The Biology of Malus domestica Borkh. (Apple)
2. Identity

2.1 Name

Malus domestica Borkh. (USDA-ARS 2012).

2.2 Family

Rosaceae (rose family) (USDA-ARS 2012).

2.3 Synonym(s)

Synonyms for M. domestica are: M. malus (L.) Britton, nom. inval., M. pumila auct., M. pumila Mill. var. domestica (Borkh.) C. K. Schneid., M. sylvestris auct., M. sylvestris (L.) Mill. var. domestica (Borkh.) Mansf., and Pyrus malus L. (USDA-ARS 2012).

2.4 Common name(s)

M. domestica is commonly known as apple in English, and pommier commun in French (USDA-ARS 2012).

2.5 Taxonomy and genetics

The genus Malus belongs to the rose family (Rosaceae) which includes over 100 genera and 3000 species distributed worldwide, most commonly in temperate regions (Velasco et al. 2010). Species of agronomic importance include: almond, apple, apricot, cherry, peach, pear, plum, quince, raspberry, sour cherry, sweet cherry, and strawberry (Shulaev et al. 2008; Webster 2005a; Westwood 1993) . Other non-edible species with ornamental value include: rose, hawthorn, potentilla, cotoneaster, and pyracantha (Shulaev et al. 2008).

The Rosaceae family has traditionally been divided into four subfamilies on the basis of fruit type. These include: Rosoideae (e.g., Rosa, Fragaria, Potentilla, Rubus, fruit an achene, x = 7, 8 or 9); Prunoideae (e.g., Prunus, fruit a drupe, x = 8); Spiraeoideae (e.g., Spirea, fruit a follicle or capsule, x = 9), and; Maloideae (e.g., Malus, Pyrus and Cotoneaster, fruit a pome, x = 17) (Luby 2003; Shulaev et al. 2008). More recent phylogenetic analyses have resulted in the reclassification of Rosaceae into three subfamilies, namely Dryadoideae (x = 9); Rosoideae (x = 7) and Spiraeoideae (x = 8, 9, 15 or 17), with each of the latter two further divided into supertribes, tribes and subtribes. With less emphasis on fruit type, taxa formerly included in Prunoideae and Maloideae (including apples and pears) were reclassified into Spiraeoideae (Potter et al. 2007; Shulaev et al. 2008). Another recent nomenclatural change includes the former Maloideae and Spiraeoideae in a new subfamily, Amygdaloideae (see below), based on requirements in the 2012 edition of the International Code of Nomenclature for Algae, Fungi and Plants (USDA-ARS 2012; USDA-NRCS 2012). Determination of phylogenetic relationships within the Rosaceae is complex and ongoing but regardless of which classification scheme is used, apples and pears belong to the same subfamily, and along with a handful of other closely related genera, are distinct in having a haploid (x) number of 17 chromosomes (x = 17). They are thought to be of allopolyploid origin, having originated from ancient hybridizations between species in the Prunoideae (x = 8) and the Spiraeoideae (x = 9), followed by chromosome doubling. The original hybrids would have been sterile and only after chromosome doubling would they have formed fertile allopolyploids (Hancock et al. 2008; Luby 2003; Way et al. 1990; Webster 2005a).

Like the family it belongs to, the genus Malus is diverse and complex, and there are considerable challenges in species delimitation due to hybridization, polyploidy, and apomixis (Luby 2003). The primary centre of species richness and diversity is in southwest China and Central Asia, with several species ranging eastwards to Manchuria and Japan, and others ranging westwards to Europe (Ferree and Carlson 1987; Ignatov and Bodishevskaya 2011; Luby 2003) . A secondary centre is in North America, with four native species (see Section 5). The number of species included in the genus is still a subject of debate, with different treatments recognizing as few as 8 to as many as 78 primary species, depending on the rank given to certain taxa, and the acceptance of reported hybrids (Hancock et al. 2008; Harris et al. 2002; Jackson 2003; Luby 2003; Rieger 2006; Robinson et al. 2001) . Most species in the genus can be readily hybridized, and many hybrid species, derived naturally or artificially, are recognized (Hancock et al. 2008; Luby 2003). A list of 58 species and hybrid species currently recognized in the taxonomy database of the U.S. Department of Agriculture Germplasm Resources Information Network (GRIN) is provided in Appendix 1. Most apple species are diploid (2n = 2x = 34) but higher somatic numbers exist (e.g., 51, 68, 85) and several cultivated types are triploid (Hancock et al. 2008).

The cultivated apple, M. domestica (also sometimes designated M. x domestica Borkh. to indicate its hybrid origin), is thought to be the result of initial domestication followed by inter-specific hybridization (Hancock et al. 2008; Luby 2003; Mabberley et al. 2001; Qian et al. 2010) . It is not a naturally evolved species; rather it has been collected, transported, hybridized and selected by people over millennia (Ferree and Carlson 1987). Its primary wild ancestor is thought to be M. sieversii (Ledeb.) M. Roem., whose range is centered at the border of western China and the former Soviet Union (Hancock et al. 2008; Luby 2003). This has been supported by recent genetic analysis (Velasco et al. 2010). Other species which are thought to have contributed to the genetic background of M. domestica are: M. orientalis Uglitzk. of Caucasia, M. sylvestris (L.) Mill. from Europe, M. baccata (L.) Borkh. from Siberia, M. mandshurica (Maxim.) Kom. ex. Skvortsov from Manchuria, and M. prunifolia (Willd.) Borkh. from China. It is likely that these species hybridized with cultivated apples as they were spread by people (Hancock et al. 2008; Juniper et al. 1999). A number of species are also known to have contributed to the M. domestica complex in modern breeding programs, including: M. floribunda Siebold ex Van Houtte, M. x micromalus Makino, M. x atrosanguinea (hort. ex Spath) C. K. Schneid., M. baccata, M. zumi (Matsum.) Rehder and M. sargentii Rehder (Hancock et al. 2008).

