Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 93

Regeneration of shoots from existing meristems of Annona spp. can be achieved after explant browning (Rasai et al., 1994) and contamination (Tazzari et al., 1990) are controlled, depending on genotype. Annona spp. retain most of their morphogenic potential during the juvenile stage; however, rooting capacity disappears almost completely following phase change, although differences do occur among species and genotypes. Difficulty of rooting is typical of horticulturally interesting Annona selections, which are at the adult stage when selection has been made.

Nair et al. (1984a), working with atemoya, developed a protocol using nodal explants from juvenile trees. Multiple shoot initiation (six to seven axillary shoots per culture) was obtained using semi-solid Murashige and Skoog (1962) (MS) medium supplemented with 8.9 μM benzyladenine (BA); however, shoots were miniaturized, and it was necessary to elongate them in a medium with low cytokinin content before rooting was feasible. Rooted plantlets were transplanted to soil and were acclimatized, with an 80% survival rate. Rasai et al. (1993) developed a system to stimulate partial autotrophy in atemoya seedlings grown in vitro, using high irradiance, high relative humidity and low sucrose concentration in the culture medium. Tazzari et al. (1990) worked with juvenile and mature nodal sections of A. cherimola but were able to establish only juvenile explants in vitro. Encina et al. (1994) developed a protocol for micropropagation from nodal explants of juvenile A. cherimola ‘Fino de Jete’. After sprouting and multiplication of axillary shoots on MS basal medium supplemented with either 0.7 μM BA or 1.4 μM zeatin, 95% rooting efficiency was achieved in three steps (Encina, 1992): (i) pretreatment of in vitro shoots for 7 days on semi-solid MS medium containing activated charcoal (0.1% w/v); (ii) root induction on semi-solid MS medium supplemented with 500 μM indolebutyric acid (IBA), 58.4 mM sucrose and 200 mg/l citric acid for 10 days (7 days in darkness followed by 3 days of continuous light); and (iii) elongation of roots in half-strength semi-solid MS medium. Following rooting, the acclimatization rate reached 100% (Encina et al., 2001a,b). Padilla (1997), using a modification of this protocol, succeeded in initiating shoot multiplication with adult explants from 18-year-old cherimoya ‘Fino de Jete’, while rooting and acclimatization were about 30% less efficient (Padilla and Encina, 2004).

Micropropagation is being adapted to other selected cherimoya cultivars and rootstocks. A new system for aseptic establishment of explants is under study and involves a modified micrografting protocol in cascade, which increases the sprouting and shoot development of all cultivars assayed (Padilla and Encina, 2011). Although photoautotrophy does not improve the efficiency of micropropagation (Encina et al., 1999a), inoculation of micropropagated plantlets with arbuscular mycorrhizal fungi (Glomus spp.) improves vegetative growth and development of plantlets in the glasshouse and substantially reduces acclimatization time in juvenile (Azcón-Aguilar et al., 1994a,b, 1996; Padilla and Encina, 2005) and mature plants (Padilla and Encina, 2005).

Lemos and Blake (1996a) reported the micropropagation of A. squamosa from axillary shoots from 3-year-old trees. Following culture of explants on semi-solid woody plant medium (WPM) (Lloyd and McCown, 1981) containing 0.5 mg/l silver thiosulfate and 9 μM BA, four shoot buds per explant elongated. Preconditioning of shoots for 2 weeks on WPM (Lloyd and McCown, 1981) with 10 g/l activated charcoal was required before rooting on WPM containing 43 μM naphthaleneacetic acid (NAA) or 39 μM IBA, and rooting percentages improved if sucrose was replaced with 47% galactose. Following acclimatization, 80% of plantlets survived transfer to soil.

Lemos and Blake (1996b) developed a micropropagation protocol for juvenile and adult A. muricata. Axillary shoots on nodal explants were multiplied on WPM containing 0.5 μM NAA and 8.9 μM BA. Shoots were preconditioned on WPM with 3% activated charcoal and rooted in liquid WPM containing 21.5 μM NAA and 1% galactose. Approximately 70% rooting occurred under these conditions and all rooted plantlets were acclimatized.