Flowering Lemna minor.

Each plant has two leaves and a single root.  The pollen-bearing anthers are white and easily seen in the photo.  Each flower has two stamens and a single style.  The style is shorter and more difficult to see.   See a close-up photo of flowering Lemna by Wayne Armstrong, and Ludmila V.Tsatsenko and N.G. Malyuga's excellent photos of flowering in several Lemna species.
 

Photo from the work of Elias Landolt (1986), Biosystematic Investigations in the family of  duckweeds (Lemnaceae) vol. 2, p.516.  Photo used with permission.

 

Flowering Wolffia, A Scanning Electron Micrograph (SEM)

Micrograph of Wolffia australiana (Benth.) den Hartog & van der Plas, courtesy of Patrick Denny, IHE, Delft Netherlands, published in Bernard, Bernard & Denny (1990) Bull. Torrey Bot. Club 117:18-26. For color photos of flowering Wolffia, [ link ].
MF, mother frond DF, daughter frond S, stomata VB, ventral bulge	Pi, pistil A1, A2, anthers (two lobes) DL, dehiscence line (where the anther splits open to release pollen)	Scale bar = 0.25 mm

Flowering
of duckweed species has been studied exensively.  Various duckweed species flower either on long days or on short days.  Lemna gibba, for example is a long-day species, while Lemna paucicostata 6746 is a short-day species.  This flowering behavior (photoperiodism) is regulated by the phytochrome system.  Duckweeds have been used also to uncover chemical factors that stimulate or inhibit induction of flowering.  Using this system Charles Cleland and coworkers discovered that salicylic acid induces flowering under certain conditions (1).

Self-pollination of Lemna flowers is generally prevented by a delay in the receptivity of the stigma, through which the pollen tube must grow.  This is a common mechanism for preventing selfing in higher plants.  However, these plants are not self-incompatible, since two plants vegetatively derived from the same clone can be crossed.  By such repeated pollinations Janet Slovin and her colleagues (USDA-ARS, Beltsville, Maryland) have isolated inbred lines of L. gibba that are suitable for genetic research (2).

(1) Cleland, C.F. (1985) Chemical control of flowering in the long-day plant Lemna gibba G3.  Biologia Plantarum 27(4/5):392-397.
(2) Tam, Y. Y., J. P. Slovin and J. D. Cohen. 1995. Selection and characterization of alpha-methyltryptophan-resistant lines of Lemna gibba showing a rapid rate of indole-3-acetic acid turnover. Plant Physiology 107: 77-85.

According to Elias Landolt (1986), duckweed fruits range from over 2 mm (Spirodela) down to less than 0.4 mm.(Wolffia and Wolffiella).  They ripen just a few weeks after fertilization.  The fruit has a dry skin (pericarp) and contains one or a few seeds.  Fruits of Lemna gibba, for example, may have from one to five seeds.  In most species, the fruit burst open to release their seeds.
 

Right:  Photographs of Lemna gibba fruit by Ludmila V.Tsatsenko and N.G. Malyuga.  Click on an image for a larger view.  Their website contains many more excellent photos and diagrams.

The World's Smallest Fruit! - by Wayne Armstrong, Palomar College, California.
 
"Two of the smallest species of Wolffia in the world are the Australian Wolffia angusta and the Asian/African Wolffia globosa. In fact, both species are so small that it is difficult to distinguish between the size of their fruits. Since the entire plant body of these two species is less than one millimeter long (less than 1/25th of an inch), the tiny mature fruit takes up most of its parent plant body. The fruit of W. angusta is only 0.25 mm long (1/100th of an inch) and weighs about 70 micrograms (1/400,000 of an ounce). One can get an idea of how small these fruits really are when you consider that an average, single, cubical grain of ordinary table salt (NaCl) is about 0.3 mm on a side and weighs about 60 micrograms. This fruit is smaller than the individual cells of many plants and animals, and is more than 4 billion times lighter than a massive world class pumpkin."
[ follow link to the complete article and photos ]

Seeds
develop after pollination as in other flowering plants.  They sink to the bottom of the pond or culture vessel since they do not contain the air spaces (aerenchyma) that provide buoyancy to vegetative duckweeds.  There are 1-5 seeds in each fruit, which may be smooth or ribbed.  The dormant seeds (unlike turions) are resistant to drying.
 