Throughout its history of cultivation, more than ten thousand cultivars of M. domestica have been developed, although many of these are now lost (Qian et al. 2010; Rieger 2006; Velasco et al. 2010; Way et al. 1990) . Currently, about 100 cultivars are grown commercially, the most popular worldwide including: 'Fuji', 'Delicious', 'Golden Delicious', 'Gala' 'Granny Smith', 'Idared', 'Jonagold', 'Braeburn', 'Cripps Pink', 'Jonathan', 'Elstar' and 'McIntosh' (Belrose 2012; Jackson 2003). The majority of cultivars are diploid (e.g., 'Fuji', 'Delicious', 'Golden Delicious', 'Gala', 'Granny Smith', 'Jonathan', 'McIntosh'), while some are triploid (e.g., 'Jonagold') (Hampson and Kemp 2003; Westwood 1993) and a few are tetraploids (e.g., 'Antonovka Ploskaya', 'Wealthy Tetraploidnyi', 'Papirovka Tetraploidnaya', 'McIntosh Tetraploidnyi') (Sedov and Makarkina 2008).

Taxonomic position (USDA-ARS 2012; USDA-NRCS 2012):

Kingdom: Plantae (plants)
Subkingdom: Tracheobionta (vascular plants)
Superdivision: Spermatophyta (seed plants)
Division: Magnoliophyta (flowering plants)
Class: Magnoliopsida (dicotyledons)
Subclass: Rosidae
Order: Rosales
Family: Rosaceae (rose family)
Subfamily: Amygdaloideae
Tribe: Maleae
Subtribe: Malinae
Genus: Malus
Species: Malus domestica Borkh.

2.6 General description

M. domestica is a small- to medium-sized, much-branched, deciduous tree with a single trunk and a broadly spreading canopy. Wild trees can reach 10-15 m in height, while cultivated trees are generally 2-5 m tall (in cultivation, tree size and shape are heavily dependant on rootstock and training system) (CABI 2012; Flora of China editorial committee 1959+; Rieger 2006) . Roots consist of a horizontal layer of permanent, thickened, spreading scaffold roots less than 50 cm from the surface, and numerous vertical 'sinkers' descending to an impermeable layer or water table (Jackson 2003). Young stems and twigs are somewhat tomentose (hairy), while older branches are glabrous (smooth) (Bailey and Bailey 1976; CABI 2012; Webster 2005a) . Buds are purplish brown, ovoid and densely hairy (Flora of China editorial committee 1959+). Leaves are alternate, elliptic-ovate, rounded at the base, 4-13 cm long x 3-7 cm wide, with irregularly saw-toothed margins, and usually hairy beneath (CABI 2012; Rieger 2006; Webster 2005a). Flowers are usually terminal on spurs (although they may grow laterally from one year old wood in some cultivars), borne in groups of 4-6, in inflorescences that have variously been described as corymbs, corymbose racemes, cymes, and false cymes (CABI 2012; Jackson 2003; Rieger 2006). Flowers are typically 3-4 cm in diameter, with 5 petals varying from white to deep pink, 5 sepals, about 20 stamens in 3 whorls (10 + 5 + 5) with yellow anthers, and a pistil comprised of five styles united at the base (Figure 1) (Flora of China editorial committee 1959+; Hancock et al. 2008; Jackson 2003). The pedicel and calyx are usually woolly, and the calyx is persistent in the fruit (Webster 2005a). Fruit is an ellipsoid to obovoid globe-like pome indented at the base and usually also at the apex; usually more than 5 cm in diameter and weighing 200-350 grams (Figure 1). It may vary in colour, from uniformly red, green, or yellow, or bi-coloured, such as striped or blushed red on a yellow or green background. Each fruit contains a cortex of (edible) flesh between the skin and the core line, and a central core of fleshy pith with a papery capsule of five fused carpels. Each carpel generally contains two seeds. Seeds are smooth, shiny, and chestnut brown (Jackson 2003; Rieger 2006).

M. domestica closely resembles four other Malus species present in Canada (see Section 5), but can be distinguished primarily by its fruit which are significantly larger (6-12 cm in diameter), and by its branches which do not usually have thorns (Gleason and Cronquist 1991) (note that M. domestica plants grown from seed may have thorns in their juvenile state). The native crabapple species M. coronaria (L.) Mill. and M. fusca (Raf.) C. K. Schneid. both have thorny branches; M. coronaria has glabrous leaves and hypanthium, pink flowers fading to white with pink or salmon-coloured anthers (as compared to M. domestica's white flowers tinged with pink, and yellow anthers), and small fruit (2-3 cm in diameter), while M. fusca has yellow to purplish-red fruits about 1 cm in diameter (Gleason and Cronquist 1991; Scoggan 1979). The introduced crabapple species M. baccata and M. prunifolia both have glabrous pedicels, leaves and hypanthium, and small, tart fruit. M. baccata has glabrous twigs, a deciduous calyx, and fruit about 1 cm in diameter, while M. prunifolia has pubescent twigs, a persistent calyx, and fruit about 2 cm in diameter (Gleason and Cronquist 1991).