21 June 2023
Hot or not? A tale of plant heat production
A new study examines plants with the power to keep themselves warm, and the mystery of massive variation in this curious heating behaviour.
Mammals get hot. From the great whales of the sea to fairly average sized humans, to the tiniest pygmy shrew, our ability to keep our bodies warm indefinitely makes us special above all other life… doesn’t it?
Straight away, the birds have something to say about it. They’ve been keeping warm with their own metabolism for ages, very likely since the time of their split from reptilian dinosaurs. If we look beyond animals that can keep warm on a permanent basis, the list of heat producers grows even longer. Many fish can heat their muscles for periods of vigorous activity, while flying insects such as moths and bees warm up from the incredible energy needed to expand and contract flight muscles thousands of times per second.
Plants, however, fade into the background of our thermal world. A cold, unfeeling form of life whose temperature is at the whim of the weather any individual plant finds itself in? Not exclusively.
In fact, a few plants are very capable at warming some of their features to many degrees above the surrounding temperature. A fresh study co-led by Cyrille Claudel at the University of Hamburg and Kew’s Director of Science, Alexandre Antonelli, tells us there’s much more to hot plants than we think.
Why get hot?
We already know a little bit about plants that can make their own heat, a process known as thermogenesis.
In the family Araceae, containing plants known as the arums, heat production occurs in the flowers. Some species can generate flower temperatures as high as 45oc, even when the surrounding air temperatures are considerably brisker.
Botanists and horticulturists have long pondered over why. Perhaps warmth serves as a defence from freezing during a cold snap? Maybe it plays a role in easing flower unfolding?
The leading explanations are tied to a key player in plant reproduction – animal pollinators.
Producing heat may help attract pollinators capable of sensing the infrared spectrum, something invisible to our mammal eyes. Higher temperatures make certain chemicals used in insect attraction, called volatiles, more volatile, making them easier to spread. Warmth may be a reward for arriving pollinators keen to heat up without spending their own dearly earned energy reserves.
Despite this knowledge, until now there’s never been a study to examine thermogenesis across an entire clade of plant life, nor an attempt to understand how this unusual trait is maintained over the course of evolution.
Heating up with Amorphophallus
Cyrille and colleagues dug deep into an Araceae subgenus: Amorphophallus, known for their unusual and sometimes mighty flowers. Amorphophallus includes one of the most famous plants of all, the titan arum, an enormous plant that like many of its close relatives, pollinates by emitting a foul smell. This death-essence tricks insects looking for something dead or dying on which to lay their eggs, into becoming non-consenting pollen delivery drones.
The team selected 80 Amorphophallus species for close investigation. Most flower unpredictably and so patience was crucial, with the study taking a whopping ten years to complete in full. While most species studied grow in the botanical garden Hamburg, much of the data collected was only made possible thanks to Stephen Jackson, a world-leading Amorphophallus horticulturist who grew and examined some of the otherwise inaccessible species in this study. The team also carried out measurements in the Gothenburg botanical garden, Sweden.
Temperature probe experiments revealed that the majority of those tested – 64 of 80 species – do possess the ability to heat their flower structures at least 1.5oc above the surrounding air temperature. Among 16 of these however, 10oc or hotter increases in flower temp were observed, with one species (A.longituberosus) achieving a toasty 21.7oc over its surroundings.
New thermal camera imagery shown below captures the extraordinary patterns of this heating behaviour in a few of the species studied, in a way never seen before.
An evolution question
What we see here, within one genus of plant, is extensive diversity in heat-producing behaviour. The only thing that unites them is that heat is produced in exclusively male, pollen-producing flowers, and the appendix – the site from which lots of pollinator attracting chemicals are released. Broad variation in the scale of the temperature increases achieved by different plants is matched with unequal lengths of time that the heat is maintained, some for as little as a few hours, others as long as a month.
Moreover, why do some species show no warming whatsoever? Heat-production is costly, and so must bring a significant advantage for it not to be lost on the timescale of evolution. Has this proven the case for those species, or did they never have the power to begin with?
Examining the genetic history of the 80 species, Cyrille, Alex and their colleagues suggest that the ability to heat their flowers arose early in Amorphophallus’ separation from Araceae, with the genes that make heat production possible probably widespread through all of these species.
If this does turn out to be the case, a likely scenario is that the variation in heat-production between (and within) species can be explained by differences in how these identical genes are expressed, as opposed to different species having evolved heat production genes separately.
These, however, are the questions and theories of tomorrow, rather than the answers of today.
Warming up our knowledge
A long road of discovery lies ahead. Can anything explain the high variation in temperature patterns between species? How has this highly costly trait evolved and persisted, particularly in countries where average air temperatures are already particularly high? Do we actually understand the benefits that heated flowers bring, via pollination or otherwise?
This study, the largest of its kind so far, has laid the foundations for such questions to be investigated in detail, using Amorphophallus and comparisons to other plant clades as a case study.
Investigating these unusual phenomena in nature gives us more insight into the complexity of how our network of global ecosystems function. The more we understand, the better equipped we are to preserve Earth’s biodiversity in the long run.
A keen reminder of the urgency of such research is that numerous members of the 80 Amorphophallus species in this study are currently teetering on the brink of extinction. What abilities gifted to us by our natural world are we losing every day, abilities perhaps key to our future, without ever realising they were under our noses?
Plants are heating themselves up in ways we’re only now starting to understand, and we’re hot on the trail of new discoveries that may yet change our future for the better.