Understanding the ecophysiology of trophic transition with micropropagated terrestrial orchids for impriving restoration success

Orchids are now one of the most threatened plant groups in the world; the result of both natural and anthropogenic causes (Kull et al., 2006). New integrated protocols to progress orchid conservation are desperately needed, with emphasis on both propagation of threatened species and successful reintroduction of vigorous and acclimatized seedlings. The need for application of ecophysiological principles to improve the ability of the prolific micropropagation system to deliver more robust and resilient orchid plants for restoration has never been more critical. This study set out to investigate abiotic (temperature and light) and biotic (fungal symbionts) influences on in vitro seed germination, seedling development and photosynthetic capacity of four terrestrial orchid species Caladenia latifolia, C. huegelii, Microtis media and Pterostylis sanguinea from south-west Western Australia. Germination was higher and development of seedlings faster overall in all test species in symbiotic compared with asymbiotic media treatments. Pterostylis sanguinea seeds demonstrated the best response (among species tested) to asymbiotic germination on ½ MS (Murashige and Skoog basal medium) with from 40 to 53% of germinated seeds reaching developmental stage 3 up to stage 5 in light or dark incubation (at 20 ^oC) respectively. Illumination had no effect on fungal symbiont growth across all species, however incubation temperature treatments (10, 15, 20 and 25 ^oC) affected fungal growth rate. Growth of the fungal symbionts of Caladenia huegelii, M. media and C. latifolia was lower at 10 ^oC, but the cumulative radial growth rate of the P. sanguinea fungal symbiont reached 64 cm2 after only two weeks at all temperatures tested, including 10 ^oC. The study highlights differences in symbiotic and asymbiotic germination and early protocorm development in vitro between co-occurring herbaceous terrestrial Australian orchid taxa in response to variations in basal media, temperature and light. Whole seedling CO₂ exchange (i.e. photosynthetic activity) was analysed with Caladenia huegelii, Caladenia latifolia, Microtis media and Pterostylis sanguinea grown with and without fungal symbionts in response to both LED and fluorescent lamps, and with added nitrogen source (N). Results strongly indicated that photosynthesis at the early stages of orchid growth may be more affected by the presence and absence of fungus than by light source, light quantity or available N. This study demonstrates the difficulties in dealing with species that possess a complex life history and further studies will be needed to translate these findings into improving the survival of terrestrial orchid seedlings from in vitro propagation to the glasshouse environment. However, it is hoped that the study will be of benefit to the conservation of Australian terrestrial orchids specifically and terrestrial species generally.