1. Overview of Induction Methods
Agarwood (resinous wood) formation in Aquilaria is a defense response triggered by injury, microbial infection, or chemical elicitors. Induction methods are classified into three main categories:
- Physical induction – mechanical wounding or injury.
- Chemical induction – application of chemicals/elicitors to stimulate resin biosynthesis.
- Biological induction – inoculation with fungi or bacteria that trigger defense pathways.
Each method has advantages, limitations, and applications.
2. Comparative Table
| Feature | Physical Induction | Chemical Induction | Biological Induction |
|---|---|---|---|
| Mechanism | Mechanical injury (drilling, chiseling) induces wound response | Application of chemical elicitors (e.g., methyl jasmonate, NaCl, plant hormones) triggers defense metabolism | Microbial infection (fungi/bacteria) activates defense pathways and secondary metabolite synthesis |
| Examples | Drilling holes, nailing, girdling, burning | Jasmonates, salicylic acid, hydrogen peroxide, plant extracts | Fusarium oxysporum, Lasiodiplodia theobromae, Aspergillus spp. |
| Resin Quality | Moderate; may be variable and uneven | Moderate to high, depends on elicitor type and concentration | High; often richest in sesquiterpenes and chromones |
| Resin Yield | Low to moderate | Moderate; can be enhanced with repeated applications | High; microbes often penetrate wood more effectively |
| Time to Resin Formation | Slow (months to years) | Moderate (weeks to months) | Moderate to fast (weeks to months) |
| Advantages | Simple, low-cost, no chemicals or microbes | Controlled induction; less labor-intensive than microbial inoculation | Produces high-quality resin; mimics natural process |
| Limitations | Labor-intensive; stress may damage tree; low yield | Requires optimization; possible phytotoxicity | Risk of uncontrolled infection; requires sterile inoculum; complex management |
| Application Scale | Individual trees or small plots | Plantation or research scale | Plantation, commercial, and experimental scale |
| Environmental Impact | Minimal | Chemical residues possible | Low if controlled; risk if pathogenic fungi spread |
3. Mechanistic Notes
- Physical induction
- Causes local tissue damage → triggers wound-induced defense pathways → deposition of resin in heartwood.
- Often results in patchy resin formation.
- Chemical induction
- Elicitors such as methyl jasmonate or salicylic acid activate secondary metabolite pathways, especially sesquiterpene and chromone biosynthesis.
- Allows precise control of timing and concentration.
- Biological induction
- Certain fungi (e.g., Fusarium oxysporum, Lasiodiplodia theobromae) infect wounded tissue → tree responds with defense metabolite accumulation.
- Microbial enzymes may aid resin diffusion into wood.
- Considered most effective for high-quality agarwood in commercial plantations.
4. Integrated Strategies
- Combination approaches often yield the best results:
- Physical wounding + microbial inoculation → fast and high-quality resin.
- Chemical elicitor sprays after minor wounding → uniform induction.
- Precision induction:
- Holes drilled in a systematic pattern → fungal inoculation → resin deposition along defined zones.
- Allows controlled harvest without killing the tree.
5. Recommendations for Plantation Use
| Method | Best for | Notes |
|---|---|---|
| Physical | Small-scale, experimental | Low cost, easy to apply, but lower yield |
| Chemical | Medium-scale plantations | Requires trained personnel; repeated application may be needed |
| Biological | Large-scale commercial plantations | High-quality agarwood; must use certified, non-pathogenic inoculum |