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  1 DUCKWEED: A tiny aquatic plant with enormous potential for agriculture and environment   CHAPTER 1: Introduction   AQUATIC HABITATS    A considerable proportion of the world's surface is covered by saline waters, and the land areas from which the salts of the sea mostly srcinated are continuously leached of minerals by the run-off of rain water. Aquatic habitats abound; these may be temporary following rains or permanent largely through impediments to drainage . From the beginning of time these aquatic habitats have been harvested for biomass in many forms (food, fuel and building materials) by animals and man. From the time of the Industrial Revolution and with the onset of intensive land use enormous changes occurred. Agriculturists harvested both water and dry lands for biomass and minerals were applied to stimulate biomass yields, the aquatic habitats often became enriched (or contaminated) and water bodies were more temporary because of water use in agriculture or were lost through drainage or the establishment of major dams for irrigation, human water supplies and/or hydro-electric power generation. On the other hand other human activities, created aquatic areas for such purposes as the control of soil erosion, for irrigation, storage of water, sewage disposal and industrial waste storage or treatment and for recreational use.    Aquatic habitats have, in general, degenerated throughout the world because of pollution by both industry and other activities. Human activities have, in general, resulted in much higher flows of minerals and organic materials through aquatic systems, often leading to eutrophication and a huge drop in the biomass produced in such systems. The lack of dissolved oxygen in water bodies, through its uptake by microbes for decomposition of organic compounds, produces degrees of anaerobiosis that results in major growth of anaerobic bacteria and the evolution of methane gases.   Despite this in the areas of high rainfall, particularly in the wet-tropics, there remain major aquaculture industries, which vary from small farmers with 'manure fed' ponds producing fish through to large and extensive cultivation of fish and shellfish that are replacing the biomass harvested from the seas. The distribution of global aquaculture is shown in Figure 1. Fish production from ocean catches appear to be reduced, but production from farming practices are increasing which clearly demonstrates how important aquaculture is (and will become) in protein food production. This trend is illustrated by the trend in world prawn (shrimp) production shown in Figure 2.   Figure 1: Distribution of global aquaculture (Source: FAO 1989)   Figure 2: The changing pattern of world prawn production for human consumption (FAO 1989)    Although traditional or staple crops can be produced from water bodies and in many situations traditional people often harnessed these resources, the aquatic habitat has been considered too costly and to difficult to farm other than for extremely high value crops such as algae harvested for high value materials such as b -carotene or essential long-chain fatty acids. Intensive aquaculture (hydroponics) of crops in highly mechanised farms have been developed but require highly sophisticated management systems and are expensive.   Throughout the world, and particularly in Asia, farmers have harvested naturally produced aquatic plants for a number of purposes including animal feed, green manure and for their family feed resources. The best known of these include the free floating plants; water lettuce (Pistia), water hyacinth (Eichhcornia), duckweed (Lemna) and Azolla and some bottom growing plants.    Azolla, which is a member of the fern family grows extensively in association with nitrogen fixing bacteria, which allows it to produce on waters low in N but containing phosphorus. Azolla has been comprehensively discussed by van Hove (1989).   In recent years a commonly occurring aquatic plant, duckweed , has become prominent, because of its ability to concentrate minerals on heavily polluted water such as that arising from sewage treatment facilities. However, it has also attracted the attention of scientists because of its apparent high potential as a feed resource for livestock (Skillicorn et al., 1993; Leng, et al., 1994). Duckweed grows on water with relatively high levels of N, P and K and concentrates the minerals and synthesises protein. These are the nutrients which are often critically deficient in traditional fodders and feeds given to ruminants and to pigs and poultry particularly where the former depend on agro-industrial byproducts and crop residues.   The growing awareness of water pollution and its threat to the ecology of a region and agriculture per se has also focussed attention on potential biological mechanisms for cleansing water of these impurities making it potable and available for reuse. In general, water availability is becoming a primary limitation to expanding human activities and also the capacity of agricultural land to feed the ever increasing population of the world.    Another pressure that has stimulated interest in aquatic plants has been the over-use of fertilisers, particularly in Europe that has led to contamination of ground water supplies that can no longer be tolerated.   ECOLOGICAL CONSIDERATIONS   In the early 1960's a number of scientists warned of the pending shortage of fossil fuels, the expanding population and the potential for mass-starvation from an inability of agriculture to produce sufficient food.   