Some plants live in environments where fire is a frequent enemy to survival. Unless you’re big enough to outgrow the risk of fire with tough bark like Sequoia, the obvious answer is to lie dormant in the soil until the fire has passed, helpfully leaving lots of nice nutrients available for germinating seeds. But if you’re buried in the soil, you need to know when the time for germination comes before you get out-competed by species which are quicker off the mark.
In 1990, de Lange and Boucher reported the landmark discovery that aerosol smoke and aqueous smoke-water could promote the germination of Audouinia capitata, a rare South African species. Subsequently, smoke has been shown to promote the germination of many other species, many of which were previously difficult to germinate. Smoke-stimulated germination has extensive implications for horticulture, weed control, conservation and habitat restoration. Much effort has focused on determining the chemical(s) in smoke responsible for stimulating germination.
Karrikinolide (KAR1, or 3-methyl-2H-furo[2,3-c]pyran-2-one if you prefer) was once considered by some to be the sole chemical responsible for all smoke-stimulated germination. Tersonia cyathiflora, an Australian fire ephemeral with an obligate requirement for smoke to germinate, is unresponsive to known smoke chemicals such as KAR1. Most related species are fire ephemerals that germinate predominantly after fire, often in large numbers, and live for only a few years, thereafter persisting as seeds in the soil seed-bank until a subsequent fire. In addition to T. cyathiflora, other fire ephemerals in the Gyrostemonaceae, including Gyrostemon racemiger and G. ramulosus, germinate in response to smoke following a period of burial. However, the response of Gyrostemon seeds to KAR1 had not yet been tested previously. Germination stimulation of T. cyathiflora by plant-derived smoke-water, but not KAR1, suggests that there may be other chemical(s) in smoke-water that promote the germination of certain species. Glyceronitrile (2,3-dihydroxypropanenitrile) was recently isolated from smoke-water and stimulates the germination of a number of species, including various Anigozanthos spp. that are also unresponsive to KAR1. This chemical contains nitrogen in addition to carbon, hydrogen and oxygen, and is proposed to operate through the release of cyanide.
New research published in Annals of Botany tested whether seeds of Gyrostemon racemiger and G. ramulosus respond to plant-derived smoke-water and/or KAR1; and, secondly, whether seeds of G. racemiger were stimulated to germinate by glyceronitrile or nitrogen oxides. The results show that these species respond to smoke-water but not to KAR1 or to glyceronitrile. However, making smoke-water by burning cellulose alone showed that germination is stimulated by a compound composed only of carbon, hydrogen and oxygen alone. Whether the smoke-stimulated germination response is an ancient trait or has arisen from convergent evolution has been debated. Both smoke and KAR1 responsiveness are phylogenetically widespread which gives some support to this being an ancient trait.
Comparison of germination responses of Anigozanthos flavidus (Haemodoraceae), Gyrostemon racemiger and Gyrostemon ramulosus (Gyrostemonaceae) to smoke-water and the smoke-derived compounds karrikinolide (KAR1) and glyceronitrile. (2013) Annals of Botany 111(3): 489-497.
A major germination-promoting chemical in smoke-water is 3-methyl-2H-furo[2,3-c]pyran-2-one (karrikinolide, KAR1). However, not all species that germinate in response to smoke-water are responsive to KAR1, such as Tersonia cyathiflora (Gyrostemonaceae). In this study, a test was made of whether two Gyrostemon species (Gyrostemonaceae) that have previously been shown to respond to smoke-water, respond to KAR1. If not, then the smoke-derived chemical that stimulates germination of these species is currently unknown. Recently, glyceronitrile was isolated from smoke-water and promoted the germination of certain Anigozanthos species (Haemodoraceae). Whether this chemical promotes Gyrostemon racemiger germination is also examined. Furthermore, an investigation was carried out into whether these species germinate in response to smoke-water derived from burning cellulose alone. Gyrostemon racemiger and G. ramulosus seeds were buried after collection and retrieved in autumn the following year when dormancy was alleviated and seeds had become responsive to smoke-water. Anigozanthos flavidus seeds were after-ripened at 35 °C to alleviate dormancy. Gyrostemon and Anigozanthos seeds were then tested with ‘Seed Starter’ smoke-water, KAR1, glyceronitrile and cellulose-derived smoke-water. Although Gyrostemon racemiger, G. ramulosus and A. flavidus were all stimulated to germinate by ‘Seed Starter’ smoke-water, none of these species responded to KAR1. Gyrostemon racemiger germination was not promoted by glyceronitrile. This is in contrast to A. flavidus, where glyceronitrile, at concentrations of 1–500 µM, promoted germination, although seedling growth was inhibited at ≥400 µM. Maximum A. flavidus germination occurred at glyceronitrile concentrations of 25–300 µM. Some Gyrostemon germination was promoted by cellulose-derived smoke-water. KAR1 and glyceronitrile, chemicals in smoke-water that are known to stimulate germination in other species, did not promote the germination of G. racemiger. This suggests that other chemical(s) which promote germination are present in smoke, and may be derived from burning cellulose alone.
At this point, it’s time to reward the plant scientists “of a certain age” who will have been humming this all the way through reading this post:
- Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Ann Bot (2010) 105(4): 627-635
- Seed-coat Dormancy in Grevillea linearifolia: Little Change in Permeability to an Apoplastic Tracer after Treatment with Smoke and Heat. Ann Bot (2008) 101(5): 623-632
- The Changing Window of Conditions that Promotes Germination of Two Fire Ephemerals, Actinotus leucocephalus (Apiaceae) and Tersonia cyathiflora (Gyrostemonaceae). Ann Bot (2005) 96(7): 1225-1236