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Aquatic Plants

All land plants have evolved from aquatic ancestors. Species from nearly one hundred flowering plant families, along with some ferns, mosses, and liver-worts, have reinvaded the water. Many land plants can tolerate flooding for some time. A good definition of aquatic plants is therefore difficult. The term is normally used for plants that grow completely underwater or with leaves floating on the surface. Parts of the shoot, particularly flowering stems, will often grow up above the water. Most aquatic plants live in freshwater—in lakes, ponds, reservoirs, canals, or rivers and streams. These habitats are very different in water depth, flow rates, temperature, acidity and alkalinity (pH), and mineral content. Some aquatic plants live in river mouths, with an ever-changing mixture of fresh- and saltwater, and a few (such as sea grasses) live completely submerged in the sea. Aquatic plants can be free-floating (e.g., water hyacinths) or rooted to the bottom of the pond or stream (e.g., water lilies). The most important grain crop in the world, rice, is an aquatic plant.

Water supply is generally not a problem for aquatic plants. They do not need waterproof cuticles or a lot of woody tissue to keep them erect. Life underwater, however, is a challenge. All green plants need oxygen and carbon dioxide. These gases cannot diffuse easily from the air down through the water. Most of the alterations in physiology and structure in water plants are adaptations to solve these gas exchange problems. Many water plants develop large internal air spaces—aerenchymatous tissues—in their roots and shoots. These air spaces make the tissues buoyant and help them float. Green leaves, photosynthesizing underwater in the light, release oxygen that can be temporarily stored in the air space and later used for cellular respiration. If the plant grows or floats to the surface, oxygen can also easily diffuse down through the air spaces. One of the water lilies, Nuphar, has even developed a ventilation system to circulate air from the leaves floating on the water surface down to its roots in the mud. The rapid growth of shoots to the surface—called depth accommodation growth—seen in rice seedlings and many other aquatic and amphibious species, is driven by the shoot buoyancy and a buildup of carbon dioxide and the naturally produced gas, ethylene, in the tissues under water. Plant organs in the mud at the bottom of lakes survive long periods without much oxygen.

Aquatic plants require carbon dioxide for photosynthesis. The amount of carbon dioxide dissolved in water depends on the pH and temperature. Aquatic plants may acidify their leaf surfaces. This causes carbon dioxide to be liberated from carbonate and bicarbonate salts dissolved in the water. Underwater leaves are often finely divided, with a large surface area. Unlike land plants, they may have chloroplasts for photosynthesis in the surface cell layer—the epidermis.

The amount of light available for photosynthesis declines with depth (especially in dirty water), and the light quality (the proportion of red, far-red, and blue light) is also altered. Some species (e.g., Potamogeton, Sagittaria) can produce underwater, floating, or emergent leaves, each with different shapes and structures all on the same plant—a phenomenon called heterophylly. This is a response to light quality and the amount of a plant hormone, abscisic acid (which is at slightly higher levels in emergent shoots).

Most aquatic plants flower and set seed. Many of them, however, can also grow rapidly and reproduce vegetatively. Aquatic plants, particularly free-floating species, can colonize the surface of a water body very quickly. If the water dries up, plants can produce a variety of tubers and resting buds (turions) that will persist in the mud until the water returns. These abilities make aquatic plants some of the most troublesome and persistent weeds in the world, particularly in tropical and subtropical countries. There is, understandably, considerable reluctance to put herbicides into rivers and lakes. A wide variety of control methods, including mechanical harvesting and the introduction of fish and other animals to eat them, have been attempted. The canal system of late nineteenth-century Britain was clogged with introduced Elodea canadensis; the water hyacinth, Eichhornia crassipes, has spread from tropical South America to waterways in Africa, Asia, and North America; and Salvinia auriculata, a free-floating aquatic fern, rapidly covered the 190-kilometer lake behind the Kariba Dam on the Zambezi River in Africa in the 1960s. The Everglades and waterways of the southeastern United States have been repeatedly invaded by quickly spreading, alien aquatic plants.

Many aquatic habitats, and the aquatic plants that live in them, however, are under constant threat from pollution and from drainage for urban and industrial development across the globe. Aquatic plants can help remove pollutants and purify our water supplies. They are also a vital part of a fully functional aquatic ecosystem for fish and other wildlife.