The prophesies have proved wrong in the short term, largely because of the extent of the then undiscovered fossil fuel, but also because of the impact of the development of high yielding crop varieties, particularly of cereal grain. The Green Revolution whilst increasing cereal crop yields faster than human population increase has had serious side effects such as increased erosion and greater water pollution in some places and a huge increase in demand for water and fertiliser. Fertiliser availability and water use are often highly dependent on fossil fuel costs. Water resources in many of the world's aquifers are being used at rates far beyond their renewal from rainfall (see World Watch 1997).    At the present time it appears that potentially the application of scientific research could maintain the momentum for increased food production to support an increasing world population, but it is rather obvious that if this is to occur it must be without increased pollution, and with limited increases in the need for water and fertiliser and therefore also fossil fuel.   GLOBAL WARMING, FOSSIL FUEL AND NUTRIENT RECYCLE NEEDS.   Global warming has now been accepted as inevitable. It is now a major political issue in most countries. Governments are now considering the need to reduce the combustion of fuels which contribute most to the build up of greenhouse gases and thus the increase in the thermal load that is presently occurring. A second problem for fossil fuel devouring industries is the potential for scarcer, and therefore, more costly oil resources in the near future. As Fleay (1996) in his book The decline in the age of oil has pointed out there have been no major discoveries of oil in the last ten years. This suggests that we have already discovered the major resources. Many of the oil wells are approaching or have passed the point at which half the reserves have been extracted. At this stage the cost in fuel to extract the remaining fuel increases markedly. The need to reduce fuel combustion and the potential for large increases in costs of extraction of oil from the major deposits all indicate major increases in fuel costs and the need to stimulate alternative energy strategies for industry and agriculture alike.   Fuel is a major economic component of all industries, and in particular, industrialised agriculture. Therefore food prices are highly influenced by fuel prices. The energy balance for grain production has consistently decreased with mechanisation as is illustrated by the fuel costs for grain production which is approaching 1MJ in as fossil fuel used in all activities associated with growing that crop to 1.5MJ out in the grain. A major component of the costs are in traction, fertiliser, herbicides and water use, particularly the energy costs of irrigation.   In recent times, a movement has begun to examine a more sustainable future for agriculture, particularly in the developing countries. The need in developing countries of Asia and Africa where most of the world's population lives and where population growth is the highest is to:      decrease population growth      maintain people in agriculture      and produce an increasing amount of food in a sustainable way   This suggests that small farmers need to be targeted and that farming should be integrated so that fertiliser and other chemical use is minimised together with lowered gaseous pollution. At the same time a country must ensure its security of food supplies. In the 1998 financial crisis in Asia, the small farmer was seriously effected because of the relatively high cost of fuel. This is bound to have serious effects on food production in the next few years if fertiliser applications are restricted. This will show up as a decline in crop yields over the next few years.   The problem of decreasing world supplies of fuels, increased legislation to decrease use of fossil fuel to reduce pollution, and the economic disincentive to use fertilisers in developing countries  2 indicates to this writer that there is a massive need to consider a more integrated farming systems approach, rather than the monocultures that have developed to the present time.   Integrated farming systems use require three major components to minimise fertiliser use:      a component where nitrogen is fixed (e.g. a legume bank)      a component to release P fixed in soils for plant use when this is limiting      a system of scavenging any leakage of nutrients from the system.   It also requires incorporation of animals into the system to utilise the major byproducts from human food production.   Duckweed aquaculture is an activity that fits readily into many crop/animal systems managed by small farmers and can be a major mechanism for scavenging nutrient loss. It appears to have great potential in securing continuous food production, particularly by small farmers, as it can provide fertiliser, food for humans and feed for livestock and in addition decrease water pollution and increase the potential for water re-use.   The production and use of duckweed is not restricted to this area and there is immense scope to produce duckweeds on industrial waste waters, providing a feed stock particularly for the animal production industries, at the same time purifying water.   In this presentation, duckweed production and use, particularly in small farmer systems, is discussed to highlight its potential in food security, particularly in countries where water resources abound and have been misused. On the other hand, duckweed aquaculture through its water cleansing abilities can make a greater amount of potable water available to a population living under arid conditions, providing certain safeguards are applied.   