Plant biology · Level 4

Terpenes: The Other Half of Quality

4.2 · 7 min read

Two growers, same THC number on the lab report. One jar smells like cut grass and the other like lemon peel and diesel. People will pay double for the second one and tell you it “hits different.” They’re not imagining it — but most of what they think is happening isn’t. This lesson is about the half of quality that the THC percentage never captures, and about being honest where the science still is.

What You Need to Know

Terpenes come from their own family of enzymes

In Lesson 1 you saw cannabinoids branch from one mother molecule. Terpenes work on a parallel logic, with their own gene family. Allen and colleagues sequenced the lot: 55 terpene synthase genes in Cannabis sativa — the most complete catalogue to date, well up on the 33 found in earlier work. They sort into three subfamilies:

  • TPS-b — monoterpene synthases (the lighter, C10 terpenes: myrcene, pinene, limonene).
  • TPS-a — sesquiterpene synthases (the heavier, C15 terpenes: caryophyllene, humulene).
  • TPS-c — diterpene synthases (a minor group).

Which of those 55 genes a given plant switches on — and how hard — decides the smell. In Purple Kush, three genes (TPS1, TPS18, TPS5) dominate the output, but 16 genes each contribute at least 1% of the total. Different cultivar, different subset firing, different aroma.

Seb’s Corner. The smell of your plant is a gene-expression fingerprint. It is not magic, not “the soil,” not the full moon. When two phenotypes from the same pack smell different, you’re seeing different terpene synthase genes expressing at different strengths. That’s also why selecting for smell in a breeding programme works — you’re selecting which of those 55 genes you carry forward.

From terpene synthase genes to a strain's smell A cannabis plant carries about 55 terpene synthase (TPS) genes, but only a subset switches on in any given plant. The genes that fire decide which terpenes are made, and those terpenes decide the aroma. Two phenotypes from the same seed pack can express different genes and so smell completely different. You can select which genes carry forward, but you cannot make the plant build a terpene it has no gene for. Why two plants from one pack smell different The smell is a gene-expression fingerprint — which terpene genes fire, not the soil or the moon 55 TPS genes a subset switches on ■ expressed different pheno = different subset terpenes made Myrcene Limonene Pinene Linalool the subset that fired the smell earthycitruspinefloral the aroma fingerprint Select by smell and you're selecting which genes carry forward. But you can't bottle a terpene the plant has no gene to make.

Two pathways feed the terpenes

Both papers confirm the dual-pathway system. The plant builds terpene precursors two ways:

  • The MEP pathway, in the plastids, makes GPP (C10) → monoterpenes.
  • The MEV pathway, in the cytosol, makes FPP (C15) → sesquiterpenes.

You don’t act on these directly. But knowing there are two separate supply lines explains why a plant can be heavy in monoterpenes and light in sesquiterpenes, or the reverse — they’re built in different compartments from different feedstock.

How a cannabis plant builds its terpenes Cannabis builds terpenes on two separate supply lines in two compartments. The MEP pathway in the plastids makes geranyl diphosphate (GPP, a ten-carbon molecule), which becomes the lighter monoterpenes — myrcene, limonene and pinene — via the TPS-b gene subfamily. The MEV pathway in the cytosol makes farnesyl diphosphate (FPP, a fifteen-carbon molecule), which becomes the heavier sesquiterpenes — beta-caryophyllene, humulene and bisabolol — via the TPS-a subfamily. Across the plant 55 terpene synthase genes exist; which subset a cultivar switches on decides its smell. How a plant builds its terpenes Two supply lines in two compartments — and 55 genes deciding which terpenes a plant fires MEP pathway in the plastids PP GPP — C10 Monoterpenes lighter, volatile · made by TPS-b Myrcene Limonene Pinene geranyl diphosphate → C10 terpenes MEV pathway in the cytosol PP FPP — C15 Sesquiterpenes heavier, deeper · made by TPS-a β-Caryophyllene Humulene Bisabolol farnesyl diphosphate → C15 terpenes 55 terpene synthase genes — which subset a cultivar switches on, and how hard, sets its smell After Allen et al. (2019), PLOS ONE; dual-pathway biosynthesis per Chacón et al. (2022), Biomedicines.
Two supply lines build two terpene families — the light monoterpenes (myrcene, limonene, pinene) from GPP in the plastids, the heavier sesquiterpenes (caryophyllene, humulene, bisabolol) from FPP in the cytosol — and of the plant's 55 synthase genes, the subset it switches on decides the smell.

