Temporal range: Cretaceous–Recent
|N. nucifera (sacred lotus)|
Nelumbo is a genus of aquatic plants with large, showy flowers. Members are commonly called lotus, though "lotus" is a name also applied to various other plants and plant groups, including the unrelated genus Lotus. Members outwardly resemble those in the family Nymphaeaceae ("water lilies"), but Nelumbo is actually very distant to Nymphaeaceae. "Nelumbo" is derived from the Sinhalese word Sinhalese: නෙළුම් neḷum, the name for the lotus Nelumbo nucifera.
There are only two known living species of lotus; Nelumbo nucifera is native to Asia and is better-known. It is commonly cultivated; it is eaten and used in traditional Chinese medicine. This species is the floral emblem of both India and Vietnam.
There is residual disagreement over which family the genus should be placed in. Traditional classification systems recognized Nelumbo as part of the Nymphaeaceae, but traditional taxonomists were likely misled by convergent evolution associated with an evolutionary shift from a terrestrial to an aquatic lifestyle. In the older classification systems it was recognized under the biological order Nymphaeales or Nelumbonales. Nelumbo is currently recognized as a only living genus in Nelumbonaceae, one of several distinctive families in the eudicot order of the Proteales. Its closest living relatives, the (Proteaceae and Platanaceae), are shrubs or trees.
The leaves of Nelumbo can be distinguished from those of genera in the Nymphaeaceae as they are peltate, that is they have fully circular leaves. Nymphaea, on the other hand, has a single characteristic notch from the edge in to the center of the lily pad. The seedpod of Nelumbo is very distinctive.
The Cronquist system of 1981 recognizes the family but places it in the water lily order Nymphaeales. The Dahlgren system of 1985 and Thorne system of 1992 both recognize the family and place it in its own order, Nelumbonales.
A unique property of the Nelumbo genus is that it can generate heat, which it does by using the alternative oxidase pathway (AOX). This pathway involves a different, alternative exchange of electrons from the usual pathway that electrons follow when generating energy in mitochondria, known as the AOX, or alternative oxidase pathway.
The typical pathway in plant mitochondria involves cytochrome complexes. The pathway used to generate heat in Nelumbo involves cyanide-resistant alternative oxidase, which is a different electron acceptor than the usual cytochrome complexes. The plant also reduces ubiquitin concentrations while in thermogenesis, which allows the AOX in the plant to function without degradation. This alternative pathway is used by the plant more often than the typical cytochrome pathway, and the excess heat production during this process allows the plant to heat itself. Thermogenesis is restricted to the receptacle, stamen, and petals of the flower, but each of these parts produce heat independently without relying on the heat production in other parts of the flower.
Several theories have been put forth about the function of thermogenesis, especially in an aquatic genus such as Nelumbo. The most common theory posits that thermogenesis in flowers attracts pollinators, for a variety of reasons. Heated flowers may attract insect pollinators; as the pollinators warm themselves while resting inside the flower, they deposit and pick up pollen onto and from the flower. The thermogenic environment might also be conducive to pollinator mating - pollinators may require a certain temperature to reproduce in, and by providing an ideal thermogenic environment, the flower is pollinated by mating pollinators. Others theorize that heat production facilitates the release of volatile compounds into the air to attract pollinators that are flying over water, or that the heat is recognizable in the dark by thermo-sensitive pollinators. None have been conclusively proven to be more plausible than the others.
After anthesis, the receptacle of the lotus transitions from a primarily thermogenic to a photosynthetic structure, as seen in the rapid and dramatic increase in photosystems, photosynthetically involved pigments, electron transport rates, and the presence of 13C in the receptacle and petals, all of which assist in increasing photosynthesis rates. After this transition, all thermogenesis in the flower is lost. Pollinators do not need to be attracted once the ovary is fertilized, and thus the receptacle's resources are better used when it is photosynthesizing to produce carbohydrates that can be put toward plant biomass or fruit mass.
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