Enhanced Maturation and Desiccation Tolerance of White Spruce [Picea glauca (Moench) Voss] Somatic Embryos: Effects of a Non-plasmolysing Water Stress and Abscisic Acid

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Abstract
A non-plasmolysing moisture stress effected by polyethylene glycol (PEG) was beneficial when applied to maturing white spruce (Picea glauca) somatic embryos for the following reasons. An osmotic treatment of 5.0–7.5% PEG stimulated a threefold increase in the maturation frequency. The osmotically treated somatic embryos displayed higher dry weights and lower moisture contents than the controls, indicating a greater accumulation of storage reserves. Moisture contents of mature, osmotically-treated, hydrated somatic embryos were 40–45%, in contrast to 57% for the non-osmotically treated controls. Desiccation was achieved by placing the somatic embryos in a range of relative-humidity environments. No clear trend for the effect of PEG on survival of desiccated somatic embryos was observed; mean survival values ranged from 34 to 62% when somatic embryos from all osmotic treatments were desiccated for 14 d at 81% relative humidity. Following this desiccation treatment, somatic embryos from all osmotic concentrations had moisture contents of 26–31%, similar to the 32% recorded for unimbibed zygotic embryos. After imbibition, moisture contents for these zygotic and somatic embryos were in the order of 60%. Somatic embryos matured with PEG remained quiescent during desiccation due to their low initial moisture contents, and gave rise to plantlets of normal appearance. Gradual desiccation of the somatic embryos directly following maturation with abscisic acid (ABA) was crucial to survival during desiccation. A plasmolysing water stress effected by sucrose at osmotic potentials similar to PEG was detrimental to somatic embryo maturation, thereby emphasizing the importance of the choice of osmoticum.

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