Toluidine blue O is a polychromatic dye, i.e. it absorbs
different colours depending on the nature of its chemical binding with
the components of the tissue. It is positively charged at a pH of 4.4.
At this pH, it will bind to carboxylated polysaccharides such as pectins
in the cell walls and it will colour them pink. Compounds containing
benzene rings, such as lignin, will be coloured green (lignin in the
xylem tissue for example) or turquoise (lignin of the sclerenchyma). The
colour will depend upon the particular chemical structure of the compound.
At a lower acidic pH, toluidine blue gives only a blue or green
colour. The pectins at that pH do not carry negative charges since
their carboxyl groups would be complete (COOH); therefore they cannot react
with the dye. Only the lignin with its benzene rings will do so, hence
the green colour.
With toluidine blue at a higher basic pH (11.1), on the contrary, the
stained tissue is mostly dark pink since most (if not all) pectins will
be charged. This will hinder the green colour of the lignin. At this
pH, the dye penetrates easily the resin used to embed plant tissue. It
is quite useful to stain fixed tissue.
I hope this makes sense to you. You can find more about
Toluidine Blue in the book entitled "The study of Plant structure:
Principles and selected methods" by O'Brien and McCully, Termarcarphi Pty
One last thing: two weeks ago, I went to a marsh with my
students. We brought back leaves of cattails. These are easy to section
by hand since they are quite stiff. One can section transversally or cut
the leaves in half longitudinally. Stained with Toluidine blue (pH 4.4),
the sections were absolutely gorgeous. One could see parenchyma cells
anastomosing forming very large aerenchyma, and beautiful lignified tissues
(especially sclerenchyma). If you have the chance, loot at that same
tissue under a fluorescent scope, it is magnificent. I should stop now,
I could continue to talk about these leaves for quite a long time.
Have all a nice Fall term.
Frederique C. Guinel
Department of Biology
Wilfrid Laurier University
Ontario, Canada, N2L 3C5