Mangroves, the iconic trees that line tropical and subtropical coastlines, face a double threat: high salinity and frequent drought. If their internal water transport system fails, the trees die. They display diverse perforation-plate types and pit structures. However, it remains unclear to what extent these anatomical features contribute to their adaptation to coastal environments.

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

The researchers measured 27 hydraulic, anatomical, and pit traits in eight mangrove species from Hainan Island, southern China. Four species had simple perforation plates (SI) and four had scalariform (ladder-like) perforation plates (SC). Despite long-standing theories that perforation plate type strongly influences hydraulic efficiency, they found no significant difference in water conductivity or embolism resistance between the two groups.

They found that all eight mangrove species showed high resistance to xylem embolism but low water transport efficiency, compared with global woody plant data. There was a clear tradeoff between safety and efficiency across species.

Species with higher 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, although perforation plate morphology did not have a direct effect on hydraulic function, it was tightly linked to pit architecture. SC species had larger pit membranes and apertures but fewer pits per area, whereas SI species packed many small pits densely. These pit-level differences, especially 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, not efficiency, is the evolutionary priority for mangroves surviving hyper-saline intertidal zones,” said CHEN Yajun of XTBG. “The intricate coordination of vessel anatomy, living cell allocation, and pit structure jointly shapes their resilience.”