Sean Otterspoor- 1997
Remember if you will, back to kindergarten. We learned that plants, our peaceful oxygen producing friends, were passive slaves to the whim of nature. Frost, snow, and late springs are their mortal enemy. What if I were to tell you that certain plants were actually able to produce their own heat? What if certain plants had found a way to actually regulate their internal temperature in respect to the outside environment? What if these plants used this "knowledge" to entice insects, and then use the unsuspecting beetle for it’s own perverse sexual needs. What would happen, now that a few plants could now stand up to nature? World conquest? The domination of the Earth by hordes of warm-blooded plants set on avenging the centuries of slaughter their fellow plant brothers and sisters endured for the sole reason of human consumption? Maybe not, but then you didn’t realize that in your very backyard are warm blooded plants. So lock your doors, and bring the dog inside. Pray that you can convince them that you’re a vegetarian, and that those flowers in the dining room are silk.
Why would a plant put energy into generating heat, what gain could such an expenditure of energy serve? Why wouldn’t a plant just delay its germination until temperatures were more inviting. Compared with animals, rates of respiration and heat production in plants are generally quite low. Exceptions to this rule occur among several species of the arum lily family and certain water lilies. Their flowers produce so much heat that some species are able to raise the temperature of the inflorescence as high as 35°C above the environmental temperature. Explanations for this phenomenon include volatilization of insect attractants, promotion of flowering in cold weather while protecting the flowers from frost damage, and providing a warm environment for insect pollinators (Seymour, 1991).
Seymour and his team have focused their studies of thermogenicity on Philodendron selloum, figure 1.
It is a variable species with a wide distribution through northern Argentina, Bolivia, Paraguay and southeastern Brazil, found in areas where the tree cover is thinner and the light brighter like the edges of waterways, swamps, coastal scrub or outcrops of rock. Some confusion exists over the correct name, in the past botanists have recognized both P. bipinnatifidum and P. selloum. Some have used the two names to differentiate distinctive populations but recent studies suggest that they are best regarded as a single polymorphic species (Brown, 1988).
However the raised temperature of the inflorescence does not in itself attract pollinators. In one experiment an artificially heated but scentless spadix was substituted in Arum nigrum inflorescences and did not interest insects at all. The main purpose of the heated inflorescence appears to be the volatilization and dispersion of insect attracting odors. As a side-effect, the heat creates a micro-climate and this may make insects more active once inside, so that they are more likely to come into contact with the floral sex organs and carry out the transfer of pollen. In some cases the warmth and humidity (and possibly the odor) encourages insects to use the inflorescence as a mating area( Brown, 1988). Dr. Seymour speculates that the plant maintains a temperature at 30°C because that is the temperature that beetles need to achieve a thoracic temperature greater than 30°C before they can fly, this way the plant is eliminating the need for endogenous heat production (Seymour et all, 1996).
The volatilization of odor by heat increases the distance at which the beetles can sense the plant, thereby increasing it’s chances of being pollinated. The way in which a visitor approaches a blossom tells a great deal about which secondary attractant is the more important: if the attraction is visual, the visitor will fly in a more or less straight line towards the blossom, independent of wind direction. If it is olfactory, as it is in beetles, the approach is less regular. The beetle flying against the sensory gradient will approach from downwind, and when it reaches the zone of maximum concentration it will drop suddenly to the source. As cold air is a poor conductor for odor, the heat of the inflorescence acts to boost the distance at which the scent can be perceived by the beetle, the principal pollinator of the plant (Faegri, 1979).
In the eastern skunk cabbage (Symplocarpus Soetidus), found in swampy land throughout the north east. The warmth from the spadix dissipates foul-smelling volatile amine, indole and skatole compounds to attract flies and beetles (and possibly bees). A secondary effect of this heat output is that its fist-sized ground-level inflorescences, which are not frost-proof, can melt their way through ice and snow by radiating some of the heat produced by the spadix. Regardless of near freezing air temperatures, it can raise its tissues 25°C above the surroundings. This is a remarkable feat, but even more astonishing is the fact that this high respiration rate can be maintained for around two weeks and can be regulated according to fluctuations in the ambient temperature. The colder the weather, the higher the respiration rate, and a 10°C drop in the air temperature almost doubles the oxygen consumption of the spadix. Interestingly, the minimum weight for a heat-regulating organism has been put at 2.5 g, which is the weight of the smallest spadices, and their oxygen consumption is similar to that of small mammals of the same size. The source of energy needed for this heat production is provided by massive stores of carbohydrates (Brown, 1988).
