Plant Growth Regulators (PGRs) Continue

In this section, you will study three very high-yield PGRs:

1) Cytokinins → mainly cell division + delay senescence + lateral bud growth

2) Ethylene → fruit ripening + abscission/senescence + triple response (only gaseous hormone)

3) Abscisic Acid (ABA) → stress hormone + stomatal closure + dormancy (growth inhibitor)

✅ Key to Memorise: Cytokinin = division & youth | Ethylene = ripening & shedding | ABA = stress & sleep

3. Cytokinins

What are Cytokinins?

Cytokinins are plant hormones that promote cytokinesis (cell division) and are especially important where new cells / new organs are being formed.

Discovery

  • F. Skoog observed that something from autoclaved DNA promoted division in tobacco callus.
  • Miller later identified kinetin (synthetic cytokinin).
  • First natural cytokinin: zeatin (from maize / corn kernels, coconut milk also rich).

Key Words: Skoog–Miller, kinetin, zeatin

Site of synthesis

  • Mainly root apices (rapid cell division)
  • Also: developing shoot buds, young fruits, developing seeds

Transport

  • Mostly through xylem (roots → shoots)

Functions of Cytokinins

1.Cell division (Primary function)

  • Cytokinins are essential for tissue culture.
  • Usually work best with auxin.

2.Overcome apical dominance (Opposite of auxin)

  • Auxin: keeps lateral buds suppressed.
  • Cytokinin: pushes lateral buds to grow → plant becomes bushier.

3.Delay senescence (Richmond–Lang effect)

  • Cytokinins keep leaves green longer by delaying chlorophyll breakdown.
  • They help mobilise nutrients to the treated region.

4.Promote shoot formation in tissue culture

  • High cytokinin : auxin ratio → shoots
  • Low cytokinin : auxin ratio → roots

Cytokinin Memory Line: Cytokinin = Cell division + Side buds + Stay green + Shoots (high CK:Auxin)

4.Ethylene (C₂H₄)

Why Ethylene is Special?

Ethylene is the only gaseous plant hormone and it can act as both a growth promoter and a growth inhibitor depending on tissue and stage.

Discovery idea (storage fruits)

Ripened oranges released a volatile gas that made stored bananas ripen faster → identified as ethylene.

Site of synthesis

  • Ripening fruits
  • Senescing tissues
  • Nodes/internodes and stressed tissues

Functions of Ethylene

1.Fruit ripening (Top function)

  • Causes softening (cellulase/pectinase)
  • Converts starch → sugars
  • Improves colour (carotenoids)

2.Respiratory climacteric (climacteric rise)

  • During ripening of climacteric fruits (banana, apple, tomato), respiration rises sharply.
  • This rise is linked with ripening and is triggered by ethylene.

3.Senescence + abscission (shedding)

  • Promotes ageing of leaves/flowers/fruits and their fall.

4.Triple response (in etiolated seedlings)

  • Reduced elongation (shorter stem)
  • Radial swelling (thicker stem)
  • Horizontal growth / apical hook

5.Root hair formation

  • Increases root hairs → larger surface area for absorption.

6.Special roles

  • Deep water rice: helps internode elongation so leaves stay above water.
  • Promotes flowering in pineapple; increases female flowers in cucumber.

Ethylene Memory Line: Ethylene = Ripening + Climacteric rise + Shedding + Triple response + Root hairs

5.Abscisic Acid (ABA)

What is ABA?

ABA is primarily a growth inhibitor and is called the stress hormone because it helps plants survive adverse conditions.

Nature and synthesis

  • Made in leaves, roots, developing seeds
  • Levels rise during drought, salinity, cold stress

Functions of ABA

1.Stomatal closure

  • In drought, ABA increases → guard cells lose turgor → stomata close → less water loss.

2.Seed and bud dormancy (sleep hormone)

  • ABA induces and maintains dormancy.
  • Helps seeds survive unfavourable conditions.

3.Growth inhibition

  • Slows metabolism and growth during stress.

4.Senescence & abscission

  • Has a role, but NCERT notes ethylene is the major abscission hormone.

ABA vs Gibberellin (classic antagonist pair)

  • ABA → dormancy ON
  • GA → dormancy OFF (germination)

ABA Memory Line: ABA = Drought → stomata close + Dormancy + Growth inhibition

Summary Notes

Cytokinin: cell division, lateral buds, delay senescence, tissue culture shoots

Ethylene: gaseous, fruit ripening, climacteric rise, abscission/senescence, triple response

ABA: stress hormone, stomatal closure, seed dormancy, antagonist of GA

Cytokinins, Ethylene, and Abscisic Acid

Cytokinin = “C for Cytokinesis”

  • Cell division
  • Counters apical dominance (side buds grow)
  • Keeps leaves green (delay senescence)
  • Tissue culture: High CK:Auxin → shoots

Ethylene = “E for Eating fruit”

  • Only gaseous hormone
  • Ripening + respiratory climacteric
  • Abscission + senescence (shedding)
  • Triple response (short + thick + horizontal)

ABA = “A for Avoid water loss”

  • Stress hormone
  • Stomatal closure (drought)
  • Dormancy (seed/bud)
  • Opposes GA (ABA sleeps, GA wakes)

💡 Questions and Answers

Q1: What are the physiological functions of Cytokinins, Ethylene, and Abscisic Acid (ABA)?

A1:

  • Cytokinins mainly help the plant make new cells. They increase cell division, make side buds grow (so they reduce apical dominance), and keep leaves green for longer.
  • Key Points: cell division, lateral buds, delay senescence
  • Ethylene is a gas hormone that is famous for ripening fruits. It also causes leaf/flower/fruit fall and speeds up ageing.
  • Key Points: gaseous, fruit ripening, abscission, senescence
  • ABA helps plants in stress, especially drought. It closes stomata to save water and keeps seeds dormant.
  • Key Points: stress hormone, stomatal closure, dormancy, growth inhibitor

Q2: Match the PGRs with their functions.

(a) Auxin | (i) Bolting (b) Gibberellin | (ii) Weed-free lawns (c) Cytokinin | (iii) Fruit ripening (d) Ethylene | (iv) Overcome apical dominance

A2:

  • (a) Auxin → (ii) Weed-free lawns (2,4-D)
  • (b) Gibberellin → (i) Bolting
  • (c) Cytokinin → (iv) Overcome apical dominance
  • (d) Ethylene → (iii) Fruit ripening

Q3: What is the “stress hormone” and why is it called so?

A3: The stress hormone is ABA. When the plant faces drought or stress, ABA increases and closes stomata, so the plant loses less water. It also makes seeds stay inactive (dormant) until conditions become good.

  • Key Points: ABA rises in drought → stomata close → water saved; dormancy maintained

Q4: Explain the antagonistic relationship between auxins and cytokinins in apical dominance.

A4: Auxin from the top bud keeps side buds inactive, so the main shoot grows more. Cytokinins do the opposite — they make side buds grow. So, if cytokinin level is high in a lateral bud, it can overcome the auxin effect and the plant becomes bushy.

  • Key Points: Auxin suppresses lateral buds; Cytokinin promotes lateral buds; ratio decides bud growth

Q5: What is the “respiratory climacteric” and which hormone triggers it?

A5: The respiratory climacteric is the sudden rise in respiration rate that happens when some fruits start ripening (like banana, apple, tomato). This burst is triggered by ethylene, and it helps in softening and sweetening of the fruit.

  • Key Points: climacteric rise = respiration spike during ripening; triggered by ethylene