Exogenous fulvic acid improves the structure and industrial functional properties of potato starch under drought stress

Drought stress severely restricts potato vegetative growth and deteriorates tuber starch structural and functional properties, thereby reducing its value as an industrial raw material. Fulvic acid (FA), a natural eco-friendly bio-stimulant, can effectively boost plant stress resistance and regulate crop biochemical components under adverse environments. In the present work, we screened the optimal exogenous FA application scheme via gradient concentration and growth-stage trials, confirming that 0.05 mM FA sprayed at the tuber expansion stage exerted the most significant regulatory effect. Physiological phenotyping demonstrated that FA obviously relieved drought-triggered growth retardation, lowered reactive oxygen accumulation and membrane lipid peroxidation damage, and elevated systemic antioxidant enzyme activities to maintain cell homeostasis. Regarding starch physicochemical properties relevant to industrial processing, FA optimized starch granule morphology and particle size distribution, recovered the activities of key starch-synthesizing enzymes (soluble starch synthase and starch branching enzyme), and stabilized critical pasting parameters including peak viscosity, breakdown, and setback value, which directly determine industrial processing performance. Transcriptome and metabolome joint profiling revealed that FA initiated rapid hormone-mediated transcriptional rewiring in leaf tissues, and simultaneously activated tuber MAPK signaling, phenylpropanoid and flavonoid antioxidant metabolism, as well as central carbon metabolic pathways. Multi-layer integrated omics and biochemical validation collectively verified that FA reshaped carbon partitioning and directed more photosynthates toward tuber starch biosynthesis. Overall, FA co-elevates drought adaptability and industrial starch functionality of potato, offering a feasible agronomic strategy for securing high-quality industrial starch raw materials and downstream non-food applications, particularly by improving starch suitability for paper surface sizing, where paste stability and film-forming ability are critical, under water-limited cultivation conditions.