Duckweed, a collective term for the tiny, fast-growing aquatic plants of the Lemnaceae family, often forms a dense, bright green mat on the surface of still freshwater bodies globally. These plants reproduce rapidly through budding throughout the warm season. In temperate zones, where water bodies freeze or become too cold for growth, the duckweed population does not entirely perish; instead, it employs a biological strategy to survive the winter. This survival mechanism involves transforming the plant into a specialized resting structure that remains viable until spring returns.
Duckweed’s Specialized Winter Structure
The key to duckweed’s cold-weather survival is the formation of a dense, vegetative bud known as a turion. This specialized structure is developed from the meristematic pockets of the active fronds. The plant initiates turion formation in response to environmental cues that signal the end of the growing season, such as decreasing water temperatures, reduced daylight hours, and nutrient depletion.
A significant factor in this structural change is the reduction of available nutrients, particularly phosphate, in the water. The turion is a compact, distinct frond that is much smaller and thicker than the typical summer frond. Internally, the turion is densely packed with starch grains, serving as a concentrated food reserve to fuel the plant’s metabolism during dormancy.
This structure also undergoes a physical change to facilitate sinking, often developing a darker green, brown, or purplish color due to the accumulation of pigments. Unlike the buoyant vegetative fronds, the turion lacks the internal air spaces (aerenchyma) necessary for flotation. This increased density allows the turion to detach and sink immediately to the sediment layer at the bottom of the pond or lake.
The Dormant Period Underwater
Once the turion settles onto the bottom substrate, it enters a state of innate dormancy, where growth is arrested. The plant’s metabolic activity is significantly reduced, allowing it to survive for months on its stored starch reserves with minimal energy expenditure. This adaptation is a mechanism of frost avoidance, as the turions escape the freezing temperatures and ice formation that occur at the water’s surface.
The environment at the bottom of a water body offers a stable refuge from the cold. Water reaches its maximum density at approximately 4 degrees Celsius (39 degrees Fahrenheit), meaning this warmer water settles to the deepest point, insulating the turions from colder surface conditions. The turion’s thick cell walls and protective coatings help it withstand the low-oxygen conditions that can develop in the sediment during the winter months.
This prolonged exposure to low, non-freezing temperatures is necessary for a process called “after-ripening,” which is required to break the turion’s deep dormancy. The turion remains in this quiescent state, protected by the sediment and the stable water temperature, until specific conditions signal that the growing season has returned. Without this period of cold conditioning, the turion would be unable to resume active growth.
Returning to the Surface in Spring
The transition from dormancy back to active life is triggered by environmental shifts that indicate the start of a favorable growing season. The primary signals for revival are rising water temperatures and increased exposure to sunlight penetrating the water column. As the water warms, the turion’s internal systems become active again, initiating the germination process.
The stored starch within the turion is metabolized, providing the energy needed for initial growth and the production of a new vegetative frond. A crucial physical event in this revival is the development of internal air pockets within the emerging frond. The plant begins to photosynthesize, producing oxygen gas that accumulates in these newly formed internal spaces, rapidly increasing its buoyancy.
This increase in buoyancy causes the turion to float upward from the sediment, returning the new frond to the water surface. Once the plant reaches the sunlight-rich surface, it quickly resumes the rapid vegetative reproduction cycle characteristic of duckweed. The surviving turions ensure the water body is quickly repopulated, allowing the duckweed mat to spread rapidly across the water body once more.