Abscisic acid (ABA) previously called Dormin or Abscisin mainly because of their regulatory effect on abscission and dormancy. This hormone is widespread in higher plants and is found in many different organs and tissues (both old and young) of plants. ABA induces abscission of the leaves of a wide variety of plants and fruits of some plant species

1. Plant Growth Regulators
Growth retardants(ABSCISIC ACID)
Prof. Dr./ Galal Ismail Eliwa
Head Of Pomology Department
Faculty Of Agriculture
Damietta University
E. mail/ geliwa2002@du.edu.eg

2. Discovery of Abscisic acid (ABC)
Abscisic acid (ABA) was the last major hormone to be
identified, although its existence had been forecast
for some time.
Groups lead by Addicott studying hormonal
relationships in cotton (Ohkuma et al., 1963)
And Wareing studying dormancy in woody perennials
independently identified Abscisin II in cotton and
Dormin in woody plants (Corniforth et al., 1965).
The two compounds were ultimately determined to
be identical and were given the name Abscisic acid in
1967 at a plant hormone conference in Ottawa,
Canada.
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3. ABSCISIC ACID (ABC)
 ABA is one of the hormone plant. Also called as Abscisin
& Dormin because of their regulatory effect on
abscission and dormancy.
 Growth retardant (inhibits).
 Inhibition of bud growth and shoot formation.
 Promotes dormancy.
 Closes stomata.
 Produced in response to environmental stress, such as
heat stress, water stress, salt stress.
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(C15H20O4)

4. ABSCISIC ACID
• 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.
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5. ABSCISIC ACID Function
 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.
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6. ABSCISIC ACID Function
 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 translocate 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.
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7. ABSCISSIC ACID
• Sites:
 Plastids by Mevalonic acid pathway
(The Shikimic Acid Pathway)
Most tissues, especially leaves and seeds
• Transport:
 Xylem and phloem (greater amounts).
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8. ‫دامط‬ ‫تخهيك‬‫ااألبسسك‬Biosynthesis of ABA:
•‫تكىيه‬ ‫يتم‬ABA‫مه‬Mevalonate‫خالل‬ ‫مه‬
pyrophosphateFarnesylً‫وه‬‫في‬ ‫انىسيطت‬ ‫انمادة‬
‫تكىيه‬ ‫دورة‬‫االيزوبزيىىيذاث‬isoprenoids‫وهى‬‫وفس‬
‫مه‬ ‫يتم‬ ‫انمسارانذي‬‫خالنه‬‫انجبزيهيىاث‬ ‫تكىيه‬‫انتأثيزاث‬ ‫أن‬ ‫مع‬
‫وانكيمىديىيت‬ ‫انفسيىنىجيت‬‫مه‬ ‫نكم‬GA‫و‬ABAً‫ا‬‫عمىم‬
‫متعارظت‬.
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9. • ABA appears to be an internal factor inducing
dormancy in the buds of at least some temperature
zone woody plants.
•‫إوتاج‬ ‫ويتىلف‬GA & ABAً‫عه‬‫االظاءة‬ ‫فتزة‬ ‫طىل‬‫نها‬ ‫تتعزض‬ ً‫انت‬
‫تخهيك‬ ‫يتم‬ ‫ديث‬ ‫األوراق‬GA‫لصز‬ ‫مع‬ ‫بيىما‬ ‫االظاءة‬ ‫فتزاث‬ ‫طىل‬ ‫مع‬
‫يتكىن‬ ‫االظاءة‬ ‫فتزاث‬ABAً‫ف‬ ‫تذخم‬ ‫ديث‬ ‫انبزاعم‬ ً‫ف‬ ‫تزاكمه‬ ‫ويزداد‬
‫انسكىن‬ ‫طىر‬(‫انزادت‬ ‫طىر‬)‫األوراق‬ ‫متسالطت‬ ‫األشجار‬ ً‫ف‬.
•‫األوراق‬ ‫انمتسالطت‬ ‫األشجار‬ ‫اوراق‬ ‫إسماط‬ ً‫ان‬ ‫انمزارعيه‬ ‫يهجأ‬ ‫نذنك‬
(‫وانكمثزي‬ ‫انتفاح‬)‫وتفتخ‬ ‫انسكىن‬ ‫كسز‬ ً‫ف‬ ‫نهتبكيز‬ ‫وىفمبز‬ ‫شهز‬ ً‫ف‬
ً‫انتان‬ ‫انىمى‬ ‫مىسم‬ ً‫ف‬ ‫مبكزا‬ ‫انزاعم‬(.‫تفسيز‬Bud dormancy-ABA)
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10. Abscisic acid actions
• 1- ABA- drought resistance:
 Abscisic acid is the key internal signal that facilitates
drought resistance in plants.
 Under water stress conditions, ABA accumulates in
leaves and causes stomata to close rapidly, reducing
transpiration and preventing further water loss.
 ABA causes the opening of efflux K+ channels in guard
cell plasma membranes, leading to a huge loss of this
ion from the cytoplasm.
 The simultaneous osmotic loss of water leads to a
decrease in guard cell turgor, with consequent closure
of stomata.
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ABA in roots that elongate
the root for water absorption

