Atomic Force Microscopy of the Intervessel Pit Membrane in the Stem of Sapium Sebiferum (Euphorbiaceae)

Abstract

Sapwood and juvenile wood of Sapium sebiferum (Euphorbiaceae) was collected during 2000 –2002. In air-dried vessel elements, the surface of pit membranes (PMs) in the outermost growth ring was coated with plaque-like or interstitial material that was 2–5 nm thick. This coating was phase dark and overlaid a phase bright layer of globules and reticulately arranged microfibrils (MFs) that was 25–50 nm thick. Beneath the reticulate layer there was another surface exposed during sectioning/fracturing. It had parallel MFs which appeared to be continuous with the middle lamella, and were also coated. The total thickness of the dried PM appeared to be in the range of 50–100 nm. Overwintering and heartwood PMs were encrusted with a non-microfibrillar layer that differed from the above mentioned coating. Prior to chemical treatment, specific dried, untreated PMs were located and then the sample was dismounted, treated with acidic H2O2, and observed after treatment so that before and after images could be compared. Treatment with acidic H2O2 removed some of the coating and greatly modified the fibrillar nature of the surface layer, but did not reduce its overall thickness. The native structure of sapwood PMs was observed in water. Non-dried PMs displayed two layers, each with a different type of surface. The outer layer was non-microfibrillar and covered the entire surface of the PM. The non-microfibrillar layer was extremely sensitive to mechanical perturbation by the AFM tip, and had phase characteristics similar to the coating of dried PMs. The underlying layer was thick and microfibrillar. The MFs in non-dried PMs were, like the dried MFs, phase bright but they were much more loosely intermeshed compared with those seen in dried materials. The measurable thickness (which does not represent the total thickness) of non-dried PMs frequently ranged from 90–225 nm, although a few 500 nm vertical features were measured.

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

“…The accumulation of proteins at intervessel pit membranes is hardly surprising because proteins are quite abundant in xylem sap (Buhtz et al, 2004) and likely to accumulate at pit membranes (Neumann et al, 2010). The proteins observed in intervessel pit membranes of Triadica are likely to be part of the thick membrane coatings observed in this species via atomic force microscopy (Pesacreta et al, 2005). It is unclear whether these thick coatings may be unique to this species or perhaps the family Euphorbiaceae and what their functions may be.…”

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

“…This seasonal change in electron density was suggested to be either caused by an alteration of substances already present in the pit membrane or by secondary encrustation. The latter seems most likely, as substances from the xylem sap could be filtered out and deposited onto or within the pit membrane (Côté, 1958;Schmid, 1965; Wheeler, 1981; Pesacreta et al, 2005). It would be fair to say that nanoparticles in intervessel pit membranes and on conduit walls have been hiding in plain sight for some time, although a few previous studies noted that they only appeared after OsO 4 fixation, indicating their lipid nature (Fineran, 1997;Westhoff et al, 2008).…”

Cavitation and Its Discontents: Opportunities for Resolving Current Controversies

“…The relevant pores are generally conceptualized as determined by the cellulose microfibril network, with the matrix of polysaccharides playing only an indirect role in air-seeding resistance (Sperry and Tyree, 1988), as discussed further below. Nevertheless, recent work on hydrated pit membranes suggests that the cellulose network is completely embedded within a dense hydrogel phase (Pesacreta et al, 2005;Lee et al, 2012), raising the question of whether capillary sealing is the appropriate model if the spaces between cellulose microfibrils are filled with a gel phase rather than a liquid phase.…”

“…However, the matrix may be modified during apoptosis and vessel maturation, as noted by . The exact chemical composition of the pit membrane matrix remains controversial, particularly with regard to the presence or absence of the homogalacturonan (HG) pectin species (Choat et al, 2003;Pesacreta et al, 2005). Yet, the presence or absence of HG should not be confused with the presence or absence of a pectic gel phase in the matrix.…”

“…Also in that study, Fraxinus mandshurica had no discernible pores extending through the thickness of the membrane, consistent with a report for Fraxinus americana that found resolvable pores (up to a maximum of 30 nm) in only a few membranes. Thicker pit membranes appear more likely to have a homogenous surface in SEM (Jansen et al, 2009), and atomic force microscopy (AFM) has shown a change from an apparently smooth to rough surface between hydrated and air-dried membranes (Pesacreta et al, 2005).…”