Abstract: Mangrove trees growing in subtropical and tropical regions needs to uptake water from the brackish water. Under evaporative conditions in summer, mangrove trees need to generate the gradient in water potential by achieving highly negative leaf water potential, which expose the plant hydraulic systems to the risk of hydraulic failure. Although mangrove trees are known to be water-saving species represented by the low stomatal conductance, less is understood in their responses to the seasonal or episodic evaporative events. We hypothesized that mangrove trees sensitively respond to the increased atmospheric evaporative demand and close stomata, sacrificing the carbon gain for the avoidance of hydraulic failure and the resultant mortality. To examine the hypothesis, we monitored the environments and the sap flux of mangrove trees (Bruguiera gymnorrhiza and Rhizophora mucronata), which are naturally growing in a subtropical bay in southern Japan. Results revealed low stomatal conductance relative to the photosynthetic rates under full sunlight in both species. Sap flux did not, however, exhibit downregulation but increased with increasing vapor pressure deficit, suggesting that both species managed to open stomata, and refilled the lost water due to transpiration, even on the annually most evaporative days. Leaf water potential was highly negative but was above the critical levels for hydraulic safety (turgor loss point). In the morning with wet leaf surface, clear negative sap flux from the leaves to the mainstem was observed, possibly driven by the highly negative brackish water in the rhizosphere. Nighttime transpiration was not detected, suggesting that stomata were tightly closed in order to avoid water loss without carbon gain. Mangrove trees had leaves and vessels highly tolerant to negative water potential and overcome the brackish environments, without missing the opportunity for increasing carbon gain in midday in evaporative conditions. Results suggested that mangrove forests with free access to available water sources would be a highly productive ecosystem, without severe competition with other species.
