The Biology of Malus domestica Borkh. (Apple)
3. Geographical Distribution
3.1 Origin and history of introduction
M. domestica is thought to have originated in Central Asia where its primary ancestor, M. sieversii, isnative to the foothills between western China and the former Soviet Union (Figure 2) (Hancock et al. 2008; Harris et al. 2002; Velasco et al. 2010) . Apples are the main forest tree in this region and M. sieversii is widespread in the Tien Shan mountains at elevations of 1200-1800 m (Luby 2003; Velasco et al. 2010). M. sieversii is the only wild species that shares all the characteristics of M. domestica, in terms of fruit and tree morphology. Its fruits are highly variable and display the full range of colours, forms and tastes found in cultivated apples across the world. Humans have lived and practiced nomadic agriculture in this region for thousands of years, and it is thought that prior to deliberate domestication, there may have been a long period of opportunistic gathering of apples, and unintentional planting of trees via garbage disposal (Hancock et al. 2008; Luby 2003). Bears and other vertebrates may also have contributed to dispersal (Ignatov and Bodishevskaya 2011; Juniper et al. 1999).
Ancient trade routes that linked China to the Middle East and Europe are thought to have facilitated the repeated short- and long-distance dispersal of M. sieversii to the east and west from its area of origin in Central Asia (Harris et al. 2002; Velasco et al. 2010). Travellers on foot as well as camels and pack horses are thought to have used parts of the Old Silk Road as early as the Neolithic period (~9000 BC) and the route was well established by the Bronze Age (~2500 BC), providing untold opportunities for the dispersal of fruit and seeds, either intentionally or unintentionally (Hancock et al. 2008; Harris et al. 2002; Luby 2003) . As a result of this movement, hybrids could have occurred to the east with species native to China (e.g., M. baccata, M. mandshurica, and M. prunifolia) and to the west with European species (e.g., M. orientalis and M. sylvestris)(Hancock et al. 2008; Juniper et al. 1999; Luby 2003).
Archaeological and historical evidence indicates that apples were being collected in the wild during the Neolithic and Bronze ages at sites throughout Europe (Harris et al. 2002; Juniper et al. 1999), and cultivated as early as 3,000 BC in the near East (e.g., Turkey, Syria, Iraq) (Hancock et al. 2008). Writings from the Persian Empire (e.g., Iran and beyond) indicate that apples were widely cultivated there by 500 BC. When Alexander the Great conquered the Persians around 300 BC, the cultivation of fruits was introduced to the Greek world as well, and from there to the Romans (Hancock et al. 2008; Luby 2003). Historical evidence indicates that ancient Greeks were familiar with the art of grafting, and that Roman horticulturalists used budding, grafting and rootstock techniques. By the first century AD, several apple cultivars were recorded by the Roman writer Pliny (Juniper et al. 1999). The rise of the Roman Empire spread cultivation of the domestic apple north and west into Europe, where it supplanted and hybridized with the native crabapple, M. sylvestris. Over the next several centuries, apple cultivation was maintained through the rise and spread of Christianity and Islam, particularly in the abbey gardens of Europe and the orchards of Iberia (e.g., modern day Portugal and Spain) (Hancock et al. 2008; Luby 2003).
By the 1200s, cultivated apples were becoming increasingly popular throughout Europe, in the gardens of both royalty and commoners, and by the 1600s there were at least 120 cultivars of apple described (Luby 2003). In 1826, the Royal Horticultural Society of England recognized at least 1200 varieties, and these were starting to be recognized and classified based on their desirability for different end uses (e.g., cooking apples, dessert apples, cider apples). The late 1800s and early 1900s represent the period of greatest diversity for apple cultivation in Europe, with hundreds of locally popular cultivars being grown in thousands of small orchards, and the known list of cultivars probably exceeding 2500 (Hancock et al. 2008; Juniper et al. 1999; Luby 2003).
In the meantime, European colonists had introduced M. domestica to the Americas (1500-1600s), South Africa (1650s), Australia (1788), and New Zealand (1814). By the late 1800s, it had also been introduced to southern and eastern Asia, where it supplanted the Chinese soft apple, M. x asiatica Nakai, the primary cultivated apple in that region for over 2000 years (Hancock et al. 2008; Luby 2003).
