Seed germination and seedling emergence in the extremely endangered species Rhododendron protistum var. giganteum-the world's largest Rhododendron

Big tree rhododendron, Rhododendron protistum var. giganteum (Ericaceae) is an endangered and important germplasm source with high ornamental value in China. To design appropriate germination and seedling establishment protocols for a species conservation and restoration program, we studied the effects of temperature, photoperiod, storage, and gibberellic acid (GA3) treatment on seed germination in a laboratory experiment. The effects of soil substrate and sowing depth on seedling emergence were studied in a greenhouse experiment. We hypothesized that photoperiod is a limiting factor for seed germination of big tree rhododendron, and soil type and sowing depth would affect seedling emergence. Results showed that seeds of big tree rhododendron were non-dormant, and temperature significantly affected germination percentages (GP) and mean germination time (MGT). The optimal temperatures for seed germination were relatively low (15 °C and 20 °C). High temperature negatively affected seed germination. The seeds had less than 5% of GP when incubated in complete darkness, suggesting that darkness significantly inhibits seed germination. Storage period and temperature had significant effect on GP and MGT of big tree rhododendron seeds, and the effect of their interaction on MGT was also significant. GA3 treatment increased GP and germination speed. Soil substrates did not significantly affect seedling emergence, but native soil is beneficial to seedling growth. When the seeds were sown on the soil surface, seedling emergence was higher than 80%, but it decreased with increasing sowing depth, and no seedling emerged at 3 cm soil depth. We present some suggestions for the ex situ conservation and restoration program, including seed germination and seedling artificial propagation of big tree rhododendron based on the research results.