July 2009

AQUATIC PLANTS

The world's largest and smallest floating plants are shown here.

Babies sitting on Victoria regiae	Wolffia columbiana and Lemna minor on a finger

Water lilies are our most obvious floating plants. This growth form has evolved independently in many unrelated species. So it is an example of convergent evolution. Claude Monet painted water lilies many times; Dale Chihuly sculpts them in glass.

Monet's water lilies	Chihuly's fanciful glass water lilies

And Sara Teasdale wrote a poem, "Water Lilies:"

If you have forgotten water lilies floating
On a dark lake in the afternoon shade,
If you have forgotten their wet, sleepy fragrance,
Then you can return and not be afraid

LEAVES, STEMS, & ROOTS OF SUBMERGED PLANTS

Submerged aquatic plants need even less support than floating species because water buoys them completely. Reviewing the April 2009 column on tree sun and shade leaves, what adaptations would you hypothesize for submerged aquatic plant roots, stems, and leaves? Think about availability of carbon dioxide for photosynthesis and inorganic plant nutrients in water.

How did you do? Here are comparison images that show differences in leaves and stems.

Terrestrial & submerged aquatic plants in air	Diagram of a plant with leaves in water and in air	The same plant has different leaves in air and water

The change in leaf type in the same individual plant is called heterophylly and it is an example of phenotypic plasticity.

Three hypotheses may explain the advantage of finely dissected underwater leaves.

• They may give less resistance to water flow.
• They may provide greater surface area relative to volume of the plant for absorption of nutrients and bicarbonate.
• They may cast less shade on lower leaves. These hypotheses are not mutually exclusive.

In water, the carbon dioxide (CO₂) needed for photosynthesis diffuses into the one-cell to three-cell thick leaves as bicarbonate (HCO₃). So no stomates  are needed on the leaves for gaseous CO₂ to enter.

Submerged plants do not have a problem of water loss. So their leaves do not have shiny waxes to retard water loss.

Submerged plants often have very thin roots. Why? Some researchers suggest that they need roots to get an adequate supply of nutrients from the sediments.

EMERGENT AQUATIC PLANTS

	Sections of emergent plant stems showing compartmented aerenchyma.
All emergent aquatic plants have leaves held above water by a stem and anchored in the bottom by roots. If you squeeze the stems of these plants they are squishy because they have openings called aerenchyma all the way from the leaves to the roots. Why?

If you review the first figure of the first Plant-Animals Mutualisms column you will see that during photosynthesis plants produce oxygen (O₂) in the leaves. But plants also respire and use oxygen to get energy to absorb nutrients in their roots.

To understand why roots need oxygen we need to review the concentrations of oxygen in air, water, and sediments under the water. Roughly the concentration of oxygen in the air is 20% which converts to 200,000 parts per million (ppM).

In water of about 30°C (86°F) the concentration in ppM is less but how much less? 100,000? 10,000? 1,000? 100? 10? 1?

In the sediments, where roots are, the concentration of oxygen in ppM is still lower because fungi and bacteria decompose organic matter. How much lower do you think? 100? 10? 1? 0.1? 0.001?

Actually the oxygen concentration in 30°C water is about 5 ppM. Its concentration in the sediments is zero, zippo, nada, nil, nothing, none!!

The next two images show that water lilies have continuous aerenchyma which allows you to blow bubbles through or suck air through a cut stem. In a live water lily, oxygen produced during photosynthesis in the leaves travels down the aerenchyma to the roots.

Water lily stems under water	Cross section of white water lily stem	Blowing bubbles through  stem of water lily

Floating water lilies have continuous aerenchyma, because the buoyancy of water provides support. Emergent aquatic plants, like pickerel weed or bulrush, have compartmented aerenchyma to provide structural support for the stems and leaves in the air. Actually the compartments have tiny microscopic holes big enough for oxygen to diffuse from the leaves to the roots.

THE MYSTERY OF BLADDERWORTS

Bladderworts, species of the genus Utricularia, are submerged plants with finely dissected leaves and no roots but many small spots that look like tiny bladders.

