Cell walls consist of microfibrils embedded in a matrix of pectin and lignin. Pores
are formed by
"channels" within the bundles of microfibrils, where ions and water can move freely. Carboxyl
groups with a high cation exchange capacity (CEC) line these pores. Calcium is the ion most
frequently adsorbed to these groups, as it is present in the soil solution in greater quantities than
other ions.
The Apparent Free Space (AFS) refers to the totality of ions within this diffusion
and exchange
system. It consists of two components:
- The Donnan Free Space (the volume
where cations are held by the negative charge), and
- The Water Free Space (the volume
freely accessible to ions and charged or uncharged
molecules).
Due to the properties of the adjacent tissue, the AFS is a negatively charged matrix
with
electronegative sites (carboxyl groups).
- Plants differ in the CEC of these
electronegative sites—dicots generally have a higher CEC
than monocots.
- A decrease in the pH of the soil
solution concentration lowers the effective CEC of the tissue.
- Cations bound to electronegative
sites can be replaced (displaced) by other cations.
- Divalent cations are bound more strongly
than monovalent cations.
The principles governing ion sorption in the AFS are not sufficient to explain the
selectivity of ion
uptake.
The apoplast can serve as a temporary storage pool for heavy metals such as Fe and
Zn. These, in
turn, can be mobilized by root exudates (phytosiderophores).
The predominant ions at the exchange sites are Ca, Mg, Fe, Mn, and Al. Monovalent
cations
mostly exist freely in solution
