Narrow Neck Plateau in Australia’s Blue Mountains is usually a lovely place to visit. It is, in essence, an enormous sandstone peninsula, one that juts some 10 kilometres from the main escarpment on which the town of Katoomba sits. The plateau presides over two vast valleys of eucalypt forests more than 400 metres below, and due to the curve of the land and the height of the trees, those forests seem to rise up to meet the sheer cliffs of Narrow Neck on either side. The treetops reach within 100 metres of the precipice and from there, dense clumps of plant life grow in whatever nooks and fissures they have found in the sandstone. It is, if nothing else, a fine example of the dispersing power of seeds. Plants long ago proved they could spread and thrive on dry land – here they are just showing off. There is another forest up on the plateau, too, rich with flora and fauna, much of it native, and including rare and threatened species. It forms an elongated ecosystem replete with eucalypts, heathlands and wildflowers.
But on 1 December 2019, Narrow Neck was one of the worst places you could be. A few days earlier, lightning had sparked a bushfire in the Jamison Valley to the east, and despite firefighters’ best efforts to contain the blaze, it spread quickly through the tinder-dry eucalypts, aided by a catastrophic combination of hot weather and fierce winds. Enormous flames licked up the side of Narrow Neck, consuming cliff-side foliage and filling the sky with thick, billowing plumes of grey smoke. Much like a match-tip touched to a flame, the peninsula had caught alight.
The fire threatened the towns of Katoomba, Leura, Medlow Bath, Blackheath and others, blocking any escape to the south. Meanwhile, the Gospers Mountain fire bore down from the north, merging with several other bushfires to form an unstoppable ‘mega-blaze’. At times, the flames reached 60–70 metres into the air.
In a December 2019 article by the Blue Mountains World Heritage Institute, Lucy Baranowski, a volunteer firefighter who responded to the Gospers mega-blaze, was quoted as saying:
The earth has been razed so hard, the ground is like pottery fresh out of the kiln. The trees are burnt out like matches. There’s no chance of finding injured wildlife – everything is dead and has turned to ash and dust. Nothing would have survived that fire.
Narrow Neck fared little better. Not all of the canopy was incinerated, suggesting the fire here did not burn quite as intensely as at Gospers, but it still annihilated most of the plateau, leaving scorched earth and charred tree trunks in its wake. As with many of the places impacted by bushfires that summer, a suffocating haze of smoke and drifting ash settled over the Blue Mountains and spread as far as Sydney, making the air almost unbreathable and turning the sky a dark, eerie colour. In those last days before anyone had ever heard of COVID-19, face masks and respirators were already in high demand. Seen from space, the plumes of smoke from so many fires turned eastern Australia into a picture of its former volcanic self, a glimpse of something ancient and perhaps on the precipice of extinction.
In the end, it was relentless firefighting and wet weather that caused the many fires to flicker out, with the Gospers blaze finally extinguished with the aid of torrential rains in early February 2020. I can still remember the images in the news: exhausted firefighters kneeling in the rain, arms spread wide, their weary, soot-smeared faces turned up to the sky. That summer, more than ten million hectares of land was burned, including nearly 80 per cent of the Greater Blue Mountains World Heritage Area. It was later revealed that the majority of the fires had been ignited by lightning strikes. Using satellite data, a team of ABC journalists was able to determine that the lightning strike responsible for the Gospers Mountain fire, the biggest in the country’s history, had lasted just 518 milliseconds. In those drought-stricken bushlands, catastrophe required only the slightest nudge.
The first wave of COVID-19 infections began before the fires were fully extinguished, and TV cameras panned away from the bushfire destruction to zoom in on overwhelmed hospitals overseas. Still, sombre follow-up reports on the fires detailed how thirty-three people and around three billion animals had died. According to a report released by the Wildlife and Threatened Species Bushfire Recovery Expert Panel, many of Australia’s 26,000 plant species had also burned, including 486 species that had been deemed endangered or critically endangered before the blazes even began. Some species, including Forrester’s bottlebrush (Callistemon forresterae), Betka bottlebrush (Callistemon kenmorrisonii) and grey Deua pomaderris (Pomaderris gilmourii var. cana), were ‘at imminent risk of extinction’. There were also wince-worthy estimates of how much CO2 the fires had released, somewhere in the ballpark of 830 million tonnes. Yet, here and there, vibrant green shoots were emerging from the soot-dark ground or poking out of charred stumps. In a decimated landscape that appeared incapable of ever supporting life again, here were tiny new stems straining upwards, a sign that perhaps not all was lost, that something, however small, could literally rise from the ashes. In a year that seemed absent of hope, we were grateful for the metaphor.
