Human nutrition has always been an area of high priority, especially in the present era where people are becoming more and more aware of health benefits and / or side effects of their daily food. Of course, this is aided by the progressive and advanced research in the area of human nutrition. We are now going to talk about a plant (rather a plant family), which is a good component of human diet. These plants are commonly called duckweeds with the Latin name Lemnaceae. They are free-floating aquatic monocots. There are 37 different species of duckweeds known until now and they measure from a little more than a centimeter to as small as less than a millimeter. They include the smallest and the fastest growing angiosperms. They are cosmopolitan in distribution except the arctic and Antarctic regions.
Duckweed as Food
Duckweeds, especially species belonging to the genus Wolffia, are being eaten traditionally in the Southeast Asian countries like Laos, Cambodia and Thailand. Already more than four decades back, a preliminary report by Bhanthumnavin and McGarry  showed that the protein quality of duckweeds is good. More recently, Yan et al.  carried out detailed analysis of the fat content in duckweeds. The group had presented very interesting and elaborate data but did not focus on the present context of human diet.
Our recent report  is the result of cooperation with nutritional scientists at the University of Jena (Gerhard Jahreis, Volker Boehm), University of Hohenheim (Walter Vetter, Simon Hammann) and the Thuringian State Institute of Agriculture (Matthias Leiterer). We have presented an overview of the nutritional status of plants of the whole duckweed family. There are five genera subdividing the species and that is why we selected at least one species from each of the five genera. The content of protein, fat and starch were analyzed. The results are presented in figure 1.
Nutrition Potential of Duckweed
The protein content in duckweeds was as high as 35% on a dry weight basis, when grown under suitable cultivation conditions.
Amino acid composition of these proteins was remarkably good for nutrition, meeting the WHO recommended levels (fig. 2). The amino acid profile was similar to that of Soya, however, the advantage with duckweeds is that the whole plants are used, not only the seeds, as in the case of other plants . Moreover, duckweeds do not need arable land for their cultivation, which would in turn increase the land use efficiency and, allow enough room for other terrestrial crop plants to utilize the fertile soils.
The fatty acid composition in duckweeds is also very good. The content of polyunsaturated fatty acids was between 48 and 71%. One of the highlights is the ratio of omega - 6 to omega - 3 fatty acids. According to the FAO recommendation , this ratio should not be higher than 5. In duckweeds, it was less than 1. It is important to note that this is not the case with many other plant-derived fatty acids. A comparison of the fatty acid composition of duckweeds and that of other plant seed oils has been presented in table 1.
Under nutrient rich cultivation conditions, as in the present investigation, the starch content is low, which is suitable for dietary intake. In industrial countries, this provides low-energy food and in most of Asian countries, starch is already available in abundance via the staple food rice. However, when necessary the starch content can be increased by modulating the cultivation conditions to make use of the duckweed biomass for other applications, e.g. for the production of biofuel.
The mineral composition of duckweeds can be conveniently manipulated as the in vivo mineral contents depend on the composition of the nutrient medium being used for cultivation of these aquatic plants. As an example, diet with low sodium content in comparison to potassium can easily be produced. The accumulation of trace elements can also be manipulated making it possible to have higher content of zinc, selenium, iodine or other required mineral components. Apart from minerals, these green plants have good amounts of antioxidants like lutein, zeaxanthin and tocopherols. Phytosterols need a mention especially because of their higher concentration in comparison to other plant oils. The increasing nutritional interest in phytosterols derives from the fact that phytosterols have the capacity to lower plasma cholesterol and LDL (low-density lipoprotein) cholesterol.
We continue with our project by screening more clones or strains of species belonging to the genus Wolffia, as it has been traditionally eaten in some parts of the world. Wolffia is also a chosen species because of its root-less nature, which might increase the palatability of these plants. It is interesting to note that duckweeds do not own a strong taste, other than a slight taste of a normal green plant. It is interesting because this nature allows duckweeds to be used as an ingredient in many different dishes without coming in conflict with other tastes.
We believe that in future, more and more of the nutritive properties of duckweeds would be uncovered because as of now only a few clones of the 37 different species of duckweeds have been screened in this respect. Moreover, breeding of duckweeds has not yet taken place, unlike other crop plants- some of which have been bred for many thousand years.
K. Sowjanya Sree1 and Klaus-J. Appenroth2
1 Central University of Kerala, Department of Environmental Sciences, Kerala, India
2 Friedrich Schiller University Jena, Institute of General Botany and Plant Physiology, Jena, Germany
Friedrich Schiller University Jena
Institute of General Botany and Plant Physiology
 Bhanthumnavin, K., & McGarry, M. G. (1971). Wolffia arrhiza as a possible source of inexpensive protein. Nature, 232, 495. DOI: 10.1038/232495a0
 Yan, Y., Candreva, J., Shi, H., Ernst, E., Martienssen, R., Schwender, J., et al. (2013). Survey of the total fatty acid and triacylglycerol composition and content of 30 duckweed species and cloning of a ∆6-desaturase responsible for the production of gamma-linolenic and stearidonic acids in Lemna gibba. BMC Plant Biology, 13, 201. DOI: 10.1186/1471-2229-13-201
 Appenroth, K-J., Sree, K. S., Boehm, V., Hammann, S., Vetter, W., Leiterer, M., & Jahreis, G. (2017). Nutritional value of duckweeds (Lemnaceae) as human food. Food Chemistry, 217, 266-273. DOI: 10.1016/j.foodchem.2016.08.116
 Edelman, M., & Colt, M. (2016) Nutrient value of leaf vs.seed. Frontiers in Agricultural Chemistry 4,32. DOI: 10.3389/fchem.2016.00032
 FAO (2010). Fats and fatty acids in human nutrition. Report of an expert consultation. FAO Food and nutrition paper 91. ISBN 978-92-5-106733-8