Comparative analysis of citrus seed conservation and germination between diploid and tetraploid rootstocks

Citrus rootstocks are polyembryonic and are thus propagated by seeds. Seeds are extracted at fruit maturity, partially desiccated and stored at low temperature (around 5°C) until use or diffusion. However, during this period, seeds performance regarding germination in terms of delay and percentage varies with the genotypes. The genetic homogeneity in seedlings is also a critical point for the clonal propagation depending on the zygotic embryo germination and to the chromosome doubling that may occur in nucellar embryos. Double diploid (4x) genotypes are considered to confer better tolerance to environmental constraints. The seed quality and seedling homogeneity of five pairs of diploid (2x) and 4x rootstock cultivars were investigated. Seed quality was evaluated by 7 criteria of embryonic rate and of sowing description (speed and duration of germination, number of germinated plants by seed, etc.). The genetic homogeneity was tested by analyzing the ploidy level by flow cytometer and the genetic constitution by SNP and SSR markers. According to the vigor, the delay and the duration of germination, Volka-meriana and citrumelo 1452 were the best genotypes. The ploidy rate did not impact the seed performance but modified the homogeneity of seedlings. Although the ploidy level was more stable in 4x seedlings, the percentage of zygotic plantlets was increased by tetraploidy. The degree of heterogeneity of seedlings may reach 50% in some 4x rootstock cultivars challenging their use by nurserymen. (Texte integral)