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This study analyses the influence of four citrus rootstocks, Citrus macrophylla Wester, Gou Tou Chen (Citrus hybrid), C. volkameriana Ten. & Pasq. and Cleopatra mandarin (C. reshni Hort. ex Tan.), on growth, yield and fruit quality on Lane Late navel orange. Trees were grown in a plot in Alicante (Spain), where the soil is clay loam, with pH 8.5 and electric conductivity in the 1/5 extract: 0.643 mmhos cm–1. Yield was weighed during four harvests, fruit quality was determined in 2007 and 2009. Cleopatra mandarin and Gou Tou Chen were found to be the most invigorating rootstocks for ‘Lane Late’ in heavy and calcareous soil. Trees on C. macrophylla rootstock produced the highest cumulative yield (328.81 kg tree–1), without significant differences compared to trees on Cleopatra mandarin (292.41 kg tree–1). Rootstock significantly affected fruit quality variables. C. macrophylla and C. volkameriana would appear to induce the highest fruit weight and ripening index.

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Propagation of citrus rootstocks depends upon the production of clonal plants from nucellar seedlings. This makes apomixis one of the host important traits in breeding programs for citrus rootstocks. The genetic control of apomixis was studied in a 50-tree progeny derived from the cross C. volkameriana×P. trifoliata using 69 molecular markers and bulked segregant analysis. The proportion of nucellar seedlings was estimated by isoenzymatic analysis of 25 seedlings per tree for 2 consecutive years. The type of embryony (polyembryonic versus monoembryonic seeds) was also determined for fruit-yielding trees. Separate genetic maps for each parental species were developed. The integration and comparison of these maps could be accomplished using common multiallelic segregant loci. Differences in gene synteny between the two species-specific genetic maps were shown. Important distortions in the segregation of markers at several genomic regions, some of them also involving differences in the C-methylation pattern, have been observed, especially for the pollen parent. Analysis of quantitative trait loci (QTLs) revealed the presence of six genomic positions (two in P. trifoliata and four in C. volkameriana) contributing individually up to 24% of the total variation for apomixis. Within the same species, QTLs with positive and negative allele effects were present, even in the same linkage group. One of the markers associated to apomixis (Apo2) is also associated to embryony type. Therefore, the genetic control of apomictic reproduction found in citrus (nucellar embryony) is quite complex compared to what has been reported for gametophytic apomixis. Molecular markers linked to QTLs governing apomixis will be useful to assist selection of future apomictic rootstocks for citrus varieties.

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