Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 39

Immature zygotic embryos, nucellar tissue and cotyledonary segments explants have been utilized as explants for inducing embryogenic cultures of cashew. Jha (1988) reported the induction of embryo-like structures from immature embryos excised from 1-month-old tender cashew nuts. Culture induction and production of globular protuberances was light independent; subsequent greening and growth required light and reduction of auxins in the medium. The atypical somatic embryo-like structures which had many abnormalities were termed ‘neomorphs’. Hegde et al. (1994) induced embryogenic cultures, although the somatic embryos could not be converted into plants. Embryogenic cultures were induced from nucellus by Ananthakrishnan et al. (1999). Immature seeds are surface sterilized, dissected longitudinally and placed on medium. The induction medium contained 60 g/l sucrose, 400 mg/l glutamine, 20% (v/v) coconut water and 6.78 μM 2,4-dichlorophenoxyacetic acid (2,4-D). The use of 100 mg/l ascorbic acid as an antioxidant is critical to reduce the browning during induction. The addition of 100 mg/l casein hydrolysate to medium is also beneficial. Cardoza and D’Souza (2002b) reported a simpler protocol for induction of embryogenic cultures from nucellar tissue (Fig. 2.1.2a). Medium consisting of MS with 2.07 μM picloram with 2% sucrose was used to induce embryogenic cultures. Subculture onto media of the same composition with 1 mg/l putrescine resulted in formation of globular embryos. Heart-shaped and cotyledonary-stage embryos developed on medium with 3% sucrose and 1.89 μM abscisic acid. Mature somatic embryos germinated on medium without plant growth regulators c.3 days after subculture. Fused somatic embryos (Fig. 2.1.2b) germinated (Fig. 2.1.2d). Cardoza and D’Souza (2002b) reported induction of embryogenic cultures, development and germination of somatic embryos from nucellar tissues and directly from an immature zygotic embryo (Fig. 2.1.2c). Somatic embryos were induced directly from immature zygotic embryo radicles after 5 weeks on MS medium containing 5 μM 2,4-D, 3 μM GA₃, 5 μM BA, 3% (w/v) sucrose and 0.5% (w/v) activated charcoal (Nadgauda and Gogate, 2005) and on medium with 2.07 μM picloram (Cardoza and D’Souza, 2000). Development of somatic embryos occurred at a low rate and the germination rate was low and attributed to poor development of cotyledons. Shirly and Thimmappaiah (2005) induced embryogenic cultures on Raj Bhansali (1990) medium (RB) with kinetin, spermidine and 2,4-D. Maturation was obtained on half strength MS medium with 20 μM abscisic acid, and germination occurred on RB medium with NAA, GA₃, glucose and amino acid supplements. Somatic embryos were recovered from mature cotyledonary explants on MS medium with 16.11 μM NAA and 2.22 μM BA (Sy et al., 1991).

Fig. 2.1.2. (a) Nucellar tissue on MS medium initially generated embryogenic culture and somatic proembryos. (b) Fused somatic embryos from nucellar culture. (c) Direct somatic embryogenesis from an immature zygotic embryo. (d) Precocious somatic embryo germination resulting in formation of pale green leafy structures.