Alborea: A New Mid-late Mandarin Triploid Hybrid [(Citrus clementina x C. tangerina) x (C. nobilis x C. deliciosa)]

Abstract

Citrus is one of the most important fruit crops worldwide, with more than 130 million tons produced in 2017. Mandarins represent 25% of this production (Food and Agriculture Organization, 2018) and are mainly destined for fresh consumption. The Mediterranean area is the most important region for mandarin exports, with 60% of the total volume, and Spain is the leading country. The fresh market demands high-quality, seedless fruit that can be harvested throughout the marketing season. Therefore, mandarin breeding programs worldwide are mainly aimed at obtaining new, seedless, easy-peeling varieties with an attractive fruit color and flavor, and with high organoleptic characteristics (Grosser et al., 2010; Navarro et al., 2015; Rapisarda et al., 2008).

The mandarin varietal structure in Spain has several problems. It includes satsumas [Citrus unshiu (Mak.) Marc.], clementines (C. clementina Hort. ex Tan.), and mandarin hybrids. Satsumas are harvested from the end of August to mid November. Their pollen and ovules are not viable, and thus they are seedless. Clementine varieties are the most important group of mandarins in Spain and can be harvested from mid September until the second half of January. They are self-incompatible, but their pollen and ovules are viable and, consequently, they are able to pollinate and be pollinated with other compatible cultivars. ‘Hernandina’ clementine, our latest maturing clementine, is characterized by low fruit quality when grafted onto ‘Carrizo’ citrange [C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.], which is by far the predominant rootstock in our country. Fruit peel deteriorates quickly after mid January, and in practice no clementine fruit is available in the second half of January. Mandarin hybrids, such as ‘Nova’ [C. clementina × (C. paradisi Macf. × C. tangerina Hort. Ex Tan.)], ‘Fortune’ (C. clementina × ?), ‘Murcott’ (C. reticulata Blanco × C. sinensis), and ‘Ortanique’ (natural hybrid between mandarin and C. sinensis), were introduced to our citriculture to cover the demand of late-maturing mandarins by international markets. These cultivars are self-incompatible, but their pollen and ovules are viable and they cross-pollinate with clementines, producing seedy fruit in both groups of mandarins. In addition, some late mandarin hybrids, such as ‘Fortune’, ‘Nova’, and ‘Murcott’, are susceptible to Alternaria alternata fungus, which reduces production and depreciates the fruit commercially for the fresh-fruit market. This fungus has forced the top-grafting of trees of these susceptible cultivars, particularly ‘Fortune’, and during the past few decades there has been a decrease in late-mandarin plantations and an increase in clementine plantings, mainly ‘Clemenules’. This has created an imbalance between production and market demand that has caused a drastic reduction in prices for the growers such that, in many cases, they cannot even sell their products. As a consequence of excessive clementine production, a lot of growers decided to cultivate other late mandarin cultivars, especially ‘Nadorcott’ [(C. reticulata × C. sinensis) × ?], ‘Tango’ (irradiated variety from ‘Nadorcott’ mandarin), and ‘Orri’ (irradiated variety from ‘Orah’ mandarin) and, more recently, ‘Spring Sunshine’ (irradiated variety from ‘Murcott’ mandarin), which is susceptible to Alternaria. These varieties cover the period from February to the end of April and are managed by different private companies that limit the number of plants or the cultivated area, and also impose high royalties that, in many cases, are difficult for growers to pay. Under this scenario, the recovery of a new, high-quality seedless mandarin cultivar that matures from mid December until the end of January, when there are no other high-quality mandarins in the market, and that is resistant to Alternaria is a very important objective for our citriculture.

In 1995, a triploid breeding program was started at the Instituto Valenciano de Investigaciones Agrarias (IVIA) with the objective of producing new mid- and late-maturing triploid cultivars resistant to A. alternata fungus through sexual hybridization, embryo rescue, and ploidy analysis by flow cytometry (Navarro et al., 2015). Triploid plants generally produce aneuploid gametes, thus leading to very low fertility of their pollen and ovules (Otto and Whitton, 2000). For this reason, citrus triploid hybrids can be considered sterile, producing seedless fruit, and do not induce seed formation in other varieties by cross-pollination, even in the presence of bees (Navarro et al., 2015). A routine strategy exploited for triploid citrus breeding is spontaneous female unreduced gamete formation in diploid × diploid crosses (Aleza et al., 2012; Cuenca et al., 2011, 2015), where triploid hybrids arise usually from the union of an unreduced megagametophyte formed through a second-division restitution mechanism with a haploid pollen (Cameron and Frost 1968; Cuenca et al., 2011; 2015; Esen and Soost 1971, 1973; Geraci et al., 1975; Luro et al., 2004). Triploid embryos are predominantly found in small seeds, which generally do not germinate in greenhouse conditions. Thus, embryo rescue from these small seeds is required to reach high germination rates (Aleza et al., 2010b). In addition, ploidy level determination by flow cytometry is also required in extensive triploid citrus breeding programs (Aleza et al., 2012). From this program, the mid- to late-maturing triploid varieties ‘Garbı’ [(C. clementina × C. tangerine) × (C. reticulata × C. sinensis)] (Aleza et al., 2010a) and ‘Safor’ [(C. clementina × C. tangerina) × (C. unshiu × C. nobilis Lour.)] (Cuenca et al., 2010) were released, with more than 600,000 plants commercialized until 2018.

We describe a new triploid hybrid named ‘Alborea’ mandarin [(C. clementina × C. tangerina) × (C. nobilis × C. deliciosa Ten.)] that is resistant to Alternaria alternata and characterized by the production of high-quality, seedless fruit that can be harvested from mid-December until the end of January.