Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 164

Industrialization, urbanization and the collapse of traditional oases agriculture have forced date palm growers to cultivate only a small number of elite cultivars (Krueger, 2011; Jaradat, 2015; Abul-Soad et al., 2017). To prevent genetic erosion, it is important to conserve the diversity of date palm cultivars. The date palm is a large tree, and as a monocotyledonous species, it cannot be pruned; therefore, its conservation has depended on field plantings. Offshoots, needed for traditional vegetative propagation, develop only from juvenile trees. Therefore, replanting of collections is necessary at 15–20 year intervals. As a result, there are only a few date palm germplasm collections, and most of them have limited variation. Consequently, in vitro conservation and cryopreservation are an important alternative (Bekheet et al., 2002; Bekheet, 2011).

Short- and medium-term storage can be achieved under minimal growth conditions (Bekheet, 2011; El-Dawayati, 2017; Bekheet, 2017a). Incubation of shoot tips, embryogenic cultures or clusters of somatic embryos at temperatures of 15°C provides optimal conditions for conservation and enables efficient recovery and regeneration. Sucrose provided to the media at concentrations of 9–12% functions as both a carbon/energy source and an osmotic agent. The use of abscisic acid provides an adaptive response of the tissue to the stress conditions. Due to the minimal growth conditions, subculturing can be extended for several months or years.

Synchronization of embryonic suspension cultures using 50–100 μM abscisic acid and 0–15% polyethylene glycol (Al-Khayri and Al-Bahrany, 2012) has enabled the recovery of synthetic seeds (Alwael et al., 2017), and encapsulation using 3% sodium alginate (Bekheet et al., 2005; Bekheet, 2011, 2017b) for storage following cryopreservation, and efficient exchange of germplasm (Bekheet, 2011, 2017b).

Protocols were developed for cryopreservation of proembryonic masses (Salma and Engelmann, 2017), shoot tips and embryogenic cultures (Bekheet, 2017a) and caulogenic meristems (Fki et al., 2017). These protocols involve preculturing with osmotic agents, such as high concentration of sucrose, dehydration treatments and cold-hardening treatments. These protocols enable efficient conservation of date palm germplasm, as well as international exchange of germplasm. Since cryopreservation is cost-effective, commercial laboratories can perform tests for clonal fidelity before moving to large-scale propagation from cryopreserved cultures (Fki et al., 2017).