Cortical cell walls of spruce [Picea abies (L.) Karst.] and pine (Pinus sylvestris L.) were found to contain large amounts of acid polysaccharide, which was absent from all other cell walls studied. In its staining characteristics, calcium content, extractability and structure it was similar to the pectic substance filling intercellular regions in the cortex, xylem and other tissues. The acid polysaccharide also formed a matrix embedding fungal cells of the Hartig net in mycorrhizal roots. Matrix substance in the mantle was of a different character.
The distal limit of the acid polysaccharide wall incrustation marked the transition between the differentiation zone of the root tip and the mature cortex, and coincided exactly with the distal limit of the Hartig net and fungal penetration. The endodermis and, where present, the metacutis formed other barriers. Proximally, the onset of senescence and death of cortical cells made mycorrhiza formation impossible, or terminated the existing symbiotic relationship.
Thus, ‘the mycorrhizal infection zone’, i.e. the part of the root where fungal colonization could take place, was defined by (a) a particular wall composition and (b) a physiological condition. Such limitation is consistent with the theory of mechanical fungal penetration during mycorrhiza formation-the pectins increase the cell-wall flexibility-as well as the model for host-mycobiont interaction proposed in an earlier study (Nylund & Unestam, 1982).