Storage behaviour and cryopreservation of Citrus cavaleriei, an endangered, cold-resistant species of northeast India with exceptionally large seeds.

BACKGROUND Ichang Papeda (Citrus cavaleriei H.Lev. ex Cavalerie) is a wild and endangered species of NE India that requires urgent preservation of its genetic resources. OBJECTIVE To characterize the storage physiology of the seeds and to cryopreserve the embryo and embryonic axis (EA). MATERIALS AND METHODS The morphology and storage physiology of the seeds were determined, and the cryopreservation of embryos and EA attempted using various techniques (viz., air desiccation-freezing, vitrification and encapsulation-dehydration). RESULTS Weighing up to 4 g, seeds of Citrus cavaleriei are the largest known in the genus. Based on estimates using the seed coat ratio - seed mass (SCR-SM) model there was a very high probability of seed desiccation intolerance, which was validated physiologically; seeds lost vigour on drying below 30% moisture content (MC) and no seeds germinating after drying to <12% MC. Embryos and EAs could be air dried to 25-30% MC and cryopreserved with c. 50% survival. In contrast, EA optimally exposed to PVS2 (20 min) or encapsulated, sucrose pretreated (0.5 M, 24 h) and dehydrated (6 h) had c. 40% survival after cryopreservation. CONCLUSION Citrus cavaleriei produces large, recalcitrant seeds that can, nevertheless, be cryopreserved as embryos or isolated EA after air drying to c. 25-30% MC; encapsulation-dehydration and vitrification provide alternative options for the cryopreservation of EA.