Abscisic acid (ABA) is a plant hormone that plays a key role in mediating plant responses to stress. It induces stomatal closure to reduce water loss during drought, inhibits cell growth to induce seed and bud dormancy over winter, and promotes the development of protective bud scales. ABA is produced in response to stresses like drought, cold temperatures, and soil compaction and translocates to leaves and buds to trigger adaptive responses that allow plants to survive adverse conditions.

1. 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.

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.

4.  In preparation for winter, ABA is produced in
terminal buds.This slows 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.

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.

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.

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.

9.  ABA inhibits the growth of seedlings.

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.

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.

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