In the Americas, M. domestica was first introduced by Spanish priests to missions in Chile and California in the 1500s. Spanish and Portuguese colonists continued to introduce apples to their settlements in the suitable temperate climate zones of Central and South America, and European settlers brought seeds to establish orchards in the eastern parts of the U.S. and Canada. By the 1620s, the first apple orchards were recorded in New England, and likewise during the 1600s, French colonists established orchards in Canada, along the St. Lawrence River valley and in the milder valleys of Nova Scotia and New Brunswick. As settlers moved westwards in North America, apple orchards became a requirement for homesteading, and by the late 1800s apples were grown on the west coast of North America as well (AAFC 2011; Hancock et al. 2008; Luby 2003) . Very hardy cultivars such as the Wealthy were developed in the late 1800s for cold areas of the US great plains (Luby and Fennell 2006). Eventually, a new group of new American cultivars was established, such as 'Jonathan', 'Wagener' and 'Golden Delicious', which were successful in the more extreme American climates and also subsequently did well in South Africa, Australia and the Mediterranean (Juniper et al. 1999).
By the early 1900s, the U.S. and Canada were the two largest apple-producing nations in the world. By the late 1900s, the former Soviet Union was also an important world producer, and by the turn of the (21st) century, China was the largest apple producer, with a large proportion of the crop being exported as concentrated juice. Today, world production of apples exceeds 70 million metric tons, with China, the U.S., India, Turkey, Poland, Italy, France and Iran being leading producers (FAO 2013; O'Rourke 2003). Major southern hemisphere production occurs in Brazil, Chile, Argentina, South Africa, New Zealand and Australia, much of it for export to northern hemisphere countries during their spring and summer (FAO 2013; Hancock et al. 2008; Luby 2003; O'Rourke 2003) . Although there are 6000 regionally important cultivars and land races recognized across the world, global trade is dominated by just a few cultivars; primarily 'Delicious', 'Golden Delicious', 'McIntosh' and 'Jonagold' from North America, 'Braeburn' and 'Gala' from New Zealand, 'Granny Smith' from Australia and 'Fuji' from Japan (Hancock et al. 2008; Luby 2003).
3.2 Native range
The native range of M. domestica is difficult to determine, as the species is a product of domestication and multiple hybridizations across the world over thousands of years. Its primary ancestor isnative to the foothills between western China and the former Soviet Union (Hancock et al. 2008; Velasco et al. 2010), and this is suggested by some authors to be its centre of origin (e.g., see discussion in Harris et al. 2002; Robinson et al. 2001).
Asia Possibly native to Kazakhstan, Kyrgystan, Tajikistan, Turkmenistan, Uzbekistan (Harris et al. 2002; Robinson et al. 2001).
3.3 Introduced range
Cultivated in Afghanistan, Armenia, Azerbaijan, Bhutan, China, India, Indonesia, Iraq, Iran, Israel, Japan, Jordan, Kazakhstan, Korea (Republic of), Kyrgyzstan, Lebanon, Myanmar, Nepal, Pakistan, Philippines, Syria, Tajikistan, Thailand, Turkey, Turkmenistan, Uzbekistan, Vietnam, Yemen (CABI 2012; Flora of China editorial committee 1959+; Hancock et al. 2008) . Production in tropical Asia is limited to high altitudes; in India this includes the temperate North Western Hills region and to a lesser extent the North Eastern Hills region (Papademetriou and Herath 1999).
Cultivated in Algeria, Egypt, Kenya, Madagascar, Morocco, Reunion, South Africa, Tunisia, Zimbabwe (CABI 2012).
Cultivated in Canada, Mexico, United States (CABI 2012). Also naturalized in Canada and the United States (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; USDA-NRCS 2012).
/ Cultivated in Grenada, Guatemala, Honduras (CABI 2012).
Cultivated in Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Paraguay, Peru, Uruguay (CABI 2012).
Cultivated in Albania, Austria, Belarus, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Moldova, Netherlands, Norway, Poland, Portugal, Romania, Russian Federation, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Ukraine, United Kingdom (CABI 2012; Tutin et al. 1968). Reported throughout Europe as "often escaping and occasionally naturalized" (Tutin et al. 1968).
