17Growth and Growth hormone

📚
GROWTH
▢ Definition/Concept:
  • •
    Growth = quantitative change
  • •
    Development = qualitative + quantitative change
  • •
    ↑ dry weight + permanent ↑ size/volume/cell number
▢ Sequence: Cell division → Cell enlargement → Cell differentiation
▢ Maximum Cell Elongation: Region behind root/shoot apex
▢ Major Climatic Factors:
  • •
    Light
  • •
    Temperature
▢ Growth Curve:
❖ Shape: S-shaped sigmoid curve
Image 1
❖ Seen In:
  • •
    Plant growth
  • •
    Bacterial growth in lab
▢ Measurement:
  • •
    Auxanometer
  • •
    Horizontal microscope
  • •
    Crescograph
▢ Classical Experiments:
  • •
    Boysen-Jensen
  • •
    Darwin
▢ Phases of Growth:

Table 1: Growth Phases

Phase
Keyword
Cell Activity
Lag phase
Initiating phase
Slow cell division
Log phase / Exponential phase
Maximum growth
Fast cell division
Decelerating phase
↓ growth rate
Slowing growth
Stationary phase
Constant growth
No net increase
▢ Plant Growth Nature:
  • •
    Limited
  • •
    Unlimited
  • •
    Unlocalized
  • •
    Diffused
📚
GROWTH PROMOTER PHYTOHORMONES
▢ Growth Regulators:
  • •
    Organic substances → required for growth
  • •
    Called hormones
  • •
    Term hormone → Starling
▢ Darwin Observation:
❖ Concept: Existence of growth-promoting substance
❖ Plant: Canary grass = Phalaris canariensis
▢ Nature of Hormone:
  • •
    Naturally produced organic substance
  • •
    Active in minute amount
  • •
    Produced at one site → transported → action at remote site
  • •
    Promotory / inhibitory role
  • •
    Used up after biochemical reaction
  • •
    Enzymes → unchanged after biochemical reaction
  • •
    Synthesized mainly in meristematic region
▢ Measurement:
  • •
    Earlier → bioassay
  • •
    Now → gas chromatography + immunoassay
▢ Tissue Culture: Auxin + Gibberellin + Cytokinin required
▢ Major Growth Promoters:

