Abscisic acid immunolocalization and ultrastructural changes in water-stressed lavender (Lavandula stoechas L.) plants

The relationship between the bulk abscisic acid (ABA) content, ABA compartmental redistribution, and chloroplast ultrastructural changes was studied in leaves of lavender (Lavandula stoechas L.) plants subjected to water stress. ABA was uniformly distributed in the cytosol, nucleus, chloroplasts, and cell walls of mesphyll cells in well-watered plants. In plants subjected to water stress -2.6 MPa water potential) the bulk leaf ABA increased from 900 to 3600 pmol g fresh weight. At the ultrastructural level, the first indication of this rise in ABA was a 4-fold increase in ABA immunolabeling in the cell wall in which the highest labeling values were recorded. This increase in apoplastic ABA in lavender was not attributable to ABA release from the chloroplast, because a simultaneous increase in ABA labeling was observed in both the chloroplast and nucleus (2- and 3-fold, respectively). Water stress induced a progressive increase in bulk leaf ABA concentration to 13600 pmol g fresh weight coincident, with the highest immunolabeling of ABA in the nucleus and chloroplast. Under severe water stress, the chloroplast membrane broke down, resulting in leakage of ABA from the chloroplast. The stress-induced increase of ABA in chloroplasts and nuclei may serve a function other than affecting stomatal movement.