Introduction:

Duckweeds are the smallest flowering plants. They grow as small colonies of plants floating on the surfaces of quiet bodies of water. Growing vegetatively, their multiplication can be extremely rapid, given the proper conditions. These plants are almost all leaf, having essentially no stem tissue, and only one or a few, very fine roots. In nature, duckweeds serve as food for many species of fish and aquatic birds. They can tolerate and grow under a wide range of conditions, including on water polluted with high concentrations of bacteria and some agricultural wastes. These characteristics have brought the duckweeds to the attention of environmental engineers and agriculturists alike.

A delightful, nontechnical discussion of duckweeds, has recently appeared in Pacific Discovery (Armstrong, 1989), while an article in Smithsonian (Stewart, 1989) mentions their role in advanced biological sewage treatment. A thorough technical discussion of all aspects of their biology has been published (in English) by Professor Elias Landolt of the Swiss Geobotanical Institute (Landolt and Kandeler, 1987).

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Productivity of Duckweeds:

Culley et al. (1981) and Landolt and Kandeler(1987, pp. 371-373) summarize many earlier studies demonstrating the unusually high productivity of this aquatic plant. Dry weight increases of 10-20 tons/ha/yr are the norm. Doubling times in the range of 24 hr have been observed on many occasions, a rate of increase results in 64 g/g dry weight/week, or 73 tons/ha/yr (Landolt and Kandeler, 1987, p. 371). As Landolt and Kandeler point out, the observed values are therefore about 1/3-2/3 the theoretical value. Under summer conditions in Louisiana with heavy fertilization, up to 44 tons/ha/yr have been obtained (Said et al. 1979).

Needless to say, these rates of increase will be negatively affected by diminished rates of fertilization or cold weather in the temperate zone. However, it is clear that the species of this group are capable of a level of productivity closer to that obtained with microorganisms than with other higher plants.

The high levels of fertility required to obtain this massive growth of duckweeds might seem to be an obstacle, but in fact are economically feasible, if the use of nitrogenous runoff wastes from feedlots or other ready sources are foreseen...

Tbc....

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