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, physiological, 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% GWC) coincided with peak total phenolic content (TPC), reaching 19.57 mg GAE g−1 DW. Physiological stress was initially detected at 21–22% GWC, evidenced by a decline in relative water content (RWC). Furthermore, 8–9% GWC soil moisture marked a critical physiological threshold characterized by a pronounced reduction in photosynthetic performance and elevated oxidative stress markers. Prolonged water limitation (4 weeks at 20–30% GWC) was associated with lower leaf greenness, RWC, and photosynthetic performance. Concurrently, water restriction led to distinct anatomical alterations, including reduced chloroplast abundance, starch accumulation in the stem cortex, and a reduction in stem xylem vessel diameter. These findings indicate that water deficit affects physiological adaptations, accompanied by increased bioactive compound accumulation.