Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 154

The date palm (Phoenix dactylifera L.) is a member of the Arecaceae. The genus Phoenix is comprised of at least 12 species and occurs naturally from north-western Africa to South-east Asia (Barrow, 1998; Dransfield et al., 2008; Jaradat, 2015). Many of the species are sexually compatible, and this has led to difficulties in determining the taxonomy of the genus, especially since several species are present in close geographic proximity. The Phoenix genus forms a distinct tribe (Phoeniceae) within the Coryphoideae subfamily (Barrow, 1998; Dransfield et al., 2008). The date palm is the only Phoenix species that is an important fruit crop, but some species of the genus, including the date palm, are extensively used as ornamentals (Hodel, 2009; Broschat et al., 2014), e.g. the Canary Island date palm (P. canariensis) and the dwarf date palm (P. roebelenii).

Unlike most fruit crops, date palms are perennial monocotyledonous plants. Therefore, their biology resembles that of other monocots, like grasses, more than that of typical dicotyledonous trees. The date palm has a solitary trunk that can reach up to 30 m. All organs develop from a single apical meristem that is situated in the ‘palm heart’ at the base of the crown at the top of the stem (Zaid and De Wet, 2002b; Chao and Krueger, 2007; Cohen, 2017). The leaves are large, 3–5 m long and generated at the top of the trunk in the crown. Axillary buds give rise to offshoots (suckers) during the juvenile stage of development or to inflorescences in mature trees. The date palm is a dioecious species, having female and male inflorescences on separate trees (Zaid and De Wet, 2002b; Chao and Krueger, 2007). Inflorescences are large and carry hundreds to several thousand flowers. The date palm is wind pollinated. In commercial orchards, pollen is harvested from male inflorescences and female inflorescences are hand or mechanically pollinated. The date fruit is a drupe with a single seed. The fruit naturally dries on the tree. Fruits are consumed fresh, wet or dry. Dates accumulate extremely high levels of sugars. The fresh ripe fruit can have up to 50% sugars while the naturally dry fruit can have up to 80% sugars in its pulp.

The date palm, together with the coconut palm (Cocos nucifera) and the African oil palm (Elaeis guineensis), and to a lesser extent the peach palm (Bactris gasipaes) are major agronomic palm crops. The date palm is one of the most important fruit crops in the arid regions of the Middle East and North Africa, providing staple food, beverages, ornamentals and architectural materials. In the last several centuries, date cultivation has expanded to other arid regions of the world, i.e. South America, Mexico, USA, South Africa and Australia. Global annual production of dates is c.8.46 million t (FAOSTAT, 2016). The date is extremely important for agriculture and sustainability in desert oases. Traditional oasis agriculture has been enabled by the protective shading of the date palm, enabling more sensitive trees, cereals and vegetables to survive and be productive. The date palm is well adapted to the harsh conditions of desert oases. It can survive very high temperatures, short periods of drought (Zaid and De Wet, 2002c) and low quality, as well as relatively highly saline water (Tripler et al., 2007, 2012). The date palm has important cultural significance to the three main monotheistic religions, Islam, Christianity and Judaism, and its fruit and fronds are used in religious ceremonies.

Date cultivation is still performed in many countries using traditional methods. Modern agricultural practices were applied to palm orchards only in recent decades and are still restricted to specific countries. In traditional cultivation, pollen is collected by hand, female flowers are hand pollinated and fruit bunches are thinned and harvested manually (Zaid and De Wet, 2002b; Chao and Krueger, 2007). Therefore, each tree is climbed several times annually. In most countries, climbing palm trees is done without sophisticated equipment, sometimes with a securing belt or rope, or on simple ladders attached to the trunk. In several counties, special equipment enables mechanical lifting of the workers to the crowns of the palm trees (Cohen and Glasner, 2015; Krueger, 2015).

Date palm is one of the first fruit crops to have been domesticated, c.6000 years ago, and which occurred in the fertile crescent (Zohary and Spiegel-Roy, 1975; Zohary et al., 2012; Abbo et al., 2015). Over time, many date palm clones were locally selected and were propagated vegetatively from suckers and named as cultivars. Until the 20th century, each cultivar was known only in specific locations, and many were specific to distinct desert oases (Zaid and De Wet, 2002b; Chao and Krueger, 2007). Today, with the commercial application of in vitro propagation techniques, several elite cultivars are grown in various countries.

Date palm biodiversity and genetic resources have recently been reviewed (Al-Khayri et al., 2015a,b); date palm biotechnology has been described in Jain et al. (2011), and protocols on specific methodologies have been published (Al-Khayri et al., 2017a,b). The genomics era will enable rapid development of date palm improvement.