Exogenous saponin-containing Gypsophila root extract mitigates flooding stress damage in spinach

Flooding stress associated with global climate change increasingly constrains plant growth in agricultural production. Spinach is among the vegetable crops most adversely affected by flooding stress. In addition, saponin applications are known to exert beneficial effects under various stress conditions. Accordingly, this study aimed to evaluate the dose-dependent responses of a saponin-containing Gypsophila root extract in spinach under flooding stress. Under pot conditions, spinach plants were subjected to a 10-day flooding stress and treated with three saponin-containing extract concentrations (1%, 3%, and 5%). The saponin-containing extract was applied twice as a foliar spray, once before stress initiation and again one week later during the flooding period. Flooding stress reduced key plant growth parameters in spinach, including shoot fresh and dry weights, root fresh and dry weights, and leaf area by 11%, 15%, 13%, 12%, and 31%, respectively. Saponin-containing extract applications partially mitigated flooding effects, especially by improving selected growth traits and reducing oxidative markers. Moreover, flooding stress decreased leaf relative water content (LRWC) and photosystem II quantum yield (ΦPSII), while significantly increasing oxidative stress markers, including malondialdehyde and hydrogen peroxide. Under full irrigation, the extract increased pigment contents, whereas under flooding it reduced stress-induced pigment accumulation. PCA indicated that the 5% extract treatment was positioned closest to the well-watered control under flooding conditions. However, treatment responses were parameter-specific: the 3% extract application most strongly reduced oxidative markers, whereas the 5% application showed greater improvements in selected growth traits.