Ethylene in Cannabis: the Hormone Behind Flower Sex

Ethylene is a gaseous plant hormone. It promotes female flowers. If its action is blocked, even a female plant forms male flowers. Feminized seed, meant to grow only females, rests on this. Plants also make ethylene when wounded. That gave rise to the belief that pruning causes hermaphrodites, female plants with male flowers. On females the mechanism argues against it.

What is ethylene, and where does it come from?

A gas that plants form during ripening, ageing and strain. It forms via a precursor called ACC. How much forms is set by ACC synthase. That is an enzyme, a protein that drives this step. In cannabis a gene for it sits on the X chromosome, which females carry twice. It is a candidate for controlling flower sex. You can act on the formation of ethylene or on its action. Blocking the action worked better. Silver thiosulfate (STS) worked more strongly than AVG, an agent that inhibits formation.

  • To turn females into pollen donors, use STS.

Which hormones shift the sex of the flowers?

Ethylene above all. Sprayed with STS, 80.5 to 94.7 % of the flowers on female plants became male. Plants sprayed with water formed none. Gibberellins also triggered male flowers. These are growth hormones. Given alone, they only worked on early flowers. The stress hormone methyl jasmonate changed nothing.

Does wounding cause hermaphrodites through ethylene?

On females the mechanism argues against it. Cutting does raise ethylene. But more ethylene promotes female flowers, not male ones. In a trial, pruning gave no more hermaphrodites than no pruning. Males are different: ethylene can trigger female flowers there. Whether a cut does that in cannabis has not been measured.

  • Prune females normally. Pruning is not documented as a trigger.
  • On males, check the cuts for female flowers in the days afterwards, as described in the article on stress testing.

How do STS and ethephon work?

From two sides. STS blocks the action of ethylene. A female plant then forms male flowers, and its pollen sires only females. Ethephon releases ethylene instead. A male plant then forms female flowers. Both kinds of converted flowers are fertile.

  • Use STS and ethephon only on breeding plants, never on plants meant for harvest. Ethephon is described here only as a principle.

What follows for growing?

Targeted conversion with STS or ethephon is predictable, spontaneous hermaphrodites are not. Documented triggers are cold of about 13 °C at the switch, a short day on young plants and an abrupt light switch. Whether they act through ethylene has not been measured.

  • Stay well above 13 °C around the switch to flowering.
  • Check mother and pollen donor untreated for male flowers.

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