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 (Fv/Fm), net photosynthetic rate (Pn), and dark respiration rate (Rd). 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 Pn, Fv/Fm, 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.