MATERNAL ABSCISIC-ACID TRANSPORT IN DEVELOPING EMBRYOS OF SUNFLOWER

When [2-C-14] abscisic acid (ABA) was applied to <<blocks>> excised from sunflower heads, containing achenes and leaving a layer of head tissue below the base of the achenes, its uptake by the embryo was about 2 times higher than when achenes were placed on the surface of the culture medium; this suggested that in the mother plant, preferential anatomical connections allowed maternal (exogenous) ABA to be transported to the embryo. This transport of ABA through the head tissue was possible even during late embryogenesis. Changes, with embryo age, in [2-C-14] ABA uptake by isolated embryos proved that the regulation of external ABA accumulation took place at the level of the embryo itself. During the first part of embryo development, an increasing intracellular pH of cotyledonary cells contributed to the increasing accumulation of unmetabolized ABA. Thereafter, an increasing catabolism played a major role in the decrease of ABA level. The importance of this catabolism and its particular orientation towards the formation of alkaline non-hydrolysable compounds presented the problem of the physiological significance of these conjugates.