂ exposure for 48 h led to the decline of NR, maximal PSII quantum yield (F_v/F_m), net photosynthetic rate (P_n), and dark respiration rate (R_d). The maximum rate for the post-illumination carbon dioxide burst (PIB) occurred in 48-h exposed leaves 13–15 s after darkening. Moreover, NO₂ exposure resulted in a significant increase in nitrogen percentage (from 0 to 33%) and a decrease in the macro and micro-elements of leaf surface. Spraying Na₂S aqueous solution on the leaf surfaces significantly increased the thicknesses of palisade/spongy tissue and H₂S content. Na₂S pretreatment alleviated NO₂-caused toxic effects as indicated by increased NR and higher values of P_n, F_v/F_m, and actual photochemical efficiency in light (ФPSII) compared with the control. Na₂S pretreatment had no significant impacts on PIB-based photorespiration or elements composition of a leaf surface.