The plant hormone abscisic acid (ABA) mediates plant adaptation to stress by inducing stomatal closure to reduce water loss, inhibiting cell growth and seed germination to promote dormancy, and regulating bud formation and abscission. ABA is produced in response to stresses like drought, cold temperatures, and soil compaction and translocates from roots to leaves to trigger closing of stomata and reduce transpiration. It plays important roles in enforcing bud and seed dormancy, inhibiting seedling growth, and preventing premature sprouting or germination until conditions are suitable.

1. Plant hormone Abscisic acid

2. Induces stomatal closure, reducing transpiration to prevent water loss. Inhibits fruit ripening Responsible for seed dormancy by inhibiting cell growth – inhibits seed germination Inhibits the uptake of Kinetin Down regulates enzymes needed for photosynthesis. Effects

3. Unlike animals, plants cannot flee from potentially harmful conditions like drought the approach of winter They must adapt or die. The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress. Abscisic acid (ABA)

4. In preparation for winter, ABA is produced in terminal buds.Thisslows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season. ABA also inhibits the division of cells in the vascular cambium, adjusting to cold conditions in the winter by suspending primary and secondary growth. Function

5. Abscisic acid is also produced in the roots in response to decreased soil water potential and other situations in which the plant may be under stress. ABA then translocates to the leaves, where it rapidly alters the osmotic potential of stomatal guard cells, causing them to shrink and stomata to close. The ABA-induced stomatal closure reduces transpiration thus preventing further water loss from the leaves in times of low water availability.

6. ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted. Bud dormancy

7. ABA plays a role in seed maturation, at least in some species, and also enforces a period of seed dormancy. As we saw for buds, it is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted. 2. Seed maturation and dormancy

8. ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid. 3. Abscission

9. ABA inhibits the growth of seedlings. 4. Seedling Growth

10. ABA — moving up from the roots to the stem — synergizes with auxin — moving down from the apical meristem to the stem — in prevent the development of lateral buds. The result is inhibition of branching or apical dominance. 5. Apical Dominance

11. ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration. The mechanism: ABA binds to G-protein-coupled receptors at the surface of the plasma membrane of the guard cells as well as to other receptors in the cytosol. Receptor activation produces a rise in pH in the cytosol transfer of Ca2+ from the vacuole and endoplasmic reticulum to the cytosol These changes cause ion channels in the plasma membrane to open allowing the release of ions (Cl−, organic [e.g., malate2−], and K+) from the cell. The loss of these solutes from the cytosol reduces the osmotic pressure of the cell and thus turgor. The stomata close. 6. Closing of stomata

12. Released during desiccation of the vegetative tissues and when roots encounter soil compaction. Synthesized in green fruit and seeds at the beginning of the wintering period Mobile within the leaf and can be rapidly translocated from the roots to the leaves by the transpiration stream in the xylem. Produced in response to environmental stress, such as heat stress, water stress, salt stress. Synthesized in all plant parts, e.g. roots, flowers, leaves and stems Location and timing of ABA biosynthesis