Thermogenic Plant
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Thermogenic plants have the ability to raise their temperature above that of the surrounding air. Heat is generated in the
mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
, as a secondary process of
cellular respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cell ...
called
thermogenesis Thermogenesis is the process of heat production in organisms. It occurs in all warm-blooded animals, and also in a few species of thermogenic plants such as the Eastern skunk cabbage, the Voodoo lily ('' Sauromatum venosum''), and the giant w ...
.
Alternative oxidase The alternative oxidase (AOX) is an enzyme that forms part of the electron transport chain in mitochondria of different organisms. Proteins homologous to the mitochondrial oxidase and the related plastid terminal oxidase have also been identified ...
and uncoupling proteins similar to those found in
mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
enable the process, which is still poorly understood.


The role of thermogenesis

Botanists are not completely sure why thermogenic plants generate large amounts of excess heat, but most agree that it has something to do with increasing
pollination Pollination is the transfer of pollen from an anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds. Pollinating agents can be animals such as insects, for example bees, beetles or bu ...
rates. The most widely accepted theory states that the endogenous heat helps in spreading chemicals that attract pollinators to the plant. For example, the Voodoo lily uses heat to help spread its smell of rotting meat. This smell draws in flies which begin to search for the source of the smell. As they search the entire plant for the dead carcass, they pollinate the plant. Other theories state that the heat may provide a heat reward for the pollinator: pollinators are drawn to the flower for its warmth. This theory has less support because most thermogenic plants are found in tropical climates. Yet another theory is that the heat helps protect against frost damage, allowing the plant to germinate and sprout earlier than otherwise. For example, the
skunk cabbage Skunk cabbage is a common name for several plants and may refer to: * the genus '' Lysichiton'' ** Asian skunk cabbage, '' Lysichiton camtschatcensis'', grows in eastern Asia ** Western skunk cabbage, '' Lysichiton americanus'', grows in western N ...
generates heat, which allows it to melt its way through a layer of snow in early spring. The heat, however, is mostly used to help spread its pungent odor and attract pollinators.


Characteristics of thermogenic plants

Most thermogenic plants tend to be rather large. This is because the smaller plants do not have enough volume to create a considerable amount of heat. Large plants, on the other hand, have a lot of mass to create and retain heat. Thermogenic plants are also
protogynous Sequential hermaphroditism (called dichogamy in botany) is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodit ...
, meaning that the female part of the plant matures before the male part of the same plant. This reduces inbreeding considerably, as such a plant can be fertilized only by pollen from a different plant. This is why thermogenic plants release pungent odors to attract pollinating insects.


