Development of white spruce (Picea glauca(Moench.) Voss) somatic embryos during culture with abscisic acid and osmoticum, and their tolerance to drying and frozen storage

Abstract

The limit of permeability of white spruce (Picea glauca [Moench.] Voss) somatic embryo cell walls to molecules was in the order of 30 Å. Polyethylene glycols (PEGs) and dextrans of molecular weights greater than 1000 and 6000, respectively, produced a nonpermeating (non-plasmolysing) water stress which improved embryo development. Somatic embryos converted to plantlets at frequencies of 76–84% following slow drying and storage at −20 °C for 1 year, which was similar to the 77% recorded for control somatic embryos slowly dried then germinated without freezing or storage. Culture for 7–8 weeks with medium containing abscisic acid, 3% sucrose, and 7.5% PEG 4000 yielded somatic embryos with five times the embryo storage lipid content recorded for zygotic embryos. During culture with PEG the moisture content of the somatic embryos decreased from 96% for immature suspension-cultured somatic embryos, to 47% for mature embryos. Somatic embryos cultured for 7–8 weeks survived rapid drying to 5% moisture content, and converted to plantlets at frequencies of 60–70%, but no somatic embryos survived rapid drying when cultured for only 4 weeks; however, slow drying did induce desiccation tolerance in 3-week cultured somatic embryos. Abscisic acid was important to maintain embryos in a developmental state, but ABA alone did not induce desiccation tolerance. In order to induce desiccation tolerance a water stress treatment was required. Tolerance of rapid drying coincided with moisture contents below 55%, which occurred after 5 weeks of culture in the presence of PEG 4000 and abscisic acid.