Salt–Drought Co-Stress Impairs Root Ultrastructure, Remodels Rhizosphere Bacteria, and Suppresses Peanut (Arachis hypogaea L.) Yield in Saline-Alkali Soil

Background: Peanuts (Arachis hypogaea L.) cultivated in saline-alkali areas frequently endure drought stress. Yet, mechanistic research on combined salt–drought stress for peanut growth and yield remains scarce. Methods: A pot culture experiment was conducted to investigate the impacts of short-term drought imposed at the flowering stage on peanuts grown in saline-alkali soil. We comprehensively assessed peanut agronomic traits, cell ultrastructure, physicochemical properties, hormone change, rhizobacterial community, and rhizosphere soil metabolic profiles between single salt stress and salt and drought co-stress. Results: Our study reveals that co-stress markedly suppressed peanut yield, with 100-pod weight, 100-seed weight, pods per plant, and pod yield per plant reduced by 3.16%, 12.79%, 16.65%, and 22.14% relative to salt-only treatment. Combined stress triggered more severe ultrastructural alterations, cell wall degradation, and tissue deformation. It also degraded soil quality by lowering available phosphorus, alkaline hydrolyzable nitrogen, and available potassium. Meanwhile, co-stress reduced the relative abundance of nitrogen-cycling and plant growth-promoting rhizobacteria, alongside depleted beneficial sugar metabolites in rhizosphere soil. These shifts jointly constrain peanut growth and productivity. Conclusions: Thus, it is imperative to implement timely irrigation practices to avoid drought during peanut cultivation in saline-alkali areas, particularly during the flowering stage.