Structural and functional responses of xylem in Rhizophora mucronata Lam. seedlings under drought and hypersaline conditions

Water translocation in mangrove seedlings is often affected by water stress conditions such as drought, hyper-salinities and their frequent variations. This study was therefore aimed at studying the wood anatomical responses of xylem tissue and hydraulic conductivity of Rhizophora mucronata Lam., a common species in mangrove planting, under different levels of drought [25%, ~50% and ~100% of water holding capacity (WHC)] and soil salinity [high salinity (35 psu), moderate salinity (15 psu) and freshwater (0 psu)]. As wood anatomical responses, significantly higher vessel density, vessel grouping (P<0.001) along with narrow vessel elements (P<0.001) were observed in plants grown in the 25% and 50% WHCs and high salinity treatments. All these anatomical responses are more directed towards avoidance of vessel cavitation which is commonly found under water deficit conditions. The results showed that R. mucronata plants failed to maintain efficient transportation of water when the field capacity was 50% of WHC or lower and the level of salinity was 35 psu or greater, as evident by the reduction of water conductive areas, vessel areas and hydraulic conductivity (P<0.05). Overall, water use efficiency of R. mucronata seedlings under the imposed water stress conditions has remarkably reduced and it further indicated that such imposed stress conditions directly affect the survival of planted seedlings as depicted by the significantly low survival in 25% and 50% of WHCs and high salinity. Therefore, in-depth study on lagoon hydrology including inundation levels, water depth, salinity and the selection of correct tidal positioning is highly recommended as prerequisites in mangrove planting. Keywords. Hydraulic architecture, hydraulic conductivity, mangroves, restoration, water stress.