Understanding the internal anatomy of an agarwood-forming tree is essential for resin induction, quality grading, and designing digital traceability systems. Agarwood resin develops in specific layers of wood, each with distinct structural and biochemical properties.
I. Overview of Tree Wood Anatomy
A mature Aquilaria or Gyrinops trunk contains the following anatomical layers (from outside → inside):
- Bark (Periderm)
- Phloem
- Cambium Layer
- Sapwood (Xylem: outer zone)
- Transition Zone (Infected/Wounded Zone)
- Resinous Heartwood (Agarwood Zone)
- Inner Heartwood (Non-resinous core)
Agarwood resin forms not in the sapwood, but within the xylem tissue when triggered by stress or infection.
II. Normal (Uninfected) Heartwood vs. Agarwood Resin Zones
1. Normal Heartwood (Non-Resinous Core)
In a healthy, unstressed tree:
- Heartwood is light-colored, often pale yellow or cream.
- Contains a high proportion of:
- cellulose (40–45%)
- hemicellulose (20–25%)
- lignin (25–30%)
- Aromatic compounds are minimal.
- Not commercially valuable.
Heartwood acts mainly as structural support and water storage.
III. Formation of the Resin Zone (Agarwood Heartwood)
When the tree is wounded or infected, biochemical defenses are activated in the xylem. This results in the creation of the resin zone, also called the agarwood formation front.
This zone has distinct anatomical and biochemical characteristics.
IV. Anatomical Structure of Resin Zones
1. Discolored Resin Lines and Pockets
Resin accumulates in:
- parenchyma cells
- ray cells
- vessels
- axial parenchyma
- fiber walls
These become visibly darker—brown, dark brown, or black depending on resin load.
2. Occluded Vessels (Tylose Formation)
The tree produces tyloses, balloon-like extensions that seal xylem vessels to block pathogen movement.
3. Compartmentalization (CODIT Model)
The tree forms defensive boundaries to contain infection:
- Wall 1: Blocks vertical spread
- Wall 2: Blocks inward/outward spread
- Wall 3: Blocks radial spread
- Wall 4: New defensive tissue
This produces concentric bands of resin as the infection progresses.
V. Biochemical Structure of Resin Zones
The biochemical composition changes dramatically during resin formation.
1. Sesquiterpenes (Core Aroma Compounds)
Major sesquiterpene groups found in agarwood include:
- Agarospirol
- Valencene
- Jinkoh-eremol
- Guaianes
- Eudesmanes
- Cadinanes
These compounds are responsible for the deep, woody, balsamic, and animalic oud aroma.
2. 2-(2-Phenylethyl) Chromones (PECs)
These are unique to agarwood and are responsible for:
- long-lasting scent
- incense-quality smoke
- high-value resin grades
Examples:
- Agarotetrol
- Aquaquilarone
- Baimuxinal
3. Phenolics and Phytoalexins
Produced in response to fungal invasion:
- antioxidant compounds
- antimicrobial defense
- contributors to color and density
4. Lipids and Resinous Alcohols
Provide the “sticky” texture and fuel slow combustion.
VI. Structural Zones of Resin Deposition
In cut cross-sections, three main zones are observed:
1. Outer Sapwood Zone (White/Yellow)
- No resin
- Active water transport
- High moisture
- Low aromatic compounds
2. Transition Zone (Light Brown → Dark Brown)
- Early resin accumulation
- Fungal colonization
- Increased lignification
- Oxidation of phenolics
- Beginning of chromone formation
This zone reflects the current active infection front.
3. Resin Heartwood (Dark Brown to Black)
- High sesquiterpene content
- High chromone density
- Cells saturated with resin
- Strong aroma and oil content
This is the commercial agarwood.
VII. Microanatomy Observed Under Microscope
Resin Zone Characteristics:
- Resin droplets in parenchyma cells
- Darkened fiber lumens
- Mycelial presence (fungi) in early stages
- Degraded lignin structures
- Accumulated phenolic pigments
Sapwood Characteristics:
- Open vessels
- Well-defined fibers
- No resin inclusions
VIII. Factors Influencing Resin Zone Structure
1. Fungal Strain Activity
Different fungi induce different resin patterns (linear, patchy, radial).
2. Age of the Tree
Older trees produce thicker resin zones.
3. Induction Method
- Mechanical → scattered resin pockets
- Fungal inoculation → radial uniform resin
- Chemical induction → faster but sometimes lower chromones
4. Tree Species
A. malaccensis vs. A. crassna vs. Gyrinops produce distinct biochemical profiles.
IX. Application: Why This Matters for Traceability & Grading
1. CT Scan / X-ray Verification
Resin zones can be quantified digitally.
2. Chemical Fingerprinting
Chromones and sesquiterpene ratios identify:
- species
- region
- induction method
- age of resin
3. Digital Twin Parameters
The resin zone structure becomes part of the digital agarwood passport:
- resin depth
- resin density
- resin-to-wood ratio
- biochemical fingerprint (GC-MS)
4. Grading Systems
International grading heavily depends on:
- resin color
- resin density
- anatomical uniformity