Influence of Water Limitation on Morphological, Physiological, and Anatomical Adaptations in Water Hyssop (Bacopa monnieri (L.) Wettst.)

Water hyssop (Bacopa monnieri (L.) Wettst.) is a water-demanding medicinal plant whose responses to declining soil moisture remain insufficiently characterized, particularly in relation to phenolic accumulation. This study investigated the morphological, physiolog-ical, and anatomical responses of water hyssop to water limitation and identified the soil moisture conditions associated with enhanced phenolic accumulation. Short-term water limitation (11 days at 9.06% soil moisture) maximized total phenolic content (TPC), which reached a peak of 195.69 µg mL−1. Physiological stress was initially detected at 21–22% soil moisture, evidenced by a decline in relative water content (RWC). Furthermore, 8–9% soil moisture marked a critical physiological threshold characterized by severe photosyn-thetic impairment, elevated oxidative stress, and cellular damage. Prolonged water limita-tion (4 weeks at 20–30% soil moisture) reduced leaf greenness, RWC, and photosynthetic performance. Concurrently, long-term stress resulted in anatomical adaptations in both stem and leaf, such as chloroplast depletion, starch accumulation in the stem cortex, and a reduction in stem xylem vessel diameter. These findings demonstrate that drought stress can serve as an effective elicitor to enhance bioactive compound accumulation in water hyssop, providing valuable insights for applications in controlled-stress cultivation and irrigation management.