Root structures and mycorrhiza of the achlorophyllous Voyria obconica Progel (Gentianaceae).

The genus Voyira comprises 19 achlorophyllous, mycotrophic species with reduced cormi. Roots of Voyria obconica are up to 1 cm long, 1-1.5 mm thick, succulent, brittle and radiate from the shoot base, forming a star-shaped root system. In cross section the central cylinder consists of up to 10 central vessels, surrounded by some parenchymatous cells, 5 to 7 strands of phloem and a pericycle. The cell walls of the anatomically inconspicuous endodermis are characterised by a faint suberin lamella. The cortex is divided into an inner cortex, with 3 to 5 layers of longitudinally elongated cells and a multilayered outer cortex, comprising isodiametric cells. The 2-3 cell layers of the dermal tissue also show a faint suberin lamella within their thickened cell walls. Non-parasitic, achlorophyllous plants need symbiotic interactions with mycorrhizal fungi. In V. obconica the exclusively intracellular hyphae of a single mycorrhizal fungus grows after penetration of the dermal tissue straight towards the inner cortex. Within the inner cortex the hyphae proceed parallel to the central cylinder. Branches of these straight inner cortex hyphae then colonize the outer cortex, where they form coils, swell, and eventually degenerate to amorphous clumps. Similarities and differences in root structure and mycorrhiza to the closely related Voyria tenella are elucidated. Arguments are given to call this association a special form of a Paris-type arbuscular mycorrhiza. The ecological significance of the revealed mycorrhizal compartmentation is discussed.