Thermogenesis is the word for the way living things create their own body heat. While we usually only think of birds and mammals as being warm-blooded, all complex life forms produce some heat. Small power plants in cells, called mitochondria, turn food into a biological fuel, adenosine triphosphate (ATP). But only about one-quarter of the energy from your food actually becomes ATP; the remaining escapes as heat.
Sometimes, mitochondria can turn all the energy in sugars into heat. In plants, an enzyme that can do this is even named alternative oxidase. However, only a few plants generate heat for some very clever uses.
The ‘sacred lotus’ plant (Nelumbo nucifera)is native to North and Central India and grows in ponds, lakes, and gently flowing waters. Flowering begins in early summers, at mild ambient temperatures. Its pretty flower blooms for three to four days. In this period the internal temperature of the flower is around 30-35º C even as the surrounding temperature can go down to 10º C.
Thermogenesis is triggered when the unopened flower’s petal tips turn pink. Early next day, the opening flower emits heat, which also helps in releasing an attractive scent. The lotus flower has a central cone-shaped receptacle whose flat top has 10-30 female organs, the pistils. As in other thermogenic plants, the female organs mature first. The scent attracts insect pollinators — bees and beetles — to its receptive pistils. The petals close by midday, forming a cozy, insulated chamber where insects seek shelter for the night.
Before the flower opens on the second morning, the flower’s male organs, the stamens, mature. Fed on pollen and covered with it, the insects fly out and move to other receptive plants. The system is designed to ensure cross-pollination between plants, which offers many advantages to their offspring, including greater genetic diversity and increased pest resistance. Each pistil grows into a lotus nut and the receptacle matures into a showerhead-shaped seed pod.
The flat top carrying the pistil gets about 4-5º C warmer than the rest of the flower. Calcium ions serve as the ‘on’ switch. The level of calcium in the cells of this region jumps to four-times higher when it is time to warm up. This calcium heads to the mitochondria and signals them to shift into high gear. To produce heat, large amounts of stored starch and fat are used up (Plant Physiology, 198, 2025, kiaf173).
Some plants of the Arum family also use thermogenesis to attract insects and for other strange purposes. The Eastern skunk cabbage, not related to the cabbage, grows in the colder zones of North America and gets its name from a fetid odor reminiscent of cabbage, with a hint of mustard. The flowering stalk of this plant emerges in early spring by generating heat to melt the snow covering the soil. Beetles find a warm respite in this flower, along with pollen. Even spiders notice the insect traffic and spin their webs in the vicinity of the flower.
A putrid smell is associated with the flowers of the dead horse arum lily, found in Sardinia. This plant uses heat to vigorously dissipate compounds like dimethyl disulphide, which smells like a leaking gas cylinder with a top note of garlic. Carrion-seeking blowflies find this odour totally irresistible and arrive in large numbers.
(The article was written in collaboration with Sushil Chandani, who works in molecular modelling.)