Dynamic control of osmolality and ionic composition of the xylem sap in two mangrove species

Abstract

• Premise of the study: Xylem sap osmolality and salinity is a critical unresolved issue in plant function with impacts on transport efficiency, pressure gradients, and living cell turgor pressure, especially for halophytes such as mangrove trees.• Methods: We collected successive xylem vessel sap samples from stems and shoots of Avicennia germinans and Laguncularia racemosa using vacuum and pressure extraction and measured their osmolality. Following a series of extractions with the pressure chamber, we depressurized the shoot and pressurized again after various equilibration periods (minutes to hours) to test for dynamic control of osmolality. Transpiration and final sap osmolality were measured in shoots perfused with deionized water or different seawater dilutions.• Key results: For both species, the sap osmolality values of consecutive samples collected by vacuum extraction were stable and matched those of the initial samples extracted with the pressure chamber. Further extraction of samples with the pressure chamber decreased sap osmolality, suggesting reverse osmosis occurred. However, sap osmolalities increased when longer equilibration periods after sap extraction were allowed. Analysis of expressed sap with HPLC indicated a 1:1 relation between measured osmolality and the osmolality of the inorganic ions in the sap (mainly Na, K, and Cl), suggesting no contamination by organic compounds. In stems perfused with deionized water, the sap osmolality increased to mimic the native sap osmolality.• Conclusions: Xylem sap osmolality and ionic contents are dynamically adjusted by mangroves and may help modulate turgor pressure, hydraulic conductivity, and water potential, thus being important for mangrove physiology, survival, and distribution.

Xylem Sap Surface Tension May Be Crucial for Hydraulic Safety

“…From three of six branches, side twigs were removed, and the remaining main axes (main stem length, 50-60 cm; side stem length, less than 20 cm) were completely debarked, washed with distilled water, and dried with paper towels. Branches were debarked to avoid reverse osmosis from living cells during xylem sap extraction (López-Portillo et al, 2014). Furthermore, this procedure should prevent the contamination of extracted sap with resin.…”

Mangroves provide blue carbon ecological value at a low freshwater cost

“…First, mangrove tree species growing under increasingly saline environments can have greater xylem vessel grouping and vessel density, as well as reduced vessel diameters and lengths than their low-salinity conspecifics, which is proposed to reduce the risk of embolisms associated with highly negative water potentials 45 . Second, some mangroves – those that secrete salt from their leaf and bark surfaces – can dynamically control xylem ion composition 46 to increase hydraulic conductivity 47 as water potential becomes more negative, whilst maintaining relatively stable stomatal conductance (and transpiration). This dynamic xylem sap osmolality control increases the water available to the transpiration stream without actively increasing transpiration, further reducing the risk of embolisms under salt stress 46 .…”

“…Second, some mangroves – those that secrete salt from their leaf and bark surfaces – can dynamically control xylem ion composition 46 to increase hydraulic conductivity 47 as water potential becomes more negative, whilst maintaining relatively stable stomatal conductance (and transpiration). This dynamic xylem sap osmolality control increases the water available to the transpiration stream without actively increasing transpiration, further reducing the risk of embolisms under salt stress 46 .…”

Biology and Ecology of the Halophyte Laguncularia racemosa (L.) Gaertn. f.: A Review

“…Osmotic adjustments are necessary for mangroves to resist substrate salinity and to maintain constant turgor pressure (Méndez-Alonzo et al 2016). López-Portillo et al (2014) reported that xylem sap osmolality and ionic contents affect turgor pressure, hydraulic conductivity, and water potential. Hydraulic properties can be interrupted by a break in xylem vessels.…”