Gradual long-term water stress results in abscisic acid accumulation in the guard-cell symplast and guard-cell apoplast of intact Vicia faba L. plants

As signals that integrate plant water status, abscisic acid (ABA) made within leaves and that imported from roots contribute to stomatal control. Guard cells exhibit multifaceted responses that are initiated through ABA's interaction with outward-facing plasmamembrane receptors or internal receptors. These responses include short-term regulation of ion transport that alters stomatal conductance as well as long-term regulation of gene expression. These molecular effects have been studied intensively, but the in planta guard-cell ABA contents must also be known if regulation of gas exchange by ABA is to be understood. Water was withheld from Vicia faba L. plants for up to 6 days. Over this period, psi(Leaf) dropped from -0.33 +/- 0.03 to -0.87 +/- 0.04 MPa and leaf conductance fell from 0.14 +/- 0.01 to 0.02 +/- 0.00 mol(.)m-2(.)s-1. Concomitantly, leaf ABA concentration increased from 298 +/- 43 to 6317 +/- 500 nmol(.)kg-1 (fresh mass) and leaf-apoplast ABA concentration increased from 273 +/- 37 to 4609 +/- 333 nM. The main purpose of this study was to determine the ABA content in guard cells dissected from surrounding tissue. Initially, the ABA content was 3.2 +/- 0.1 fg guard-cell pair-1, virtually all of which was symplastic. Six days after withholding of water from the plants, the guard-cell symplast ABA content increased to 10.0 +/- 1.1 fg ABA(.)guard-cell pair-1, whereas the guard-cell apoplast ABA content increased by more than 10 fold to 1.5 +/- 0.7 fg ABA(.)guard-cell pair-1 (approximately 1.5 micromolar). These results contrasted with ABA compartmentation in guard cells dissected from plants that were water stressed by a different protocol. The relationship between guard-cell ABA pools and other ABA pools and the importance of compartmentation of ABA by guard cells are discussed.