Regulation of water balance in mangroves

Abstract

Background: Mangroves are a group of highly salt-tolerant woody plants. The high water use efficiency of mangroves under saline conditions suggests that regulation of water transport is a crucial component of their salinity tolerance.

Scope: This review focuses on the processes that contribute to the ability of mangroves to maintain water uptake and limit water loss to the soil and the atmosphere under saline conditions, from micro to macro scales. These processes include: (1) efficient filtering of the incoming water to exclude salt; (2) maintenance of internal osmotic potentials lower than that of the rhizosphere; (3) water-saving properties; and (4) efficient exploitation of less-saline water sources when these become available.

Conclusions: Mangroves are inherently plastic and can change their structure at the root, leaf and stand levels in response to salinity in order to exclude salt from the xylem stream, maintain leaf hydraulic conductance, avoid cavitation and regulate water loss (e.g. suberization of roots and alterations of leaf size, succulence and angle, hydraulic anatomy and biomass partitioning). However, much is still unknown about the regulation of water uptake in mangroves, such as how they sense and respond to heterogeneity in root zone salinity, the extent to which they utilize non-stomatally derived CO2 as a water-saving measure and whether they can exploit atmospheric water sources.

Keywords: Avicennia; Mangrove; Rhizophora; WUE; aquaporins; halophyte; hydraulic anatomy; salinity tolerance; salt secretion; vapour pressure deficit; water uptake; water use efficiency.