Starch appears to be a primary source of energy in Arum plants, however due to the low respiratory exchange ratio starch can not be the primary fuel (Seymour et al,1984). Lipids are fatty or waxy substances that can be mobilized and broken down very rapidly to meet the cells' demand for energy. On the whole, plant tissues have far lower energy requirements than those of animals and so lipid metabolism is not associated with normal functions in plants (it was once observed as a brief response to wounding in potatoes). This remarkable discovery demonstrated not only that the spadix of P. bipinnatifidum utilizes lipids for heat production but also has an oxygen consumption approaching that of a flying hummingbird whose muscles have extremely high requirements. In Philodendron bipinnatifidum, lipids are contained in the sterile male florets which are situated between the male and female zones. Flowering lasts for two days and the production of heat and odor begins as soon as the spathe starts to open. Most of the time the scent is fruity and pleasant and the spadix is only about 10°C above the surrounding air. At about seven o'clock in the evening the temperature of the spadix rises and emits a pungent spicy odor, temperature levels out around 38°C but has been recorded as high as 46°C. In one experiment it was found that the spadix could still maintain these temperatures when the surrounding air was near freezing. The peak lasts for 20 to 40 minutes and then cools down around 8 p.m. The warm scent is switched on and off each evening by light-sensitive hormones and permeates the cool evening air just as the pollinators- large dusk-flying scarab beetles such as Erioscelis emarginata and Cyclocephala species - become active. The inflorescences are a big attraction and some have been seen with nearly 200 beetles covering the spadix and filling the chamber in an orgy of mating and feasting on the nutritious sterile flowers and secretions. In the melee the beetles are smeared with resin that oozes from the spathe, especially at the constriction which they have to squeeze past, and so when the male florets release pollen in the final stage of flowering, it sticks easily to their hard shiny bodies. Heat is always produced by the spadix, and usually by sterile flowers, but the odor it disperses may arise from either the spadix or the spathe (Brown, 1988).
The generation of heat is based on a respiratory process so fierce that it compares with that of a flying humming bird. It displays a extreme cyanide resistance, based on the operation of an alternative electron transport chain, that is not dependent on cytochrome C oxidase and traps little energy in the form of ATP. Because these plants maintain a regulated maximum temperature by reversible thermal inhibition of respiratory heat production, a constant temperature can be maintained. There by avoiding any shifts in the metabolic pathway that creates the heat, due to the effects of temperature on the optimal level of enzyme activity (Somero,1978). This response protects the inflorescence and attracted insects from thermal damage. Heat production equals heat loss, and there is no net phosphorylation, therefore showing heat production to be the sole reason for the high rates of respiration. There is no conservation of energy, all of the energy produced by oxidation appears as heat (Seymour,1983).
The Skunk Cabbage and the lily both use the ability to regulate their temperature to ensure they are pollinated. Though the function of the heat production is very costly to the plant, the cost is repaid in an ecological advantage of being first to grow, as demonstrated by eastern skunk cabbage (Meeuse,1988). That is all any plant or animal is really looking for, an advantage over its peers and predators all in the hope of producing the next generation.
That could be the answer to the question; how do lilies and skunk cabbage ensure they are pollinated? Through the adaptation of an alternative metabolic pathway that allows them to regulate their temperature. Thus spreading their scents and protecting their inflorescences from damage by cold weather. The production and regulation of heat, helps the lily protect itself from cold, increase the distance at which pollinators notice it, and gives it a head start on growing and establishing itself in the forest. Heat production might be wide spread in the plant kingdom, just at levels that are not as noticeable. Mitochondria are much more concentrated in plants that do produce heat than those that do not. It would take more than two pounds of corn seedling to equal the amount of mitochondria found in one lily ( Yoon,1996). The heat produced by the plant might also serve to increase the rate at which the flowers are able to develop. If the production and regulation of heat in the lily family is so advantageous, why don’t more plants engage in the practice? I do think we are far from having to fear world conquest by angry plants, but plants do seem to become less passive and more active in their environment each day. With further study who knows what we might find out?
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