Several days before Christmas 2020, about a year after the fires had torn through Narrow Neck, one of the most intriguing signs of recovery appeared. Bushwalkers noticed a few pink-and-white flowers growing on the plateau. Pictures started appearing on social media, beautiful close-ups of what, at first glance, looked like daisies, each with downy white petals and a domed pink centre. They were tiny, too – several could fit in the palm of a child’s hand. They were identified as pink flannel flowers (Actinotus forsythii), so named for those furry-looking petals, which aren’t actually petals at all: they’re bracts, modified leaves recruited, in this case, as an extra flourish to attract pollinators. Pink flannel flowers are endemic to the Blue Mountains, but even for locals they are a rare sight. In fact, they hadn’t been seen on Narrow Neck for sixty-four years. There is one story of a woman in her nineties making the trek to view the flowers. She had seen them the last time they’d grown on Narrow Neck, after a devastating bushfire tore through the same area in 1957. In all likelihood, she was now looking at their offspring.
As the weeks passed, more flowers grew. The summer was bright and warm, and mercifully lacking in mega-fires. Instead, La Niña rains came, courtesy of fiercer-than-usual trade winds pushing warm water towards the western edge of the Pacific Ocean. What began as a rare glimpse of a few pink flannel flowers had by late summer transformed into entire fields of blooms. They spread across the damaged plateau, growing in thickets, softening cliff edges, and surrounding the blackened remains of eucalypts and banksias. Remarkably, pink flannel flowers even began to appear in regions burned by the Gospers Mountain blaze. The phenomenon attracted a steady stream of local sightseers and, in due course, even made international news. But while it all seemed a delightful surprise, ecologists had been expecting it.
Pink flannel flowers are fire ephemerals, meaning their seeds only germinate following fire. Yet fire alone is not quite enough for this species. They also require a period of cool weather and rain. This unique combination of cues accounted for the flowers’ rare appearance, because unless these conditions are met, the seeds will not germinate. Indeed, it appears the seeds on Narrow Neck and elsewhere had been lying dormant for decades, just waiting.
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As seeds evolved, they developed structures that enabled them to better disperse. Some became so light and aerodynamic they could travel on the wind, others could float on water, others found ways to hitch rides with unwitting animals. In this fashion, seeds enabled plants to spatially disperse anywhere from a few centimetres to thousands of kilometres away. Dispersal is a neat strategy for species survival because it allows an organism to roll the dice and venture out as existing habitats change over time or become too competitive. There is always a risk in sending the next generation to new locations, but potentially there’s a big pay-off. Even moving a few metres out of the shade of another tree, or into it, can make all the difference in how well a new plant will fare. Likewise, shifting a population of trees further up or down a hill may provide precious access to groundwater or exposure to a slight but beneficial shift in temperature. Sometimes a suitable new habitat is kilometres away yet reachable by storm winds, a winding river or a particularly energetic albatross.
Dispersal by any of these means can be quite useful, but the ability of a plant population to establish itself on remote oceanic islands, or just a little further down a valley, was not the only trick seeds brought to the plant party. Sometimes, finding the right conditions involves staying exactly where you are and waiting for them to come to you. This is perhaps seeds’ greatest manoeuvre: the ability to disperse not just through space but also through time.
In biology, the term ‘habit’ refers to a predictable, sometimes instinctive behaviour. It can also refer to a structure. Seeds involve both. As such, the evolution of seeds in plants is sometimes referred to as the ‘seed habit’, which bestows on each new generation of plants the ability to wait for opportune conditions. Springtime, for example, brings longer days, warmer temperatures, and rainfall. For the seeds that formed and dropped before winter, these are all worth waiting for. A new plant generation may also need to coordinate its timing with the presence of certain nutrients or the arrival of pollinators. The emergence of pink flannel flowers on Narrow Neck Plateau is a perfect example of how plants use seeds to disperse their genes into the future. When at last those seeds germinated, there was plenty of light for growth because the canopy had burned away and the soil was full of fire-generated nutrients.