Cultivated in Australia, New Zealand (CABI 2012; Mabberley et al. 2001). Also naturalized in Australia (Australian National Botanic Gardens 2012) and present as a casual exotic in New Zealand (Landcare Research 1996-2012).
3.4 Potential range in North America
M. domestica is cultivated throughout temperate areas of the world, including North America (Mexico, U.S. and Canada). In the U.S., apples are grown in every continental state and commercial production occurs in 35 states, with top producers including Washington, New York, Michigan, Pennsylvania, California and Virginia (Rieger 2006; U.S. Apple Association 2012); M. domestica is also reported as naturalized in 41 states (Kartesz 2011; USDA-NRCS 2012). In Canada, M. domestica is cultivated in all provinces but not the northern territories, with major production in British Columbia (19%; 3794 ha), Ontario (37%; 7541 ha), Quebec (31%; 6232 ha), New Brunswick (1%; 255 ha) and Nova Scotia (11%; 2226 ha) (AAFC 2011). It is also reportedly naturalized in British Columbia, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979).
Cold hardiness and length of growing season requirements vary significantly between different cultivars of M. domestica, although as a rule of thumb most varieties do best in USDA Plant Hardiness Zones 4-7 (e.g., 'Delicious'; 'Duchess'; 'Gala'; 'McIntosh'; 'Enterprise'; 'Macoun'; 'Wolf River') (Hampson and Kemp 2003; Jackson 2003; Orange Pippin Ltd. 2012; Webster 2005b; Westwood 1993) . Some varieties recommended for use in North America also do well in USDA Plant Hardiness Zone 3, including: 'Honeycrisp', 'Spartan', 'Sunrise', 'Sweet Sixteen' (BCMA 2006; Orange Pippin Ltd. 2012). USDA Plant Hardiness Zones 3-7, which spans from -40°C to -12.3°C average annual minimum temperature, represent about one-third of Canada south of 60° latitude (see map at: http://planthardiness.ars.usda.gov/PHZMWeb/Images/northamerica.jpg).
In general, M. domestica is considered best adapted to the cool-temperate zone between about 35-50° latitude, in areas with high light intensity, warm days, and cool nights (Rieger 2006; Webster 2005b). It has a more northern range than many other fruit crops due to its relatively late blooming and cold hardiness (Rieger 2006). It is also grown to a lesser extent in semi-arid, subtropical and tropical areas, where irrigation, altitude, and various cultural strategies are used to overcome climatic limitations (Hampson and Kemp 2003; Westwood 1993). An example of the range of temperatures over which apples are successfully produced is provided by Jackson (2003). At the extremes, the sites presented include Poland, with winter monthly minimum temperatures of -17°C and summer monthly maximum temperatures of 30°C, and Egypt with winter minimums of 1°C and summer maximums of 43°C (Jackson 2003).
The primary climatic constraints for M. domestica include inadequate winter chilling in warmer climates, as well as summer heat stress and fruit sunburn (i.e., mild winters and hot summers), and winter freeze damage in more northerly regions (i.e., cold winters) (Jackson 2003). M. domestica requires a winter chilling period (about 1000-1600 hours at <7°C) to break dormancy, so winters cannot be too mild. If the dormancy requirement is not met, budbreak is sporadic and light, and cropping is poor. High summer temperatures (e.g., 40°C for more than a few days) can also cause tree stress, as they interfere with the ability to take up and transpire water quickly enough to cool the leaves, and can cause reduced photosynthesis, reduced fruit size and colour, and sunburned fruits (Webster 2005b). Conversely, M. domestica can also be damaged by frost, and if frost follows warm temperatures budbreak is accelerated and often premature (Rieger 2006). The level of damage caused by low temperatures is dependent on the stage of dormancy of the trees; trees are most hardy in the middle of winter and more vulnerable in the late fall and early spring. Frost-free springs are particularly important (Webster 2005b). Apple trees begin to harden in the fall from the outer shoots down the trunk, so that buds and shoots are less sensitive to frost injury than roots. Wood and buds of apple trees may be hardy to -40°C but rootstocks can only survive to about -18 °C and open flowers and young fruit can be killed by brief exposure to -2°C or colder (Palmer et al. 2003; Rieger 2006). Frost injury to flowers can reduce fruit yield by as much as 90% (AAFC 2011; Solymar 2004).