Years ago M. Alice Bryant wrote "These are wonderful plants which float upon the surface of the water by means of countless little bags full of air, joined to the seaweed like leaves." To test this hypothesis you can squeeze the plant under water to see if bubbles of air are released. They are not!

Another hypothesis is that the bladders are traps that catch live prey. The following photo shows what is apparently a prey item in a bladder. Here is a photo of a bladderwort with inset of enlarged bladder with a 2mm copepod crustacean inside

Several observations are consistent with this carnivory hypothesis. Bladderworts are only found in waters with very low nutrients and in one species the abundance of bladders increases as the level of nitrate (NO₃) nitrogen decreases.

In fact special bladders capture tiny crustaceans, like water fleas or copepods, or large protozoa, like paramecium. The trigger hairs act as a lever that distends the pliable door. This breaks the seal to the partial vacuum and water rushes in along with the prey.

Bladderwort trap capturing a water flea	Bladderwort plant with many bladder traps

This video "Carnivorous plant eats tadpole of cane toad" (www.youtube.com/watch?v=FrAE1CA4Qus) shows a bladderwort apparently "trying" to capture a tiny tadpole. I do not understand this video because once a trap is triggered it sucks in prey very quickly and closes; then it takes several minutes to transport water out of the bladder to create a negative pressure and thus reset the trap.

Bladderworts are indicators of excellent water quality with low phosphorus and especially low nitrogen. The yellow-flowered species, Utricularia foliosa, has more bladders in low nitrate waters. The purple flowered species, Utricularia purpurea, is an indicator of very low nutrient water in the interior of the Refuge marsh where the main source of nutrients is rain water.

MUTUALISM OF WATER FERN AND BACTERIUM

Another indicator of low nitrogen in water is the obligate mutualism, shorthand + / +, between the water fern Azolla carolinianum and the blue-green bacterium Anabaena azollae. Obligate mutualism is like a marriage where divorce is non-existent. Separation results in the death of one or both partners. The bacterium fixes atmospheric nitrogen for the fern and the fern provides sugars from photosynthesis for the bacterium.

Close-up of Azolla	Anabaena cells from inside Azolla

This mutualism is found in low nitrogen but moderate phosphorus waters. Here, for example, is a mat of reddish Azolla on Lake Okeechobee among spike rush

In waters of very high nutrient concentration, especially phosphorus, duckweed can be a solid mat.   Green Gator head amongst a carpet of duckweed

PLANT ZONATION: DEPTH & NUTRIENTS

The rim canal around the Refuge has high nutrient water that flows from the Everglades Agricultural Area (EAA) through Stormwater Treatment Areas (STAs). STAs have not reduced plant nutrients enough to eliminate a gradient from the high nutrient canal to the low nutrient marsh interior. Plants reflect this gradient. Emergent cattail and willow are very abundant and very tall along the high nutrient peripheral canal and spike rush and sawgrass are common and short in the low nutrient marsh interior.

Floating plant species also graduate from high nutrient areas to low.

In intermediate nutrient levels, white water lily, Nymphaea odorata,and yellow water lily, Nuphar advena, separate by water depth.

Students in shallow water habitat of white water lilies	White water lilies in flower	Yellow water lilies in flower

 

Column author Tom Poulson demonstrates the
deep water amongst yellow water lily leaves →

Dr. Tom's mantra for interactive teaching comes from Baba Dioum, a Senagalese Conservationist:

You will conserve only what you love,
You will love only what you understand,
And you will understand only what you are taught.

Tom's corollary is that you really understand only what you teach. And he tries, in his column, to emulate Einstein's dictum: "If you can't explain it simply, you don't understand it well enough."

REVIEW QUESTIONS

a. Leaves of the same aquatic plant can be broad and waxy with many stomates in air but very thin, non-waxy, and without stomates in the water.
b. Rooted aquatic plants have air tissue called aerenchyma to get oxygen from the leaves in air to the roots in bottom sediments.
c. The bladders on bladderwort are adaptations for flotation.
d. Species of aquatic plants are as good as water chemistry for detecting the degree of nutrient enrichment.