Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 155

Date palm can be propagated by seeds or by offshoots. However, the offspring generated through seeds are not true to type. The resulting progenies consist of 50% male and 50% female trees. Moreover, due to the high heterozygosity of this species, the female trees will be different and most of them will be of lower quality than their maternal parent. Propagation of female trees is therefore almost entirely performed clonally from offshoots. In recent years, micropropagation of female trees of superior cultivars has also been extensively used. However, in the Kutch region of Gujarat in India, in Peru, Mexico and other countries, large seedling populations of date palms form economically important orchards (Johnson et al., 2013). Most of these trees yield fruits of inferior quality that are only consumed locally. Male trees are usually not conserved as clones, and many male trees are seed generated (Zaid and De Wet, 2002a,b).

Most date palm cultivation still uses traditional cultivars generated from centuries of selection by farmers. It is estimated that there are at least 3000 different cultivars (Johnson, 2011; Jaradat, 2015), and most of these were selected in isolated oases throughout the Middle East and North Africa. Several constraints have limited the use of breeding for date palm cultivar improvement. The inheritance of important horticultural traits, including disease and pest tolerance, is still unknown because of the lack of segregating populations and the difficulties of performing crosses and backcrosses (El Hadrami et al., 2011a). The life cycle of the dates is long with at least 5–7 years from planting until first flowering and fruit setting. Moreover, several years are required to propagate each genotype by offshoots or in vitro. Dioecy restricts the use of backcrossing and results in 50% of seedlings being male plants. Tools for early identification of the sex of individual trees were only recently developed (Cherif et al., 2013; Dakhlaoui-Dkhil et al., 2013).

Date palm is a diploid species. The haploid genome of the date palm is c.670 Mb and is comprised of 18 chromosomes (Al-Dous et al., 2011; Al-Mssallem et al., 2013; Mathew et al., 2014). Date palm improvement is still by conventional breeding. There have been only a few date breeding programmes (El Hadrami and El Hadrami, 2009; El Hadrami et al., 2011a). Date palm breeding has several challenges and aims: enhancement of fruit quality, yield and tolerance of biotic and abiotic stresses. Two major efforts were in the USA (Nixon and Furr, 1965; Carpenter, 1979; Krueger, 2015). The goals of these projects were selections that were tolerant of cold or extreme heat, high relative humidity and rain damage, suitable for mechanical harvesting, improved fruit quality, and reduction of the tree’s vertical growth rate. The breeders also attempted to detect superior males with metaxenic characters (male effect on resulting maternal tissues of the fruit) that could be used to manipulate fruit production. By performing repeated backcrosses involving male trees with elite female cultivars, they also attempted to develop elite female inbred lines sufficiently uniform and homozygous to enable their propagation by seeds (Krueger, 2015). Some of the backcrosses were repeated up to five times; however, none of the backcrossed females became commercial cultivars. The backcrossed male trees were useful in the initial identification of the sex-determining region of the date palm genome (Al-Dous et al., 2011). Although some cultivars were selected in these breeding programmes and by growers, none of them became important in the USA or elsewhere.

The other major breeding project has been in Morocco, where the primary aim is combining good fruit quality and resistance to Bayoud disease, a soilborne disease caused by Fusarium oxysporum f. sp. albedinis (Zaid et al., 2002; El Modafar, 2010). The disease has caused the loss of >10 million date palms in Morocco during the 20th century, resulting in degradation of oases and desertification. This programme has screened >100,000 seedlings, and at least 12 resistant/tolerant cultivars with high quality fruits were identified. These were micropropagated, and some are being distributed to Moroccan farmers (Sedra, 2011b, 2015; Mazri et al., 2017).

Another breeding effort is to generate high quality trees with shorter stature by crossing elite female cultivars with males of Phoenix pusilla, a dwarf species. Following embryo rescue, multiplication, plant production and acclimatization, several trees are being screened for their vegetative and reproductive traits (Sudhersan and Al-Shayji, 2011).