Nelumbo nucifera occurs naturally around the world, in northern Australia, Papua-New Guinea, south-east Asia, Japan and India (where it is the national flower and sacred); it is also found in the Americas (as subspecies lutea). It is an attractive plant that has seen wide adoption in gardens throughout the world.
In contrast to other water lilies whose leaves generally float on the water surface, the leaves of the Lotus Lily are usually held above the water and attain diameters of 0.5 to 1m.
As the wet season advances, the prickly leaf stalks become elongated (up to 1.5m long), continuing to hold the leaves above the rising waters until the floods, when they become completely submerged and die back. Without leaves, the plant can no longer photosynthesize and must then survive on food reserves stored in the rhizome (the underground stem).
As the floodwaters recede in the season that the Bininj/Mungguy people of Kakadu call Banggerreng, new leaves are formed and flowering begins, extending into Yegge (the early dry season).
The evolution of beetles coincided with the evolution of the early flowering plants (including the ancestors of Nelumbo) and there are many examples of the dependence between flowering plants and beetles. Many plants use insects to transport pollen between flowers and provide them with a reward to keep them visiting and returning to similar flowers. Rewards provided by plants to insects are commonly food sources like nectar or special structures that are sacrificed to promote pollination. In the Lotus Lily, large quantities of pollen are produced so that some can be sacrificed as food for pollinators. These rewards only become available when the flower is fully developed and ready for reproduction.
When the flower of N. nucifera is fully developed, it has in its middle, a female reproductive part shaped like a flat-topped ice-cream cone. This structure contains 20 or so ovules (equivalent to the eggs of animals). More than 200 yellow, pollen-producing stamens (the male reproductive parts) surround the female reproductive structure and all of these are enclosed by about 20 pink petals.
It is advantageous to the genetic diversity of the species if pollen from one plant fertilizes ovules from another plant (cross-fertilization) rather than on the same plant (self-fertilization). So, a mechanism has evolved in the Lotus Lily to increase the chance of this happening.
The delicate perfume of the lotus flowers signals to distant pollinators the readiness of the flowers and the availability of rewards. As the animals get closer, the large, pink flowers raised above the water become visible and identifiable.
Early one morning, the tips of the petals open to expose the female reproductive structure and the elongated tips of the stamens (but not the pollen). The female structure and the tips of the stamens produce large amounts of perfume. Although it has not actually been observed in this species, the tips of the stamens appear to be sacrificial food; in order to eat these, pollinators would probably walk around on the female reproductive structure. If the pollinator has recently picked up pollen from another flower of N. nucifera, that pollen might be transferred to this flower and fertilization of one or more of the ovules can take place.
As night falls, the flower closes up, trapping inside it any late pollinators. Pollen is shed into the bowl of the flower to provide a reward to the pollinators which are able to wander around the flower, probably self-fertilizing the ovules.
The next morning, the flower opens into a bowl shape, now exposing the stamens (as well as the female reproductive structure). The overnight visitors are free to leave, covered in pollen from this flower. If they visit another flower of N. nucifera, some pollen may be transferred and cross-fertilization can occur. The open flowers are available to pollinators all day so that any ovules that are still unfertilized might have further opportunity (for either cross-pollination or self-pollination). The flower closes more loosely that night, again trapping potential pollinators. The petals begin to wither.
On the morning of the third day, the flower opens fully. The petals and stamens are shed are shed over the next day or so.
Whilst it is possible for pollinators to transfer pollen of one flower to fertilize ovules of the same flower, the female reproductive structure is more receptive to pollen from other plants. Aiding this is the timing mechanism of the opening of the petals which prevents self-fertilization on the first day.
The benefits to the plant are clear and there is a food reward for pollinators. But, that’s not all!
Like some other primitive plants that rely on beetles and flies for pollination, Nelumbo nucifera is a heat-generating plant. This observation was first published over a century ago but recent studies by Prof. Roger Seymour (University of Adelaide) and his colleagues have shown that these flowers can maintain temperatures more than 20°C above air temperature, regulated to within a couple of degrees of 32°C.
Beetle activity depends on temperature so a beetle that has spending the night in the warm conditions inside a lotus flower give the beetles advantages for feeding and, possibly, better digestion. However, because the flowers attract beetles, they very likely provide venues for courting and mating – Professor Seymour refers to them as ‘nightclubs for beetles’ – so that the flowers can greatly enhance the reproductive success of the beetles that use them.
Heat generation comes from the female reproductive structure and the stamens (especially the tips). The energy to generate this heat comes from the plant’s reserves so this energy cost must have a purpose.
The plant starts producing heat only when the flower is fully developed, just before it is ready for fertilization. However, most of the energy consumption goes into heating the flower during the few days it is opening and closing – when it is most ready for reproduction. Heating occurs mainly at night and the early morning, with temperatures dropping back during the afternoon.
Pollinators that have learned this cycle might still be attracted to the flower in the afternoon with the promise of spending a warm, well-fed night in the flower. Furthermore, heating spreads the perfume, better advertising the readiness of the flowers.
After fertilization, the ovules develop into seeds. By the end of the dry season, the seeds reach maturity and are ready to be shed. The female reproductive structure has become quite dry and holes appear in the top, from which fall the 2cm-long nuts containing the seeds.
The seeds are very long-lived, some seeds having been germinated hundreds of years after they were formed.
The empty, dry, female reproductive structures are often called ‘pepperpots’ and are used by florists in dried-flower arrangements.
Trampling by buffaloes severely reduced populations of the Lotus Lily but, now that buffaloes have been virtually eradicated from the Top End, the Lotus Lily has made a strong resurgence.
This photograph was taken in Kakadu National Park.
Indigenous people have several uses for Lotus Lilies: the rhizome which stores food reserves for the plant is highly nutritious; lotus seeds are ground for flour; juice (latex) from the leaf stalks is used in the treatment of diarrhoea; the large, water-repellant leaves are used as umbrellas.
Prof. Wilhelm Barthlott noted that the leaves were highly water-repellant and also remained very clean. His subsequent examination of the leaves led to the invention of self-cleaning surfaces. A self-cleaning exterior paint is already on the market and other products are in development.