Cation exchange capacity (CEC) characterizes the number of fixed
negative charges of plant cell walls and is an important parameter in
studies dealing with the uptake of ions into plant tissues, especially in
roots. Conventional methods of CEC determination use bulk tissue, the
results are the mean of many cells, and differences in the CEC of
different tissue types are masked. Energy-dispersive microanalysis (EDX)
in the transmission electron microscope allows CEC determinations on much
finer scales. Shoot and fine root tissue of Picea abies was acid
washed to remove exchangeable cations. Tissue blocks or semithin tissue
sections were loaded with 0.2 mM CaCl2, AlCl3, or
Pb(NO3)2 at pH 4.0. The amount of Ca, Al, or Pb
adsorbed to the exchange sites of cell walls was determined by EDX. The
CEC of cell walls of different tissue types was highly different, ranging
in shoot tissues from 0 to 856 mM Ca and 5.8 to 1463 mM Al (block loading)
or 4.3 to 1116 mM Ca and 0 to 2830 mM Al (section loading). In root
tissue, Pb adsorption to semithin sections yielded CEC values between 29.1
and 954 mM Pb. In most P. abies shoot tissues, the binding
capacity was clearly higher for Al than for Ca.