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: (Sinhala is the name of the language) Sinhala: නෙළුම් neḷum, the name for the lotus Nelumbo nucifera.[1]

There are only two known living species of lotus; N. nucifera is native to East Asia, South Asia and Southeast 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.

The other lotus is Nelumbo lutea and is native to North America and the Caribbean. Horticultural hybrids have been produced between these two allopatric species.

There are several fossil species known from Cretaceous, Paleogene and Neogene aged strata throughout Eurasia and North America.

Microscopic water droplets resting above the leaf surface, allowing gas exchange to continue.

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 the 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 APG IV system of 2016, recognizes Nelumbonaceae as a distinct family and places it in the order Proteales in the eudicot clade, as do the earlier APG III and APG II systems.[4]

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. The USDA still classifies the lotus family within the water lily order.[5]

The leaves of Nelumbo are highly water-repellent (i.e. they exhibit ultrahydrophobicity) and have given the name to what is called the lotus effect.[6] Ultrahydrophobicity involves two criteria: a very high water contact angle between the droplet of water and the leaf surface, and a very low roll-off angle.[7] This means that the water must contact the leaf surface at exactly one, minuscule point, and any manipulation of the leaf by changing its angle will result in the water droplet rolling off of the leaf.[7] Ultrahydrophobicity is conferred by the usually dense layer of papillae on the surface of the Nelumbo leaves, and the small, robust, waxy tubules that protrude off each papillae.[8] This helps reduce the area of contact between the water droplet and the leaf.[8]

Ultrahydrophobicity is said to confer a very important evolutionary advantage. As an aquatic plant with leaves that rest on the water's surface, the genus Nelumbo is characterized by its concentration of stomata on the upper epidermis of its leaves, unlike most other plants which concentrate their stomata on the lower epidermis, underneath the leaf.[8] The collection of water on the upper epidermis, whether that be by rain, mist, or the nearby disturbance of water, is very detrimental to the leaf's ability to perform gas exchange through its stomata. Thus, Nelumbo's ultrahydrophobicity allows the water droplets to accumulate together very quickly, and then roll off of the leaf very easily at the slightest disturbance of the leaf, a process which allows its stomata to function normally without restriction due to blockage by water droplets.[9]

A unique property of the genus Nelumbo is that it can generate heat,[10] which it does by using the alternative oxidase pathway (AOX).[11][12] 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.[13] The plant also reduces ubiquitin concentrations while in thermogenesis, which allows the AOX in the plant to function without degradation[14] 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.[15]

There are several theories 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.[10] The thermogenic environment might also be conducive to pollinator mating - pollinators may require a certain temperature for reproduction. By providing an ideal thermogenic environment, the flower is pollinated by mating pollinators.[16] Others theorize that heat production facilitates the release of volatile compounds into the air to attract pollinators 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.[17]

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.[18] 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 increase plant biomass or fruit mass.[16]

Other plants utilize thermoregulation in their life cycles. Among these is the eastern skunk cabbage, which heats itself to melt any ice above it, and push through the ground in early spring,.[19] Also, the elephant yam, which heats its flowers to attract pollinators. In addition, the carrion flower, which heats itself to disperse water vapor through the air, carrying its scent further, thus attracting more pollinators.

The sacred lotus, N. nucifera, is sacred in both Hinduism and Buddhism.[2]