Grafting Onto a Tolerant Rootstock Enhances Grapevine Tolerance to Combined Salt and Drought Stress

Salt and drought (SD) stress often occur simultaneously, posing severe challenges to grapevine cultivation. Grafting resistant rootstocks is considered a simple and effective method for enhancing grapevine tolerance to environmental stress. This study assessed the performance of different resistant seedlings to SD stress, including the self-rooted seedlings of the SD-sensitive grapevine cultivar 'Shine Muscat' (SM), self-rooted seedlings of the highly SD-tolerant cultivar 'Thompson Seedless' (TS), and grafted seedlings SM/TS. We examined changes in growth, photosynthetic performance, osmotic regulatory substances, and antioxidant capacity under SD stress. Our findings indicated that SD stress reduced leaf relative water content, disrupted internal leaf structure, reduced spongy tissue thickness, and inhibited grapevine growth. The three test grapevine leaves exhibited a significant SD stress-induced Na⁺/K⁺ imbalance accompanied by a substantial reactive oxygen species (ROS) buildup. The ascorbate-glutathione (AsA-GSH) cycle was disrupted, and the activities and gene expression levels of monodehydroascorbic reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were significantly reduced, along with decreases in ascorbate/dehydroascorbic acid (AsA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG) ratios. SM sustained the most severe injuries, followed by the SM/TS, then the TS. Grafting scion SM onto the tolerant rootstock TS significantly increased osmotic adjustment substances and K⁺ content, enhanced antioxidant enzyme activity and the AsA-GSH cycle, improved photosynthetic capacity, reduced Na⁺ content, and decreased ROS accumulation in SM leaves under SD stress, thereby strengthening tolerance. This research established a theoretical basis for improving grapevine tolerance to SD stress through grafting plants onto resistant rootstocks.