Table 1: Growth Promoter Phytohormones

Feature
Auxin
Gibberellin
Cytokinin / Kinetin
Main Action
Cell enlargement + differentiation
Cell enlargement + differentiation
Cell division
History
  • •
    Discovered by F.W. Went
  • •
    Test → Avena coleoptile curvature test
  • •
    First observed by Charles Darwin in Phalaris canariensis
  • •
    First isolated from human urine
  • •
    Discovered by Yabuta + Hayashi
  • •
    Yabuta + Suniki → isolated Gibberellin A + B
  • •
    First isolated from fungus Gibberella fujikuroi / Fusarium moniliforme
  • •
    Fungus group → Deuteromycetes
  • •
    Disease → Bakanae / foolish seedling disease of rice
  • •
    Effect → abnormally tall rice seedling
  • •
    First isolated by Miller + Skoog
  • •
    Source → old stock yeast DNA / denatured DNA
  • •
    Accidental isolation
Bioassay / Test
  • •
    Avena / oat coleoptile curvature test
  • •
    Bioassay = living tissue used to study biologically active substance
  • •
    Amylase test
  • •
    Dwarf maize / dwarf pea test
  • •
    Cereal endosperm test
  • •
    Carrot root phloem test
  • •
    Richmond-Lang test → ageing test
  • •
    Excised leaves remain fresh in \(H_2O\) + cytokinin
Examples / Source / Structure
◉ Natural Site: Apical meristem
◉ Natural Auxins:
  • •
    Auxin A / Auxientriolic acid → human urine
  • •
    Auxin B / Auxienolonic acid → corn grain oil
  • •
    Heteroauxin → re-examined human urine by Kogl et al.; also from Rhizopus culture by Thimann
  • •
    True natural auxin = IAA = Indole-3-acetic acid
  • •
    Heteroauxin ≈ IAA
  • •
    Natural auxin synthesis → tryptophan
  • •
    Zinc → important for auxin synthesis
  • •
    Transport → basipetal polar transport: shoot apex → base
◉ Synthetic Auxins:
◈ Adventitious Root Formation:
  • •
    NAA = Naphthalene acetic acid
  • •
    IPA = Indole-3-propionic acid
  • •
    IBA = Indole-3-butyric acid
◈ Weed Killers:
  • •
    2,4-D = 2,4-dichlorophenoxy acetic acid
  • •
    2,4,5-T = 2,4,5-trichlorophenoxy acetic acid
◉ Synthesis Sites:
  • •
    Young leaves
  • •
    Buds
  • •
    Seeds
  • •
    Root tips
◉ Important Form: \(GA_3\) = Gibberellic acid
◉ Chemical Identity: Kinetin = 6-furfuryl amino purine
◉ Natural Cytokinin:
◈ Zeatin / Coconut milk factor:
  • •
    Pure natural cytokinin
  • •
    Most active cytokinin
  • •
    Isolated from immature maize grain by Letham
◉ Synthesis Sites:
  • •
    Endodermis
  • •
    Root tips
  • •
    Shoot tips
Function
  1. Cell elongation
  2. Rooting
    1. High auxin in root → lateral roots ↑ + main root elongation ↓
    2. IBA → most potent root initiator
  3. Apical dominance → lateral bud suppression by apical bud
    1. Terminal bud IAA → apical dominance
    2. Cutting lawn grass top → profuse lateral growth
  4. Cell wall material synthesis
  5. 2,4-D + 2,4,5-T → weed killers
    1. Agent orange = 2,4-D + 2,4,5-T + dioxin; Vietnam war defoliant
  6. Usually inhibits flowering
    1. Litchi + pineapple → flowering by 2,4-D / NAA
    2. Pineapple flowering also promoted by ethylene
  7. Parthenocarpy → seedless fruit production
  8. Controls abscission of leaves/fruits, especially fruits
    1. Abscission mainly by ABA + ethylene
  9. Maintains dormancy of potato tubers, corms, rhizomes
  10. Maleic hydrazide methyl ester → prevents potato sprouting
  11. Seasonal cambium activity + cell division ↑
  12. Phototropism + geotropism
  13. IAA →photoperiodism
  14. Nodule formation in legumes
  1. Internode elongation and Leaf expansion
    1. Genetically dwarf plant → tall after gibberellin
    2. Dwarf pea/maize → normal size by intermodal growth stimulation
    3. Sugarcane → sugar yield ↑ by internode length ↑
  2. Bolting + flowering in rosette plants, e.g., cabbage
    1. Bolting = promoted suppressed growth
  3. Parthenocarpy when auxin fails
    1. Pome fruits → apple, pear
    2. More effective than auxin for parthenocarpy
  4. No effect on roots
  5. Long-day plants → flowering under short-day condition
    1. Short-day plants → flowering inhibited
  6. Seed germination
    1. Breaks dormancy of potato tuber + tree buds
    2. Dormancy maintained by ABA
  7. Monoecious cucumber → male flower induction
    1. Ethylene → female flower induction
  8. Fruit number + size ↑, e.g., grapes
  1. Primary function → cell division
    1. Vascular cambium cell division → by auxin
  2. Cell enlargement + cotyledon/foliage leaf expansion
  3. Morphogenesis → shoot initiation
    1. Auxin = cytokinin → callus formation
    2. Auxin high + cytokinin low → root initiation
    3. Auxin low + cytokinin high → shoot initiation
  4. Lateral bud growth ↑
    1. Counteracts apical dominance
  5. Interfascicular cambium formation ↑
    1. Delays senescence / ageing
    2. Prevents leaf fall
    3. Storage of leafy vegetables, flowers, fruits
  6. Short-day plants → flowering under long-day condition
  7. Chlorophyll retention
📚
GROWTH INHIBITOR PHYTOHORMONES
▢ Major Growth Inhibitors:

Table 1: Growth Inhibitor Phytohormones

Feature
ABA
Ethylene
Full Name / Synonyms
Abscisic acid / Abscisin II / Dormin / Dormant hormone / Stress hormone
Ethylene / Ripening hormone
Main Role
Bud dormancy + resting stage
Senescence / ageing + fruit ripening
Examples / Structure / Source
  1. Mildly acidic hormone
  2. Formed from xanthophyll + mevalonic acid
  3. Produced in mature stem, leaf, fruit, seed
  4. Most abundant formation → chloroplast
  5. Can be produced from isolated chloroplast
  6. Transport → diffusion to all plant parts
  1. Precursor → methionine
  2. Only gaseous hormone
  3. Synthesis promoted by \(O_2\)
  4. Synthesis inhibited by \(CO_2\)
  5. Produced in root + shoot meristem
  6. Large amount in senescing tissues + ripening fruits
  7. Connected with climacteric respiration
Functions
  1. Natural growth inhibitor
  2. Senescence hastened
  3. Leaf fall / abscission
  4. Maintains bud + seed dormancy
  5. Stomatal closure → anti-transpirant
  6. Functionally antagonistic to gibberellins
    1. Blocks gibberellin action → starch to sugar conversion inhibited
  7. Cambium activity inhibited
  8. Protection from frost, injury, desiccation
  1. Fruit ripening + dehiscence
  2. Generally inhibits flowering
    1. Pineapple + litchi → flowering
  3. Monoecious cucumber → female🚺 flower induction
  4. Abscission promoted; prevented by auxin
  5. Epinasty → downward bending of leaves
  6. Senescence promoted; prevented by cytokinin
  7. Transverse stem growth ↑
    1. Node swelling
    2. Longitudinal growth ↓
  8. Adventitious root formation
  9. Breaks dormancy of different organs but not lateral buds
📚
OTHER PLANT HORMONES
▢ Caline / Formative Hormone:
❖ Definition: Hormone responsible for root, shoot, leaf formation

