Physiological Responses of Two Submerged Macrophytes to Pulsed Aerial Exposure: Contrasting Photoprotection and Oxidative Defense

Pulsed aerial exposure (PAE) is a major stressor for submerged macrophytes in tidal river networks, yet species-specific physiological mechanisms remain unclear. We conducted a 7-day PAE mesocosm experiment using Hydrilla verticillata (L.f.) Royle and Vallisneria denseserrulata (Makino) Makino to compare their photosynthetic performance, pigment retention, and oxidative defense. Hydrilla maintained lower baseline Y(II) and reliable EC50s (64.6–85.7 min/d). In contrast, Vallisneria showed higher baseline Y(II) but a transient Y(NPQ) increase peaking at 120 min/d that subsequently collapsed, greater pigment loss (total chlorophyll −68% vs. −44%), and highly uncertain or out-of-range EC50s. Although antioxidant enzyme activities were higher in Vallisneria, MDA accumulation was comparable between species. Principal component analysis (87.14% total variance) revealed that PC1 (42.03%) separated pigments from antioxidant enzymes, and PC2 (26.46%) captured photochemical efficiency. PC3 (18.65%) further indicated a transient oxidative burst and subsequent defense breakdown in Vallisneria under severe stress. These results suggest that Hydrilla exhibits a low light-capture strategy to mitigate PAE pressure, while Vallisneria exhibits an inducible defense that becomes overwhelmed. The quantitative parameters provide baseline data for macrophyte growth models, though field validation is required to confirm long-term ecological implications.