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Applying large amounts of potash fertilizer in apple orchards for high apple quality and yield aggravates KCl stress. As a phytoalexin, resveratrol (Res) participates in plant resistance to biotic stress. However, its role in relation to KCl stress has never been reported. Herein we investigated the role of Res in KCl stress response of Malus hupehensis Rehd., a widely used apple rootstock in China which is sensitive to KCl stress. KCl-stressed apple seedlings showed significant wilting phenotype and decline in photosynthetic rate, and the application of 100 μmol Res alleviated KCl stress and maintained photosynthetic capacity. Exogenous Res can strengthen the activities of peroxidase and catalase, thus eliminating reactive oxygen species production induced by KCl stress. Moreover, exogenous Res can decrease the electrolyte leakage by accumulating proline for osmotic balance under KCl stress. Furthermore, exogenous Res application can affect K+/Na+ homeostasis in cytoplasm by enhancing K+ efflux outside the cells, inhibiting Na+ efflux and K+ absorption, and compartmentalizing K+ into vacuoles through regulating the expression of K+ and Na+ transporter genes. These findings provide a theoretical basis for the application of exogenous Res to relieve the KCl stress of apples.

Salinity-alkalinity stress can remarkably affect the growth and yield of apple. Strigolactone (SL) is a class of carotenoid-derived compounds that functions in stress tolerance. However, the effects and mechanism of exogenous SL on the salinity-alkalinity tolerance of apple seedlings remain unclear. Here, we assessed the effect of SL on the salinity-alkalinity stress response of Malus hupehensis seedlings. Results showed that treatment with 100 μM exogenous SL analog (GR24) could effectively alleviate salinity-alkalinity stress with higher chlorophyll content and photosynthetic rate than the apple seedlings without GR24 treatment. The mechanism was also explored: First, exogenous GR24 regulated the expression of Na+/K+ transporter genes and decreased the ratio of Na+/K+ in the cytoplasm to maintain ion homeostasis. Second, exogenous GR24 increased the enzyme activities of superoxide, peroxidase and catalase, thereby eliminating reactive oxygen species production. Third, exogenous GR24 alleviated the high pH stress by regulating the expression of H+-ATPase genes and inducing the production of organic acid. Last, exogenous GR24 application increased endogenous acetic acid, abscisic acid, zeatin riboside, and GA3 contents for co-responding to salinity-alkalinity stress indirectly. This study will provide important theoretical basis for analyzing the mechanism of exogenous GR24 in improving salinity-alkalinity tolerance of apple.

Background: Applying large amounts of potash fertilizer in apple orchards for high apple quality and yield aggravates KCl stress. As a phytoalexin, resveratrol (Res) participates in plant resistance to biotic stress. However, its role in relation to KCl stress have never been reported. Results: Malus hupenensis is a widely used apple rootstock in China and is sensitive to KCl stress. In this study, 100 µmol Res was exogenously applied to alleviate KCl stress in M. hupenensis seedlings. The seedlings treated with Res had higher chlorophyll content and photosynthetic index than those without Res treatment. Moreover, the molecular and physiological mechanisms of Res in ion toxicity, osmotic stress, and oxidative damage induced by KCl stress were also investigated. First, exogenous Res affects K+/Na+ homeostasis in cytoplasm by enhancing K+ efflux outside the cells, inhibiting Na+ efflux and K+ absorption, and compartmentalizing K+ into vacuoles. Second, this compound could respond to osmotic stress by regulating the accumulation of proline. Lastly, this polyphenol functions as an antioxidant that strengthens the activities of POD and CAT thus eliminates the reactive oxygen species production induced by KCl stress.Conclusions: Taken together, these results reveal that resveratrol alleviates the KCl salinity stress of M. hupenensis seedlings by regulating K+/Na+ homeostasis, osmotic adjustment, and reactive oxygen species scavenging.