Photosynthetic performance in mangrove species under salt stress: mesophyll conductance as a key limitation

Mangroves are vital components of coastal blue carbon ecosystems due to their high carbon sequestration capacity, offering a nature-based strategy for climate change mitigation and adaptation. However, their photosynthetic carbon assimilation is highly susceptible to increased salinity. Previous studies have shown that the net photosynthesis rate (Anet) in mangrove plants under salt stress was limited by stomatal conductance (gs) and biochemical factors, but the role of mesophyll conductance to CO2 (gm)-a diffusion component increasingly highlighted as a significant constrain on photosynthesis in various plant species-has not been explicitly considered. In this study, we revisit the physiological mechanisms underlying photosynthetic response of mangrove plants to salt stress. We experimentally examined variations in a comprehensive set of photosynthetic parameters (i.e., with gm included) and leaf structural components in two common coastal woody species of southern China, Kandelia obovata Sheue, H.Y. Liu & J. Yong and Aegiceras corniculatum (L.), across different salinity gradients. Our results demonstrate that both species exhibited optimal photosynthetic performance at 10‰ salinity; however, A, gs and gm significantly declined with increasing salinity level. However, maximum carboxylation rate (Vcmax) did not decrease significantly in K. obovata, while it showed a significant decline in A. corniculatum. Photosynthetic limitation analysis showed that gm was the dominant limiting factor across salinity treatments, except in Kandelia obovata at 20‰ salinity. In K. obovata, the decline in gm correlated with reductions in chloroplast surface area exposed to intercellular airspace per unit leaf area (Sc/S), whereas no such structural relationship was observed in A. corniculatum. Overall, our results demonstrate that increased mesophyll resistance to CO2 diffusion was a primary cause of photosynthetic decline under salt stress, with species-specific structural regulation of gm. These findings enhance our understanding of mangrove responses to salinity and providing guidance for species selection and management strategies to maintain productivity and carbon sequestration in coastal blue carbon ecosystems under future climate change.