Birds and mammals are vertebrates able to maintain constant body temperature. Thermoregulation is an integral part of homeostasis, which is kept by closely interacting nervous, circulatory and excretory systems. When the body temperature rises too much, thermoregulatory center in the brain transmit electric impulses to the skin (sweat glands release sweat, which cools the skin through evaporation) and blood vessels – these are involved in the heat exchange between the body’s internal and external medias.
Excess heat is transferred to the surroundings of the body, the skin becomes red and the body temperature returns to normal after a short time. When the body temperature falls, the reverse processes take place – the vascular lumen is reduced, heat emission and sweat excretion slow down and the goose bumps cover the skin. Animal thermo-regulative system is well studied and understood. There may be a question – can other organisms carry out such a systematic exchange of heat? It appears so and the responsible organisms are plants!
Thermogenic plants are plants that are able to emit the heat in order to elevate temperature of surrounding air around the relevant parts or the cavities of the plant. Major portion of such plants belongs to the family of Araceae. Almost all thermogenic plants are larger than typical herbaceous plants. Some thermogenic plants can maintain as much as 30°C higher temperature than the surrounding air temperature for a period of time – usually for a few days. This characteristic has been quite mysterious for most researchers, as it was unknown, how the plants produce heat, or what are the reasons for such mysterious evoliutionary adaptation. Unambiguous answers to these and some similar questions are still unknown, but the hypotheses are knocking at the nature’s door.
At present, the prevailing hypothesis states that thermogenesis accelerates pollinator-attracting compounds evaporation and their diffusion in the air. This increases the efficiency of pollination – pollinators are lured faster as chemical signals disperse in the air faster. The hypothesis is supported by the plants in the genus of Amorphophallus, growing in the tropical regions. Thermogenic Amorphophallus plants spread chemical compounds, which odor resembles rotten meat, more efficiently due to faster diffusion rates at higher temperatures. Carrion smell attracts specific insects, such as flies, which while flying from one flower to another carry the pollen and play the crucial role of pollinators in the plant life cycle.
Another hypothesis, which is supported by fewer scientists, argue that thermogenic plants by generating heat protect themselves from frost. Scientists give the eastern skunk cabbage (Symplocarpus foetidus) as an example illustrating this concept, a plant, which grows in North America. This plant blooms in early spring, and the heat released by the plant melts the surrounding snow cover. However, a number of botanists believe that the primary purpose is to accelerate compounds, causing an unpleasant odor of the plant, diffusion, but not melting the snow. In addition, this hypothesis does not explain the plants growing in the tropics, where frost protection is unnecessary.
Another hypothesis explaining the function of thermogenesis says that the plant is a direct insect luring tool, which provides some heat for small pollinators. Of course, small animals face real challenge maintaining optimum temperature of vital functions. According to this hypothesis, the insects are directly provided with heat necessary for vital functions by the thermogenic plant. However, this hypothesis raises the most doubts and by significant proportion of scientists is simply discarded.
Giant water lily (Victoria amazonica), known for the water floating leaves, which can reach a diameter of three meters, can boast for very specific coordination between thermogenesis and circadian rhythm. In the evening, the ring of white petals opens, the temperature rises by 10°C and scarab-luring fruity scent spreads. At night, the petals close and imprison the crowd of insects. Next afternoon petals open again, but this time the petals are dyed in faint pink – scarabs leave water lily as it is not luring anymore. In addition, on the second day the temperature does not rise so blurry color and mild odor do not lure new scarabs. This ensures cross-pollination – scarabs start looking for another water lily that emits a pungent odor and is dyed in more visible white color. This repeating cycle increases pollination efficiency.
More detailed thermogenic plant studies have shown that the heat is produced by special thermogenic cells that were observed to have greater number of mitochondria. Mitochondria are essential for some aerobic cellular respiration phases, of which the most important is electron transfer, occurring at the protein complexes embedded in the inner mitochondrion membrane. It turned out that thermogenic cells’ electron transport system differs by one enzyme – cytochrome c oxidase (COX) is replaced by an alternative oxidase (AOX). This modified electron transport system is an expensive pleasure – an alternative transfer system does not produce ATP and releases energy in the form of heat immediately thus eliminating ability to storage excess energy.
Written by Edvinas Stankunas