Unit 3 | Growth And Development of Horticultural Crops | MSc Horticulture - 2nd Year 3rd Semester

HORMA - 301

Growth And Development of Horticultural Crops
UNIT III: Assimilate Partitioning & Plant Growth Regulators

📚 Contents

1. Assimilate Partitioning During Growth
2. Influence of Water
3. Influence of Mineral Nutrition
4. Biosynthesis of Plant Hormones
5. Growth Inhibitors and Morphactins
6. Role of Promoters and Inhibitors

1. Assimilate Partitioning During Growth and Development

Definition: Assimilate partitioning refers to the distribution of photosynthetically produced carbohydrates and nutrients (assimilates) from source organs (mature leaves) to sink organs (roots, fruits, flowers, actively growing tissues).
This movement occurs mainly via the phloem.

1.1 Source and Sink Concept

Source	Sink
Mature, fully expanded green leaves	Areas demanding assimilates: Growing tissues (apex, buds), Roots, Fruits.
Export sugars (mostly sucrose)	Storage organs (tubers, bulbs)
Become active sources after saturation	Sink Strength = ability + capacity + activity of an organ to attract assimilates.

1.2 Factors Affecting Assimilate Partitioning

• A. Developmental Stage:
  Early stage → roots and shoots act as strong sinks
  Reproductive stage → flowers and fruits dominate
  Maturity stage → storage organs gain priority
• B. Hormones:
  Auxin increases sink activity of developing fruits
  Cytokinin promotes nutrient flow to meristems
  GA enhances sink strength in fruits
  ABA promotes seed filling
• C. Environmental Conditions:
  Light intensity alters source strength
  Water stress → reduces phloem transport
  Temperature affects metabolic activity
• D. Crop Type:
  Perennials store assimilates in roots/stems seasonally
  Annuals focus on rapid reproductive sink filling

1.3 Partitioning During Growth Phases

• Vegetative Phase: Priority: young leaves, roots, stems. High demand for carbohydrates and nitrogen.
• Reproductive Phase: Fruit set and seeds become the strongest sinks. Vegetative growth slows.
• Maturation Phase: Assimilate accumulation in seeds or storage tissues. Decline in sink activity.

2. Influence of Water on Growth and Development

Water influences plant growth in multiple ways:

2.1 Cell Expansion and Turgor

Turgor pressure drives cell enlargement. Water deficit → smaller leaves, reduced elongation.

2.2 Photosynthesis

Water stress closes stomata → ↓ CO₂ uptake. Severe stress → photo-oxidative damage. Reduced chlorophyll under prolonged deficit.

2.3 Mineral Uptake

Water is essential for movement of nutrients in the soil. Deficit reduces root growth → reduced uptake.

2.4 Growth Reduction Under Water Stress

• Reduced leaf area
• Increased ABA levels → stomatal closure
• Delayed flowering and poor fruit set

2.5 Excess Water (Waterlogging)

• Reduced soil aeration → root suffocation
• Reduced respiration in roots
• Accumulation of toxic substances (ethanol, lactic acid)
• Leaf yellowing, stunting, root rot

3. Influence of Mineral Nutrition on Growth and Development

Nutrient availability alters growth, development, morphology, and hormone synthesis.

3.1 Role of Major Elements

• Nitrogen (N): Essential for chlorophyll, amino acids. Promotes leaf growth. Excess → lush vegetative growth, delayed flowering.
• Phosphorus (P): Energy transfer (ATP). Root growth, early vigor. Essential for flowering/fruiting.
• Potassium (K): Regulates stomatal movement. Improves stress tolerance. Enhances fruit quality, sugar content. Deficiency → weak stems, scorching.

3.2 Role of Secondary Elements

• Calcium: Cell wall stability, meristem development.
• Magnesium: Central element in chlorophyll.
• Sulphur: Protein formation.

3.3 Role of Micronutrients

• Iron: Chlorophyll synthesis
• Zinc: Auxin metabolism
• Boron: Cell wall formation, pollen tube growth
• Manganese: Enzyme activation
• Copper: Lignification

4. Biosynthesis of Plant Hormones

Hormones regulate growth, differentiation, morphogenesis, and stress responses.

