Effect of polyethylene glycol and mannitol on somatic embryogenesis of pigeonpea, Cajanus cajan (L.) Millsp.
A protocol for high frequency somatic embryogenesis (SE) of pigeonpea, Cajanus cajan (L.) Millsp. was worked out. The age of the source seedlings for explants and type of explants were found to influence the callusing response. Concentration of phytohormones and growth factors were optimized for developing embryogenic callus consisting of globular cells with dense cytoplasm. Embryo axis or callus derived from such explants, when exposed to dehydration stress imposed by polyethylene glycol and osmotic stress created by D-mannitol were found to produce high frequency somatic embryos. Embryo axis from 3-day old sprouts or 28-day old callus derived from the embryo axis explants incubated for 4 h either in 4% polyethylene glycol (w/v) or 0.7 M mannitol were found optimal for SE. After exposure to stress incubation, explants or callus were cultured on semisolid Murashige and Skoog (MS) + 2,4-dichlorophenoxy acetic acid (2,4-D) (5 μM) + glutamine (0.03 mM) and the cultures formed only globular and heart shaped somatic embryos. The early stage somatic embryos subsequently developed into torpedo and cotyledonary somatic embryos in MS liquid medium supplemented with 2,4-D (2 μM), abscisic acid (3 μM) and glutamine (0.03 mM). The somatic embryos were converted into plantlets in hormone-free half-strength MS semisolid medium and subsequently established in garden soil.
Key words: Abscisic acid, glutamine, osmotic stress, polyamines, water stress.