CHAPTER 2: The plant and its habitat   I NTRODUCTION   Duckweed is the common name given to the simplest and smallest flowering plant that grows ubiquitously on fresh or polluted water throughout the world. They have been, botanical curiosities with an inordinate amount of research aimed largely at understanding the plant or biochemical mechanisms. Duckweeds have great application in genetic or biochemical research. This has been more or less in the same way that drosophila (fruit flies) and breadmoulds have been used as inexpensive medium for genetic, morphological, physiological and biochemical research.   Duckweeds are small, fragile, free floating aquatic plants. However, at times they grow on mud or water that is only millimetres deep to water depths of 3 metres. Their vegetative reproduction can be rapid when nutrient densities are optimum. They grow slowly where nutrient deficiencies occur or major imbalances in nutrients are apparent. They are opportunistic in using flushes of nutrients and can put on growth spurts during such periods.   Duckweeds belong to four genea; Lemna, Spirodela, Wolfia and Wolffiella. About 40 species are known world wide. All of the species have flattened minute, leaflike oval to round fronds from about 1mm to less than 1cm across. Some species develop root-like structures in open water which either stabilise the plant or assist it to obtain nutrients where these are in dilute concentrations.   When conditions are ideal, in terms of water temperature, pH, incident light and nutrient concentrations they compete in terms of biomass production with the most vigorous photosynthetic terrestrial plants doubling their biomass in between 16 hours and 2 days, depending on conditions. An idea of their rapid growth is illustrated by the calculation that shows that if duckweed growth is unrestricted and therefore exponential that a biomass of duckweed covering 10cm2 may increase to cover 1 hectare (100 million cm2) in under 50 days or a 10 million fold increase in biomass in that time.   Obviously when biomass doubles every 1-2 days, by 60 days this could extend to a coverage of 32ha. In natural or farming conditions, however, the growth rate is altered by crowding, nutrient supply, light incidence and both air and water temperature in addition to harvesting by natural predators (fish, ducks, crustaceans and humans).   In addition to the above limiting factors there also appears to be a senescence and rejuvenation cycle which is also apparent in Azolla.   Vegetative growth in Lemna minor exhibits cycles of senescence and rejuvenation under constant nutrient availability and consistent climatic conditions (Ashbey & Wangermann, 1949). Fronds of Lemna have a definite life span, during which, a set number of daughter fronds are produced; each of these daughter fronds is of smaller mass than the one preceding it and its life span is reduced. The size reduction is due to a change in cell numbers. Late daughter fronds also produce fewer daughters than early daughters.    At the same time as a senescence cycle is occurring an apparent rejuvenation cycle, in which the short lived daughter fronds (with half the life span of the early daughters) produce first daughter fronds that are larger than themselves and their daughter fronds are also larger, and this continues until the largest size is produced and senescence starts again. This has repercussions as there will be cyclical growth pattern if the plants are sourced from a single colony and are all the same age. Under natural conditions it is possible to see a mat of duckweeds, apparently wane and explode in growth patterns.   The cyclic nature of a synchronised duckweed mat (i.e. all the same age) could be over at least 1 month as the life span of fronds from early to late daughters can be 33 or 19d respectively with a 3 fold difference in frond rate production (See Wangermann & Ashby, 1950).   The phenomena of cyclical senescence and rejuvenation may cause considerable errors of interpretation in studies that examine, for example, the response of a few plants to differing nutrient sources over short time periods.   In practice this cycle may be responsible for the need to restock many production units after a few weeks of harvesting. In Vietnam, with small growth chambers the duckweed required reseeding every 4-6 weeks (T.R. Preston personal communication) to be able to produce a constant harvestable biomass growing on diluted biogas digestor fluid. There is also the possibility in such systems of a build up in the plant of compounds that eventually become toxic or at least diminish their growth rate.   Root length appears to be a convenient relative measure of frond-age.   The senescence-rejuvenation cycle is increased by high temperatures through a decrease in individual frond life span but there is a concomitant increase in daughter frond production so that the biomass of fronds produced in a shorter life span is the same.   The rejuvenation cycle appears to be unaffected by either light density or temperature.   The cyclical changes appear to be mediated by chemicals secreted by the mother frond and growth patterns may be modified greatly by harvesting methods which mix water, wind effects and shelter as well as light intensity and temperature.   