1 Actinidiaceae 1.1 Actinidia spp. Kiwifruit Manuel Rey,1,2 Yolanda Ferradás,3,4 Óscar Martínez1 and María Victoria González3 1Departamento Biología Vegetal y Ciencia del Suelo, Facultad de Biología, Campus Universitario, Universidad de Vigo, Vigo, Spain; 2CITACA, Agri-Food Research and Transfer Cluster, Campus da Auga, Universidad de Vigo, Ourense, Spain; 3Departamento Biología Funcional, Facultad de Farmacia, Universidad de Santiago de Compostela, Campus Sur, Santiago de Compostela, Spain; 4Present address: Instituto de las Ciencias de la Vid y del Vino (Gobierno de La Rioja-Consejo Superior de Investigaciones Científicas-Universidad de La Rioja), Logroño, Spain 1. Introduction stamens release sterile pollen because viability is lost by pro- grammed cell death (PCD) during microspore development 1.1. Botany and history (Coimbra et al., 2004; Falasca et al., 2013). Floral evocation occurs during one growth season while The genus Actinidia, commonly known as kiwifruit, is included in floral initiation occurs in the following season (Walton et al., the Actinidiaceae family together with Clematoclethra (Franch.) 1997). During the reproductive process, pollen dispersion Maxim. and Saurauia [Saurauja] Willdenow. This genus com- takes place by wind and, mainly, bees (Costa et al., 1993). prises c.54 species and 75 taxa (Wang and Gleave, 2012; Huang Some Actinidia species are characterized by a high repro- et al., 2013). Paleobiology studies have shown that Actinidia is at ductive success, being able to produce one fruit per flower least 20–26 million years old (Qian and Yu, 1991). Actinidia plants (Biasi and Costa, 1984). Abundant secretions released into the are perennial, deciduous, climbing and/or scrambling vines. intermembrane space throughout the pistil by transmitting They are often reticulate polyploids with a basic chromosome tissue cells are involved in pollen nutrition and guidance, dis- number of x = 29 (McNeilage and Considine, 1989). The genus is appearing after pollen tube passage (González et al., 1996). unusual for the extensive variation in ploidy, meaning that dip- Despite its reproductive success, kiwifruit has a short ef- loid, tetraploid, hexaploid and even octoploid individuals can be fective pollination period which is limited by the stigmatic re- found (Ferguson and Huang, 2007). All Actinidia species appear ceptivity relying on stigmatic papillar integrity (González to be dioecious, which means that a single genetic determinant et al., 1995a,b). This short effective pollination period seems controls sex with a unique set of X/Y chromosomes together to be due to PCD processes occurring in the secretory tissue, with a sex-neutral (XX)n chromosome set, where n depends on which are eventually accelerated by pollination (Ferradás the ploidy levels (McNeilage, 1997; Testolin et al., 1999; Fraser et al., 2014). et al., 2009). Sometimes gender-inconstant variants can occur as Actinidia fruits are defined as berries, with great diversity in well; in this case, staminate flowers usually are smaller than pistil- size, shape, hairiness, skin toughness and palatability, external and late flowers (McNeilage, 1991a,b). Flowers are usually arranged internal colour, flavour, flesh chemical composition and storage in cymes, although they sometimes can appear solitary. Identity of capacity (Ferguson and Huang, 2007). Kiwifruits have a high nu- floral organs is determined by the (A)BCE-like floral model, tritional value and are an excellent source of vitamins C and E, where the A function is unclear (Varkonyi-Gasic et al., 2011). The folate and potassium, with high antioxidant capacity (Wang et al., staminate flowers in male vines develop viable pollen and a rudi- 1996; Ferguson and Ferguson, 2003; Drummond, 2013). mentary ovary without ovules. On the other hand, the pistillate Actinidia chinensis is the most important crop species of the flowers in female vines bear a functional ovary, which is formed by genus, although Actinidia arguta, Actinidia kolomikta or the fusion of many carpels, leaving the radiating styles free. The Actinidia eriantha also have some importance (Ferguson and © CAB International 2020. Biotechnology of Fruit and Nut Crops, 2nd Edition (eds R.E. Litz, F. Pliego-Alfaro and J.I. Hormaza) 1 2 M. Rey et al. Huang, 2007). Planchon (1847) described kiwifruit as a unique for the future of this crop. Controlling excessive vegetative species. Further studies revealed differences in the level of vigour and improving the percentage of fruit set are im- ploidy and in morphological characteristics among individuals portant. Italy has been utilizing the clonal rootstock ‘D1’ for (Liang, 1975; Liang and Ferguson, 1984), and two separate calcareous soils (Vizzotto et al., 1999), and ‘Bruno’ and species