Seeding and compound formation: what happens to cannabinoids and terpenes when a bud sets seed

Overview

When a female cannabis plant sets seed, the concentration of active compounds in its buds drops measurably – in resin-rich drug types by an order of magnitude of 60 to 75 %, in fibre hemp sometimes not at all. The relative ratio of the compounds to one another largely stays the same, which is why a seeded sample still reflects the profile of a genetic line, but not its potency.

Why the plant forms these compounds at all

Cannabinoids and terpenes are secondary metabolites: substances that do not serve the basic metabolism but defence, signalling and the protection of tissue. They are not formed for consumption but because they increase the plant’s chances of survival and reproduction.

Protection against herbivores is best documented

In the field, the cannabinoid content of the leaves correlates negatively with feeding damage – cannabinoid-free plants were in some cases stripped bare within two weeks, while neighbouring plants containing cannabinoids remained almost unharmed. In an artificial insect diet, the acid forms CBDA and CBGA reduced the growth and survival of larvae in a dose-dependent way from a share of 0.1 %; a diet with 1 % CBDA killed all 45 test larvae within three days.

It is remarkable that insects lack the canonical cannabinoid receptors CB1 and CB2 – the effect runs via other target structures. That cannabinoid biosynthesis arose independently in several plant lineages also speaks for a genuine ecological function and against a by-product of metabolism.

What has become of the UV hypothesis

For a long time the explanation was that cannabis forms more THC at high altitudes to protect itself against UV radiation. However, several controlled studies found no increase in cannabinoid yield or concentration under additional UV-B irradiation. This does not refute the photoprotection hypothesis, but it no longer carries the explanation alone; antimicrobial action and protection against drying out are also documented.

Where the active compounds are formed and stored

Biosynthesis and storage take place in glandular trichomes, the glandular hairs that sit most densely on the female inflorescences. What is commonly regarded as “resin” is the content of these gland heads.

The two trichome types

Stalked trichomes carry a head of twelve to sixteen secretory cells, sessile ones about eight. In the mature bud, the stalked ones make up most of the glands and carry the highest cannabinoid content. A refinement from more recent work belongs here: the terpene profiles of the two types differ markedly – stalked ones dominated by monoterpenes, sessile ones shifted more towards sesquiterpenes – whereas the cannabinoid composition is fairly similar. The difference lies rather in the quantity, because larger heads hold more. Stalked trichomes apparently develop from precursors that appear sessile.

The bract carries the peak value

The perigonal bract – the bract that envelops the pistil and, after pollination, the seed – carries the highest cannabinoid content of any single part of the plant and at the same time the highest and most even trichome density. The seed thus develops in precisely the most resin-rich tissue of the plant.

The unfertilised state is a reproductive safeguard

In wild populations and in hemp fields, female flowers are almost always well supplied with pollen; there the style-stigma portion measures around 3 mm on average and is split in two. If pollination fails to happen, it expands to over 8 mm, branches several times and becomes sturdier – cannabis is one of the few species that actively enlarges its female sexual organs during prolonged virginity.

The resin-rich sinsemilla inflorescence belongs to this state of open, not yet completed reproduction. A distinction is important here: the studies document the expansion of the organs, not “more potent buds”. That unfertilised flowers carry more active compound follows from the drop in concentration after fertilisation, not from the growth of the stigmas.

What fertilisation changes

Resin-rich drug types

In drug types grown indoors, fertilisation lowered the total cannabinoid concentration markedly: in THC-rich strains by around 75 % on average, in CBD-rich ones by around 60 % – regardless of the type of the pollen donor. Profile and relative ratios were basically preserved; individual compounds even increased depending on the donor.

Fibre hemp reacts differently

In the fibre hemp variety FINOLA, total CBD remained unchanged, while the acid form CBDA was halved. Seeding therefore does not switch production off but shifts it within its precursor pool sizes. There is no fixed percentage – the effect is specific to the chemovar and to the compound, and reliable only for resin-rich drug types.

Terpenes follow two separate pathways

The monoterpenes fall in parallel with the cannabinoids, because both share the precursor GPP and are formed in the plastid. Sesquiterpenes behave independently, as they arise in the cytosol from FPP. The distilled yield of essential oil drops by about half through pollination, although this value combines cannabinoids, terpenes and other volatiles and does not report them separately.