A genuinely strange fact: roots make terpenes too

Allen’s team found 10 monoterpene synthase genes that are highly root-specific — 6 of them express only in roots. Most growers think of terpenes as a flower thing. They’re not. The roots run their own distinct monoterpene programme, probably for soil defence and microbial signalling. It doesn’t change your harvest, but it should change how you think: the plant is a whole system, not a bud on a stick.

The big five aren’t the whole story

Everyone names myrcene, limonene, pinene, linalool, caryophyllene. Chacon and colleagues catalogue the secondary terpenes — bisabolol, guaiol, nerolidol, geraniol, fenchol and more — present at tiny amounts (typically under 0.1% by mass, versus 0.5–3% for the primaries) but biologically active. The honest catch: most labs don’t even report them, because reference standards don’t exist for many. So the “full profile” of most cultivars is genuinely unknown.

Seb’s Corner. When a menu lists a strain’s “terpene profile,” it’s listing the handful the lab had standards for. The minor terpenes — the ones doing some of the most interesting chemistry — are usually invisible on the report. Absence on a COA is not absence in the jar.

Primary versus secondary terpene abundance Over 200 terpenes have been identified in cannabis, but a handful dominate the aroma. Terpenes make up about 3 to 5 percent of dry flower mass. Primary terpenes each sit around 0.1 to 1.5 percent, while secondary terpenes are typically below 0.1 percent — ten to thirty times less. A single terpene like myrcene can range hugely, from 0.12 to 14.8 milligrams per gram. A handful of terpenes do most of the smell Over 200 exist, but the majors dwarf the rest — terpenes are 3–5% of dry flower Primary (major) e.g. myrcene, limonene 0.1–1.5% each Secondary the long tail < 0.1% 0%0.5%1.0%1.5% concentration in dry flower (% by mass) One terpene swings wildly between plants — myrcene ranges 0.12 to 14.8 mg/g. That spread, not the total, is what makes two buds smell completely different.
Myrcene — skeletal structure (C₁₀H₁₆), a monoterpene; aroma: earthy, herbal, clove Myrcene C₁₀H₁₆ monoterpene · earthy, herbal, clove Limonene — skeletal structure (C₁₀H₁₆), a monoterpene; aroma: bright citrus Limonene C₁₀H₁₆ monoterpene · bright citrus α-Pinene — skeletal structure (C₁₀H₁₆), a monoterpene; aroma: fresh pine α-Pinene C₁₀H₁₆ monoterpene · fresh pine Linalool — skeletal structure (C₁₀H₁₈O), a monoterpenoid; aroma: floral, lavender Linalool C₁₀H₁₈O monoterpenoid · floral, lavender β-Caryophyllene — skeletal structure (C₁₅H₂₄), a sesquiterpene; aroma: pepper, spice β-Caryophyllene C₁₅H₂₄ sesquiterpene · pepper, spice Humulene — skeletal structure (C₁₅H₂₄), a sesquiterpene; aroma: woody, hoppy Humulene C₁₅H₂₄ sesquiterpene · woody, hoppy α-Bisabolol — skeletal structure (C₁₅H₂₆O), a sesquiterpenoid; aroma: soft, floral α-Bisabolol C₁₅H₂₆O sesquiterpenoid · soft, floral Nerolidol — skeletal structure (C₁₅H₂₆O), a sesquiterpenoid; aroma: woody, citrus Nerolidol C₁₅H₂₆O sesquiterpenoid · woody, citrus Geraniol — skeletal structure (C₁₀H₁₈O), a monoterpenoid; aroma: rose, floral Geraniol C₁₀H₁₈O monoterpenoid · rose, floral Fenchol — skeletal structure (C₁₀H₁₈O), a monoterpenoid; aroma: camphor, lime Fenchol C₁₀H₁₈O monoterpenoid · camphor, lime Guaiol — skeletal structure (C₁₅H₂₆O), a sesquiterpenoid; aroma: woody, rose Guaiol C₁₅H₂₆O sesquiterpenoid · woody, rose β-Eudesmol — skeletal structure (C₁₅H₂₆O), a sesquiterpenoid; aroma: woody, green β-Eudesmol C₁₅H₂₆O sesquiterpenoid · woody, green

The entourage effect, audited

Here’s where Dave keeps it honest. The “entourage effect” is the claim that cannabinoids and terpenes work together to shape the experience. Chacon’s review is even-handed: some evidence supports it, some refutes it. A few terpenes do appear to modulate CB1/CB2 receptor activity; others act on entirely separate targets. There’s also a real biochemical link — high-CBGA chemotypes correlate with elevated bisabolol, guaiol, and eudesmol, suggesting the cannabinoid and terpene pathways share regulatory wiring.

