What is "karrikin"

Karrikins are a group of plant growth regulators found in the smoke of burning plant material. For many years smoke from wildfires or bushfires was known to stimulate the germination of seeds. In 2004, butenolide was shown to be responsible for this effect. Later, several closely related compounds were discovered in smoke, and are collectively known as karrikins. Six karrikins have been discovered in smoke and are designated KAR, KAR, KAR, KAR, KAR and KAR, but KAR to KAR are the most active. The butenolide part of the compound is a 5-membered lactone ring while the other part of the karrikin compound is a 6-membered pyran ring.