Mastering ISR and SAR: Unlocking the Secrets to Better Terpenes and Quality in Cannabis

Cannabis growers are always looking for ways to push their plants to new heights—richer flavors, stronger aromas, and frostier buds. Induced Systemic Resistance (ISR) and Systemic Acquired Resistance (SAR) are powerful plant defense systems that not only protect plants but can also enhance secondary metabolites like terpenes and cannabinoids. By understanding and leveraging these natural processes, you can create a grow environment that produces top-shelf cannabis.

Let’s dive deep into the science, explore fascinating stress responses, and uncover practical applications that indoor growers can use.

What Are ISR and SAR?

Induced Systemic Resistance (ISR) and Systemic Acquired Resistance (SAR) are two complementary mechanisms plants use to defend themselves against stress. Here’s how they work:

Induced Systemic Resistance (ISR)

ISR is like an immune system workout. It’s activated when beneficial microbes colonize the plant’s roots. These microbes signal the plant to prepare for potential threats, priming its defenses before an actual attack occurs.

• Examples of ISR in Action:
  • Bacteria to the Rescue: Beneficial bacteria like Bacillus subtilis in the root zone can trigger ISR, making the plant less appealing to pests and more resistant to pathogens.
  • Fungal Friends: Mycorrhizal fungi form symbiotic relationships with roots, improving nutrient uptake and signaling the plant to strengthen its defenses.

Systemic Acquired Resistance (SAR)

SAR is the plant’s version of a 911 response. It’s activated after an actual attack—such as a pest chewing on leaves or a pathogen infecting a cell. SAR uses signaling molecules like salicylic acid to warn the entire plant of the attack, prompting it to ramp up protective chemicals like phytoalexins and terpenes.

• Examples of SAR in Action:
  • Pathogen Attack: When a fungus infects one leaf, SAR signals the rest of the plant to bolster its defenses, making it harder for the pathogen to spread.
  • Pest Defense: If a caterpillar chews on a leaf, SAR activates compounds like jasmonates, which repel or deter further feeding.

The Role of Stress Responses in Terpene Production

Plants respond to stress not just to survive but to thrive. Stress signals can lead to increased production of secondary metabolites, including terpenes, flavonoids, and cannabinoids, which serve as defense compounds. Here are some fascinating stress responses:

1. Pest Attacks Trigger Chemical Defenses

When pests feed on a plant, it releases volatile organic compounds (VOCs) to signal other plants and even attract predators of the pests (e.g., parasitic wasps). In cannabis, this can result in a change in terpene composition, producing stronger aromas to repel attackers.

2. Pathogen Resistance Through Phytoalexins

Phytoalexins are antimicrobial compounds produced in response to pathogens. In cannabis, this could manifest as increased resin production to trap and isolate pathogens.

3. UV-B Radiation and Trichome Production

UV-B light damages plant cells, prompting them to produce trichomes filled with resin and terpenes as a protective shield against harmful radiation.

4. Drought and Stress Hormones

Mild drought stress triggers the release of abscisic acid (ABA), which not only helps the plant conserve water but can also increase the production of secondary metabolites, enhancing the plant’s aroma and flavor.

Practical Applications of ISR and SAR in Cannabis Growing

Here’s how you can apply these natural processes to your indoor grow for better results:

1. Use Beneficial Microbes to Trigger ISR

Microbes like Bacillus subtilis and Trichoderma species are key activators of ISR. They colonize the root zone, signaling the plant to prepare for stress while improving nutrient uptake.

• How to Use: Apply microbial inoculants weekly to keep ISR active. Products like Recharge combine beneficial bacteria and fungi into one easy-to-use solution.
• Expected Results: Healthier plants, improved resilience, and potentially enhanced terpene production.

2. Activate SAR with Aspirin

Salicylic acid, a key signaling molecule in SAR, is easily accessible through aspirin. It primes the plant’s defenses to respond more effectively to potential attacks.

• How to Use: Dissolve one 325 mg aspirin tablet in a gallon of water and apply as a foliar spray during veg and early flower.
• Expected Results: Improved resistance to pathogens and a potential boost in terpene production.

3. Controlled Environmental StressUV-B Light Exposure

• What It Does: UV-B light stimulates trichome and terpene production as a defense mechanism.
• How to Use: Add UV-B lamps to your grow room for 2–3 hours daily during the last 2–3 weeks of flowering.
• Expected Results: Higher resin content and richer terpene profiles.

Drought Stress

• What It Does: Mild drought stress triggers the release of secondary metabolites to protect the plant.
• How to Use: Allow the soil to dry slightly more than usual between waterings during late flower, but avoid wilting.
• Expected Results: Enhanced terpene concentration and denser trichomes.

Sound Stimulation

• What It Does: Sound waves in specific frequencies can mimic environmental cues, activating stress responses that promote growth and metabolism.
• How to Use: Experiment with frequencies between 100–500 Hz or play natural sounds like bird songs or running water for a few hours daily.
• Expected Results: Potential improvements in terpene production and overall plant health.

Silica: A Complementary Tool for Stress Management

While not a direct activator of ISR or SAR, silica strengthens plant cell walls, helping them better tolerate environmental stresses.

• How It Works: Absorbed as monosilicic acid, silica improves structural integrity and water-use efficiency.
• How to Use:
  • Add silica to your nutrient solution at 2–5 mL per gallon.
  • Apply as a foliar spray at 0.5–1 mL per gallon during veg and early flower.
• Expected Results: Stronger stems, improved stress tolerance, and better overall plant health.

Balancing Defense and Growth

While activating ISR and SAR can enhance quality, over-stimulating these pathways can divert energy away from growth and yield. Here’s how to strike the right balance:

1. Moderation is Key: Avoid excessive stress or frequent elicitor applications.
2. Timing Matters: Apply stress techniques and elicitors during appropriate growth stages to maximize benefits.
3. Monitor Plants: Watch for signs of over-stress, such as slowed growth or discoloration, and adjust accordingly.

Final Thoughts: Growing Smarter with ISR and SAR

ISR and SAR aren’t just fascinating plant responses—they’re tools that can help growers produce premium cannabis. By combining microbial support, targeted elicitors, and carefully applied environmental stress, you can unlock your plants’ full potential for terpenes, resin, and flavor.

DGC, what’s your favorite technique for boosting terpenes and resin? Have you tried UV-B, microbes, or sound stimulation? Share your tips and results with the community, and let’s keep growing together! 🌿