Correlative light and scanning electron microscopy of the same sections gives new insights into the effects of pectin lyase on bordered pit membranes in Pinus radiata wood

Bordered pits are structures in the cell walls of softwood tracheids which permit the movement of water between adjacent cells. These structures contain a central pit membrane composed of an outer porous ring (margo) and an inner dense and pectin-rich disc (torus). The membrane is overarched on each side by pit borders. Pits may be aspirated, a condition where the torus seals against the pit border, effectively blocking the pathway between cells. In living trees this maintains overall continuity of water conduction in xylem by sealing off tracheids containing air. Drying of timber results in further pit aspiration, which reduces wood permeability to liquid treatment agents such as antifungal chemicals. One possible way to increase permeability is by treating wood with pectin lyase to modify or remove the torus. The effectiveness of this treatment was initially evaluated using light microscopy (LM) of toluidine blue stained wood. Pectic material is coloured pink-magenta with this stain, and loss of this colour after treatment has been interpreted as indicating destruction of the torus. However, correlative light (LM) and scanning electron (SEM) microscopic observations of identical areas of toluidine blue stained sections revealed that many unstained pits had intact but modified tori when viewed with SEM. These observations indicate that LM alone is not sufficient to evaluate the effects of pectin lyase on pit membranes in wood. Combining LM and SEM gives more complete information

► Bordered pits were examined by light (LM) and scanning electron (SEM) microscopy.
► LM of toluidine blue-stained native bordered pit membranes shows pink-magenta tori.
► There are no pink-magenta tori by LM after pectin lyase treatment.
► SEM of the same section shows presence of tori after treatment.
► LM of treated tori indicates loss; correlated SEM and LM suggest chemical change.