Mangroves, the iconic trees that line tropical and subtropical coastlines, face two threats: high salinity and frequent drought. If their internal water transport system fails, the trees die. They exhibit various types of perforation plates and pit structures. However, the extent to which these anatomical features contribute to their adaptation to coastal environments remains unclear.

In a study published in Tree Physiology on April 24, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators revealed that mangroves enhance hydraulic safety (rather than efficiency) through reinforced anatomical structures and pit architecture to adapt to saline environments. They built narrower, stronger vessels and adjust their microscopic pit structures to prevent fatal embolisms under extreme saline and dry conditions.

The researchers measured 27 hydraulic, anatomical, and pit traits in eight mangrove species from Hainan Island in southern China. The species were divided into two groups based on vessel perforation plate type: four species with simple perforation plates (SI) and four with scalariform (SC, ladder-like) perforation plates.

Despite long-standing theories that perforation plate type strongly influences hydraulic efficiency, the researchers found no significant difference in water conductivity or embolism resistance between the two groups.

Instead, all eight mangrove species shared a common hydraulic strategy of high resistance to xylem embolism coupled with low water transport efficiency when compared with global woody plant datasets. Across species, the researchers observed a clear trade-off between hydraulic safety and efficiency.

Species with greater embolism resistance had narrower vessels, thicker vessel walls, higher fractions of axial parenchyma (living cells that may help repair embolised conduits), and more negative minimum water potentials.They also maintained wider hydraulic safety margins, meaning they are less likely to suffer catastrophic failure during droughts.

Unexpectedly, perforation plate morphology was tightly linked to pit architecture, despite having no direct effect on hydraulic function. SC species had larger pit membranes and apertures, but fewer pits per area. In contrast, SI species had many small pits packed densely together. These differences at the pit level, especially in aperture size and shape, helped explain variations in hydraulic safety.

The study systematically reveals the anatomical basis that allows mangroves to maintain the integrity of their xylem water transport system under combined hypersaline and drought stress. It provides important insights into how woody plants in coastal ecosystems adapt hydraulically to extreme environments.

"Our findings highlight that enhanced hydraulic safety, rather than efficiency, is the evolutionary priority for mangroves surviving hypersaline intertidal zones," said CHEN Yajun of XTBG. "The intricate coordination of vessel anatomy, living cell allocation, and pit structure jointly shapes their resilience."