Above left:  Fruit of Lemna gibba with three seeds. Photographed by Ludmila V.Tsatsenko and N.G. Malyuga.	Above right:  Cutaway drawing of the fruit of Lemna gibba with seeds.  Drawing from Watson, L., and Dallwitz, M. J., see below.
. Above:  Dried seed of Lemna gibba Photogrpahed by Darryl Ubick, California Academy of Sciences.	Color photographs of duckweed fruits and the seeds, by Ludmila V. Tsatsenko, Kuban State Agicultural University, Krasnodar, Russia. View scanning electron microscope (SEM) images of dried seeds of Lemna turionifera and Lemna gibba from the University of California, Berkeley. Drawings of embryos and seeds of Lemna and Wolffia from Watson, L., and Dallwitz, M. J. (1992 onwards). ‘The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval.’ Version: 15th October 1998.  http://delta-intkey.com/.

Turions are dormant vegetative buds.
 

Above:  Turions and mother fronds of Spirodela polyrrhiza Photograph courtesy of Prof. Cheryl C. Smart

Turions have a relatively high starch content and lack the large aerenchyma (air pockets) of vegetative duckweeds, so they sink.  This allows turions to overwinter in the debris or mud on the bottom. They are not more resistant to freezing or desiccation than normal fronds. Not all species of duckweeds form turions.  For example, only one species of Lemna will form turions, Lemna turionifera.  Turions are rootless and can often be recognized by their darker green color and smaller size than vegetative fronds of the same species. In Wolffia, the turions are very small, spherical in shape and light green in color. Formation of turions  in the duckweed Spirodela polyrrhiza L. is known to be regulated by phytochrome (1) and is stimulated by the plant hormone abscisic acid (ABA, 2).  However, turion formation in Spirodela is not regulated by day-length, i.e. it is day-neutral (3).

Andrew Trewavas (University of Edinburgh) and Cheryl Smart (ETH, Zürich) characterized the induction of turions by ABA.  The regulation of turion formation by phytochrome has been studied intensively by Klaus-J. Appenroth (Jena).

According to Smart and Trewavas, there is a sensitivity window in frond development when a young frond can be diverted into forming a turion rather than a mature frond.  A turion can only be formed only if cell separation in the mesophyll has not yet started.  That process starts when the frond is about 0.7mm long.  Therefore, if a daughter frond of Spirodela has a length >0.7mm it cannot form a turion.  Once induced to form turions, a mother frond will continue to form many turions until the growth conditions are changed.  Smart and Trewavas also discovered that ABA maintains dormancy only when it is kept in the medium.  If turions induced by ABA are washed free of the ABA, they germinate immediately.  However, photoperiodically (phytochrome) induced turions will remain dormant in the dark and require a period of rest or a cold treatment before germination.

ABA can exist as two stereoisomers (enantomers).  Both the (+)- and the (-)-enantomers of ABA are equally effective in inducing formation of turions (2).  This morphogentic induction is antagonized by another class of plant hormones, the cytokinins.  Cytokinins restore vegetative growth to turions.  The genes for D-myo-inositol-3-phosphate synthase (4) and a novel basic peroxidase (5) are up-regulated during induction of turions by ABA.  The up-regulation of the peroxidase gene is attenuated by cytokinins (4).

Electrical currents are generated by turions during light-induced germination and growth (6). These currents are measured using a very sensitive vibrating probe electrode.  Following a light pulse, substantial changes in direction and magnitude of currents were observed (6).  Phytochrome also regulates induction of alpha-amylase in turions of Spirodela (7).  alpha-Amylase is the important enzyme that breaks down starch into the simple sugar, glucose, which provides energy to the germinating turion.

