Cryopreservation without vitrification suitable for large scale cryopreservation of orchid seeds

Abstract

Background: Orchids are under threat from human activities and climate change, with populations limited to small geographic hotspots. This makes them ideal candidates for ex situ conservation. Orchid seeds are desiccation tolerant, but often have poor longevity in seed banks and cryopreservation of orchid protocorms is complex and expensive. Therefore, simple methods for large-scale storage programs are essential to store orchid seeds of different life forms. Seeds of five species representing epiphytic, lithophytic and terrestrial orchids from the Central Highlands of Madagascar were studied to find a simple and effective system of cryopreservation. The use of a vitrification solution prior to cryopreservation to improve survival was investigated, as well as the use of symbiotic and asymbiotic germination media to maximise germination after cryopreservation. Using the filter paper packet method, dried seeds were stored in vapour phase above liquid nitrogen and recovered after thawing with both symbiotic and asymbiotic media. Results: The study revealed that cryoprotection is not essential for the species in this study, which represented a range of lifeforms. Vitrification generally led to a decrease in germination post cryopreservation. The use of a symbiotic germination medium post cryopreservation was found to be successful in the species in which it was tested. However, the use of an asymbiotic medium was successful for all the species in this study. Conclusions: Vitrification was not essential for the species in this study as the orchid seeds were already ultralow temperature and desiccation tolerant. However, further studies using more species are required to validate this approach. This may be an ecophysiological or genetic trait of these species. Therefore, this form of dry seed cryopreservation could form part of ex situ orchid seed conservation using a standard method. The methods developed here will store greater genetic diversity compared to what can be achieved with protocorms and are suitable for both asymbiotic and symbiotic recovery after cryopreservation. This will help reduce the time and cost of ex situ conservation, and help develop universal protocols for large genera, compared to custom protocols required for protocorm cryopreservation.