The increased death rate of duckweed mats exposed to direct sunlight has been recognised in work in Bangladesh where workers are set to cool duckweed mats by splashing them with water from below the surface and in Vietnam, Preston (personal communication) observed that the incidence of showers stimulated very rapid growth of duckweed in small ponds.   Duckweeds appear to have evolved, so as, to make good use of the periodic flushes of nutrients that arise from natural sources. However, in recent times they are more likely to be found growing in water associated with cropping and fertiliser washout, or down stream from human activities, particularly from sewage works, housed animal production systems and to some extent industrial plants.   TAXONOMY   For the many purposes related in this publication, the selection of duckweed to farm will depend on what grows on a particular water body and the farmer has little control over the species present. The various duckweeds have different characteristics. The fronds of Spirodela and Lemna are flat, oval and leaf like. Spirodela has two or more thread-like roots on each frond, Lemna has only one. Wolffiella and Wolfia are thalloid and have no roots; they are much smaller than Spirodela or Lemna. Wolfia fronds are usually sickle shaped whereas Wolffiella is boat shaped and neither has roots. Differentiation and identification is difficult and is perhaps irrelevant to the discussion. This is mainly because the species that grows on any water is the one with the characteristic requirement of that particular water and the dominant species will change with the variations in water quality, topography, management and climate, most of which are not easily or economically manipulated   MORPHOLOGY AND ANATOMY   The structure of the fronds of duckweed is simple. New or daughter fronds are produced alternatively and in a pattern from two pockets on each side of the mature frond in Spirodela and Lemma. In Wolffiella and Wolfia only one pocket exists. These pockets are situated in Spirodela or Lemna close to where the roots arise. Each frond, as they mature, may remain attached to the mother frond and each in turn, under goes this process of reproduction.   In all four genea each mother frond produces a considerable number of daughter fronds during its lifetime. However, after six deliveries of daughter fronds, the mother frond tends to die. Colonies produced in laboratory or naturally are always spotted with brown dead mother fronds.   The bulk of the frond is composed of chlorenchymatous cells separated by large intracellular spaces that are filled with air (or other gases) and provide buoyancy. Some cells of Lemna and Spirodela have needle like raphides which are presumably composed of calcium oxalate.   The upper epidermis in the Lemna is highly cutinized and is unwettable. Stomata are on the upper side in all four genea. Anthocyanin pigments similar to that in Azolla also form in a number species of Lemnacae. Both Spirodela and Lemna have greatly reduced vascular bundles.   Roots in both Spirodela and Lemna are adventitious. The roots are usually short but this depends on species and environmental conditions and vary from a few millimetres up to 14cm. They often contain chloroplasts which are active photosynthetically. However, there are no root-hairs.   Photo 1: The various species of Lemnaeca relevant to this publication    3 The plant reproduces both vegetatively and sexually, flowering occurs sporadically and unpredictably. The fruit contains several ribbed seeds which are resistant to prolonged desiccation and quickly germinate in favourable conditions.   DISTRIBUTION   The Lemnacae family is world wide, but most diverse species appear in the subtropical or tropical areas. These readily grow in the summer months in temperate and cold regions; they occur in still or slowly moving water and will persist on mud. Luxurious growth often occurs in sheltered small ponds, ditches or swamps where there are rich sources of nutrients. Duckweed mats often abound in slow moving backwaters down-stream from sewage works.   In the aquatic habitat of crocodiles and alligators, duckweeds often have luxurious growth on the nutrients from the excrement of these reptiles and the local zoo can often provide a convenient source of duckweed for experimental purposes (see Photo 2).   Some species appear to tolerate saline waters but they do not concentrate sodium ions in their growth. The apparent limit for growth appears to be between 0.5 and 2.5% sodium chloride for Lemna minor    When the aquatic ecosystem dries out or declining temperatures occur, duckweeds have mechanisms to persist until conditions return that can support growth. This occurs through late summer flowering, or the production of starch filled structures or turin which are more dense than the fronds so the plants sink to the bottom of the water body and become embedded in dried mud.   The four species of Lemnacae are found in all possible combinations with each other and other floating plants. They are supported by plants that are rooted in the pond. They effect the light penetration of water resources and depending on their coverage of the area they can prevent the growth of algae or plants that grow emersed in water. They provide habitat and protection for a number of insects that associate with the plant but they appear to have few insects that feed on them. The main predators appears to be herbivorous fish, (particularly carp), snails, flatworms and ducks, other birds may also feed on duckweeds but reports are few in the literature. The musk rat appears to enjoy duckweeds and the author suggests that many other animals may occasionally take duckweeds such as pigs and ruminants.   