Climatic tolerance varies significantly among cultivars of M. domestica. For example, 'McIntosh' and 'Antonkova' are considered outstanding for winter hardiness and can withstand the very low winter temperatures of continental climates (e.g., in Canada). By contrast, large-scale production in countries such as Egypt is based on varieties 'Anna' and 'Dorset Golden', which have been selectively bred for warmer climates and require less winter chilling to achieve budbreak (e.g., 500 hours at 7°C) (Rieger 2006).
Another important factor related to climate suitability is length of growing season, and this also varies greatly between cultivars of M. domestica. On average, apples reach maturity about 120-150 days after flowering, but some cultivars may mature in as little as 70 days and others may require as long as 180 days (Rieger 2006). Time to maturity varies with temperature (e.g., warmer temperatures reduce time to maturity) so that it varies for a given variety from place to place; however, rankings of "early" or "late" varieties relative to each other are fairly consistent (Jackson 2003). Short-season cultivars tend to have a wide climatic tolerance; they do well in colder, northerly apple-producing regions such as Canada (e.g., 'McIntosh') and may also be grown as early-season crops in countries like New Zealand and France (e.g., 'Cox') (Jackson 2003). Long -season cultivars like 'Braeburn', 'Fuji', 'Cripp's Pink', and 'Granny Smith' cannot generally be grown successfully in northern areas, and do best in the milder climates of the southern hemisphere (Hampson and Kemp 2003; Jackson 2003).
M. domestica responds positively to sunlight, and increased solar radiation generally results in increased light penetration into the tree canopy, and increased potential for photosynthesis. Cloud cover associated with rainfall is noted as a negative factor reducing the availability of solar radiation in many fruit-growing areas (Jackson 2003). Jackson (2003) provides the range of solar radiation during a 5-month growing season for five different apple-growing areas, with Wilhelminadorp, the Netherlands at 2.50 GJ/m and Davis, California with 4.13 GJ/m at the extremes. Rieger (2006) notes that the red skin colour of the fruit of many varieties is stimulated by sunlight; however overexposure of the fruit to sun can cause sunscald.
M. domestica is relatively drought intolerant, and not suited for cultivation in very arid areas unless abundant supplies of fresh water are available for irrigation (Webster 2005b). In Canada, summer droughts can negatively affect tree health and fruit production, and excessive direct sunlight and a thin ozone layer can also lead to fruit scorch, a phenomenon most often recorded in British Columbia, but also occasionally in Ontario (AAFC 2011). Conversely, high humidity and rainfall are also problematic, as the moisture can promote infections and diseases (e.g., apple scab, fire blight), and heavy rainfall can negatively affect soil structure and aeration. In general, M. domestica requires about 50-60 cm of rainfall or irrigation distributed throughout the growing season, and trees grown in drier climates experience less disease pressure (Rieger 2006; Webster 2005b).
M. domestica can be grown in a variety of soil types but performs optimally in deep well-drained, loamy soils with a pH of 6-7 and high organic matter (AAFC 2011; Rieger 2006; Solymar 2004). Sandy soils produce less vigorous growth and require more irrigation, as they have less organic matter and are prone to leaching. Clay soils are higher in organic matter and produce more vigorous growth, but provide poor drainage and are less suitable for root growth (AAFC 2011). Like most fruit crops, M. domestica is largely intolerant of poorly drained soils (Rieger 2006). Orchards are often located on hillsides, allowing cool air to flow down the slope, and avoiding spring frosts and waterlogged soil conditions (Webster 2005b). Ideally, the slope should be graded between 4-8 % and face south, for greatest exposure to the sun (AAFC 2011; Webster 2005b).
Outside of cultivation, M. domestica has naturalized in many parts of the world, as in North America, where it grows in abandoned pastures, clearings, roadsides, and borders of woods (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; Stover and Marks 1998; USDA-NRCS 2012).
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