Plant biology · Level 3 Advanced deep-dive

How a plant actually builds potency

2.5a · 9 min read · optional

What You Need to Know

Here’s what catches most growers off guard: your plant doesn’t produce THC. It produces THCA — tetrahydrocannabinolic acid — which is a completely different molecule that becomes THC only when you heat it. That distinction isn’t semantic. It rewires how you think about genetics, potency ceilings, and why no amount of nutrient tweaking will make a 15% strain test at 25%.

Understanding the biosynthetic pathway — how the plant assembles cannabinoids from raw precursors — is essential for advanced growing. It explains why potency is locked in genetically, why you can’t “feed your way” to higher THC, and why some cultivars have hard limits that no technique can break through. Tahir’s team at the University of Windsor mapped the complete chain from primary metabolites to final cannabinoids, showing the exact genetic and enzymatic bottlenecks that determine what your plant can produce.

The Science

CBGA — skeletal structure (C₂₂H₃₂O₄), a cannabinoid acid; aroma: parent of all CBGA C₂₂H₃₂O₄ cannabinoid acid · parent of all

Tahir’s team reviewed the complete biosynthetic pathway of cannabinoids, from primary metabolites all the way through to the final decarboxylated compounds you consume. Here’s the chain:

Step 1 — The building blocks. Two pathways supply the raw materials. The MEP pathway (in plastids) produces geranyl pyrophosphate (GPP), which is a 10-carbon terpenoid building block. The polyketide pathway produces olivetolic acid (OLA) from hexanoyl-CoA — a 12-carbon phenolic compound. These two pathways operate in the trichome cells and draw on primary metabolic carbon.

Step 2 — The universal precursor. An enzyme called aromatic prenyltransferase (APT) fuses GPP with olivetolic acid to produce cannabigerolic acid (CBGA). CBGA is the mother of all cannabinoids. Every single cannabinoid in cannabis — THC, CBD, CBC, and all their variants — starts as CBGA. If you’ve heard of CBG flower, that’s flower from a plant that stops at this stage because it lacks the enzymes to convert CBGA further.

Step 3 — The fork in the road. Three competing enzymes act on CBGA to produce the three major cannabinoid acids: THCA synthase converts CBGA to THCA. CBDA synthase converts CBGA to CBDA. CBCA synthase converts CBGA to CBCA (cannabichromenic acid). These three enzymes compete for the same substrate (CBGA). Your plant’s genetics determine which enzyme it expresses most, and that determines whether your cultivar is THC-dominant, CBD-dominant, or a hybrid.

Step 4 — Decarboxylation. The acidic cannabinoids (THCA, CBDA, CBCA) lose their carboxyl group (-COOH) when exposed to heat, light, or time. THCA becomes THC. CBDA becomes CBD. This is a non-enzymatic chemical reaction — the plant doesn’t do it. You do it when you light a joint, heat an oven, or leave buds in storage for months. Fresh plant material contains almost exclusively the acid forms.

The genetic bottleneck: THCA synthase and CBDA synthase are encoded by genes at a single genetic locus. They share 84% amino acid sequence identity — they’re essentially the same enzyme with a few critical mutations that change their substrate specificity. A plant that is homozygous for the THCA synthase gene (BT/BT) produces predominantly THCA. A plant homozygous for CBDA synthase (BD/BD) produces predominantly CBDA. A heterozygous plant (BT/BD) produces both. This is why crossing a THC cultivar with a CBD cultivar produces offspring with varying THC:CBD ratios — they inherit different combinations of these alleles.

Why you can’t feed your way to higher THC: The maximum THCA concentration your plant can produce is determined by the expression level and catalytic efficiency of its THCA synthase enzyme, which is genetically fixed. Environmental factors (light, nutrients, water) can help the plant reach its genetic ceiling by keeping it healthy and productive, but they can’t raise the ceiling. It’s like a speed limit — you can tune your car to go the speed limit more easily, but the limit itself is painted on the sign. Every module in this curriculum that shows “no NPK effect on cannabinoid concentration” or “no light effect on potency percentage” is telling the same story: the biosynthetic capacity is set by the DNA.