Time dispersal is also about avoiding the wrong conditions. The height of summer in some parts of the world can kill an emerging seedling as swiftly as the dead of winter in others. Plants adapt their strategies and use seeds to sidestep hostile circumstances. Being bound by the laws of physics just like the rest of us, a seed cannot actually flicker forwards in time, of course. Instead, it hunkers down and protects the embryo so that it might grow at some favourable point in the future.
This is what Christina Walters loves about seeds. At the USDA in Fort Collins, she investigates how seeds survive long periods of time in soil and in storage, and how such knowledge can be used to improve seed preservation. She tells me that seeds ‘are the basis for the next generation, the basis for gene flow’. Though they are full of potential, most seeds spend much of their existence trying to prevent germination. This might sound counterintuitive but it actually makes a lot of sense. To better understand this, let’s take a quick look at what germination entails.
Germination always begins with a drink. During a process called imbibition, water seeps into the seed and a whole raft of metabolic processes begins, involving gene transcription, enzyme reactions and more. The tight little plant embryo starts to grow and elongate, and that tiny embryonic root, the radicle, nudges its way out until at last it breaks through the seed coat. The emerging sprout has a long way to go before it can start photosynthesising, so this is where those food stores come in handy, fuelling early growth as the radicle edges downwards into the soil and the young seedling unfurls itself into the light.
Germination is perilously irreversible. A seedling, having emerged, cannot curl back into a seed and wait a little longer. A plant will likely die if it germinates in the wrong season, when the weather is too hot or too cold, when the length of the day is all wrong, or when a killer frost or a bushfire is likely. Timing is everything. For this reason, seeds must endure the wrong conditions while waiting for the right ones. ‘They have this ability to handle some of life’s biggest challenges and take it on the chin,’ says Walters. In aid of this, all seeds are equipped with some kind of physical protection. It might be a hard seed coat, a fruit or a seed pod, perhaps even a hard cone or a spikey ball. But the ability to survive for long periods of time and to know precisely when to germinate often requires more than just protection.
Let’s say a seed falls onto the soil in autumn and sprouts in spring. Why did it not germinate after the first rain it experienced back in autumn? Why do pink flannel flowers not grow on Narrow Neck Plateau soon after every downpour? How do seeds take the measure of their surroundings so astutely? How do they get it right?
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Save for a rare few people in this world, of whom I am jealous, most of us do not fall asleep the second we lie down in bed at night. No matter how still we might be, sleep may be some way off. The difference between an inactive seed and a dormant seed is a bit like the difference between just lying still and being properly asleep. They might look the same from the outside, but they’re not. A viable seed that is simply inactive but not dormant can germinate at any time in the presence of water, but a dormant seed will avoid germination even then, and can only germinate after dormancy is broken.
Mark Ooi is a plant ecologist at the University of New South Wales in Sydney, and he’s particularly interested in the dormancy of fire ephemerals like the pink flannel flower. Dormancy is protective, says Ooi. ‘The way I see it, dormancy is there to prevent germination when there aren’t any suitable conditions for the seedling.’ Indeed, more than 50 per cent of wild plants make seeds that are capable of dormancy, and outside tropical environments, its the substantial majority. According to Ooi, seeds achieve dormancy in a variety of ways. Some seeds are physically dormant, wherein a physical barrier – like a tough, impermeable seed coat – prevents water from seeping in and hydrating the seed’s interior. These tough coats also restrict the supply of oxygen to the embryo, where it would trigger the premature production of a variety of germination signals and could also cause damage. It is only after the seed coat is breached that the seed can become hydrated and germinate.
The seeds of the Canna indica plant are a prime example of physical dormancy. Commonly known as Indian shot or African arrowroot, Canna indica has tall, leafy, green stalks tipped with bright crimson, vermilion or sometimes yellow flowers with a shape reminiscent of irises. They belong to the same genus as Canna compacta Roscoe and both species have very durable seeds. Recall how viable 600-year-old Canna compacta Roscoe seeds were found in a rattle necklace in Santa Rosa de Tastil? Although it’s not known exactly how long Canna indica seeds can live, we do know they’re incredibly tough. Canna indica have tiny, dark, spherical seeds just several millimetres in diameter. With their shape and their smooth hard coat, they really do resemble gunshot pellets. The plant’s common name of ‘Indian shot’ is rumoured to have its origins in the First War of Indian Independence in 1857, also known as the Sepoy Mutiny. Legend has it that when soldiers ran out of lead shot for their rifles, they used Canna indica seeds precisely because they were so small, round and incredibly hard. Though it is difficult to verify the story, it’s certainly plausible. During an episode of the BBC documentary How to Grow a Planet, the show’s host, Professor Iain Stewart, loaded a shotgun cartridge with Canna indica seeds, then fired at a target made of thin wood. Shown in dramatic slow motion, the Canna indica seeds pierced the target and embedded in a large block of ballistics gel just behind it – the seeds prised from the gel were unscathed. Canna indica seeds did not evolve to become backup ammunition, of course. The strong seed coat is there simply to keep water away from the seed embryo so it can remain dormant for long periods of time.
