Mangrove resilience to salinity: Anatomical and biochemical traits of leaves on the Eastern Coast of Sri Lanka

Mangroves are unique coastal ecosystems that thrive in saline environments. Understanding their anatomy and physiology is essential for assessing their adaptability to environmental change. This study examined the leaf anatomy, chlorophyll content, and proline levels of three common mangrove species (Avicennia marina, Excoecaria agallocha, and Lumnitzera racemosa) across three sites; Sathurukondan (Site 1), Thampalagamam (Site 2) and Panama (Site 3)  along the eastern coast of Sri Lanka during rainy and dry periods from 2023 to 2024. Water and soil salinity were recorded, and leaf anatomy was assessed based on cell layers and tissue thickness. Chlorophyll content was measured in the field using a SPAD chlorophyll meter, while proline content was quantified in the Laboratory using the acid ninhydrin method. Data were analyzed using Pearson correlation and one-way ANOVA. Results showed clear spatial and rainy and dry period variation in salinity, with the highest levels at Panama (site 3) and the lowest at Sathurukondan (site 1) in both periods. Thickness of the laminar, palisade layer, and water storage tissue (WST) differed significantly among species (p < 0.05), with L. racemosa exhibiting the greatest values. The thickest spongy layer was observed in E. agallocha, which also showed higher chlorophyll and proline levels across all sites and periods. A. marina exhibited anatomical stability with an unusual increase in proline during the rainy period. Most anatomical traits were positively correlated with salinity, except for palisade thickness and the palisade/spongy ratio. The WST thickness increased notably at high-salinity sites, while chlorophyll content was negatively correlated with salinity, and proline showed a positive correlation. Overall, this study highlights how the selected mangrove species adjust to fluctuating salinity through coordinated anatomical and physiological responses. These findings provide useful markers for assessing mangrove resilience and may support restoration and conservation efforts under salinity stress.