How cannabis makes THC and CBD Simple precursors form CBGA, the parent cannabinoid. THCA synthase turns CBGA into THCA and CBDA synthase turns it into CBDA. Heat then converts these acid forms into THC and CBD. The cannabinoid pathway Where the THC and CBD in your flower actually come from Olivetolic acid + GPP the raw stock CBGA the "mother" cannabinoid THCA synthase CBDA synthase THCA acid form · not active CBDA acid form · not active heat · decarb heat · decarb THC the active form you feel CBD the active form you feel Genetics fix which synthase dominates — that's set in the seed. The plant makes the acids; heat finishes the job, which is why raw bud does little until it's smoked, vaped or decarbed. Source: Tahir et al. (2021), Journal of Cannabis Research.
Genetics fix which synthase dominates; the plant makes the acid forms, and heat finishes the job — which is why raw bud does little until it's smoked, vaped or decarbed.
How total THC is calculated from THCA Raw cannabis holds its THC as THCA, the acid form, which is not active. Heat drives off carbon dioxide and converts THCA to THC. Because that lost carbon dioxide is weight, total THC equals THCA times 0.877 plus any free THC. A bud testing 22 percent total THC has almost no active THC until it is heated. Raw bud barely gets you high — yet THCA has to be heated into THC, and it loses weight on the way THCA acid form — what's in raw bud not active heat — decarboxylation drives off CO₂ THC active form — after heat gets you high Total THC = (THCA × 0.877) + THC the 0.877 is the weight lost as CO₂ when THCA becomes THC A "22% total THC" bud is almost all THCA — near-zero active THC until you heat it.
THCA — skeletal structure (C₂₂H₃₀O₄), a cannabinoid acid; aroma: raw, pre-heat THCA C₂₂H₃₀O₄ cannabinoid acid · raw, pre-heat CBDA — skeletal structure (C₂₂H₃₀O₄), a cannabinoid acid; aroma: raw, pre-heat CBDA C₂₂H₃₀O₄ cannabinoid acid · raw, pre-heat CBC — skeletal structure (C₂₁H₃₀O₂), a cannabinoid; aroma: minor CBC C₂₁H₃₀O₂ cannabinoid · minor Δ9-THC — skeletal structure (C₂₁H₃₀O₂), a cannabinoid; aroma: psychoactive Δ9-THC C₂₁H₃₀O₂ cannabinoid · psychoactive CBD — skeletal structure (C₂₁H₃₀O₂), a cannabinoid; aroma: non-intoxicating CBD C₂₁H₃₀O₂ cannabinoid · non-intoxicating CBG — skeletal structure (C₂₁H₃₂O₂), a cannabinoid; aroma: the 'mother' molecule CBG C₂₁H₃₂O₂ cannabinoid · the 'mother' molecule

How To Apply This

  • Accept that your cultivar’s potency ceiling is genetic. No nutrient, light spectrum, stress technique, or additive will make a 15% THC strain test at 25%. Your job is to help the plant reach its ceiling — not raise it. Every previous module has shown that the controllable variables (light, nutrients, water) affect yield, not potency percentage.

  • Understand the THCA → THC conversion when reading lab results. Most lab reports provide “total THC” which is calculated as: Total THC = (THCA × 0.877) + THC. The 0.877 factor accounts for the mass lost during decarboxylation. If a lab reports 22% total THC, the actual THC in the raw bud is close to zero — it’s almost all THCA that will convert when heated.

  • If you’re interested in breeding, know that THC:CBD ratio inheritance is relatively simple. It’s controlled primarily by a single locus with codominant alleles. Crossing two high-THC plants produces high-THC offspring. Crossing high-THC with high-CBD produces a 1:1 mix of mixed-ratio offspring. This is one of the more predictable traits in cannabis breeding.

  • Work with your genetics’ actual ceiling. If you’re running a properly lit, properly fed, properly watered setup and the potency plateaus, the genetics are the bottleneck. The solution is better genetics, not more expensive nutrients.

