Abstract
In nature, orchid seeds obtain the nutrients necessary for germination by degrading intracellular fungal structures formed after colonization of the embryo by mycorrhizal fungi. Protocols for asymbiotic germination of orchid seeds typically use media with high concentrations of soluble carbohydrate and minerals. However, when reintroduced into the field, seedlings obtained via asymbiotic germination have lower survival rates than do seedlings obtained via symbiotic germination. Tree fern fiber, the ideal substrate for orchid seedling acclimatization, is increasingly scarce. Here, we evaluated seed germination and protocorm development of Cyrtopodium glutiniferum Raddi cultivated in asymbiotic media (Knudson C and Murashige & Skoog) and in oatmeal agar (OA) medium inoculated with the mycorrhizal fungus Epulorhiza sp., using non-inoculated OA medium as a control. We also evaluated the performance of tree fern fiber, pine bark, eucalyptus bark, corncob and sawdust as substrates for the acclimatization of symbiotically propagated plants. We determined germination percentages, protocorm development and growth indices at 35 and 70 days of cultivation. Relative growth rates and the effects of substrates on mycorrhizal formation were calculated after 165 days of cultivation. Germination efficiency and growth indices were best when inoculated OA medium was used. Corncob and pine bark showed the highest percentages of colonized system roots. The OA medium inoculated with Epulorhiza sp. shows potential for C. glutiniferum seedling production. Corncob and pine bark are promising substitutes for tree fern fiber as substrates for the acclimatization of orchid seedlings.
Highlights
Starch is the main source of carbon for immature orchid seeds
The seeds of C. glutiniferum germinated in both asymbiotic media (KC and MS) and in the inoculated oatmeal agar (OA) medium (Fig. 1)
At 70 days, we observed protocorms at stage 5 in the inoculated OA medium, whereas the protocorms in the KC and MS media developed only to stage 3 (Fig. 1B). This indicates that the OA medium inoculated with Epulorhiza sp. was more efficient in promoting the germination and in vitro development of C. glutiniferum than were the asymbiotic media tested
Summary
Starch is the main source of carbon for immature orchid seeds. During the seed formation phase, starch is replaced by proteins, lipids and complex reserve sources, such as cellulose and hemicellulose (Rasmussen 1995). In natural conditions, symbiotic association enables seed germination in orchids (Dearnaley 2007). Symbiotic fungi colonize the parenchyma cells near the suspensor of the embryo of the orchid seed (Peterson et al 2004). The intracellular structures (pelotons) formed by the fungus within these cells are degraded by the embryo to obtain the nutrients required for its germination and development until the establishment of the adult plant, a phenomenon known as “mycoheterotrophy” (Rasmussen 1995; Peterson et al 2004)
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