4.1 Biosynthesis of Auxins (IAA)

Main natural auxin: Indole-3-acetic acid (IAA)

Biosynthesis Pathways:

• Tryptophan-dependent pathways (IPA pathway is MAJOR route)
• Indole-3-acetonitrile (IAN) pathway
• Indoleacetamide (IAM) pathway
• Tryptophan-independent pathway

Sites of Synthesis: Shoot apices, Young leaves, Developing seeds.

4.2 Biosynthesis of Gibberellins (GAs)

Precursors: Derived from isoprenoid pathway.

Acetyl-CoA → Mevalonic acid → ent-kaurene → GA intermediates → Active GA

Sites: Young leaves, Embryos, Root tips, Developing seeds.

4.3 Biosynthesis of Cytokinins

Natural cytokinin: Zeatin

Key enzyme: Isopentenyl transferase (IPT)

Adenine nucleotides → Isopentenyl adenine → Zeatin

Sites: Root apical meristem, Developing embryos. (Transported mainly roots to shoots).

4.4 Biosynthesis of Abscisic Acid (ABA)

Pathway: Synthesized from carotenoids (xanthophylls).

Violaxanthin → Neoxanthin → Xanthoxin → ABA

Sites: Mature leaves (stress), Root tissues, Seeds (dormancy).

4.5 Biosynthesis of Ethylene

Key Enzyme: ACC oxidase (ACO)

Methionine → SAM → ACC → Ethylene

Sites: Ripening fruits, Senescing leaves, Meristems.

4.6 Biosynthesis of Brassinosteroids (BRs)

Precursors: Derived from campesterol.

Campesterol → Castasterone → Brassinolide

Sites: All growing tissues (Young shoots, expanding leaves, seeds).

5. Growth Inhibitors and Morphactins

5.1 Growth Inhibitors

Compounds that slow or stop growth.

• Examples: Abscisic acid, Coumarins, Phenolic acids, Jasmonic acid.
• Actions: Inhibit cell division, Delay germination, Promote dormancy, Restrict elongation.

5.2 Morphactins

Definition: Synthetic growth regulators derived from fluorene-9-carboxylic acid.

• Effects: Strong inhibition of auxin transport, Disturbs apical dominance, Produces abnormal branching (witches’ broom), Alters phyllotaxy.
• Uses: Controlling unruly growth in ornamentals, Inducing dwarfing.

6. Role of Plant Growth Promoters and Inhibitors

6.1 Growth Promoters

Auxins	Apical dominance, Root initiation, Vascular differentiation, Fruit set (parthenocarpy).
Gibberellins	Stem elongation, Breaking seed dormancy, Fruit enlargement (grapes), Bolting.
Cytokinins	Cell division, Delay senescence, Tissue culture proliferation, Shoot initiation.
Brassinosteroids	Cell elongation & division, Stress tolerance, Pollen tube growth.

6.2 Growth Inhibitors

Abscisic Acid (ABA)	Induces seed dormancy, Mediates drought response (stomatal closure), Promotes leaf abscission.
Ethylene	Fruit ripening, Leaf senescence, Abscission, Triple response in seedlings.
Morphactins	Inhibit auxin transport, Induce branching disorders, Growth suppression.

📚 REFERENCES

Buchanan B, Gruissem W & Jones R. 2002. Biochemistry & Molecular Biology of Plants. Wiley.

Epstein E. 1972. Mineral Nutrition of Plants. Wiley.

Fosket D.E. 1994. Plant Growth and Development. Academic Press.

Leopold A.C. & Kriedemann P.E. 1985. Plant Growth and Development. McGraw-Hill.

Salisbury F.B. & Ross C.W. 1992. Plant Physiology. 4th Ed. Wadsworth Publ.

Peter K.V. 2008. Basics of Horticulture. New India Publishing Agency.

Roberts J.A., Downs S.J., Parker P. 2002. In Plants (I. Ridge, Ed.). Oxford Univ. Press.

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