To Root or Not to Root

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Duckweed Roots

Patterns of Root Growth

To Root or Not to Root:  Spirodela, Landoltia and Lemna develop roots.  The maximum number initiated per frond depends on the species. Wolffiella and Wolffia form no roots, and roots may occasionally be absent in the other groups. Lemna fronds make just one root, while Spirodela and Landoltia fronds initiate more.  According to Landolt (1986) Spirodela can have as many as 21 roots per frond.  We do not know what environmental factors govern the number of roots initiated.

The regulation of root length depends on environmental conditions (Landolt, 1986).  Except in extreme conditions, low levels of nitrogen, phosphate or trace minerals encourage longer roots, while high fertility results in very short roots, or even an absence of root.  Far-red light shortens roots, suggesting that phytochrome is involved.  Gibberellins may also have a role.

Photo:  A view from below, roots of Spirodela as collected from a wetland.  The reflection at the water surface makes some of the roots appear to grow upward.  Debris from the wetland still clings to some roots but will fall off as the roots grow in culture.

The structure of duckweed roots is very simple.  Unlike most plants, duckweed roots do not show secondary growth or branching, and do not sprout root hairs.  As a result, duckweed roots are very slender, less than 0.5 mm.  Like other primary roots, they can be divided into four zones:  the root cap, the meristematic region, the elongation zone, and the zone of mature cells.

What is the function of the duckweed root?

That question may seem obvious if one knows plant physiology, but duckweeds are exceptional.  Research has shown that duckweeds mostly absorb water and nutrients through the lower surface of the frond, not the roots.

An experiment you can try:

Paul Gorham (1941) tried painting the lower surface of Lemna fronds (but not the roots) with lanolin, a soft fat.  The thin lanolin coating prevented the fronds from making direct contact with the water.  He found that coated fronds grew very slowly and their roots elongated more than the roots of control plants.  For another control, he painted some fronds with lanolin on the upper surface.  Those fronds were unaffected.  Another experiment showed that Lemna will dry up if the fronds are held above the water but with their root tips still submerged.

In the water, long duckweed roots become entangled with one another.  If one tries to gently pick up one plant or frond colony, often the result is to pick up several others whose roots are entangled with the first.  In nature, this helps duckweeds to form loosely- connected mats on the water surface.  Landolt (1986) points out that the a duckweed root provides stability to help keep the individual plants orientated rightside-up in the water as they encounter wind and water turbulance.  The root is like a sea-anchor.  A sea anchor does not touch bottom, but prevents a sailor's boat from drifting in the wind.  In the same way, the duckweed root slows the plant's movement when the wind blows.

Duckweed roots are very sticky.  This is key to another root function:  dispersal.  Ducks and other aquatic birds land among duckweeds and eat the fronds.  When they do, some

of the fronds may adhere to their feathers.  This allows duckweeds to hitch a ride on the bird to the next pond.  In this way, duckweeds spread from one place to another.

Test this experiment yourself with a duckweed frond that has one or more long roots:

Pick up a frond with a stainless steel spoon or spatula.  Tip the spoon and let the water drain off for just a minute.  Try gently shaking the frond off.  Did it still cling?  Take a paper towel or tissue and gently wick the water from the root.  Does it still cling?  Cut the root(s) off the frond and repeat the experiment.  Can you think of a way to measure the force it takes to pick up a frond clinging to a smooth surface? The Charms of Duckweed. John Cross, Missouri Botanical Garden 2002
Gorham, P.R. (1941) "Measurement of the response of Lemna to growth-promoting substances." Amer. Jour. Bot. 28: 98-101.
Landolt, E. 1986. The family of Lemnaceae - a monographic study. Vol 1. In: Biosystematic Investigations in the Family of Duckweeds (Lemnaceae). Veroff. Geobot. Inst. ETH, Stiftung Rubel. No. 71.

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