S02 - Session O6 - Behavior of seed tolerance to desiccation among Citrus species: a crucial step for genetic resources cryopreservation

Authors: Francois Luro *, Isabelle Tur, Olivier Pailly

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 (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 loss as soon as the seed WC drops below 25%). For varieties moderately sensitive 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%) allows to obtain the highest germination rates 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 a review of seed behaviors against desiccation of different citrus species and their hybrids