‘Safor’ Mandarin: A New Citrus Mid-late Triploid Hybrid

Abstract

World mandarin production rose from 18.3 million tons to 27.9 million tons between the years 2000 and 2007 (FAO, Food and Agriculture Organization, 2009). In Spain, 87% of mandarin production in the last 5 years has been destined to the fresh fruit market (Intercitrus, Interprofessional Citricola Espanola, 2008), which demands high-quality, seedless fruits throughout the marketing season. Therefore, the production of seedless varieties is very important. Triploid plants are generally considered an evolutionary dead end, because they generally give rise to aneuploid gametes with very low fertility (Otto and Whitton, 2000). Predominantly trivalent and a high number of bivalent and univalent associations are formed during meiosis of citrus triploid hybrids (Frost and Soost, 1968). Moreover, abortion of the megasporogenesis in the period between the embryo-sac first divisions and the fecundated egg cell is common (Fatta Del Bosco et al., 1992). For this reason, citrus triploid hybrids are generally sterile, although they can occasionally produce fruits with very few seeds and induce seed formation in fruits of other cultivars. Citrus triploid hybrids can be obtained by means of 2x · 2x hybridizations through the production of unreduced gametes by the female parent (Esen and Soost, 1971). The frequency of unreduced gamete formation depends on the genotype (Esen and Soost, 1971; Luro et al., 2004). Triploid embryos are preferentially found in seeds between onethird and one-sixth smaller than normal seeds and these small seeds generally do not germinate under conventional greenhouse conditions. Embryo rescue from these small seeds is required to reach high germination rates (Navarro et al., 2002). Luro et al. (2004) proposed that Second Division Restitution is the mechanism controlling unreduced gamete formation in clementines, whereas in sweet orange, Chen et al. (2008) proposed First Division Restitution to be the mechanism involved. In these two genetic mechanisms, only one part of the maternal heterozygosity is transmitted to the triploid hybrid and the rate of maternal heterozygosity varies among the loci in relation with the rate of single crossing over between the centromere and a given locus (Ollitrault et al., 2008). Ploidy level determination by histological methods is too laborious for large-scale analyses. However, ploidy level can be accurately determined relatively rapidly in large populations by flow cytometry (Ollitrault and Michaux-Ferriere, 1992). Embryo rescue and flow cytometry are two indispensable techniques for extensive triploid citrus breeding programs. These techniques allow the efficient recovery of plants from embryos contained in small seeds and enable the ploidy level of regenerated plantlets to be determined quickly and easily with just a small piece of leaf while the plants are still in the test tube (Navarro et al., 2002). In Spain, there are numerous problems associated with the production season of mandarin cultivars, which include satsumas [Citrus unshiu (Mak.) Marc.], clementines (C. clementina Hort. ex Tan.), and mandarin hybrids. Satsumas and clementines are traditionally harvested from the beginning of September until mid-February. Satsumas produce seedless fruits because they have sterile pollen and ovules. Clementines, the most widely grown mandarins in Spain, are self-incompatible and also produce seedless fruits if grown in isolation. However, their pollen and ovules are viable, and consequently they are able to pollinate and be pollinated by other sexually compatible cultivars. As a result of the demand for lateseason mandarins by international markets, several midand late-maturing mandarin hybrids were introduced in Spanish citriculture. These hybrids are self-incompatible, but their pollen and ovules are viable and crosspollinate with clementines, producing fruits with seeds in both groups of cultivars, which causes substantial economic losses. With a view to solving this problematic situation, a triploid breeding program was started in Spain in 1996. The main objective was to produce new midand late-maturing triploid cultivars through sexual hybridization, embryo rescue, and ploidy analysis by flow cytometry (Navarro et al., 2002). Recently we released the late-maturing triploid variety Garbi [(C. clementina·C. tangerina) · (C. reticulata · C. sinensis)] (Aleza et al., 2010), which reaches optimum maturation during the second half of March. This variety of mandarin is replacing ‘Fortune’ mandarin (C. clementina · C. tangerina), which reached a peak production of 300,000 tons but is currently being replaced rapidly as a result of its high susceptibility to Alternaria alternata. Another problem is the low fruit quality of ‘Hernandina’ clementine, our latest maturing clementine, when grafted on ‘Carrizo’ citrange (Citrus sinensis · Poncirus trifoliata), which is by far the most predominant rootstock. Fruit peel deteriorates quickly after mid-January, and in practice, no clementine fruits are available by February. In this article, we describe a new triploid hybrid named ‘Safor’ mandarin [(C. clementina · C. tangerina) · (C. unshiu · C. nobilis)] characterized by its high quality, seedless fruits, and mid-late ripening season.