Seasonal Changes in the Levels of Free and Bound Abscisic Acid in Blackcurrant (Ribes nigrum) Buds and Beech (Fagus sylvatica) Buds

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Abstract
Free and bound abscisic acid (ABA) levels in blackcurrant and beech buds have been determined by an extraction procedure involving several thin-layer chromatography purification stages and final bioassay in the wheat coleoptile straight-growth test. In both species the highest level of free ABA occurs in the autumn at about the time of onset of winter dormancy. The free ABA content then declines throughout the winter months reaching its lowest value just before bud burst. These results strengthen the view that free ABA plays an important role in the induction and maintenance of winter dormancy. The bound ABA level increases in both species throughout the autumn and winter until halfway through the bud swelling phase and then declines. It is proposed that the bound ABA accumulates in the buds because it cannot be translocated or further metabolized during the winter months. The ratios of free to bound ABA were plotted and the resulting curves show a distinctive annual cycle with the highest free ABA/ bound ABA ratio (F/B) occurring in the autumn and the highest bound ABA/ free ABA ratio (B/F) coinciding with bud burst. A gradual increase in the B/F ratio during bud swelling may indicate a feed-back reaction between free and bound ABA which maintains the free ABA at a sufficiently high level to put a gentle brake on growth. At bud burst the B/F ratio falls very sharply and a period of very rapid growth ensues.

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