Examples

Thermogenic plants are found in a variety of families, but
Araceae The Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe (or leaf-like bract). Also ...
in particular contains many such species. Examples from this family include the
dead-horse arum ''Helicodiceros muscivorus'', the dead horse arum lily, is an ornamental plant native to Corsica, Sardinia and the Balearic Islands. It is the only species in the genus ''Helicodiceros''. Within the family Araceae the plant is part of the subfa ...
(''Helicodiceros muscivorus''), the
eastern skunk cabbage ''Symplocarpus foetidus'', commonly known as skunk cabbage or eastern skunk cabbage (also swamp cabbage, clumpfoot cabbage, or meadow cabbage, foetid pothos or polecat weed), is a low-growing plant that grows in wetlands and moist hill slopes of ...
(''Symplocarpus foetidus''), the
elephant foot yam ''Amorphophallus paeoniifolius'', the elephant foot yam or whitespot giant arum, is a tropical plant native to Island Southeast Asia. It is cultivated for its edible tubers in Southeast Asia, South Asia, Madagascar, New Guinea, and the Pacific ...
(''Amorphophallus paeoniifolius''), elephant ear (''
Philodendron selloum ''Philodendron'' is a large genus of flowering plants in the family Araceae. , the Plants of the World Online accepted 621 species; other sources accept different numbers. Regardless of number of species, the genus is the second-largest member o ...
''), lords-and-ladies (''
Arum maculatum ''Arum maculatum'', commonly known as cuckoopint, jack-in-the-pulpit and other names (see common names), is a woodland flowering plant species in the family Araceae. It is native across most of Europe, as well as Eastern Turkey and the Caucasus. ...
''), and voodoo lily (''
Typhonium venosum ''Sauromatum venosum''''Sauromatum venosum''
Plants of the World Online; Kew Sc ...
''). The
titan arum The titan arum (''Amorphophallus titanum'') is a flowering plant in the family Araceae. It has a large unbranched inflorescence; a tall single leaf, branched like a tree; and a heavy tuber which enables the plant to produce the inflorescence. ' ...
(''Amorphophallus titanum'') uses thermogenically created water vapor to disperse its scent—that of rotting meat—above the cold air that settles over it at night in its natural habitat. Contrary to popular belief, the
western skunk cabbage ''Lysichiton americanus'', also called western skunk cabbage (US), yellow skunk cabbage (UK), American skunk-cabbage (Britain and Ireland) or swamp lantern, is a plant found in swamps and wet woods, along streams and in other wet areas of the Pac ...
(''Lysichiton americanus),'' a close relative from the family Araceae, is not thermogenic. Outside Araceae, the sacred lotus (''
Nelumbo nucifera ''Nelumbo nucifera'', also known as the pink lotus, sacred lotus, Indian lotus, or simply lotus, is one of two extant taxon, extant species of aquatic plant in the Family (biology), family Nelumbonaceae. It is sometimes colloquially called a ...
'') is thermogenic and
endothermic An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics, it is a thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, ...
, able to regulate its flower temperature to a certain range, an ability shared by at least one species in the non-photosynthetic parasitic genus ''
Rhizanthes ''Rhizanthes'' is a genus of four species of parasitic flowering plants in the family Rafflesiaceae. They are without leaves, stems, roots, or photosynthetic tissue, and grow within the roots of a few species of ''Tetrastigma'' vines. The genus i ...
'', ''
Rhizanthes lowii ''Rhizanthes lowii'' is a species of parasitic flowering plant without leaves, stems, roots, or photosynthetic tissue. It grows on the roots of the ''Tetrastigma'' vine. It includes the specimens with the largest measured flowers in '' Rhizanthes ...
''.


Heat production

Many endothermic plant species rely on
alternative oxidase The alternative oxidase (AOX) is an enzyme that forms part of the electron transport chain in mitochondria of different organisms. Proteins homologous to the mitochondrial oxidase and the related plastid terminal oxidase have also been identified ...
(AOX), which is an enzyme in the mitochondria organelle and is a part of the electron transport chain. The reduction of mitochondrial redox potential by alternative oxidase increases unproductive respiration. This metabolic process creates an excess of heat which warms thermogenic tissue or organs. Plants containing this alternative oxidase are unaffected by the effects of cyanide because AOX acts as electron acceptor collecting electrons from
ubiquinol A ubiquinol is an electron-rich (reduced) form of coenzyme Q (ubiquinone). The term most often refers to ubiquinol-10, with a 10-unit tail most commonly found in humans. The natural ubiquinol form of coenzyme Q is 2,3-dimethoxy-5-methyl-6-poly p ...
while bypassing the third electron complex. The AOX enzyme then reduces oxygen molecules to water without the presence of a proton gradient which in turn is very inefficient yielding a drop in free energy from Ubiquinol to oxygen which is released in heat.{{cite journal , vauthors = Ito K, Ogata T, Seito T, Umekawa Y, Kakizaki Y, Osada H, Moore AL , title = Degradation of mitochondrial alternative oxidase in the appendices of Arum maculatum , journal = The Biochemical Journal , volume = 477 , issue = 17 , pages = 3417–3431 , date = September 2020 , pmid = 32856714 , doi = 10.1042/BCJ20200515 , pmc=7505559 , doi-access = free


References

Heat transfer Plant physiology Plants by adaptation