The coat-enhanced dormancy mechanism of western white pine (Pinus monticola Dougl. ex D. Don) seeds is mediated by abscisic acid homeostasis and mechanical restraint

The dormancy mechanism of western white Pine (Pinus monticola Dougl. ex D. Don) seeds was determined to be coat-enhanced; sequential removal of tissues surrounding the embryo - the seed coat, nucellar membrane/cap and megagametophyte - showed that all of these contribute to dormancy maintenance. Transfer of seeds to germination conditions following moist chilling of the seed decreased the mechanical restraint of the enclosing seed tissues. Coincidentally, the embryo growth potential increased, as demonstrated by an increased mechanical strength of the embryo during moist chilling and germination. and its reduced sensitivity to highly negative osmotic potential. The seed coat and underlying structures also influence the ability of embryos to metabolize abscisic acid (ABA). Unlike the intact dormant-imbibed seed, which maintained high levels of ABA in both the embryo and megagametophyte. removal of structures surrounding tire embryo decreased ABA levels and promoted ABA metabolism and/or transport. ABA levels in the embryo and megagametophyte were inversely correlated with increases in the germination capacity of seed populations resulting from treatments to remove tissues surrounding the embryo (e.g. the seed coat and other tissues). Changes in ABA metabolites showed no clear relationship to the germination capacity of seed Populations. which may have been due to the further flux of phaseic acid, dihydrophaseic acid, and 7 '-hydroxy ABA to downstream metabolites and/or transport of ABA and metabolites. This Study provides insight into the dormancy mechanism of western white pine seeds, and underscores tire importance of the surrounding seed tissues in maintaining ABA homeostasis in the embryo and megagametophyte.