Résumé
In genebanks, citrus genetic resources are often maintained as grafted scions on rootstocks in field or greenhouse settings. Despite many precautions, collections are at risk of being lost due to pathogen attacks or to climatic disasters. Many citrus cultivars such as sweet oranges, grapefruits, limes and lemons, citrus used as rootstocks or many mandarins are apomictic with polyembryonic seeds allowing clonal propagation by seedling. Seed preservation at low temperature (4°C) is possible but cannot maintain germination properties over a year. Reducing seed moisture (or water content, WC) prior to long-term storage in liquid nitrogen (-196°C) is a crucial step for the effective success of the process. The seed tolerance to desiccation is variable according to the species. For example, all citrus varieties related to citron (Citrus medica), such as lemons and limes, can be considered as orthodox (i.e., the seeds are dry-hardy and tolerant to low temperature). Varieties of trifoliate orange (Poncirus trifoliata) and kumquat (Fortunella spp.) are very sensitive to high desiccation degree (germination capacity was fully lost as soon at seed WC <25%). For varieties with moderately sensitivity to desiccation, such as mandarins or sweet oranges, the control of WC by incubating seeds in a closed environment with constant relative humidity (between 75 and 81%) enables the highest germination level after cryopreservation. A better understanding of seed behavior during water flux through the cell membrane would be beneficial to improve seed preparation before freezing in liquid nitrogen. We report here the seed behaviors to desiccation and to liquid nitrogen exposure of different citrus species and their hybrids, and the implication of electrolyte leakage in desiccation sensitivity.