The mechanism: redirection and termination

After fertilisation, carbon is preferentially allocated to seed production rather than to the formation of flowers and cannabinoids. This is not a weakening of the plant but a form of phenotypic plasticity – the resources go where reproductive success is now being decided.

The end of open growth

Cannabis has an indeterminate inflorescence: the shoot tip does not end in a terminal flower but continuously forms new units of a reduced leaf, two bracts, two single flowers and a side shoot. The transformation into a compact bud begins in the first days under short days and requires short days throughout. That pollination ends this open growth has been described as a model for Arabidopsis and is transferred to cannabis.

Where no seed grows, the active compound stays

A ploidy study supports the resource explanation from the other side: seed set lowered flower biomass and cannabinoid concentration, while triploids, which remain low in seeds, retained more cannabinoids under pollen exposure. Without pollen there was no difference between the ploidy levels – it is the seed that draws off the resources, not the contact with pollen.

Botanically, the cannabis seed is a fruit

What is traded as a seed is an achene: a dry, single-seeded fruit that does not open when ripe and whose seed is attached at one point to the hardened fruit wall. It forms from the ovary of a pollinated female flower. The thick fruit wall is an adaptation to dispersal by animals; on average a force of around 12 N was needed to crack an achene, with wild forms tending to have thicker walls than domesticated varieties.

What this means for a lab test

Because the relative ratios are preserved after fertilisation, a seeded sample still reflects the qualitative fingerprint of a genetic line – which cannabinoids and terpenes are present in which ratio, in other words the chemotype. For a representative potency figure, by contrast, seeded material is unsuitable: in drug types the absolute values are markedly lower, and the seed mass dilutes the sample further. A clean test needs bract or flower material, an explicit statement that the material is seeded and ideally an unseeded reference sample of the same genetics.

Maturity and CBN

A late harvest shifts THC oxidatively towards cannabinol (CBN), and growing practice links a high share of amber trichomes with a heavier effect described as narcotic. However, the human evidence for CBN as a sedative in its own right is thin; the milder character of aged material could just as well be due to the breakdown of terpenes. The observation is widespread, the causal attribution weakly supported.

“Seeded gives the full effect” – what is true about it

As a statement about potency, the sentence is refuted: in drug types, seeding brings less active compound, not more. As a statement of experience, it can nevertheless be explained. Traditional cultivation in the countries of origin was seeded anyway, because the sinsemilla technique is a modern method that only spread in the West from the 1970s; charas and ganja come from cannabis grown seeded and selected for resin over generations. Seeded plants also often stand longer, which shifts maturity towards CBN, and using the whole plant produces a different experience from consuming buds alone. So the yardstick of comparison is different, not the chemistry.

The history of the technique itself is also less certain than it is usually told: the term and its spread in the West from the mid-1970s are documented, while the frequently cited origin in Sinaloa in Mexico rests mainly on popular sources. Unpollinated flowers occurred earlier, sometimes by chance, sometimes in growing cultures of South and Southeast Asia.

Why two cuttings are no proof

Secondary metabolites vary strongly with light, ripening time and location. Two clones in the same room are therefore not a clean comparison – the microclimate is not homogeneous, and draughts, distance to the light and spacing between plants also play a part. That is exactly why the primary literature isolates the seeding effect differently: the same genetics fertilised versus unfertilised, in some cases with genetically identical, induced pollen. Anyone who wants to test the effect themselves needs this set-up, not two plants side by side.

What remains open

The function of cannabinoids has not been conclusively settled – protection against herbivores is currently best documented, the UV hypothesis has not been refuted, and the two do not exclude each other. Equally open is the size of the fertilisation effect beyond the resin-rich drug types. And one widespread expectation cannot be reconciled with the data: that the resin protects the seed and a seeded plant must therefore form more of it. The framework holds – cannabinoids are concentrated close to the seed, and selection for seed survival is discussed as a hypothesis. But the plant protects the seed by building it itself and embedding it in the bract that is the most resin-rich anyway, not by ramping up flower resin after fertilisation.