高度な菌類依存の観点からみたラン科植物の祖型ヤクシマランの解剖的および生態的特質

• Research objective: Apostasioideae, the basal-most subfamily of orchids, comprises two genera, Apostasia and Neuwiedia, which occur in forests in tropical and subtropical parts of Asia, Australia and the Southwest Pacific. A single species occurs in southern Japan, Apostasia nipponica, which is morphologically similar to the very widespread A. wallichii. The former can be distinguished from the latter on account of its considerably smaller stature (up to 15 cm versus up to 1 m tall), as well as differences in leaf shape and dimensions (ovate-lanceolate and up to 4 cm long, versus linear and up to 50 cm long) and the number of flowers in the inflorescence (fewer than ten versus up to 30). Both are perennial, green, partial myco-heterotrophs that form mycorrhizal root tubercles with ectomycorrhizal fungi. Apostasia nipponica has been reported to receive ≥84% of its carbon from its mycobiont, presenting an interesting topic for comparison with A. wallichii. However, anatomical and ecological features of either species that may be important to the myco-heterotrophic habit remain largely unstudied; the same is true with respect to the possibility of asexual propagation. Because A. nipponica is an endangered species in Japan, knowledge of its ecological characteristics would be useful for its conservation. • Methods: We examined the habitat, growth habit, phenology, anatomical and physiological characteristics of tuberous root of A. nipponica, and compared our findings with descriptions of A. wallichii. • Results: Plants of A. nipponica live for at least five years and have a sparse, scattered or clumped occurrence in forests dominated by ectomycorrhizal trees belonging to the Fagaceae. Although plants may decrease in number as a result of wild boar rooting behavior, populations can establish beside man-made trails and at other sites subject to disturbance. Fruits remain green on the stem for about 10 months, with the present year’s capsules undergoing development only once those from the previous year are detached. Underground parts comprise a rhizome and tuberous roots that lie within the A-horizon of the soil, and account for considerably more biomass than above-ground parts. Tuberous roots adopt a clavate form and have a warty surface, via which mycobionts enter and colonize the cortex. The cortex is partitioned into five discrete zones and zone 3 (digestion cell layer) is composed of exceptionally large cells, as has been described for A. wallichii. The tuberous roots of A. nipponica are thicker than those of A. wallichii (5 mm diameter in the former versus 3.5 mm in the latter), which is attributed to approximately double the number of cell layers of zone 2 (host cell layer). In fact, the host cell layer accounts for approximately 53% of the cross-sectional area of A. nipponica roots, as compared with 34% in A. wallichii roots. Our observations confirm that A. nipponica can propagate through division of both the rhizome and tuberous roots, representing the first report of clonal propagation in the Apostasioideae. • Conclusions: Apostasia nipponica appears to exhibit increased dependence on mycotrophy as compared with A. wallichii, as evidenced by both reduction in above-ground biomass relative to rootstock biomass and increased proportion of the host cell layer in tuberous roots. We infer that these features facilitate the establishment of stable associations with ectomycorrhizal mycobionts present in Fagaceae-dominated forests. Although the species is capable of both asexual propagation and sexual reproduction, habitat disturbance is an important limiting factor for population growth and survival at some sites. Tuberous roots function as storage organs. Crickets, the known seed-dispersal agent, may play a crucial role in the removal of capsules from the previous year, thereby facilitating the maturation of those formed in the present year. The occurrence of this species at disturbed sites suggests the potential for cultivation by embedding seeds below leaf litter around the base of Fagaceae trees.