Duckweed species are the most miniature flowering plants with minute sizes from 0.5 millimeters to less than two centimeters. They are commonly known as water lentils or water lenses, a subfamily of flowering aquatic plants. Allied to Lemna, this is a genus of eight species of floating aquatics distributed over most of the tropical to warm-temperature regions of the world. They are among the most miniature known flowering plants, having minuscule, bright green plant bodies or ‘fronds’, from which emerge very fine, hair-like roots. The fronds mass together to cover large areas of still water, each one producing a minute green flower in a central cavity. The growth of common duckweed species, including Ammonia, is one of the significant constituents of domestic fat duckweed (Lemna gibba L.), common duckweed wastewater (Lemna minor L.), star duckweed (Lemna trisulca L.), great duckweed [Spirodela polyrhiza (L.) Schleiden], turion duckweed (Lemna turionifera Landolt), and spotless watermeal [Wolffia arrhiza (L.) Horkel ex C.F.H. Wimmer], on different types of wastewater. An invasive species is commonly known as the least duckweed (Lemna minuta Kunth).
Wolffia arrhiza is a species found in Eurasia, Africa, and Australia under the family of Lemnaceae. Lemnaceae consists of two subfamilies (Lemnoidea and Wolffioideae), with four genera (Spirodella, Lemna, Wolffia, and Wolffina). It is commonly known as duckweed. Duckweeds are monocotyledonous, categorized as small, structured floating aquatic plants that grow on calm surface waters. It is a semi-evergreen, sometimes dying off in less favorable months, but often producing its tiny green flowers all year round if conditions are good. The small and free-floating aquatic plant leaves are rounded one millimeter long, and each plant can spread out to cover about one meter in diameter.
Lemnaceae show a worldwide geographic and climatic distribution ranging from cold temperate to tropical regions except for waterless deserts and permanently frozen polar regions. In arid and extremely wet areas (Malaysia, Iceland, and others), the natural occurrence of duckweed is also rare. Most species, however, are found in moderate climates of subtropical and tropical zones. The small floating vascular plants grow on still, nutrient-rich fresh, and brackish waters. The family of Lemnaceae consists of four genera: Spirodela, Lemna, Wolffia, and Wolffiella, with a total of about thirty-seven species worldwide. Often introduced to ponds as food for waterfowl or used as cold-water aquarium plants, they vary in hardiness depending on the species, but are generally quickly grown in any still water with sun or part-shade.
Duckweed species have been cultivated in Asia for human and animal nutrition for many generations. As the plant body consists predominantly of photosynthetically active non-structural tissues, duckweed species generally display rapid growth in nutrient-rich waters, with doubling times as low as 1.24 days. Duckweeds also tend to have a nutritionally desirable composition of protein and poly-unsaturated fatty acids. Notably, the potential to grow duckweed as a food source during long-term space missions has begun to be explored. Further, duckweeds may also be exploited as a source of starch, which can be used to produce bioethanol, and this may be an especially pertinent use for duckweeds grown on media that make them unfit for animal and/or human consumption.
Compared to soil-based systems, the cultivation of aquatic plants as a biofuel source can reduce competition with food crops and facilitate more outstanding biomass production per hectare. Given the high nutritional value and phytoremediation potential, it is perhaps not surprising that there is growing interest in multipurpose bioreactor-based systems that facilitate large-scale duckweed cultivation (more than a tonne dry-biomass per several 100 m2 of available growth surface area). Apart from its uses as a feed for livestock or bio-product production, but also recovers nutrients from selected wastewaters. Large-scale duckweed cultivation has generally occurred outdoors, e.g., in shallow ponds, lagoons, or canal-based systems.
Duckweed is very sensitive to wind and, therefore, unsuitable for wastewater treatment in very windy regions. Duckweed is blown in drifts to the shore of the ponds, where it piles up and subsequently dies. If plants are not redistributed, which requires manual labor, it will lead to decreased treatment efficiency due to incomplete coverage of the pond surface. A complete duckweed cover has to be maintained to suppress algal growth, nutrient competition, and the development of odor and mosquito breeding. Lemnaceae are very sensitive to water currents. The natural habitat of the free-floating plants is stagnant or almost quiescent water bodies. Lemnaceae can withstand higher water currents when larger ones like Eichhornia or Phragmites protect the plants. Lemnaceae can withstand a movement velocity of 0.1 m/s in water bodies without rooted plants. Therefore, a sufficiently low flow velocity has to be considered in duckweed treatment systems designed as plug-flow.
In some instances, protective structures, such as greenhouses and polytunnels, are used by commercial companies to encapsulate duckweed ponds for the provision of a hybrid ‘semi-indoor’ controlled environment. These outdoor cultivation systems are relatively low-cost and have the advantage of simplicity and scalability. However, there are a variety of situations where full indoor growth systems are beneficial. For example, duckweed-mediated pharmaceutical production must occur in sterile, highly controlled cultivation conditions where plants are not exposed to human pathogens. Indoor systems are also of interest, for example, in urban farming, where a nutritious duckweed salad is grown close to urban consumers, or in industrial settings, where continuous year-round growth is required. Indoor systems allow control of temperature, air circulation, light spectrum, photoperiod, humidity, and other climatic factors that optimize production.
The duckweed genus Wolffia has a valuable nutrient profile compatible with human consumption and is generally found in more tropical regions of Asia and America. The low fiber content and high nutritional value of duckweed make it a quality feed or feed component for animals and possibly also for humans. Because of its high moisture and nitrogen content can also be used as organic fertilizer in agriculture by direct land application or via composting. The application of duckweed as fish feed is the most frequent and best-studied use of duckweed. Moreover, duckweed is also known as a feed for ducks, chickens, freshwater prawns, pigs, edible snails, horses, and ruminants like castles and sheep. However, information on these applications is scarce. Its advantage over other protein sources is that it is characterized by better availability and absorption of amino acids, including lysine, methionine, and vitamins. It is rich in leucine, threonine, valine, isoleucine, and phenylalanine.
Its use in folk medicine has been scientifically substantiated. It exhibits sialagogic, carminative, expectorant, diaphoretic, diuretic, and anticarcinogenic properties. For more than a decade, duckweed has been successfully used in biotechnology to produce biologically active compounds of importance in dietetics, phytotherapy, and Phyto-cosmetics.
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