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Seminar - Tracing water movement in plant tissues

This video is the presentation by Prof Martin Canny from ANU - please contact Tamara Howlett for more information

This presentation took place at the NWGIC on Tuesday 18 August 2009.

Prof Martin Canny from ANU presented a seminar titled 'Tracing water movement in plant tissues'

Summary

Tracing water
It is basically impossible to trace water by the movement of solutes dissolved in it, because of the size and unique nature of the water molecule.  Various approximations to some of the pathways can be achieved by selection of appropriate tracer molecules.

Mature xylem
The easiest pathway to follow is that of mature xylem tracheids or vessels.  Dyes that bind to cell walls, such as basic fuchsin, tend to block their own movement through pit membranes, but have the advantage of giving clear images of those elements they pass through.  They cannot be trusted in tracheids, or for long distance movement through vessels.  Of the non-wall-binding dyes, two are especially valuable: sulphorhodamine G (SR) and pyrene trisulphonate (PTS).  Both are fluorescent and can be detected in very small amounts. 
The non-binding dyes diffuse in cell walls away from the xylem paths, and so yield fuzzy images in the microscope.  This property can be utilized to measure the rates of diffusion of the dyes in the cell walls.  Such rates have been found to be very low in comparison with diffusion in water. 

Sites of entry to the symplast (flumes)
When water moves out of the xylem and into the symplast, it is virtually impossible to follow its further progress except with labelled water (e.g. D2O).  The site where the transfer occurs can be located by the accumulation of the dye that is left behind in the apoplast (sumps).  The sparingly soluble SR is especially useful for locating these flumes.

Other useful properties of SR
SR is dissociated into ions at neutral pH and so does not penetrate cell membranes.  At ph 4.5 it is undissociated, and does enter living cells.  Thus it can be used as an identifier for those cells that excrete H+ ions, i.e., especially cells with active proton-ATPases, which are engaged in accumulating solutes like amino acids.  Clusters of such cells have been found localized in leaf teeth, hydathodes, the extended bundle sheath of legume leaves, and in the xylem of pine needles. 

Images of tracer dyes
High resolution images of the location of these tracer dyes can be achieved by anhydrous freeze-substitution and resin embedding, but this is a demanding and tedious procedure.  Quick and low-resolution images can be obtained by cutting hand sections of the fresh material in paraffin oil. 

Location of vessel end walls and flumes
Small particles in suspension in water can be used to locate those places in the xylem where water passes through primary cell walls of pit membranes.  The particles are left behind, like the dyes at cell-membrane sumps.  Such particle sumps can also be used to estimate the size of the pores in the pit membranes from the maximum particle diameter that can penetrate the membranes.

Particles used in these experiments include India ink, latex paint and colloidal gold. 

For more information please contact Dr Suzy Rogiers: suzy.rogiers@dpi.nsw.gov.au