Divergent adaptations to salinity fluctuations in mangroves: opportunistic photosynthetic leveraging vs. conservative stress resistance in Aegiceras corniculatum and Bruguiera gymnorhiza.

Large-scale mangrove mortality associated with salinity fluctuations is frequently reported. These fluctuations are caused by climatic extremes or hydraulic engineering projects. However, the adaptation processes of mangrove plants to salinity fluctuations remain poorly understood. We subjected the salt-secreting species Aegiceras corniculatum and the salt-excluding species Bruguiera gymnorhiza to daily salinity fluctuations, and investigated their responses in terms of gas exchange, PSII photochemical efficiency, and osmotic regulation. A. corniculatum demonstrated an opportunistic strategy by efficiently utilizing optimal salinity conditions (5-15‰) during early salinity fluctuations (Day 1). It rapidly modulated stomatal conductance to enhance photosynthetic rate. At the same time, it employed inorganic ion mediated osmotic adjustment to maintain water uptake capacity and sustain stomatal opening. Notably, even when photosynthetic activity was suppressed under high salinity stress (25‰), this species successfully exploited subsequent low salinity periods (5‰) to fully restore photosynthetic performance within a defined recovery period (by Day 35). In contrast, B. gymnorhiza adopted a conservative approach. It responded to rising salinity by promptly reducing both stomatal conductance and photochemical efficiency to prevent excessive water loss, although this occurred at the expense of photosynthetic rate. B. gymnorhiza maintained stable leaf inorganic ion concentrations and osmotic potential. This strategy effectively avoided ion toxicity but resulted in reduced water absorption capacity. Consequently, photosynthetic inhibition persisted during transient low salinity intervals. These findings highlight that opportunistic species thrive in dynamic estuarine environments through energy efficient adaptations involving stomatal regulation and inorganic ion homeostasis. In contrast, conservative strategies, while protective against stress, demonstrated limited resilience to salinity fluctuation regimes.