But “it’s real chemistry” is not the same as “we’ve proven the effect a marketer is selling you.” The current state, said plainly: it’s mixed, emerging, and nowhere near settled. We do not make claims about what any of this does in a person — that’s a hard rule and it’s also just intellectually honest. What we can say: the terpene profile is real, it’s heritable, and it’s a legitimate quality axis. The rest is “promising, unproven,” and anyone telling you otherwise is ahead of the data.

How To Apply This

  • Grow terpene-first as well as yield-first. Smell is a quality driver buyers reward. Select phenotypes by aroma, not just by trichome count, and you’re selecting gene expression you can carry forward.
  • Protect terpenes in handling. They’re volatile. Heat, light, rough handling and over-drying drive them off — the same enemies as Lesson 1’s THCA, for the same reason. A cool, slow, dark cure preserves the aroma you grew.
  • Pick genetics for the profile you want. If you want a citrus or fuel nose, that’s a TPS-b/TPS-a expression pattern set by the genetics. Buy for it.
  • Read a COA for what it omits. Treat the listed terpenes as a partial picture. Don’t assume a strain is “flat” because the lab only reported three.

Watch Out For

  • Entourage claims dressed as fact. “These terpenes enhance the effect” is a hypothesis with mixed support, not a settled result. Note the uncertainty; never make a medical claim.
  • Chasing terpenes with bottles. No additive reliably “increases terpenes.” Like cannabinoids, the ceiling is genetic; your job is to reach it through plant health and clean handling, not to exceed it.
  • Confusing strong smell with strong effect. Aroma intensity and cannabinoid content are separate axes. A pungent plant isn’t automatically more potent.
  • Trusting menu terpene percentages as complete. Secondary terpenes are routinely unmeasured.

Quiz

1. How many terpene synthase genes did Allen et al. identify, and what determines a cultivar’s smell?

2. Name the two precursor pathways and the terpene class each feeds.

3. What surprising tissue, besides flowers, runs its own distinct monoterpene programme?

4. Why are many “secondary” terpenes missing from lab reports?

5. State the honest current status of the entourage effect.

Sources

Allen, K. D., et al. (2019). Genomic characterization of the complete terpene synthase gene family from Cannabis sativa. PLOS ONE, 14(9), e0222363. https://doi.org/10.1371/journal.pone.0222363. CC-BY 4.0.

Chacon, F. T., Raup-Konsavage, W. M., Vrana, K. E., & Kellogg, J. J. (2022). Secondary terpenes in Cannabis sativa L.: Synthesis and synergy. Biomedicines, 10(12), 3142. https://doi.org/10.3390/biomedicines10123142. CC-BY 4.0.

Next: Lesson 3 — Spectrum engineering, and the far-red and UV reality check.

Common questions

What are terpenes and why do they matter?

Terpenes are the aromatic compounds that give each strain its smell and flavour, from lemon peel to diesel to cut grass. They are a big part of perceived quality, which is why two jars with the same THC number can be worth very different money.

Are terpenes set by genetics or by growing?

The profile is set by genetics, through which terpene-synthase genes the plant expresses, so if you want a citrus or fuel nose you buy for it. Your growing and curing decide how much of that aroma survives to the jar.

How do I keep my buds smelling strong?

Protect the terpenes in handling, because they are volatile and driven off by heat, light, rough handling and over-drying. A cool, slow, dark dry and cure preserves the aroma you grew, the same care that protects potency.

Does a lab report tell me all the terpenes?

No, treat a certificate of analysis as a partial picture. Labs often report only a handful of terpenes, so a strain is not flat just because the sheet lists three. Read a COA for what it measured, and trust your nose for the rest.

References

  1. Allen et al. (2019). Genomic characterization of the complete terpene synthase gene family from Cannabis sativa. PLOS ONE, 14(9):e0222363.
  2. Chacón et al. (2022). Secondary Terpenes in Cannabis sativa L.: Synthesis and Synergy. Biomedicines, 10(12):3142.

Want the full story, in print? It's all in Grow Good Bud — and the kit to do it is at Dublin Indoor Gardening.