Seed development in cotton: feasibility of a hormonal role for abscisic Acid in controlling vivipary

In maturing cotton (Gossypium hirsutum L.) fruits, embryos acquire the capacity to germinate in vitro about 16 days before fruit maturity and dehiscence. Vivipary is believed to be prevented by abscisic acid (ABA) originating in the seed coat and diffusing to the embryo (the Ihle-Dure hypothesis). Although endogenous ABA levels are much greater in embryos than in seed coats during the period of germinability, in «donor-receiver» experiments movement of (14)C-ABA is strongly polar in favor of the embryo. Compartmental efflux analysis showed that embryos contain 90% of their ABA in a vacuole-like compartment and an insignificant amount in a cytoplasm-like compartment. In contrast, seed coats have only 60% of their ABA in the «vacuole» and a much greater fraction than embryos in the «cytoplasm». As a result, efflux across the plasma membranes of seed coat cells is much faster than from embryo cells. Increasing external pH strongly inhibits ABA uptake by isolated seed coats and embryos, indicating a role of pH gradients in its partitioning (i.e. ABA tends to be transferred from acidic to alkaline compartments). Aqueous extracts of seed coats are much more acidic than those of embryos. This difference, presumably originating in the «vacuoles», can account for the different intracellular distributions of ABA in the two tissues and therefore can account for the polarity of ABA diffusion between tissues. The results implicate intracellular pH gradients in the control of ABA movement between seed coat and embryo. Demonstration of the feasibility of inward ABA movement, despite apparently unfavorable diffusion gradients, provides direct support for the Ihle-Dure hypothesis.