The mechanisms of refilling of xylem conduits and bleeding of tall birch during spring

Abstract

Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.

A non-invasive plant-based probe for continuous monitoring of water stress in real time: a new tool for irrigation scheduling and deeper insight into drought and salinity stress physiology

“…The daily increase in the P p values has been observed in a huge number of crop species (e.g., wheat, maize, canola (Brassica napus), grapevine, citrus, olive, banana, forest trees etc. ; Westhoff et al 2008, Rüger et al 2010b, 2013, Bramley et al 2012) and can clearly be correlated with increasing loss of leaf turgor following watering termination. The upward shift in the baseline signal, i.e.…”

“…Even though patch pressure, P p , measurements are performed on a single leaf level, whereas the balancing pressure values, P b , measured with the pressure chamber data are based on multiple leaves, there is generally a quite good overall correlation between both parameters ( Figure 2; Westhoff et al 2008, Bramley et al 2012. Concomitant direct measurements of the cell turgor on many plant species (using the microcapillary cell turgor pressure probe) have revealed that the dependency of both parameters on cell turgor pressure can be fitted by Eq.…”

“…Electric potential changes have also been detected recently between the base of the stem and upper leaf of avocado trees (Persea americana) in response to drought, irrigation and diurnal light/dark regimes (Fromm 2006, Davies 2006, Gurovich and Hermosilla 2009). Gurovich and others discuss propagation of the electric signals through the tree, but ignore that pressure propagation through the xylem vessels and tissue is most likely the primary event that leads subsequently to electrical changes on the cellular level (see also Westhoff et al 2008Westhoff et al , 2009b. No doubt, concomitant probe and electrical potential measurements will contribute to the elucidation of signals causing stomata closure, i.e., primary hydraulic event and the secondary electrical responses.…”

Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory

“…There have been various earlier observations of hydrophobic interior surfaces in xylem conduits based on a variety of methods, including microscopic stains (Laschimke, 1989;McCully et al, 2014), electron microscopy (Schneider et al, 1999;Wagner et al, 2000), experiments with organic solvents (Zimmermann et al, 2004;Westhoff et al, 2008;McCully et al, 2014), and observations of wetting angles in conduits (Zwieniecki and Holbrook, 2000;Kohonen, 2006;Brodersen et al, 2010;McCully et al, 2014). This body of evidence for plant species ranging from conifers to angiosperms, including monocots and eudicots, clearly shows that xylem conduit walls are partly hydrophobic.…”

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

“…However, critics have pointed to seemingly contradictory evidence, such as the existence of hydrophobic compounds in xylem (Schneider et al, 2003;Rösch et al, 2004;Zimmermann et al, 2004;Westhoff et al, 2008). These valid observations must be accounted for to fully understand how plants transport water via the cohesion-tension mechanism.…”