Response of Photosynthetic Characteristics to Different Salicylic Acid Concentrations in Relation to Waterlogging Resistance in Zanthoxylum armatum

Zanthoxylum armatum (Rutaceae) is an important spice and a traditional medicinal plant in southwest China. Owing to its shallow root system, it is highly susceptible to waterlogging stress. To evaluate how an exogenous salicylic acid (SA) application reduces photosynthetic damage in Z. armatum under waterlogging stress conditions, the content of photosynthetic pigments and gas exchange and chlorophyll fluorescence parameters were studied under different SA concentrations (0, 1 and 2 mM) and different numbers of waterlogging days (1, 2 and 3 d) to identify the mechanisms underlying the seedlings' waterlogging response. The results showed that waterlogging significantly affects the photosynthetic characteristics in leaves. An exogenous SA application increased the contents of photosynthetic pigments as well as the transpiration rate (Tr), net leaf photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), actual quantum efficiency (fPSII), photosystem II (PSII) maximum quantum efficiency (Fv/Fm), PSII photochemical efficiency in light (Fv'/Fm'), and electron transport rate (ETR), whereas the treatment decreased the intercellular carbon dioxide concentration (Ci), non-photochemical quenching coefficient (NPQ) and minimum fluorescence (Fo). Therefore, the SA application helps improve the photosynthetic efficiency and reduce photodamage to Z. armatum in the event of waterlogging stress.