Acacias provide another good example of physical dormancy in seeds, says Cathy Offord, a conservation biologist at the Royal Botanic Garden in Sydney. ‘What’s stopping acacias, generally, from germinating’, she says, ‘is that they have this hard seed coat that needs to be breached so water can get inside them.’ There are well over 1300 species in the Acacia genus, and they are found in tropical, subtropical and warm temperate regions of the world. Around 1000 of these are native to Australia, where they are referred to, collectively, as wattles. Acacias span a variety of habitats, but the tough seed coat seems to serve them well. In fact, to germinate the seeds of Australia’s national botanical emblem, the golden wattle (Acacia pycnantha), it is often recommended to douse them briefly in boiling water. Similar suggestions abound for Canna indica. Intriguingly, it is precisely because the Canna indica plant really likes water that such a tough barrier is in place. Canna indica are native to the tropics and the plant grows best in warm, damp soil. The hard seed coat prevents the seeds from germinating unless the weather is warm and there is adequate water available.
Physical dormancy occurs in eighteen families of angiosperms, whose seed coats are impermeable to water thanks to an outer layer of dead cells that contain, and are covered by, numerous water-repellent molecules, such as lignin, suberin and a variety of waxes. Lignin provides rigidity (it’s a key component of wood) while suberin is water-resistant (it’s the main constituent of cork). So how does water eventually find its way through such a barrier? Well, for one thing, tough seed coats might simply break down as the result of damage, such as that incurred while passing through the digestive tract of an animal. But the best way for water to get in is through a microstructure called a water gap, which is sort of a necessary weakness found in most physically dormant seeds. There are around two dozen different types of water gaps, but in general they function ‘like a little plug’, says Carol Baskin. When the water gap is firmly in place, water cannot pass into the seed. ‘What has to happen is specific environmental cues have to open a water gap on the seed,’ she tells me. ‘When it opens or falls apart, there’s a tiny hole. This is where the water gets in.’
Most water gaps are temperature-sensitive. ‘Some become sensitive when it’s hot and dry in the summer and others become sensitive when its cold and wet in the winter,’ says Baskin. The change in temperature causes slight shifts in the microscopic structures of the water gap. It is a marvel of anatomical engineering at the cellular level. Baskin says some ‘beautiful work’ was done in Australia demonstrating that a two-step process is needed to open the water gap on some physically dormant seeds. In this case, she says, ‘the hot summer days make the seeds sensitive, and then the cooler, wet days cause the water gap to actually open. But if you just take the seeds and put them in wet conditions, nothing happens. You can’t fool them!’
Yet, for most seeds, dormancy isn’t afforded by a tough outer layer but is guided by changes inside the seed itself. This is called physiological dormancy, and there are a number of different ways in which it plays out. First of all, there are cases where the embryo inside the seed hasn’t finished growing yet. This, says Baskin, is an example of why the ‘baby in a box with its lunch’ analogy, while evocative and often useful, needs to be modified a bit. Most seeds do indeed contain immature plants. In other words, they contain ‘differentiated’ plant embryos in which many different cells have already taken on specific roles – some are locked in to form root tissues while others are already following a cellular fate as leaf cells. The result is a tiny plant inside the seed. But some seeds, like those produced by orchids, are at a much earlier stage. Their embryos are undifferentiated, says Baskin. ‘They’re just a little blob of cells.’
Cathy Offord works with a number of tropical orchids that, she says, have ‘tiny, tiny seeds that are like dust’. She explains that orchid seeds not only lack a nutrient-rich endosperm, ‘they just have a few cells that are the embryo, and they need to grow before the seeds can germinate.’ Those cells have to divide and divide again, differentiating along the way, forming those little plant structures. When the baby plant is ready, then they can germinate.