were recognized, A. chinensis and Actinidia deliciosa, ‘Kaimai’ are important in New Zealand (Oliveira and Fraser, with diploid and hexaploid individuals, respectively (Liang and 2005). ‘Bruno’ seedlings have been used to support vines Ferguson, 1984). Currently, they are recognized as two varieties that grow rapidly and with greater productivity (Lawes, of the same species, A. chinensis var. chinensis and A. chinensis 1990). ‘Kaimai’, a clonal rootstock of Actinidia hemsleyana var. deliciosa, due to the presence of clines between both var- Dunn, stimulates doubling the number of flowers on each ieties, as well as extensive introgressive hybridization where shoot of the scion (Wang et al., 1994b). they cohabit (reviewed in Ferguson, 2016). The Actinidia genus is distributed throughout much of BREEDING ACCOMPLISHMENTS Analyses of A. arguta and eastern Asia, with most species and intraspecific taxa occurring . different rootstock–scion combinations of A. arguta have dem- in south central China (Liang, 1983). In 1904, some seeds of onstrated that both rootstock and scion affect actinidin activity were introduced into New Zealand (Ferguson, A. chinensis and free sugar and organic acid content in fruit (Boyes et al., 2004), with the establishment of the first commercial plantation 1996, 1997). Interspecific rootstocks of Actinidia can have a in the 1930s (Ferguson and Stanley, 2003; Ferguson and major effect on the amount of vegetative growth on A. chinensis Huang, 2007). New Zealand was responsible for the domesti- var. chinensis ‘Hort16A’ vines, dramatically altering plant size, cation and commercialization of kiwifruit. The inherent or- leaf area and scion vigour (Clearwater et al., 2006, 2007). Thorp ganoleptic, nutritional and storage qualities make kiwifruit a et al. (2007) reported that interspecific clonal rootstocks had a widely accepted and popular fruit crop for producers and con- substantial effect on the accumulation and concentration of sumers. For these reasons, commercial kiwifruit growing areas inorganic nutrients in the fruit, leaves and stem sap of ‘Hort16A’ have expanded rapidly in recent decades. In 2017, the global vines. The use of rootstocks of other species of Actinidia, e.g. kiwifruit growing areas had reached over 247,794 ha, with A. hemsleyana, A. eriantha and Actinidia rufa, reduced the level China (165,728 ha), Italy (26,403 ha), New Zealand (11,705 of floral abortion and increased bud burst in ‘Hayward’ (Wang ha) and Iran (10,771 ha) as the main producers accounting for et al., 1994a). The flower-promoting rootstocks have more starch about 87% of world kiwifruit plantings; global kiwifruit pro- grains, more and larger xylem vessels and idioblasts with raphides duction represents 0.62% of total production for major fruit and mucilage (Wang et al., 1994b). Powell and Santhanakrishnan crops (FAOSTAT, 2017). A. chinensis var. deliciosa is the most (1986) showed that ‘Hayward’ on mycorrhiza-infected rootstocks commercialized kiwifruit, and female ‘Hayward’ is the main had increased shoot length and fruit production. cultivar (Ferguson, 1999). However, different species and var- ieties have been increasing in importance in the last few years. 1.2.2. Scions 1.2. Breeding and genetics Major breeding objectives. Fruit size has been an important criterion for selecting wild species from China with commer- cial potential (Ferguson and Huang, 2007). Kiwifruit could Actinidia is a genus of recent domestication. Initially, the kiwifruit make an important contribution to human health (Singletary, industry was based on cultivars of A. chinensis var. deliciosa, and since 1975, ‘Hayward’ has been the dominant cultivar (Ferguson 2012; Skinner et al., 2013); the fruits have outstanding nutritional qualities and are a good source of vitamin C, minerals, folic acid and Huang, 2007). Another important cultivar is A. chinensis var. and fibre (Ferguson and Stanley, 2003). However, it can trigger chinensis ‘Hort16A’, known commercially as Zespri® Gold kiwi- fruit (Wang and Gleave, 2012). The rapid expansion of kiwifruit allergic responses in some consumers (Lucas et al., 2003). Other cultivation has revealed the need to obtain new cultivars or do- important breeding objectives include time of maturity and mestication of new species that are better adapted to different ripening, handling and storage (Ferguson, 1990). growing conditions. A distinctive feature of the genus is the struc- Diseases can affect the production of kiwifruit orchards. tured reticulate pattern of diploids, tetraploids, hexaploids and Reported kiwifruit diseases are mostly of fungal origin, i.e. octoploids in diminishing frequency, associated with geographic Armillaria novae-zelandiae in New Zealand (Horner, 1992), separation of ploidy races