Lichtgedanken 02

Editorial 38 B A C K G R O U N D The smallest flowering plants in the world have been eaten for many generations in East Asia (Thailand, Myanmar, Laos), and are standard products in the form of »water eggs« (khai-nam). Water lenses were also a traditional item on the menu of the Mayans in Gua- temala where they were known as »water corn« (Xim Ha). Due to their high protein content, ideal amino acid composition and low fat content, water lenses are particularly valuable from the perspective of nutrition physiology. To be consumed, water lenses must grow in clean water as they quickly ab- sorb substances in the water – pollutants, for example – in large quantities and store them. On the other hand, this circumstance is also exploited, and water lenses are used specifically in order to biologically cleanse polluted waters and waste water. In water treatment plants , water lenses can reli- ably remove nitrogen and phosphorous compounds from the water as well as heavy metals, pesticides or dioxins. The tiny plants also offer further interesting deployment options thanks to their huge propensity to reproduce, for example as raw material for the biofuel ethanol . This is because water lenses produce on average around five times more biomass than maize per hectare per year. And they require no land for cultivating food or fodder and can be more easily harvested than algae, for example, which are already being used today in the production of biofuel. The family of water lens plants ( Lemnaceae ) has five genera with 37 species worldwide. In Central Europe, for example, the lesser duckweed ( Lemna min- or ), the gibbous duckweed ( Lemna gibba ) as well as the star duckweed ( Lemna triscula ) are all native species. The greater duckweed ( Spirodela polyrhiza ) is also frequently found in these parts on stagnant or slowly flowing waters. There is also the so-called rootless spotless watermeal ( Wolffia ) which is pri- marily indigenous to warm and tropical latitudes and therefore relatively rare in Germany. They are considered to be the smallest flowering plants in the world. Members of the Asian watermeal species Wolffia globosa are only 0.7 to 1.5 millimetres in size, spherical in shape and they grow without roots floating on the water. They reproduce vegetatively – by means of asexual cell division – and they do so at such a rate that they are able to cover entire water surfaces within the shortest space of time. Water lenses are the fastest growing flowe- ring plants that exist: Up to six tons of biomass grow from one gram within three weeks under ideal conditions. However: For such turbo growth, the tiny green plants need consistently warm temperatures. For any production of water lenses on a major scale, subtropical or tropical areas are much better suited than Germany or Europe. In nature, water lenses are an important staple for many fish and waterfowl. But they are also already playing a role in human nutrition. For example, water lenses