Seb’s Corner (Level 2+)

The enzymology of cannabinoid biosynthesis has advanced significantly since Tahir’s 2021 review, but the core pathway remains well-established. THCA synthase (THCAS) is a 545-amino acid, FAD-dependent oxidoreductase that catalyses an enantiospecific oxidative cyclisation of CBGA. The crystal structure (PDB: 3VTE, Shoyama et al. 2012) reveals a buried active site with covalently bound FAD, anchored by His114 and Cys176. CBDAS shares 84% sequence identity with THCAS, and the primary difference in catalytic specificity is attributed to whether the enzyme abstracts a proton from the terminal methyl (CBDAS) or the hydroxyl (THCAS) group of CBGA, directing the cyclisation product. A single amino acid mutation — A414V in THCAS — creates an analogue with threefold higher CBDA production, demonstrating the evolutionary knife-edge between these two pathways. For growers, the practical takeaway is that chemotype (THC-dominant vs CBD-dominant) is one of the most genetically tractable traits in cannabis, controlled by a small number of well-characterised genes. This is why seed banks can reliably label chemotype ratios. Potency within a chemotype (e.g., 18% vs 25% THC in two different THC-dominant cultivars) is likely polygenic, involving variation in THCAS expression levels, trichome density, trichome maturation timing, and GPP supply — all of which are harder to select for and explain why “high potency” seed claims are less reliable than chemotype claims.

Watch Out For

  • The nutrient trap: Products claiming to “unlock hidden potency” or “maximise cannabinoid expression” are marketing fiction. No NPK, micronutrient, or additive formula changes the genetics you’re working with. If the ceiling is 20%, that’s the ceiling.

  • Decarboxylation confusion: Lab reports showing “22% THC” don’t mean 22% of the fresh bud is THC. Most of that is THCA waiting for heat. Understand the conversion math before comparing results.

  • Breeding oversimplification: THC:CBD ratio is simple to inherit, but achieving specific potency targets within a chemotype requires multiple generations of selection for expression levels and supporting traits. High potency isn’t a single-gene trait.

  • Fresh vs cured potency myths: Some THCA converts during drying and curing (a few percent), but the major decarboxylation happens when you consume it. Don’t confuse curing effects with the true potency change.

Quiz

1. Your lab report says “Total THC: 22%.” In the fresh, uncured bud, how much is actually THC?

2. THCA synthase and CBDA synthase are best described as:

3. (True/False) Cranking nutrients or light can raise your cultivar’s genetic potency ceiling.

4. Cross a high-THC strain (BT/BT) with a high-CBD strain (BD/BD). What do the F1 mostly make?

5. Why is breeding for “high potency” harder than breeding for “THC-dominant chemotype”?

Common questions

If THCA isn't psychoactive, why does raw cannabis have any effect?

THCA has its own pharmacological activity — anti-inflammatory and anti-emetic properties have been reported — but it doesn't activate the CB1 receptor that produces the "high." Raw cannabis juice or raw flower won't get you intoxicated. You need heat to convert THCA to THC for psychoactive effects.

Why do some CBG strains exist? Did they breed out THCA synthase?

Essentially, yes. CBG-dominant cultivars have mutations or absent copies of both THCA synthase and CBDA synthase genes. The plant produces CBGA normally but has no enzyme to convert it further, so CBGA accumulates. When decarboxylated (by heat or time), CBGA becomes CBG. This is a relatively new chemotype that breeders have been selecting for.

Can I make my plant produce more CBGA to convert to either THC or CBD?

CBGA supply is rarely the bottleneck. In most cultivars, CBGA is converted to THCA or CBDA as fast as it's produced. The bottleneck is the synthase enzyme capacity (determined by gene expression) and the supply of precursors from the MEP pathway (determined by photosynthetic carbon flux). Increasing light increases total precursor supply, which is why yield goes up — but the ratio of cannabinoids stays the same because the enzyme ratios don't change.

Does decarboxylation happen during drying and curing?

Slowly, yes. Over weeks of drying and months of curing, some THCA converts to THC spontaneously. THCA and THC can also oxidise to CBN (cannabinol), which is why very old cannabis has a "sleepy" effect — it's accumulated CBN. Proper storage (cool, dark, airtight) slows both decarboxylation and oxidation, preserving the acidic forms until you're ready to heat them.

References

  1. Tahir et al. (2021). The biosynthesis of the cannabinoids. Journal of Cannabis Research, 3:7.

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