It would appear that, through evolutionary trial and error, orchids arrived at the bare minimum a seed could be equipped with and still function as a means to disperse the next generation of genes. It’s not a good gambit to send one’s offspring into the world so woefully ill-equipped, so orchids play the numbers, releasing tens of thousands, or even millions, of these ‘dust seeds’ at a time. As perennials, they ultimately produce many millions of seeds in their lifetimes. ‘I tell my students this is the shotgun approach,’ Baskin tells me. The seeds are so light that ‘they blow everywhere’, and there are so many that some are bound to land somewhere with good conditions. She adds: ‘It makes sense to me that it’s a strategy to produce a lot of offspring with the idea that hopefully a few would find a little spot to grow.’
Dormancy due to not-quite-finished embryo development isn’t limited to microscopic seeds. Take, for example, the seeds of the Coffea arabica and Coffea canefora plants, which you may know as ‘coffee beans’. In coffee farming, the time between seed planting and seed germination is around two to three months, largely because the seed embryos require that time to grow before they are ready to sprout.
Of course, the majority of seeds in the world contain a well-developed plant embryo. They have seed coats, but oxygen can get in, and water, too. Nevertheless, they don’t germinate right away, sometimes not for years and years. These seeds have a few cellular tricks that allow them to wait until conditions are just right. Mostly, this comes in the form of an intricate network of biochemical interactions that monitor and respond to changes in the environment. ‘There is still a little bit of physiological activity going on in seeds, they are alive but they’re also basically sleeping,’ says Offord. ‘I think you could describe it as a seed sleeping with one eye open.’
Over time, as seeds slip in and out of dormancy, genes can wink on and off and biomolecules break down while others are made. There are too many genetic and cellular components involved to describe here, especially as each one interacts with something else, leading to a bewildering series of pathways and connections – enough to make geopolitics look simple by comparison. Yet, much of it comes down to a balance between two key hormones: gibberellins (GAs) and abscisic acid (ABA). GAs promote germination – recall that Elaine Solowey sprinkled some gibberellic acid (hormone) on the ancient date palm seeds to help wake them up, and it worked. ABA, however, blocks germination. So, high levels of abscisic acid tip the scales towards dormancy. Conversely, when abscisic acid levels are low and levels of gibberellins are high, the scale tips towards germination.
Environmental cues work by placing a proverbial finger on those scales. It might be a change in temperature, humidity or soil pH, or the presence of certain species of fungi. Light is another big one. There are some seeds for which light is the central cue, says Baskin. ‘They require light to germinate, [and] as long as they’re buried in the soil where it’s dark, they can be there for years and years until some movement of the soil brings the seeds to the surface,’ she explains. ‘[Seeds] may just sit there waiting for the right environmental cue and those cues can be compounds from fire, it could be ethylene which comes from flames, [there are] all kinds of cues that seeds can perceive in the habitat. They can even perceive differences in day and night temperature. For example, if some seeds are buried in the soil and they get moved to the surface, the greater [temperature] difference between day and night can tell them, “Oh, we’re on the surface, this is okay, we can germinate!”’
Environmental cues alter the balance between ABA and GAs. A good example of this is the seed that drops onto the soil – or is planted – in autumn and germinates in spring. For many seeds like this, it’s not just a matter of putting up with winter weather. They actually require several weeks of cold temperatures, because while it starts out with high levels of ABA, the cold prompts the activation of enzymes that gradually break down ABA. There are other species, called winter annuals, that follow a different temperature regime. In this situation, high temperatures cause ABA production to increase and GAs to decrease, which inhibits germination during summer. When cooler temperatures approach, the scales tip the other way: ABA levels decrease while GA levels begin to rise, setting the stage for germination in autumn. Temperature is not the only signal, of course – seeds often require a combination of specific cues.
‘There’s this vast range of behaviours within seeds just in dormancy,’ says Offord, adding that a lot still remains unknown. For any one seed, it’s not always easy to predict what kind of dormancy it will have, if it’s there at all. That said, there are a few interesting trends. Plants within the same genus tend to share dormancy characteristics. Meanwhile, seeds of many rainforest species are non-dormant, making it difficult to wait out bad conditions. For any seed that can enter dormancy, each is equipped with mechanisms finely tuned to respond to specific cues, so when those cues begin to appear, they tug a seed out of slumber like an alarm clock that starts softly and gradually rises to a crescendo. But just because a seed comes out of dormancy doesn’t necessarily mean it will germinate. It comes down to whether the seed determines it’s really worth getting out of bed.
