Xylem Hydraulic Structure And Function In Mangroves

Mangroves grow in tropical and subtropical intertidal zones and provide important ecological services.High tensile force(low water potentials)in the xylem due to the special saline environment could bring a relatively high risk of cavitation for mangrove species.A comprehensively mechanism study between structure and function in mangrove plants was needed to fill the gap.It is unclear how the xylem structure influence their hydraulic performance and ecological adaptation in mangroves.We collected the mangrove species from China(Hainan)and Australia(New South Wales and Queensland)and studied the hydraulic structure and function of xylem from the following three aspects:(1)The hydraulic structure and function of xylem in nine woody mangrove plants in Hainan were measured and analyzed.All mangrove plants evolved strong resistance to embolism and low stem hydraulic conductivity to adapt the special saline environment(low water potential).And the hydraulic “safetyefficiency” tradeoff(R2 = 0.85,p < 0.001)was found among species studied here.In the present study,the hydraulic conductivity increased with increasing vessel diameter,but resulted in the higher risk of xylem embolism.Increased outer pit aperture fraction also related to increasing connectivity between vessels,resulted in a high hydraulic conductivity.These results support the “pit area hypothesis” for understanding embolism formation.Besides,we found that species with higher axial parenchyma fraction had higher hydraulic safety,which might relate to the adjustment of osmotic potential and possible embolism repair,with reducing hydraulic conductivity.The fibers can affect vessel embolism resistance by supporting the adjacent vessel wall by improving mechanical strength,as well as increasing the sapwood capacitance.Furthermore,fiber wall thickness is strongly correlated to the wood density.The anatomical traits of mangrove plants are closely related to their hydraulic functions.Both sides showed the adaptations of mangroves to intertidal habitats.(2)There is a synergistic correlation between stem xylem and leaves among mangrove plants,and the hydraulic efficiency and safety of stem xylem are vital factors affecting the morphological and photosynthetic traits of leaves,which are coordinated for a better adapting to the special intertidal habitat.In the present study of nine mangrove species,higher stem hydraulic conductivity is able to support larger and thinner leaf to capture solar energy.The stem with higher embolism resistance also support the construction of leaves with higher carbon investment,which also higher resistant to stress.The stem and leaf can jointly resist to environmental stress and enhance resistance of the entire plant to stress.The stem hydraulic conductivity directly influences the leaf net photosynthetic rate,indicating that xylem hydraulic system is an important factor sustaining the photosynthetic capacity of mangrove plants.Due to the special habitats of mangrove plants,the synergistic correlation between stems and leaves are quite necessary for overall physiological function of plants facing multiple stress such as high temperature,high irradiance and high salinity.In order to maintain the safety and efficiency of conducting water and to support the leaf photosynthetic rate,mangrove plants meet the correlation of morphological and functional traits via the cooperation between stem and leaf.(3)Mangrove plants face challenges from warming and altered rainfall patterns associated with global climate change.Intraspecific variation in hydraulic traits may allow a mangrove species to acclimate to changing climatic conditions.In this section,variation in plant hydraulic traits of two widespread mangrove species growing across a latitudinal gradient was quantified.The xylem hydraulic structure and function of Avicennia marina and Aegiceras corniculatum,across three sites spanning a latitudinal gradient of 17.45° in eastern Australia were investigated.Both species were highly resistant to xylem embolism and that there was significant intraspecific variation in hydraulic traits between sites.The highest embolism resistance and sapwood-specific hydraulic conductivity were found at the lowest latitude site that had the highest mean annual temperature and precipitation.There are different hydraulic plasticity between two species.A.marina showed no differences in vessel size and density among sites.It has other special features such as successive cambia enhancing its ability to adapt to a large environmental gradient.In contrast,A.corniculatum showed higher vessel densities at lower latitudes.There was a significant and positive correlation(R2 = 0.72,p < 0.05)between hydraulic conductivity and embolism resistance across species and sites,suggesting the absence of a tradeoff between hydraulic efficiency and safety.Both embolism resistance and hydraulic conductivity were negatively correlated with wood density but positively with vessel wall reinforcement.This study reveals that these two widespread mangrove species were adapted to warmer climates by enhancing both hydraulic efficiency and safety.In conclusion,through investigating the xylem hydraulic structure and function of mangrove species and their ecological adaptability,the effects of xylem structure on water conducting efficiency and hydraulic safety of mangrove species were clarified.As well as the tradeoff between hydraulic efficiency and safety among mangrove species.Coordination was found between stem xylem and leaves of mangroves and the hydraulic function of stem influences the photosynthetic capacity of the leaves.We also found the pattern how hydraulic system of xylem in mangrove species changes along the latitudinal gradient,both hydraulic efficiency and safety increased with increasing temperature(decreasing latitude),which could provide indication for predicting the response of mangrove species to global climate change.