Warm stratification and optimised temperatures improve conservation of the endangered orchid, Caladenia robinsonii (Orchidaceae)

Context. Maximising seed germination and seedling development is critical for conservation of endangered plants around the world. Orchidaceae is one of the most threatened plant families and can be one of the most difficult to propagate ex situ. Three critical, but potentially limiting, factors are important for orchid germination, namely, conducive conditions, 'ready-to-germinate' seed and effective mycorrhizal fungi. Aims. Our aim was to improve poor germination in vitro of a recalcitrant Australian endangered orchid, Caladenia robinsonii, and to predict the potential impacts of climate change on this species. Methods. Three experiments were conducted to optimise germination in C. robinsonii, with a focus on temperature. In Experiment I, on the basis of meteorological data, three constant temperatures (15 degrees C, 20 degrees C and 27 degrees C) were tested. In Experiment 2, the optimal constant temperature was compared with diurnally varying temperatures of 22 degrees C/18 degrees C (12/12 h), with and without warm stratification at 30 degrees C/27 degrees C (12/12 h) for 1 week. In Experiment 3, the same diurnally varying temperature and warm stratification were tested using multiple orchid mycorrhizal fungal isolates cultured from wild and re-introduced populations of C. robinsonii. Key results. Without warm stratification, germination was greatest at 20 degrees C (21%), but only 4% of seedlings developed to the green-leaf stage, whereas fungal growth was greatest at 27 degrees C. Stratification increased germination (79%) and development to the green-leaf stage (47%), but more so with subsequent incubation under constant 20 degrees C than diurnal 22 degrees C/18 degrees C. Fungal isolate affected total germination (53-69%) and development to the green-leaf stage (26-41%); isolates from the wild population were less effective than were those from re-introductions. Conclusions. Warm stratification and specific seasonal temperatures significantly improved germination, both factors being typical of seeds with physiological dormancy.