Table 1: Calines

Hormone
Function
Rhizocaline
Root formation
Caulocaline
Shoot formation
Phyllocaline
Leaf formation
▢ Traumatic Acid / Wound Hormone: Injured part → callus formation
▢ Florigen:
  • •
    Flowering hormone
  • •
    Produced in leaves
▢ Vernalin: Flowering under cold condition
📚
IMPORTANT TERMS
▢ Vernalization:
  • •
    Low temperature treatment: \(0-4^\circ C\)
  • •
    Chilling → early flowering
  • •
    Seen in winter annuals, e.g., wheat
  • •
    Cold stimulus perceived by shoot meristem
  • •
    Photoperiodism light stimulus perceived by leaf
▢ Senescence:
  • •
    Ageing process
  • •
    ↑ entropy
  • •
    Cellular breakdown
  • •
    ↓ homeostasis
  • •
    ↑ metabolic failure
  • •
    Replication + transcription errors
▢ Etiolation: Green plant in dark → yellow
▢ Phytochrome System:
  • •
    Photomorphogenetic pigment
  • •
    Light detection
▢ Phytotron: Device for growing plants under controlled condition/environment
Q1.
Which hormone delays leaf senescence and helps overcome apical dominance?
📅CEE MBBS 2025
Q2.
Which hormone promotes adventitious rooting?
📅CEE MBBS 2024B
Q3.
Bolting in rosette plants is promoted by:
📅CEE MBBS 2023
Q4.
Cytokinin primarily promotes:
📅CEE MBBS 2022
Q5.
The ratio of which two hormones controls shoot versus root formation in plant tissue culture?
📅CEE MBBS 2022
Q6.
Auxin is used commercially to:
📅CEE BDS 2024
Q7.
IAA is commonly biosynthesized from:
📅CEE BDS 2023
Q8.
ABA is particularly important for:
📅CEE BDS 2023
Q9.
Auxin is best associated with:
📅CEE BDS 2023
Q10.
The hypothetical flowering stimulus induced by cold treatment is called:
📅CEE BDS 2023
Q11.
Gibberellins promote:
📅CEE Nursing 2024
Q12.
Cytokinin chiefly stimulates cell:
📅IE 2009
Q13.
Gibberellins were discovered through studies on:
📅MOE 2063
Q14.
Which hormone was isolated following Japanese investigations of the bakanae fungus?
Q15.
The rooting hormone among the following is:
📅MOE 2003
Q16.
Auxins are:
📅MOE 2057
Q17.
A lever auxanometer measures:
📅MOE 2055
Q18.
The growing apex produces substances classified as:
📅MOE 2054, 2055
Q19.
Which hormone is most closely associated with cytokinesis?
📅IOM 2006, 2055
Q20.
Certain genetically dwarf plants can elongate after application of:
📅IOM 1999
Q21.
Cutting a shoot apex often promotes branching because it:
📅IOM 1999
Q22.
Reserve-food mobilization during cereal seed germination is stimulated by:
📅BPKIHS 2000
Q23.
Seed dormancy is promoted by:
📅BPKIHS 2006
Q24.
Organogenesis from callus is influenced by:
📅BPKIHS 1996
Q25.
The classical hypothetical flowering stimulus is:
📅BPKIHS 1996
Q26.
Auxin is best categorized as a:
📅BPKIHS 1997
Q27.
Marked internode elongation or bolting is particularly induced by:
📅IE 2001
Q28.
Plants perceive directional blue light through:
📅IE 2007
Q29.
Kinetin is a:
📅BPKIHS 2015
Q30.
The flowering signal historically called florigen is:
📅BPKIHS 2013
Q31.
Which substance is commonly used to suppress sprouting in stored potatoes?
📅BPKIHS 2012
Q32.
Light generally has no direct essential role in:
📅BPKIHS 2012
Q33.
Which hormone can promote maleness in cucurbit flowers?
📅IOM
Q34.
Stomatal closure is induced by:
📅MOE 2014
Q35.
A standard bioassay for auxin is:
📅MOE 2010
Q36.
Auxins are plant growth regulators involved in:
📅MOE 2057