12. Abscisic acid actions
• 2- ABA- Seed Dormancy:
 ABA plays a major role in seed dormancy
 During seed maturation, ABA levels increase
dramatically.
This inhibits germination and turns on the
production of proteins that enable the
embryo to survive dehydration during seed
maturation
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13. Abscisic acid actions
• 2- ABA- Seed Dormancy:
 As dormancy can only be broken by specific
environmental cues, it ensures that a seed will
germinate only under suitable conditions of
moisture, light and temperature
 The breaking of dormancy is associated with
a decline in the level of ABA.
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14. ABA prevents or delays the germination of seeds.
• (vivipary) ‫الثمزة‬ ‫داخل‬ ‫البذور‬ ‫إنبات‬ ‫ظاهزة‬-
•-‫تعني‬‫النبتة‬ ‫عن‬ ‫االنفصال‬ ‫قبل‬ ‫اإلنبات‬‫األم‬
-‫الظاهرة‬ ‫هذه‬ ‫لحدوث‬ ‫تؤدي‬ ‫أن‬ ‫لها‬ ‫يمكن‬ ‫التي‬ ‫األسباب‬ ‫ابرز‬:
1-‫النباتي‬ ‫النضج‬ ‫مرحلة‬ ‫تتعدى‬ ‫الثمرة‬ ‫ترك‬((over mature)‫الرتفاع‬ ‫يؤدي‬
‫المحيط‬ ‫الجيالتيني‬ ‫الغالف‬ ‫واذابه‬ ‫حموضتها‬‫بالبذور‬
2-‫األبسيسيك‬ ‫حمض‬ ‫الطبيعي‬ ‫الهرمون‬ ‫انخفاض‬((abscisic acid‫في‬‫الثمار‬
‫الرطبة‬ ‫البيئة‬ ‫داخل‬ ‫السكون‬ ‫كسر‬ ‫تستطيع‬ ‫البذور‬ ‫بالتالي‬ ، ‫الناضجة‬‫والنمو‬ ‫للثمرة‬
‫البذور‬ ‫تجفيف‬ ‫دون‬ ‫من‬ ‫كاشتال‬
3-‫إلنبات‬ ‫كمانعة‬ ‫تعمل‬ ‫والتي‬ ‫الثمرة‬ ‫ب‬ٌ‫ل‬ ‫في‬ ‫الموجودة‬ ‫الكيماوية‬ ‫المواد‬ ‫في‬ ‫خلل‬ ‫حدوث‬
‫الثمار‬ ‫داخل‬ ‫البذور‬(‫الكومارين‬ ‫مثل‬ ‫المشبعة‬ ‫غير‬ ‫الالكتونات‬ ‫مجموعة‬–
(Coumarin )،‫إزالة‬ ‫عند‬ ‫تأثيرها‬ ‫يزول‬ ‫الكيماوية‬ ‫المواد‬ ‫هذه‬ ‫أن‬ ‫المعروف‬ ‫من‬
‫وغسلها‬ ‫الثمرة‬ ‫من‬ ‫البذور‬
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17. Abscisic acid actions
• 3- ABA- Bud dormancy:
 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.
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18. Abscisic acid actions
• 4- ABA- Abscission:
 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.
It promotes senescence through leaf abscission,
degeneration of excised leaves and acceleration
of decomposition of chlorophyll.
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19. Abscisic acid actions
• 5- ABA- chemical thinner:
• ABA has appeared on the horizon as a new and
potentially useful chemical thinner.
• It has been shown to be an effective thinner on
both apples and pears (Greene, 2007, 2009).
• The mode of action has not been defined but
undoubtedly, closing of stomata, thus
restricting carbohydrate supply will prove to be
a contributing factor.
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20. Abscisic acid actions
• 6- ABA- Freezing resistance:
 Elevated ABA levels are associated with
increased freezing resistance.
 ABA appears to mediate a plant’s response
to environmental stresses, such as freezing
by regulating gene expression.
 Certain genes are switched on by ABA while
others are switched off.
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21. Physiological effects of Abscisic acid
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1. Stomata
Closure
Water shortage brings about increase
in ABA level, leading to stomata
closure as a response to water stress.
2. Growth Inhibiters ABA inhibits shoot growth but
has less effect on root growth
3. GA
Counteracts
ABA counteracts the effect of
gibberellins on alpha amylase synthesis
in germinating seeds.
4. Induced
Dormancy
ABA affects induction or maintenance of
dormancy in seeds.

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