No abstract

Embryos from dormant and stratified Fraxinus americana seed were incubated with (S)-2-"C-abscisic acid (ABA) under a variety of conditions. Both dormant and stratified embryos rapidly metabolize abscisic acid to phaseic acid, dihydrophaseic acid, and an unidentified polar metabolite apparently derived from dihydrophaseic acid. Although the stratified embryos may have an increased capacity to metabolize abscisic acid, our calculations suggest that such an increased capacity would probably not be physiologically significant. Dormant intact seeds also metabolize (S)-2-VC-abscisic acid during stratification at 5 C or incubation at 25 C. The metabolites appear to be similar to those observed in excised embryos although by 12 days of stratification a fourth metabolite is observed. More than 90% of the "C-abscisic acid was metabolized after 26 days of stratification at 5 C or after 12 days of incubation at 25 C. Stratification at 5 C leads to the breaking of dormancy while incubation at 25 C does not.The plant growth regulator ABA prevents germination when applied to embryos of Fraxinus ornus, a nondormant ash species, and to F. americana embryos from seeds whose dormancy has been overcome by stratification (5). The concentration of ABA in the seeds and pericarp of F. americana has been shown to decrease during the course of stratification to approximately that found in the nondormant F. ornus (6). These latter results suggested the possibility of ABA metabolism during the stratification process. While ABA has been shown to be metabolized rapidly in several other plants including tomato and bean (4, 11), its metabolism in dormant tissues has not been reported. We were interested in the extent to which ABA is metabolized by dormant seeds, the identity of the metabolites and in the possibility that the stratification process might result in an increased capacity for ABA metabolism. In order to answer these 'Eva Galson, Eugene Tinelli, and Daniel Walton dedicate this paper to the memory of our colleague and friend, Ernest Sondheimer. We shall sadly miss his companionship, creativity, and infectious enthusiasm for research.questions about ABA metabolism, dormant and stratified excised F. americana embryos and endosperm were incubated with 2-"`C-ABA. 2-w"C-ABA metabolism was also followed in intact seeds during a stratification period at 5 C sufficient to break dormancy, and during an incubation period at 25 C which does not result in the breaking of domancy. All of this work was done with the naturally occurring (S) enantiomer of ABA, and in some cases with the (R) enantiomer also. Relatively large scale incubations of embryos with (RS)-2-uC-ABA were also done in order to obtain sufficient quantities of metabolites for identification by chemical and spectral methods.MATERIALS AND METHODS Short Term Incubations. F. americana seeds (1971 crop from Herbst Bros., Brewster, N.Y.) were depericarped by hand and then imbibed in tap water for 24 hr. After the embryos were excised, they were shaken for 24 hr in 25-ml round b...