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One might have thought that seeds, once dormancy is broken by a long winter or a warm summer, could then germinate at any point thereafter with a bit of rain. However, it’s now clear this isn’t necessarily the case. Carol Baskin tells me that in one study, she and Jerry Baskin buried a huge number of seeds in pots of soil and kept them in a greenhouse on the campus of the University of Kentucky, to keep them out of reach of hungry squirrels. They – the Baskins, not the squirrels – buried seeds for up to 100 species, she says. ‘We kept continuous records of temperature, so we knew how many hours of heat, how many hours of cold.’
The findings were interesting. Take the case of ragweed, which has seeds that mature in autumn, lie dormant in winter and usually germinate in spring. ‘They require light to germinate so if they go through the winter, and dormancy is broken, and if they are [then] exposed to light in the spring, they germinate,’ says Baskin. But if they’re in the dark, they won’t germinate, she says. And here’s the interesting thing: if they’re exposed to light later on, when temperatures begin to warm up for summer, they won’t germinate. What has happened, says Baskin, is that there’s a brief window during which the seeds came out of dormancy and had an opportunity to germinate, but the conditions weren’t right – it was too dark – so they went back into dormancy.
It’s an important survival mechanism for ragweed, she says. ‘Spring is the time to germinate because the plant needs the whole summer to grow and make more seeds. If it doesn’t get the opportunity to germinate in the spring, it is kind of useless to germinate later because it can’t reproduce.’ So ragweed seeds have a mechanism to deal with that: ‘The increasing temperature as summer approaches sends the seeds back into dormancy.’ And what about other species like Arabidopsis that usually germinate when it gets cool in the autumn? They have a mechanism too, but it works in reverse. ‘If for some reason those seeds don’t germinate in the autumn’, says Baskin, ‘as it gets really cold the seeds go back into dormancy. So what we have really learned is there are two things that have to happen: one, the right conditions have to be present for dormancy to be broken, and even if the dormancy is broken the seeds may not germinate, because the environmental cues are not present.’
There are often combinations of cues that need to be met. Cool temperatures might break dormancy in a particular seed, but without ample spring rain or longer days, it won’t risk emerging and will drop back into dormancy. Only when dormancy is broken again will there be another chance. That’s just the nature of it, says Mark Ooi. ‘You can come out of dormancy and be sensitive and ready to germinate if all the other cues are there.’ And if they’re not? ‘You go back into dormancy and you need to go back through that cycle again.’
Many seeds ride this rise and fall of slumber, a sinuous harmonic wave extending outwards in time. Each one can only exit this resonance at certain points, and only with the right cues. Baskin clarifies that this type of physiological dormancy, which is guided by the seesaw of ABA and GAs, is referred to as ‘non-deep’ physiological dormancy. In other words, the seeds can enter dormancy, but it’s not deep. As you can probably guess, there are some seeds that can and do enter deep dormancy, but exactly what’s going on inside these seeds is still a bit of a mystery.
‘We really don’t know all that much about deep physiological dormancy,’ says Baskin. ‘If seeds have non-deep physiological dormancy and you treat them with gibberellic acid, it’s very likely to promote germination, but if they have deep physiological dormancy, GA doesn’t do anything. To me that sort of indicates that there’s something different going on with the deeply dormant ones.’ But this phenomenon has not been well studied, she adds, because these species are uncommon. Nevertheless, she doesn’t think their seeds can cycle in and out of dormancy the same way many other seeds do. She says that ‘once they have lived long enough for the dormancy to be broken, I don’t think they can go back into dormancy … the seed is committed. It has to germinate or die.’
Whatever the species or the type of dormancy, teasing apart the cues that break dormancy from the cues that trigger germination can be tricky, says Ooi, because a lot of these cues might happen at the same time. He suspects the pink flannel flower seeds on Narrow Neck had been going through cycles of non-deep dormancy for many years. They probably came out of dormancy many times and could have germinated. But something was missing – they needed all the cues, not just some of them. It’s sort of like a botanical combination lock in which seeds check off environmental cues until they have the full set required by their species.
