Improvement of salt tolerance and resistance to Phytophthora gummosis in citrus rootstocks by controlled hybridization

The sustainability of Mediterranean citriculture depends largely on the use of rootstocks that provide a better adaptation to biotic and abiotic constraints, as well as a good graft-compatibility with commercial cultivars. In the absence of rootstocks meeting all these criteria, the management of the available diversity and the selection of the desirable traits are necessary. In the present work, we conducted a controlled pollination program involving four different rootstock genotypes that belong to the germplasm collection of INRA Morocco. These genotypes included citrumelo 'Winter Haven' (Citrus paradisi Macf. x Poncirus trifoliata L. Raf.), Sunki mandarin (Citrus sunki), Cleopatra mandarin (Citrus reshni), and pomelo (Citrus grandis). The breeding program resulted in a population of 199 seedlings, 31 of which expressed marked morphological traits of the male parent. Molecular characterization of these 31 genotypes using flow cytometry and microsatellite markers helped to determine their ploidy level and to confirm the parental origin of the zygotic seedlings. On the other hand, the phenotypic characterization of the potential hybrids by early screening tests revealed heterogeneous responses to salinity and Phytophthora attacks. An interesting combination of salt tolerance traits (chloride exclusion, high relative growth, high stomatal conductance) with resistance to Phytophthora gummosis was observed in two citrumelo hybrids, H56 and H92, which seem therefore to be qualified for use in salt and Phytophthora affected regions as a replacement for the CTV-susceptible rootstock, sour orange.