The appearances of duckweed species not previously seen in areas of Europe have been attributed to global warming and/or a strong indication of rising water temperature throughout the world from global warming (Wolff & Landolt 1994).   HISTORY OF DUCKWEED UTILIZATION   This is a most difficult area to review since much of the information is by way of the popular press or is only mentioned in scientific papers. However, after a lecture given at the University of  Agriculture and Forestry in Ho Chi Minh City in which the potential of duckweed biomass for animal production was discussed, as a novel concept, the writer was most chastened to find that duckweed was used extensively by local farmers as feed for ducks and fish and there was a flourishing market for duckweed.   The duckweed based farming system in Vietnam depended largely on manure and excrement being collected in a small pond where some eutrophication takes place; the water from this pond runs into a larger pond about 0.5m deep on which duckweed grows in a thick mat. This was harvested on a daily basis and immediately mixed with cassava waste (largely peelings) and fed to ducks which were constrained in pens on the side of the pond or lagoon (see Photo 3). The ducks were produced for the local restaurant trade.   In Taiwan, it was traditional to produce duckweeds for sale to pig and poultry producers.   There are reports that Wolfia arrhiza, which is about 1mm across has been used for many generations as a vegetable by Burmese, Laotions and Northern Thailand people. Thai's refer to this duckweed as Khai-nam or eggs of the water and it was apparently regarded as a highly nutritious food stuff. It could have been a valuable source of vitamins particular of vitamin A to these people. This would have been particularly important source during the long dry season of Northern Thailand when green vegetables may have been scarce. It is also a good source of minerals, again its phosphorous content could have been vital in areas where there are major deficiencies, such as occurs in Northern Thailand.   There are references in the literature to duckweed as both a human food resource and as a component of animal and bird diets in traditional/small farmer systems in most of South Asia.   CHAPTER 3: Nutrient requirements of duckweed   INTRODUCTION   Like all photosynthetic organisms, duckweeds grow with only requirements for minerals, utilising solar energy to synthesise biomass. They have, however, the capacity to utilise preformed organic materials particularly sugars and can grow without sunlight when provided with such energy substrates. In practice the ability to use sugars in the medium as energy sources is irrelevant, as in most aquatic systems they do not exist. However, they could be of some importance where industrial effluent's need to be purified and duckweed is considered for this process (e.g. waste water from the sugar industry or waste water from starch processing).   Most research on nutrient requirements have centred on the need for nitrogen, phosphorus and potassium (NPK). However, like all plants, duckweeds need an array of trace elements and have well developed mechanisms for concentrating these from dilute sources. From the experience of the Non-Government Organisation PRISM (based in Colombia, Maryland, USA, see chapter 6) in Bangladesh, it appears that providing trace minerals through the application of crude sea salt was sufficient to ensure good growth rates of duckweeds in ponded systems. However, considerable interest has been shown by scientists in the capacities of duckweed to concentrate, in particular, copper, cobalt and cadmium from water resources where these have economic significance.   Mineral nutrients appear to be absorbed through all surfaces of the duckweed frond, however, absorption of trace elements is often centred on specific sites in the frond.   The requirements to fertilise duckweeds depends on the source of the water. Rainwater collected in ponds may need a balanced NPK application which can be given as inorganic fertiliser or as rotting biomass, manure or polluted water from agriculture or industry. Effluent's from housed animals are often adequate or are too highly concentrated sources of minerals and particularly because of high ammonia concentration may need to be diluted to favour duckweed growth. Run-off water from agriculture is often high in P and N but the concentration may need to be more appropriately balanced. Sewage waste water can be high or low in N depending on pretreatments but is almost always adequate in K and P. Industrial waste water from sugar and alcohol industries for example are always low in N.   Little work has been done to find the best balance of nutrients to provide maximum growth of duckweed. The duckweed has been provided with mechanisms that allow it to preferentially uptake minerals and can grow on very dilute medium. The main variables that effect its growth under these circumstances are light incidence and water and air temperatures.   The growth rate and chemical composition of duckweed depends heavily on the concentration of minerals in water and also on their rate of replenishment, their balance, water pH, water temperature, incidence of sunlight and perhaps day length. Its production per unit of pond surface also depends on biomass present at any one time.   