Abstracts of published results on tropical achlorophyllous, mycotrophic plants (Voyria, Gentianaceae). Emphasis on morphology, anatomy, mycorrhiza and ecology.

Imhof, S. (1997):

Phylogenetic tendencies of mycorrhizal structures in achlorophyllous gentians

In: E. Smets, L. P. Ronse Decraene, E. Robbrecht (eds), 13th Symposium Morphology, Anatomy and Systematic, Leuven. Scripta Bot. Belgica 15: 84.

Abstract:

The morphology of arbuscular mycorrhiza (AM) in members of the Gentianales differs considerably. In this order, a trend to enhanced mycorrhizal dependency of the host and increased effectiveness of taking advantage from the association exists. This is expressed on the one hand by the ancient intercellular form of AM, facultatively occurring in Asclepiadaceae [Untch & Weber, Beitr. Biol. Pflanzen 69:129-140], and on the other hand in the intracellular hyphal coils found in the obligately mycotroph Gentianaceae [Demuth & Weber, Angew. Bot. 64: 247-252 (1990)] and Loganiaceae [Tiemann et al., Beitr. Biol. Pflanzen 68: 311-321 (1994)], representing a more advanced form of AM. In Apocynaceae both forms of AM were found, suggesting to represent an intermediate evolutionary stage [Weber et al., Bot. Acta 108: 525-534 (1995)]. This path of AM-evolution can be pursued further on in the more advanced, achlorophyllous gentians Voyria.

Plants of Voyria truncata, morphologically and anatomically most similar to its autotrophic relatives, also show a similar AM morphology. The advanced character of this mycorrhiza, however, manifests in the increased intensity of fungal infestations of the root, the fast degeneration of the hyphal coils within 15 cell passages and, most important, the missing arbuscles. Since fungal haustoria (arbuscles) in roots of achlorophyllous host are of questionable function, their missing is interpreted as a logical consequence of progression.

Voyria tenella, morphologically and anatomically one of the most advanced species in the genus, exhibit an entirely different course of growth of its root fungus. Root penetrating hyphae initially grow directly through the multilayered cortex parenchyma towards the longitudinally elongated cells of the 1-2 innermost cortex layers outside the endodermis.There, the hyphae proceed along the central cylinder. Hyphal branches subsequently colonize the root cortex from the inside. Leaving the inner cortex layers, however, the hyphae start to coil, swell up and eventually collapse. This unique pattern of mycorrhiza is interpreted as an intraradical fungus-garden. By this means, a sustained benefit of the fungal penetrations is achieved, considered to be a substantial progress in taking advantage of the mycorrhiza.

Since the mycorrhiza of V. tenella is an AM too, our present notion of host 'influence' on the morphology of its endophyte has to be reappraised. In V. tenella it attains a yet unknown quality.


Publicationlist

Last revised on 13 February 2001 by Stephan Imhof
Email: imhof@mailer.uni-marburg.de