Somatic embryos of Picea abies behave like isolated zygotic embryos in vitro but with greatly reduced physiological vigour

Use of somatic or synthetic seed is of potential interest as a micropropagation tool for superior conifer genotypes. However, whether naked or encapsulated, the most important criterion is that the asexual embryo or somatic seed must be able to simulate the zygotic embryo or true seed. This study attempted a comparison of the somatic embryo of Norway spruce (Picea abies) with its zygotic counterpart. A major finding was that the percentage germination of fresh or somatic embryos partially desiccated at relative humidities of 97% and 63% to moisture contents approaching those of the seed, was substantially lower. It further declined rapidly with either cold storage and/or encapsulation. This sensitivity to drying suggests that the somatic embryo/seed may behave either as an orthodox seed with limited ability to withstand desiccation or as a recalcitrant seed that cannot survive drying below a moisture content that is relatively high. As the megagametophyte contains > 80% of the seed’s total lipid and protein reserves, the carbon and amino acid pools available to the zygotic embryo far exceed those accessible to the somatic embryo. The conclusions are: (1) in Norway spruce the somatic seed, lacking a suitable artificial megagametophyte, does not simulate the true seed, and (2) the somatic embryo, if it behaves like an isolated zygotic embryo cultured in vitro, does so with greatly reduced physiological vigour.