WATER TEMPERATURE   Duckweeds grow at water temperatures between 6 and 33° C. Growth rate increase with water temperature, but there is an upper limit of water temperature around 30° C when growth slows and at higher temperature ceases. In open lagoons in direct sunlight duckweed is stressed by high temperature created by irradication and in practice yields are increased by mixing the cooler layers of water low in the pond and splashing to reduce surface temperature of the duckweed matt.   WATER, pH   Duckweed survives at pH's between 5 and 9 but grows best over the range of 6.5-7.5. Efficient management would tend to maintain pH between 6.5 and 7. In this pH range ammonia is present largely as the ammonium ion which is the most readily absorbed N form. On the other hand a high pH results in ammonia in solution which can be toxic and can also be lost by volatilisation.   MINERAL CONCENTRATIONS   Duckweeds appear to be able to concentrate many macro and micro minerals several hundred fold from water, on the other hand high mineral levels can depress growth or eliminate duckweeds which grow best on fairly dilute mineral media. There is a mass of data on the uptake by duckweed of micro-elements which can be accumulated to toxic levels (for animal feed). However, their ability to concentrate trace elements from very dilute medium can be a major asset where duckweed is to be used as an animal feed supplement. Trace elements are often deficient in the major feed available to the livestock of small resource poor farmers. For example, in cattle fed mainly straw based diets both macro and micro mineral deficiencies are present.   Duckweeds need many nutrients and minerals to support growth. Generally slowly decaying plant materials release sufficient trace minerals to provide for growth which is often more effected by the concentrations of ammonia, phosphorous, potassium and sodium levels. There is by far the greatest literature on the requirements of duckweed for NPK and the ability of the plant to concentrate the requirements of micro nutrients from the aquatic medium is usually considered not to be a limitation. In the work in Bangladesh by PRISM, crude sea salt was considered to be sufficient to provide all trace mineral requirements when added to water at 9kg/ha water surface area when duckweed growth rates were high at around 1,000kg of fresh plant material/day.   Photo 2: Duckweed accumulation in the crocodile lagoon in Havana Zoo, Cuba.   Photo 3: Duckweed growing as part of an integrated farming system in Vietnam    4 WATER DEPTH   Depth of water required to grow duckweed under warm conditions is minimal but there is a major problem with shallow ponds in both cold and hot climates where the temperature can quickly move below or above optimum growth needs. However, to obtain a sufficiently high concentration of nutrients and to maintain low temperatures for prolonged optimal growth rate a balance must be established between volume and surface area. Depth of water is also critical for management, anything greater than about 0.5 metres poses problems for harvesting duckweeds, particularly by resource poor farmers. Whereas, where water purification is a major objective in the production of duckweed, it is impractical to construct ponds shallower than about 2m deep.   Incident sunlight and environmental temperatures are significant in determining the depth of water as undoubtedly duckweed is stressed by temperatures in excess of 30° C and below about 20° C growth rate is reduced.   In practice, depth of water is probably set by the management needs rather than the pool of available nutrients and harvesting is adjusted according to changes of growth rate, climate changes and the nutrient flows into the system.   REQUIREMENTS FOR NPK AND OTHER MINERALS   Duckweeds evolved to take advantage of the minerals released by decaying organic materials in water, and also to use flushes of minerals in water as they occurred when wet lands flooded. Duckweeds now appear to have the potential to be harnessed as a commercial crop for a number of purposes.   Water availability is likely to limit terrestrial crop production particularly of cereals in the coming years (see World Watch 1997). Water purification and re-use particularly that water arising from sewage works, industrial processing and run-off from irrigation appears to be mandatory in the future, both to reduce pollution of existing water bodies and to provide reusable water for many purposes including that required by humans in some places as drinking water.   Nitrogen requirements   Duckweeds appear to be able to use a number of nitrogenous compounds either on their own or through the activities of associated plant and animal species. The ammonium ion (NH4+) appears to be the most useful N source and depending on temperatures duckweeds continue to grow down to extremely low levels of N in the water. However, the level of ammonia N in the water effects the accretion of crude protein in the plant (see Table 1).   The value of duckweed as a feed resource for domestic animals increases with increasing crude protein content. In studies at the University of New England, Armidale, Australia, the crude protein content of duckweed growing on diluted effluent from housed pigs increased with increased water levels of N from about 15% crude protein with trace levels of N (1-4mg N/l) to 37% at between 10-15mg N/l. Above 60mg N/l a toxic effect was noticed perhaps due to high levels of free ammonia in the water. Whilst few experiments have been undertaken on the optimum level of ammonia required, these results give a guide-line for the levels of N to be established and maintained in duckweed aquaculture to obtain a consistently high crude protein level in the dry matter.   