Most Citrus species are apomictic via the somatic embryony of seeds enabling clonal propagation by sowing. However, Citrus seeds are known to be semi- or non-orthodox, i.e. susceptible to dehydration. As part of the IBISA “Cryoveg” project, 2009-2012, designed to develop a cryopreservation methodology, seeds were tested for their tolerance of desiccation (prior to cryogenics) to identify water contents that would maintain viability after freezing (Engelman et al. 2009). Citrus fruits related to Citrus medica (citrons) are highly tolerant, while Poncirus trifoliata are extremely susceptible to dehydration. For the other species, a moisture content of between 10 and 15% gives germination rates of over 25% (Luro et al. 2011). Given these results, it is considered that around ¾ of the collection could be preserved in the form of frozen seeds. Work has been undertaken to elucidate tolerance/susceptibility mechanisms. They do not seem to be linked systematically to a change in membrane permeability. Lipid composition is being studied with IRD in Montpellier and some other potential determinants, such as oxidative stress or LAE protein synthesis, are being explored.

By defining a sub-sample of the collection that is representative of diversity (core collection), it should be possible to focus preservation and agronomic characterization efforts on the core of essential genotypes. It is based on knowledge of how the diversity and phylogeny of species is structured, and different algorithms seeking to maximize genetic diversity. Working with IVIA (Spain), a methodological study was carried out to compare different algorithms in defining the core collection, using genetic markers on the IVIA and INRA-CIRAD mandarin collections. Methods based on genetic distances and on maximizing allelic diversity are the most appropriate (> 90% of overall allelic diversity conserved with 30 accessions), but with a low coincidence level (<30%; Garcia Lor et al. 2017).

References cited

• Engelmann, F.; Balsemin, E.; Barreneche, T.; Chatelet, P.; Chauvin, J.-E.; Couturon, E.; Curk, F.; Dantec, M.-A.; Dantec, J.-P.; Dussert, S.; Feugey, L.; Froelicher, Y.; Fouilhaux, L.; Gamiette, F.; Grapin, A.; Grisoni, M.; Guérif, P.; Guyarder, A.; Label, A.; Luro, F.; Moulin, B.; Muller, M.; Peyrière, A.; Prigent, Y.; Renard, M.; Roux-Cuvelier, M.; Roques, D.; Rubens, S.; Sapotille, J.; Souchet, C.; Teyssedre, D. 2009. Cryopreservation of French plant genetic resource collections (CRYOVEG) [Abstract]. Cryobiology 59 (3):p. 411
• Garcia-Lor A., F. Luro, P. Ollitrault and L. Navarro (2017) Comparative analysis of core collection sampling methods for mandarin germplasm based on molecular and phenotypic data. Annals of Applied Biology DOI: 10.1111/aab.12376
• Luro F., Tur I. and Dussert S. Oral presentation: Evaluation of Citrus seed tolerance to desiccation and liquid nitrogen storage: towards citrus germplasm cryopreservation. XIII Eucarpia Symposium on Fruit Breeding and Genetics Warsaw, Poland September 11-15, 2011.