Nocturnal sap flow as compensation for water deficits: an implicit water-saving strategy used by mangroves in stressful environments

As part of the plant water-use process, plant nocturnal sap flow (Q _n) has been demonstrated to have important ecophysiological significance to compensate for water loss. The purpose of this study was to explore nocturnal water-use strategies to fill the knowledge gap in mangroves, by measuring three species co-occurring in a subtropical estuary. Sap flow was monitored over an entire year using thermal diffusive probes. Stem diameter and leaf-level gas exchange were measured in summer. The data were used to explore the different nocturnal water balance maintaining mechanisms among species. The Q _n existed persistently and contributed markedly over 5.5%~24.0% of the daily sap flow (Q) across species, which was associated with two processes, nocturnal transpiration (E _n) and nocturnal stem water refilling (R _n). We found that the stem recharge of the Kandelia obovata and Aegiceras corniculatum occurred mainly after sunset and that the high salinity environment drove higher Q _n while stem recharge of the Avicennia marina mainly occurred in the daytime and the high salinity environment inhibited the Q _n. The diversity of stem recharge patterns and response to sap flow to high salinity conditions were the main reasons for the differences in Q _n/Q among species. For Kandelia obovata and Aegiceras corniculatum, R _n was the main contributor to Q _n, which was driven by the demands of stem water refilling after diurnal water depletion and high salt environment. Both of the species have a strict control over the stomata to reduce water loss at night. In contrast, Avicennia marina maintained a low Q _n, driven by vapor pressure deficit, and the Q _n mainly used for E _n, which adapts to high salinity conditions by limiting water dissipation at night. We conclude that the diverse ways Q _n properties act as water-compensating strategies among the co-occurring mangrove species might help the trees to overcoming water scarcity.