Fully submerged plants do not have stomata, which are tiny pores on the surface of leaves that allow gas exchange. Stomata are essential for most terrestrial plants, as they help them regulate water loss and carbon dioxide uptake. However, for aquatic plants that live completely underwater, stomata are unnecessary and even disadvantageous, as they would cause excessive water intake and reduce buoyancy. Therefore, fully submerged plants have evolved different adaptations to survive and thrive in aquatic environments.
How do fully submerged plants photosynthesize?
Photosynthesis is the process by which plants use light energy to convert water and carbon dioxide into sugars and oxygen. This process requires both light and carbon dioxide, which are abundant in the air but scarce in the water. Therefore, fully submerged plants face the challenge of obtaining enough light and carbon dioxide to sustain their growth and metabolism.
One way that fully submerged plants cope with low light levels is by having thin and transparent leaves that allow more light to penetrate. Some plants also have air-filled spaces in their stems and leaves, called aerenchyma, that help them float closer to the water surface where more light is available. Additionally, some plants can adjust their pigment composition and chloroplast distribution to optimize their light absorption and photosynthetic efficiency.
Another way that fully submerged plants cope with low carbon dioxide levels is by using bicarbonate, a dissolved form of carbon dioxide, as a carbon source. Bicarbonate is more abundant and stable in water than carbon dioxide, but it requires more energy to convert into sugars. Some plants have special enzymes, called carbonic anhydrases, that catalyze this conversion. Other plants have symbiotic relationships with bacteria or algae that can fix carbon dioxide or bicarbonate for them.
How do fully submerged plants respire?
Respiration is the process by which plants use oxygen to break down sugars and release energy. This process requires oxygen, which is less available in water than in air. Therefore, fully submerged plants face the challenge of obtaining enough oxygen to support their cellular functions and avoid anaerobic respiration, which produces toxic byproducts.
One way that fully submerged plants cope with low oxygen levels is by having aerenchyma, which provide a continuous gas-filled pathway from the roots to the leaves. This allows oxygen produced by photosynthesis in the leaves to diffuse down to the roots and other tissues. Some plants also have specialized roots, called pneumatophores, that protrude above the water surface and act as snorkels to capture atmospheric oxygen. Furthermore, some plants can store oxygen in their cells or tissues, such as starch grains or vacuoles, and use it when needed.
What are some examples of fully submerged plants?
Fully submerged plants are found in various aquatic habitats, such as ponds, lakes, rivers, streams, wetlands, and oceans. They can be classified into two main groups: vascular plants and non-vascular plants. Vascular plants have specialized tissues, such as xylem and phloem, that transport water and nutrients throughout the plant body. Non-vascular plants lack these tissues and rely on diffusion and osmosis for water and nutrient uptake.
Some examples of vascular fully submerged plants are:
- Eelgrass (Zostera spp.): A marine plant that forms dense meadows in shallow coastal waters. It has long and ribbon-like leaves that can grow up to 2 meters in length. It is an important habitat and food source for many aquatic animals, such as fish, crabs, shrimp, sea turtles, and manatees. It also helps stabilize the sediment and improve water quality by filtering nutrients and pollutants.
- Hornwort (Ceratophyllum spp.): A freshwater plant that has no roots and floats freely in the water column. It has whorled and forked leaves that bear small needle-like projections, called spines. It can produce oxygen bubbles that make it buoyant and help it disperse. It is a popular aquarium plant, as it can tolerate a wide range of water conditions and provide shelter and food for fish and invertebrates.
- Watermilfoil (Myriophyllum spp.): A freshwater plant that has feathery and finely divided leaves that resemble a milfoil. It has a slender and branching stem that can grow up to several meters in length. It can reproduce both sexually and asexually, by producing seeds or fragments. It is an invasive species in many regions, as it can form dense mats that block light and oxygen, and interfere with navigation and recreation.
Some examples of non-vascular fully submerged plants are:
- Green algae (Chlorophyta): A diverse group of aquatic organisms that have chlorophyll and other pigments that give them a green color. They can be unicellular or multicellular, and have various shapes and sizes. They can perform photosynthesis and respiration, and some can also fix nitrogen. They are the primary producers and the base of the food chain in many aquatic ecosystems. They also contribute to the oxygen and carbon cycles, and play a role in bioremediation and biofuel production.
- Red algae (Rhodophyta): A diverse group of aquatic organisms that have phycobilins and other pigments that give them a red color. They can be unicellular or multicellular, and have various shapes and sizes. They can perform photosynthesis and respiration, and some can also produce calcium carbonate. They are the most abundant and diverse group of seaweeds, and are found in both marine and freshwater habitats. They are used as food, medicine, and industrial products, such as agar and carrageenan.
- Brown algae (Phaeophyta): A diverse group of aquatic organisms that have fucoxanthin and other pigments that give them a brown color. They are exclusively multicellular, and have various shapes and sizes. They can perform photosynthesis and respiration, and some can also store iodine. They are the dominant group of seaweeds in cold and temperate waters, and can form large and complex structures, such as kelp forests. They are used as food, fertilizer, and industrial products, such as alginates and laminarin.
Conclusion
Fully submerged plants are plants that live completely underwater and do not have stomata. They have evolved different adaptations to cope with the challenges of aquatic environments, such as low light, carbon dioxide, and oxygen levels. They are found in various aquatic habitats, and can be classified into vascular and non-vascular plants. They have ecological and economic importance, as they provide food, shelter, oxygen, and other services for many aquatic organisms and humans.