| Abstract | The availability of ABA and tissue content of (+)-ABA were investigated, when using different starting concentrations of (±)-ABA to stimulate white spruce somatic embryo maturation. ELISA analysis of tissue (+)-ABA in white spruce somatic embryos during maturation showed that tissue content of (+)-ABA rose proportionally to the concentration of fed ABA (between 12 and 60μmol·L⁻¹ (±)-ABA). Irrespective of the starting concentration of fed (±)-ABA, peak values in tissue (+)-ABA were obtained after 3–7d. Following this there was a steady decline from approximately 12 to 28 d, after which tissue (+)-ABA remained at relatively constant low levels. The levels of tissue (+)-ABA under each starting concentration of (±)-ABA were very similar to one another by 28 d. There was no significant difference in tissue (+)-ABA content between embryos and embryo-free tissue fractions. Metabolism of fed (±)-ABA followed a similar pattern to (+)-ABA content, being proportional to the initial concentration. Metabolites of (+)-ABA were present in the medium in the first 6d, metabolism of (+)-ABA is likely to have depleted it from the medium between 12 to 28 d, irrespective of the initial concentration of (±)-ABA in the medium. These data indicate that the high levels of exogenous (±)-ABA that are needed to stimulate white spruce somatic embryo maturation, most likely have their major stimulatory effect on maturation during the first 12 d of the 6 to 8 wk culture period, there being no substantive difference among the concentrations in the period of (+)-ABA availability. To improve the yield of mature somatic embryos it will either be necessary to increase the abundance of responsive immature somatic embryos during this 12 d period, or to extend the period of availability of (+)-ABA to the embryo culture through periodic addition of (±)-ABA. |
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