Adaptations to Aquatic Environments

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Aquatic plants evolved from terrestrial plants. Like whales and other marine mammals, aquatic plants evolved from land back to aquatic habitats. Aquatic plants modified terrestrial features to withstand emerged, submerged, or floating conditions.

Types of Challenges

Emergents:
Aeration of Roots:

• Formation of aerenchyma: large open spaces between cells, which is important to carry oxygen down to the root zone.
• Formation of prop roots. (Red Mangrove) http://www.barbadosbirds.com/redmng.html
• Formation of pnuematophores. (Black Mangrove) http://faculty.valencia.cc.fl.us/tklenk/bkmgrv.htm
• Anaerobic respiration: plants will form ethylene (an incomplete respiration compound that can be broken down later forming fatty acids), then more aerenchyma tissue and adventitious tissue. The plant elongates and forms fatty acids.  Ethylene is a common gas in swamps due to decay.  The soil is anaerobic and has reducing conditions.
• Lacunae - specialized cells that form big openings allowing gas to transport in and out.  Cattails have lacunae.

Reproduction: There are two common modes of reproduction sexual and vegetative.  Sexual reproduction is rare; vegetative is more commonly used.
Sexual Reproduction:

• Flowers are above the water surface
• Gametes are dispersed in large numbers and close to each other to increase the chances of pollination.  However this is not successful during low tides.

Vegetative Reproduction:   

• Fragmentation, pieces break off and float away to another location where they get embedded in the substrate.
• Rhizomes: Rhizomes are underground shoots from the stem of a plant that are connected to the root system and used for reproduction.  http://www.wetland.org/kids/rhiz.htm
• Stolon: specialized type of horizontal aboveground shoot, a colonizing organ that arises from an axillary bud near the
  base of the plant.

Seed germination: Plants have different strategies for seeds. Major problems are:

• Timing of seed production to occur during the non-flood season either by delayed or accelerated flowering.
• Timing with flower formation to get good fertilization and seed formation. Above water or exposed flowers are affected by tides and flood cycles.
• Production of buoyant seeds that either float or sink.
• Seeds germinate while still attached to the plant.  Example spider plants
• Longevity – some plants can survive thousands of years. Example Chinese lotus >1,000 years. http://hledu.nhltc.edu.tw/elotus.html

Photosynthesis: Gas exchange: As the water gets deeper, the wavelength of light gets shorter until it’s gone. The red and blue wavelengths are lost, and the green (not so good for photosynthesis) remains. Adaptations include:

• Wetland plants often use C4 biochemical pathway of photosynthesis instead of C3.

-C4 provides a possible pathway for recycling CO2 from cell respiration
-plants using C4 have low photorespiration rates and the ability to use even the most
intense sunlight efficiently.
-C4 plants more efficient than C3 plants in rate of carbon fixation and amount of water
used per unit carbon fixed.

Salinity:

• Barriers prevent or control the entry of salts -root and leaf cell membrane act like ultra filters.
• Organs specialized to excrete salts -selectively remove certain ions from the vascular tissue of the leaf.

Submerged fresh water plants:

Fertilization:

• Vallisneria produce a coiled peduncle (female), which straightens out so the stigma can reach above the water surface. The spathe (male) also straightens out so its petals break loose and float on the surface. Its three leaves and anthers form a sailboat. The spathe floats along until hopefully it bumps into a stigma. http://www.mascota-mania.com/acuariofilia/plantas/054.html
• Ceratophyllum: uses a strategy of hydrophily: the male releases pollen into the water where it floats, water logs, until it sinks, hopefully landing on a female plant. http://www.tropica.dk/21.htm

Photosynthesis:

Algal blooms can block the sunlight and nutrients to submerged plants. It also filters out red light loss.

Adaptations:

• Use photopigmens more chlorophil, carbon, etc.
• C₄ vs. C₃ – more plants will use C₄ because C₄ has lower photorespirate rates and use intense sunlight efficiencly such as emergent plants.
• Halophytes - plants can withstand high salinity mechanism. Barriers keep salts at root level and only lets fresh water in. Plant organs excrete salt. Plant absorbs salt at the roots and excretes it from above. Example Mangroves, Salt Marsh plants

Aeration of Roots –

Oxygen is transmitted from the leaves to the roots and rhizomes by lacunae (air spaces forming channels in leaves, stems, and roots). Lacunae also have a structural role. Lacunae take up about 60% of the plants volume.

An experiment was done to demonstrate the oxygen gradient in plants. It was found that a plant has 20% oxygen in its leaves, 15% in its stem, 10% in the root parts, and only 2- 5% in the root hairs. The oxygen is taken in from the air by photosynthesis and travels through the plant and out the root hairs.

When low oxygen levels are present, plants use other mechanisms to adjust for respiration. Aquatic plants can respire anaerobically. This has been shown experimentally by bubbling N2 or O2 into the water with rhizomes, and then measuring the ethanol production. At <3% O2, ethanol is produced by Typha, Scirpus, Nuphar, and others. Some aquatic plants have developed air roots along their stems for respiration in water. Aquatic trees have developed pnuematophores, which are extensions of the root system reaching above the water level. Pnuematophores take in oxygen through small holes at their tips.

Other challenges that aquatic plants must adapt to include: flooding, desiccation (drying out) nutrient uptake, and vegetative reproduction.