The apoplast: A kinetic perspective An ESR study of manganese binding in plant tissue

The apoplast was defined by Münch (1930) as the non-living (extracellular) component of the plant, in contrast to the living (cytoplasmic) symplast. As such the apoplast was considered to make up a physical continuum through which water and solutes might freely move by diffusive or bulk flow throughout the plant. The cell walls that constitute the apoplast may be made up of a number of chemically and physically distinct layers. The middle lamella is the first-formed layer, deposited following cell division, and is rich in galacturonic acid residues. The dissociation of the uronic acid carboxyl groups generates the negatively charged carboxylates that are responsible for the cation-binding properties of cell walls. Upon the middle lamellae are layered the primary and secondary cell walls that are composed of cellulose microfibrils embedded in a matrix of pectins, a diverse group of xyloglucans (the hemicelluloses) together with structural proteins (Cosgrove, 1997). The cell-wall matrix, in contrast to the cellulose microfibrils, provides numerous porous regions where the hydraulic conductivity is high compared to more ordered semi-crystalline regions within the cellulose microfibrils. Traditional wisdom has suggested that the bulk flow or diffusion of water is reduced to various extents by regions of the apoplast where cell walls are impregnated with non-permeable substances such as suberin and lignin, as for example in the endodermis.