Ever wonder what that green layer is that sits on the top of freshwater ponds or lakes? This layer often consists of hundreds small free-living plants collectively termed duckweed. Duckweed is a common name given to several species in the subfamily Lemnaceae. This group of plants has a number of remarkable morphological features that allow them to remain aquatic and floating. While each plant has no apparent stems or leaves in the traditional sense, duckweeds have a leaf-like thallus structure that is only 3 cells thick on average. Interspersed in the thallus is a number of air pockets called aerenchyma cells that allow plants to float. Although duckweed primarily reproduce asexually, they do produce tiny flowers with either two stamens or a pistil. The 3mm flower of one species, Wolffia, is the smallest known flower in the world. These morphological features made duckweeds difficult to place in the angiosperm tree of leaf; molecular data has now placed duckweeds in the Araceae family – a family that is characterized by a floral feature (a spadix) that duckweeds do not possess.
Ducksweeds have the potential to be remarkably useful to humans. Several species have proven to be exceptional plants for phytoremediation of nitrate, copper, cadmium, lead and/or zinc from freshwater (Stomp 2005). These plants occupy a critical role in aquatic ecosystems by blocking sunlight as well as providing cover and food for other pond residents. Scientists are also beginning to think about the bioenergy or pharmaceutical potential of many duckweed species. Some species exhibit extremely rapid growth, doubling in as little as 30-36 hours. After recognizing this potential, the genome of Spirodela polyrhiza (giant duckweed) was recently sequenced with U.S. Department of Energy funding (Wang et al. 2014). Interestingly, this genome sequence uncovered only 19,623 genes – the least in any sequenced plant- despite a genome size that is similar to the model plant Arabidopsis thaliana (>27,000 genes).
--Nic Kooyers
Stomp, AM. 2005. The duckweeds: a valuable plant for biomanufacturing. Biotechnol Annu Rev. 11:69-99.