In most practical situations the approach to growing duckweed is to find the dilution of water where N is not limiting growth and supports high levels of crude protein in the plant. This is usually done by an arbitrary test. Serial dilutions of the water source with relatively pure water (rain water) is carried out and duckweed seeded into each dilution and weight change recorded after, say, 4 weeks. In this way the appropriate N concentration is established.    A useful indicator of whether conditions in the pond are appropriate for growth of duckweed (Lemna spp) of high protein content in the length of the roots. Many experimental observations (Rodriguez and Preston 1996a; Nguyen Duc Anh et al   1997; Le Ha Chau 1998) have shown that over short growth periods there is a close negative relationship between root lenght and protein content of the duckweed and with the N content of the water. Data taken from the experiment of le Ha Chau (1998) are illustrated in Figure 4. In most small-scale farmsituations it is not feasible to determine the protein content of the duckweed that is being used; nor can the nutrient content (especially nitrogen) of the water be estimated easily. To determine the root length of duckweed is a simple operation and and requires neither equipment nor chemicals. By monitoring this characteristic, the user can have an indication of the nutritive corrective measures when the lenght of the roots exceeds about 10mm.   In duckweed aquaculture a source of N essential and in many start-up systems, based on water effluent from sewage or housed animals, the project has been considered by pre-treatments that denitrify the water and reduce ammonia concentrations. Most forms of aeration in sewage works are highly efficient in de-nitrification of waste waters, but this process compounds pollution peoblems. for instance where the effluent is high in P this promotes the growth of algae that fix N. In Australia the contamination of river systems with phosphorus often led to massive blooms of blue-green algae that rae toxic to humans and animals.    Although there is an association of N fixing cyanobacteria with duckweeds, these are certainly not important from a standpoint of farming duckweeds. Duong and Tiedje (1985) were able to demonstrate that duckweeds from many sources had heterocystous cyanobacteria firmly attached to the lower epidermis of older leaves, inside the reproductive pockets and occasionally attached to the roots. They calculated that N fixation via these colonies could amount to 3.7-7.5kg N per hectare of water surface in typical Lemna blooms, but the association of cyanobacteria with Lemna trisulca was 10 times more effective.   Probably, under most practical situations ammonia is the primary limiting nutrient for duckweed growth and the establishment of the optimum level for maximum growth of duckweeds needs research, particularly in the variety of systems the plant may be expected to grow. The effects of time and lowering of N content of sewage water on yield and crude protein content of duckweed is shown in Figure 5 and Figure 6.   From recent research it appears that duckweed require about 20-60mg N/l to grow actively and from two studies [(those of Sutton & Ornes, (1975) compared with those of Leng et al., (1994)] it is apparent that there is a complex relationship between, the initial composition of the duckweed used in research and the level of nutrients required.   Stambolie and Leng (1993) showed with duckweeds harvested from a backwater of a river and with an initial low crude protein content, it was only when the duckweed protein increased to the highest level that rapid growth of biomass commenced (i.e. at 3 weeks after introduction to the water) (Figure 6) even though by that time the N content of the water had declined to levels that were below the optimum that appears to be necessary for maximum protein levels (Figure 3).   In the work of Sutton and Ornes (1975), however, duckweed of a higher protein content was initially used and growth rate again peaked at about the third week (Fig. 6) but by this time the crude protein content had declined to below 15%. This apparent opposite result can be rationalised if there is a stress factor involved which requires 3 weeks to overcome, and under these circumstances its growth may be considered   Table 1: The composition of duckweed harvested from a natural water source or grown on waters with minerals enriched (Leng et al. 1994)   Crude Protein   Fat   Fibre    Ash   Source   (%DM)   (%DM)   (%DM)   (%DM)   Natural lagoon   25-35   4.4   8-10   15   Enriched culture   45   4.0   9   14   Figure 3: The influence of the concentration of N in culture water on crude protein in duckweed (Spirodela spp) grown on diluted effluent from a piggery. The P levels in water varied from 1.2-6.1 mg P/litre (Leng et al., 1994).   Figure 4: Relationship between root length and protein content in duckweed (Lemna minor) (Le Ha Chau 1998)   Figure 5: The effect of N level in culture water on growth of duckweed and its crude protein content.The experiments were conducted on duckweed collected from a billabong down stream from a sewage works. The sewage water used in the incubation was taken from that flowing into the sewage works prior to denitrification processing. The pond were 2.5m. (Stambolie & Leng, 1994)  
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