SEASONAL-VARIATIONS IN ABA EXCHANGE BETWEEN THE APICAL BUD AND THE UNDERLYING STEM OF BEECH - COMPARISON WITH NUTRIENTS

The amount of endogenous abscisic acid (ABA) in the apical bud and the distribution of label from [H-3]ABA, [H-3]sucrose and [H-3]threonine between the bud and the underlying stem of beech twigs (Fagus sylvatic a) has been evaluated at periodic dates during the year. These three m olecules, applied to the stem 6 cm under the bud base, were preferenti ally transported upwards in all experiments. It was assumed that the a ccumulation of [H-3]ABA depended predominently upon the sink capacitie s of the tissues (the membrane sink trapping ABA as a weak acid). In A pril the bursting bud contained little endogenous ABA, but strongly ac cumulated label from radioactive ABA, sucrose and threonine. From May to July endogenous ABA amounts rose in the bud, which was then an acti ve sink for labelled ABA and sucrose while containing only small quant ities of label from [H-3]threonine. The sink capacity of the bud for A BA decreased in late September and was low in winter. From spring to a utumn the stem tissues were accumulating the nutrients used for cell g rowth, and subsequently for reserves, from the transport pathway (meta bolic sink). Therefore, in contrast to ABA, the transport of nutrients towards the bud (especially amino acids) could be limited early in th e season. This situation would lead to correlative inhibition reducing the axial growth of the scaly bud, and could contribute to the induct ion of dormancy.