Agri Kultuur December / Desember 2018 - Page 11

accumulate large amounts of minerals in their tissues. It is been reported that fibre and ash contents are higher and protein content lower in duckweed colonies with slow growth. Duckweeds are rich source of nitrogen, phosphorous, potassium and calcium. The concentration of N and P in duckweed tissues depend on the amount of N and P in the water, up to a threshold concentration that has not been clearly defined. Above this threshold, there is little increase in the tissue. Some researchers suggested that under lagoon conditions, 20-30 mg/l TKN might be required to maintain a crude protein level above 30 percent. The crude protein content of duckweeds grown on various nutrient solutions ranges from 7 to 45 percent of the plant dry weight, depending on the nitrogen availability. When conditions are good, duckweed contains considerable protein, fat, starch and minerals, which appear to be mobilized for biomass growth when nutrient concentrations fall below the critical levels for growth. Nutrient contents in duckweed may therefore vary according to the conditions in which it is grown. Slow growth, starvation and aging have been reported to result in protein levels as low as 7 percent DM. Fresh duckweed contained about 91-95 percent water and the moisture content is apparently not influenced by the medium under which it was grown. Duckweed species grown under nutrient-poor water or under sub-optimum nutrient conditions have crude protein contents varying between 9-20 percent, while the level varied from 24- 41 percent for duckweed species grown in nutrient-rich water. The crude protein content of duckweed seems to increase from trace ammonia concentrations to 7-12 mg N/L when crude protein reaches a maximum of about 40 percent. Similarly, the lipid content was lower AgriKultuur |AgriCulture (1.8-2.5 percent) in duckweed species grown in nutrient-poor water, while it generally varied between 3-7 percent for duckweed grown in nutrient-rich water. The medium in which duckweed was grown or the nutrient status of water did not influence the ash content of duckweed. Some researchers reported that fibre and ash contents are higher and protein content lower in duckweed colonies with slow growth. Various studies show clearly that the duckweed indeed has high quality protein. It has a better essential amino acid profile than most plant proteins and more closely resembles animal protein than any other plant proteins. The protein of duckweeds is rich in certain amino acids that are often rather low in plant proteins. The nutritional value of Lemnaceae can be compared favourably with that of alfalfa in terms of lysine and arginine, two amino acids important in animal feeds. Duckweeds are rich in leucine, threonine, valine, isoleucine and phenylalanine and are low in methionine and tyrosine. It is evident that the values for the essential amino acids compare favourably with the FAO reference pattern, except for methionine. The levels of amino acids are very similar in the various species and all the essential amino acids were generally present. Cultured duckweed has high concentrations of trace minerals and pigments, especially β-carotene and xanthophyll. Duckweeds store varying amounts of calcium as calcium oxalate crystals in the vacuoles. Calcium oxalate may be toxic in large doses and the amount should be reduced to make duckweeds more nutritious and digestible. The metabolic precursor of oxalate is Lascorbic acid (vitamin C). A study with water lettuce indicates that L-ascorbate and oxalate are synthesized within the crystal idioblast cells. 11