(1) Appenroth K-J et al., (1996) Biologia Plant. 38: 95-106.
(2) Appenroth K-J. No photoperiodoc control of the formation of turions in eight clones of Spirodela polyrhiza J. Plant Physiol. 160:1329-1334, and Appenroth K-J. et al. 1990. Phytochrome control of turion formation in Spirodela polyrhiza. Annals of Botany 66: 163-168.
(3) Smart CC, .Fleming, AJ, Chaloupkova K, and Hanke DE, (1995) The Physiological Role of Abscisic Acid in Eliciting Turion Morphogensis, Plant Physiol.108: 623-632.
(4) Smart CC; Fleming AJ (1993) Plant J. 4(2):279-93
(5) Chaloupkov´a K; Smart CC (1994) Plant Physiol. 105(2):497-507
(6) Sokolovski, S.G., Appenroth, K.-J., Weisenseel M.H. (1999) Origin of endogenously generated electrical currents in turions of Spirodela polyrhiza during photomorphogenesis. European Symposium on Photomorphogenesis.
(7) Appenroth, Klaus-J., Cyganek, Izabela and Luka, Zigmund A. (1999) Antagonistic effects of phytochrome on ß-amylase in turions of Spirodela polyrhiza. European Symposium on Photomorphogenesis.
 

Other recommended references:

A thorough study of turion formation in Spirodela can be found in four papers by Cheryl Smart and Anthony Trewavas published in the 1980's.  These papers describe their general anatomy and ABA sensitivity, the structure of turion cells at the electron microscope level, the proteins made during turion formation, and changes in ion transport that occur during turion formation.  The papers are:
Smart, CC and Trewavas, AJ. (1983) Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. I. Production and development of the turion. Plant Cell and Environment 6:507-514;
Smart, CC and Trewavas, AJ. (1983) Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. II. Ultrastructure of the turion; a stereological analysis. Plant Cell and Environment 6:515-522;
Smart, CC and Trewavas, AJ. (1984) Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. III. Specific changes in protein synthesis and translatable RNA during turion development. Plant Cell and Environment 7:121-133;
Smart, CC and Trewavas, AJ. (1984) Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. IV. Comparative ion flux characteristics of the turion and the vegetative frond and the effect of ABA during early turion development. Plant Cell and Environment 7:521-531.

A more recent summary from Professor Smart:
Smart, C. C. (1996) Molecular analysis of turion formation in Spirodela polyrrhiza: a model system for dormant bud induction. Chapter 19 In: Plant Dormancy: Physiology, Biochemistry and Molecular Biology, G.A. Lang (ed), CAB International, UK, pp. 269-281.ewavas, AJ. (1984) Plant cell and Environment 7:521-531.

Cheryl Smart writes me that the best anatomical description of turion formation is Jacobs, D.L. (1947) An ecological life-history of Spirodela polyrrhiza (greater duckweed) with emphasis on the turion phase. Ecological Monographs 17, 437-469.  She also writes:

"Actually there is only one species of Lemna described which forms turions and that is Lemna turionifera. It has been described that Lemna gibba can form turions, but this is not true. Under certain conditions, Lemna gibba forms "heavy fronds" which sink to the bottom of the flask and look a bit like turions to the untrained eye. The real turion has a very distinctive shape and structure, which can only really be clearly seen under the stereomicroscope. Spirodela polyrrhiza also forms these types of fronds from time to time, but they are quite different from real turions."

For phytochrome regulation of turion formation, see the publications of Dr. Klaus-J. Appenroth, Lehrstuhl Pflanzenphysiologie, Friedrich-Schiller-Universität Jena, Germany.  [ Dr. Appenroth's home page ] (scroll down).
 

Do you want to experiment?  Link to:  Experiments and Projects with Duckweed
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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Last revised:   July 10, 2012