Abscisic acid [(+)-ABA] content in white spruce somatic embryo tissues related to concentration of fed ABA

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 60 mu mol . 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 starring 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 Gd, 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 12d period, or to extend the period of availability of(+)-ABA to the embryo culture through periodic addition of (+/-)-ABA.