Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 9

The genus Actinidia, commonly known as kiwifruit, is included in the Actinidiaceae family together with Clematoclethra (Franch.) Maxim. and Saurauia [Saurauja] Willdenow. This genus comprises c.54 species and 75 taxa (Wang and Gleave, 2012; Huang et al., 2013). Paleobiology studies have shown that Actinidia is at least 20–26 million years old (Qian and Yu, 1991). Actinidia plants are perennial, deciduous, climbing and/or scrambling vines. They are often reticulate polyploids with a basic chromosome number of x = 29 (McNeilage and Considine, 1989). The genus is unusual for the extensive variation in ploidy, meaning that diploid, tetraploid, hexaploid and even octoploid individuals can be found (Ferguson and Huang, 2007). All Actinidia species appear to be dioecious, which means that a single genetic determinant controls sex with a unique set of X/Y chromosomes together with a sex-neutral (XX)_n chromosome set, where n depends on the ploidy levels (McNeilage, 1997; Testolin et al., 1999; Fraser et al., 2009). Sometimes gender-inconstant variants can occur as well; in this case, staminate flowers usually are smaller than pistillate flowers (McNeilage, 1991a,b). Flowers are usually arranged in cymes, although they sometimes can appear solitary. Identity of floral organs is determined by the (A)BCE-like floral model, where the A function is unclear (Varkonyi-Gasic et al., 2011). The staminate flowers in male vines develop viable pollen and a rudimentary ovary without ovules. On the other hand, the pistillate flowers in female vines bear a functional ovary, which is formed by the fusion of many carpels, leaving the radiating styles free. The stamens release sterile pollen because viability is lost by programmed cell death (PCD) during microspore development (Coimbra et al., 2004; Falasca et al., 2013).

Floral evocation occurs during one growth season while floral initiation occurs in the following season (Walton et al., 1997). During the reproductive process, pollen dispersion takes place by wind and, mainly, bees (Costa et al., 1993). Some Actinidia species are characterized by a high reproductive success, being able to produce one fruit per flower (Biasi and Costa, 1984). Abundant secretions released into the intermembrane space throughout the pistil by transmitting tissue cells are involved in pollen nutrition and guidance, disappearing after pollen tube passage (González et al., 1996). Despite its reproductive success, kiwifruit has a short effective pollination period which is limited by the stigmatic receptivity relying on stigmatic papillar integrity (González et al., 1995a,b). This short effective pollination period seems to be due to PCD processes occurring in the secretory tissue, which are eventually accelerated by pollination (Ferradás et al., 2014).

Actinidia fruits are defined as berries, with great diversity in size, shape, hairiness, skin toughness and palatability, external and internal colour, flavour, flesh chemical composition and storage capacity (Ferguson and Huang, 2007). Kiwifruits have a high nutritional value and are an excellent source of vitamins C and E, folate and potassium, with high antioxidant capacity (Wang et al., 1996; Ferguson and Ferguson, 2003; Drummond, 2013).

Actinidia chinensis is the most important crop species of the genus, although Actinidia arguta, Actinidia kolomikta or Actinidia eriantha also have some importance (Ferguson and Huang, 2007). Planchon (1847) described kiwifruit as a unique species. Further studies revealed differences in the level of ploidy and in morphological characteristics among individuals (Liang, 1975; Liang and Ferguson, 1984), and two separate species were recognized, A. chinensis and Actinidia deliciosa, with diploid and hexaploid individuals, respectively (Liang and Ferguson, 1984). Currently, they are recognized as two varieties of the same species, A. chinensis var. chinensis and A. chinensis var. deliciosa, due to the presence of clines between both varieties, as well as extensive introgressive hybridization where they cohabit (reviewed in Ferguson, 2016).

The Actinidia genus is distributed throughout much of eastern Asia, with most species and intraspecific taxa occurring in south central China (Liang, 1983). In 1904, some seeds of A. chinensis were introduced into New Zealand (Ferguson, 2004), with the establishment of the first commercial plantation in the 1930s (Ferguson and Stanley, 2003; Ferguson and Huang, 2007). New Zealand was responsible for the domestication and commercialization of kiwifruit. The inherent organoleptic, nutritional and storage qualities make kiwifruit a widely accepted and popular fruit crop for producers and consumers. For these reasons, commercial kiwifruit growing areas have expanded rapidly in recent decades. In 2017, the global kiwifruit growing areas had reached over 247,794 ha, with China (165,728 ha), Italy (26,403 ha), New Zealand (11,705 ha) and Iran (10,771 ha) as the main producers accounting for about 87% of world kiwifruit plantings; global kiwifruit production represents 0.62% of total production for major fruit crops (FAOSTAT, 2014). A. chinensis var. deliciosa is the most commercialized kiwifruit, and female ‘Hayward’ is the main cultivar (Ferguson, 1999). However, different species and varieties have been increasing in importance in the last few years.