Adjustment in leaf water relations and the maintenance of mangrove seedling growth under increasing salinity

Mangroves are ecosystems of high ecological and economic importance, particularly due to their capacity to store high amounts of carbon and stabilize soil. However, climate change and rising sea levels are intensifying salinity levels, challenging the survival of plant mangrove species, especially seedlings. Here, we evaluated the effects of different salinity concentrations on the growth and leaf water relations of Avicennia germinans (L.) L. and Rhizophora racemosa G.Mey. seedlings. Specifically, we tested whether A. germinans, due to its broader distribution, higher salinity tolerance and salt-excreting ability, would exhibit more pronounced adjustments and greater resilience to saline stress compared with R. racemosa. To this end, we conducted a greenhouse experiment, exposing 212 11-month-old seedlings (106 of each species) previously grown in freshwater to five salinity treatments over 3 months. These seedlings were analyzed for growth, embolism resistance, leaf water potential, osmotic parameters and gas exchange. Our results showed that A. germinans exhibited greater osmotic adjustment and stomatal regulation, enabling it to maintain leaf hydration and reduce the risk of embolism under high salinity. Conversely, R. racemosa adopted a more conservative strategy, with lower osmotic adjustment and stomatal regulation capacity but a higher hydraulic safety margin. Thus, we demonstrated that these species employ distinct strategies to cope with salinity, reflecting specific adaptations to their ecological distributions and salinity tolerance. These findings contribute to understanding the adaptive responses of mangrove seedlings to varying salinity conditions, with implications for the conservation of these ecosystems under predicted climate change scenarios.