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On Narrow Neck, thousands of pink flannel flowers are already preparing for the next big bushfire. Perhaps it will be another sixty-four years away, although a reprieve that long is unlikely. As global temperatures continue to rise, the frequency of severe fire conditions is predicted to increase accordingly. But for now, at least, it is quiet. The summer has come and gone, and most of autumn, too. La Niña has broken down and the days have become clear and bright. Flowers are still being pollinated by native insects, drawn towards the bright pink blooms encircled by those white bracts that look as soft as moth wings. Once fertilised, the plant will funnel resources into the development of tiny dark seeds covered in a pale fuzz of fine hairs. The seeds will drop to the soil and then they will wait. They will cycle in and out of dormancy, keeping a molecular record of the outside world. They will sense temperature, moisture and light, but not fire – not exactly. It’s not really the flames the seeds are waiting for. ‘It’s definitely the smoke,’ says Ooi.
A few years ago, he and his colleagues showed that when pink flannel flower seeds were given the right temperature regime, and the right amount of light and water, not much happened. But when they were provided these cues and also exposed to a solution of smoke-infused water, the seeds germinated really well, and without a flame in sight. ‘There are a few key compounds in smoke that seem to cause a germination response,’ Ooi says. But while there is evidence that bushfire smoke tips the hormonal scales, he is unsure if the smoke helps the seeds make more gibberellins or somehow suppresses the abscisic acid. He’s keen to find out, though. When I ask him what it is within smoke that the seeds are reacting to, Ooi explains that smoke is a rich cocktail of chemical compounds and some seeds have evolved to respond to specific chemicals produced when plants burn. Chief among these are small molecules called butanolides. In 2004, a team of scientists at the University of Western Australia showed that a curious yet unpronounceable butanolide (3-methyl-2H-furo[2,3-c] pyran-2-one) was a major driver of germination in a number of fire ephemerals. It wasn’t long before other closely related compounds were found to have a similar effect. Acknowledging that the discovery was made on the traditional lands of the Noongar people of Western Australia, the scientists named this unique group of butanolides ‘karrikins’ after the Noongar word for smoke: karrik.
During a fire, karrikins are produced by the burning of plant materials such as cellulose and sugars. They end up in the smoke and ash and eventually settle on the ground. Depending on the species, fire ephemerals wait for these karrikins or a handful of other smoke-borne molecules. Intriguingly, this is precisely how fire ephemerals avoid fire – by producing seeds that germinate exactly when the probability of the next major fire is at its lowest, which is soon after the last one. If the plants wait too long to emerge, it could be lethal for the species, says Ooi, because there won’t be enough time for seedlings to grow, flower and bear seeds before the next blaze. So, plants figured out a long time ago that nothing prevents fire quite so well as an already burned landscape. There also appears to be a close link between the timescale of fires and how quickly the seeds germinate after a fire, says Ooi. He explains that, in systems where fires occur at ten- or twenty-year intervals or more, the seeds can afford to go into longer dormancy and take their time germinating after a major fire, sometimes emerging months later, like the pink flannel flowers did. But this isn’t always the case, says Ooi: ‘In places where you have vegetation that burns really frequently, like grasslands, then you get a whole suite of different mechanisms.’
Ooi recently collaborated with ecologists Heloiza Zirondi and Alessandra Fidelis at the Universidade Estadual Paulista in Brazil to investigate fire ephemerals in the Cerrado, a large, grassy, subtropical savannah in southern Brazil. ‘It burns every couple of years,’ says Ooi. That’s a small window, so the seeds need to respond quickly. ‘Most of the species there have this mechanism where they flower straight after fire,’ he says.
Gavin Flematti from the University of Western Australia, who was involved in the discovery of karrikins, suspects this mechanism evolved sometime between sixty-five million and 145 million years ago. As he and his colleagues explain in their 2015 paper in the journal BMC Biology, ‘Seed plants could have “discovered” karrikins during fire-prone times in the Cretaceous period when flowering plants were evolving rapidly.’ Indeed, numerous charcoal deposits indicate forest fires were rife back then, due to a combination of storm-generating warmer temperatures, higher levels of atmospheric oxygen, a plethora of active volcanoes, and an abundance of fuel in the form of plants. During this time, ferns were still going strong, conifers were growing like crazy, and angiosperms were on the rise. In other words, flowering plants evolved in a fiery world, and it seems some of them contrived a workaround.
In evolution, however, nothing comes without a cost. For seeds, gaining the ability to disperse – whether through space or through time – involved trade-offs. These small, cellular compromises would go on to profoundly alter the evolution of many animals, including humans.