Salinity shifts trait-mediated photosynthetic response to flooding in salt-sensitive mangrove under sea-level rise

Background and Aims With accelerating sea-level rise, estuarine mangrove are increasingly exposed to combined stresses of prolonged flooding and elevated salinity. Although flooding duration (FD) and salinity are widely recognized as key constraints, how they interact to influence photosynthetic capacity through trait-based mechanisms remains unclear. Methods We established an in situ mesocosm experiment with Acanthus ilicifolius , a salt-sensitive mangrove species, in both oligohaline (OSZ) and mesohaline salinity zones (MSZ) of an estuary. We measured photosynthetic gas-exchange and leaf structural traits across a broad FD gradient (0.3 to 16.1 h·d⁻¹) to assess variations in net photosynthetic rate (Pn) and its potential drivers. Results Pn exhibited a nonlinear (hump-shaped) response to FD, while elevated salinity suppressed this response and shortened the optimal FD. In OSZ, Pn varied independently of specific leaf area (SLA), leaf thickness (LT) and leaf dry matter content (LDMC), but was primarily constrained by stomatal conductance (Gs) via its regulation of intercellular CO₂ concentration (Ci), as indicated by positive coordination between Pn and both Gs and Ci. However, Pn showed positive coordination with LT and LDMC in MSZ, but decreased with increasing SLA and Ci, contrary to the predictions of the leaf economics spectrum. Conclusions While mangrove can maintain photosynthesis through stomatal adjustment under prolonged flooding, elevated salinity disrupts this regulatory mechanism, triggering a shift from stomatal to non-stomatal limitations associated with altered leaf structure. These findings provide a trait-based mechanistic explanation for the physiological vulnerability of mangrove regeneration under sea-level rise.