ABSTRACT
The peripheral root tissues form a morphological, physical, and chemical complex microcosm that provides a broad selection of different habitats for a myriad of microorganisms: bacteria, actinomycetes, protozoa, and fungi. For example, the phellem tissue of Norway spruce (Picea abies) roots consists of multiple layers of thick-walled cells (sclereids) with layers of thin-walled cells in between which are filled with polyphenolic compounds (Braun and Lulev, 1969). There are no intercellular spaces in this tissue. The scales formed by the outermost phellem cells can not fall off like those on the stem. They remain in place and are gradually decomposed by microorganisms, some of which were already present as endophytes in the young tissues. These processes result in a complicated microtopography of the root surface, making it difficult to differentiate between the inside and the outside of roots. The situation is not much simpler for roots undergoing primary growth. The boundary between the root and the soil changes constantly because roots continually modify the nearby soil structure by their mechanical and metabolic activity (Foster et al., 1983). In addition, microorganisms colonize the rhizoplane, the epidermis, and the outer cortex in a nonrandom manner. Heavy microbial growth occurs in and on some individual cells while neighboring cells are almost devoid of microorganisms (Bowen and Rovira, 1976). Patchiness of microbial root coloniza tion probably reflects the uneven distribution of organic debris in the soil, which serve as food bases for the microorganisms. Many soil bacteria and fungi are able to colonize epidermal and outer cortical cells of healthy roots inter-and intracellularly. Only a comparatively small number of organisms, e.g., mycorrhizal fungi, endophytes, and pathogens, possess, however, the ability to cross the inner boundary of the rhizosphere and to colonize the inner of the root cortex (Bazin et al., 1990).
