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

The genus Voyira comprises 19 achlorophyllous, mycotrophic species with red uced cormi. Roots of Voyria obconica are up to 1 cm long, 1-1.5 mm thick, s ucculent, brittle and radiate from the shoot base, forming a star-shaped ro ot system. In cross section the central cylinder consists of up to 10 centr al vessels, surrounded by some parenchymatous cells, 5 to 7 strands of phlo em and a pericycle. The cell walls of the anatomically inconspicuous endode rmis are characterised by a faint suberin lamella. The cortex is divided in to an inner cortex, with 3 to 5 layers of longitudinally elongated cells an d 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 intrace llular hyphae of a single mycorrhizal fungus grows after penetration of the dermal tissue straight towards the inner cortex. Within the inner cortex t he hyphae proceed parallel to the central cylinder. Branches of these strai ght inner cortex hyphae then colonize the outer cortex, where they form coi ls, 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 spec ial form of a Paris-type arbuscular mycorrhiza. The ecological significance of the revealed mycorrhizal compartmentation is discussed.