Integrative physiological and transcriptomic analyses reveal key metabolic pathway responses in wax gourd (Benincasa hispida) leaves under potassium deficiency and excess.

Potassium (K) is a key nutrient for increasing yield, improving quality, and enhancing stress resistance in wax gourd (Benincasa hispida). However, the phenotypic, physiological, ecological, and molecular adaptation mechanisms of wax gourd under different K levels remain poorly understood. In this study, a field experiment was conducted with three treatments, i.e., K deficiency (K0), moderate K (300 kg K2O hm-2, K300), and excessive K (500 kg K2O hm-2, K500), to evaluate the effects of K fertilization on the morphology, physiology, and transcriptomics of wax gourd. Results showed that compared with K300, K0 and K500 reduced wax gourd yield by 25.22% and 10.38%, respectively. Fruit soluble sugar, soluble protein, titratable acidity, vitamin C, and dry matter content were all significantly decreased under K0 treatment. Both K0 and K500 damaged chloroplast ultrastructure, induced stomatal closure, and reduced photosynthetic efficiency. Under K0, the activities of Rubisco, sucrose phosphate synthase (SPS), and ADP-glucose pyrophosphorylase (AGPase) decreased by 50.79%-60.30%, 58.75%-61.11%, and 51.20%-54.00%, respectively, while photosynthetic K-use efficiency (PKUE) and K accumulation decreased by 1.54%-22.70% and 25.67%-65.95%. Collectively, K deficiency exacerbated yield and quality reduction by inhibiting photosynthesis and K+ uptake/utilization. Transcriptomic analysis revealed that under K deficiency, genes associated with photosynthesis, photosynthetic antenna proteins, nitrogen metabolism, and starch and sucrose metabolism were downregulated, whereas genes associated with antioxidant enzyme activity and plant hormone signal transduction (ABA and CTK) were upregulated. Under excess K, genes encoding various transporters, as well as those involved in starch and sucrose metabolism, were downregulated. In conclusion, among the tested K levels, K300 (300 kg K2O hm-2) showed the best overall performance in wax gourd cultivation, contributing to synergistic improvements in yield, quality, and fertilizer efficiency. This study offers both theoretical insights and practical guidance for elucidating K-stress response mechanisms and for optimizing K-fertilizer strategies in wax gourd.