Inefficient photosynthesis in the Mediterranean orchid Limodorum abortivum is mirrored by specific association to ectomycorrhizal Russulaceae

Abstract

Among European Neottieae, Limodorum abortivum is a common Mediterranean orchid. It forms small populations with a patchy distribution in woodlands, and is characterized by much reduced leaves, suggesting a partial mycoheterotrophy. We have investigated both the photosynthetic abilities of L . abortivum adult plants and the diversity of mycorrhizal fungi in Limodorum plants growing in different environments and plant communities (coniferous and broadleaf forests) over a wide geographical and altitudinal range. Despite the presence of photosynthetic pigments, CO 2 fixation was found to be insufficient to compensate for respiration in adult plants. Fungal diversity was assessed by morphological and molecular methods in L . abortivum as well as in the related rare species Limodorum trabutianum and Limodorum brulloi . Phylogenetic analyses of the fungal internal transcribed spacer (ITS) sequences, obtained from root samples of about 80 plants, revealed a tendency to associate predominantly with fungal symbionts of the genus Russula . Based on sequence similarities with known species, most root endophytes could be ascribed to the species complex encompassing Russula delica , Russula chloroides , and Russula brevipes . Few sequences clustered in separate groups nested within Russula , a genus of ectomycorrhizal fungi. The morphotypes of ectomycorrhizal root tips of surrounding trees yielded sequences similar or identical to those obtained from L . abortivum . These results demonstrate that Limodorum species with inefficient photosynthesis specifically associate with ectomycorrhizal fungi, and appear to have adopted a nutrition strategy similar to that known from achlorophyllous orchids.

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