Synthesis and movement of abscisic Acid in water-stressed cotton leaves.

Synthesis and movement of abscisic acid (ABA) into the apoplast of water-stressed cotton (Gossypium hirsutum L.) leaves were examined using pressure dehydration techniques. The exudates of leaves dehydrated in a pressure chamber contained ABA. The level of ABA in the exudates was insensitive to the leaf water potential when dehydration occurred over a 3-hour period. When leaves were rapidly dehydrated in the pressure chamber and held at a balance pressure coincident with the point of zero turgor, ABA accumulated in the leaf tissue and then in the apoplast, but only after 2 to 3 hours of zero turgor. Slow dehydration of leaves by equilibration over varying mannitol concentrations resulted in some accumulation of ABA prior to the point of zero turgor, but ABA accumulated in the tissue and apoplast most rapidly after the onset of zero turgor.When intact plants were allowed to dehydrate, stomata of leaves attached to the plant began closing as the leaf water potential decreased to -12 bars. Leaves began accumulating ABA at about -14 bars, but accumulation of ABA in the apoplast did not occur until the leaf water potential reached -16 bars. Although the apoplastic fraction of ABA should represent an "active" pool of ABA readily accessible to the guard cells, the data suggest that there may be stomatal closure prior to accumulation of ABA in the apoplast. A role for the small amounts of apoplastic ABA not dependent upon water stress is proposed for this initial stressinduced stomatal response.