Light and Nutrient Stress Interactively Regulate Phenylpropanoid Metabolism via Substrate Competition and ROS / RNS Signaling

ABSTRACT Plants allocate fixed carbon between growth and defense based on environmental constraints, yet how light and nutrient interactively regulate this trade‐off remains unclear. Here we demonstrate that high‐light/low‐nutrient combination synergistically enhances phenylpropanoid synthesis in Agastache rugosa via ROS/RNS signaling and substrate competition cascades. Plants were grown under factorial light (0% and 50% shade) and nutrient (40–160 mg kg −1 ) levels in a nested design. Path analysis revealed that nutrient supply strongly promotes free amino acid accumulation ( β = 0.789), which negatively regulates chalcone synthase activity ( β = −0.412), confirming substrate competition between protein synthesis and phenylpropanoid production. Simultaneously, high‐light increases hydrogen peroxide and inhibits nitric oxide levels ( β = −0.811), yet these ROS/RNS molecules function as complementary signaling mediators that positively regulate shikimic acid synthesis and phenylalanine ammonia‐lyase activity ( β = 0.789). This dual regulatory mechanism increased shikimic acid and chalcone synthase activity under high‐light/low‐nutrient conditions by 6‐fold and 5‐fold, respectively, than low‐light/low‐nutrient treatment, resulting in increases in flavonoid and ascorbic acid content. Principal component analysis confirmed light conditions explain 52.5% of metabolic variance, with nutrient availability modulating response magnitude. These findings establish that environmental stress combinations elicit non‐additive metabolic responses through integrated substrate competition and ROS/RNS signaling networks, providing a mechanistic framework for optimizing bioactive compound production in medicinal plants.