Plants absorb water constantly, but the physical forces driving this absorption change drastically between day and night. During the day, the primary mechanism of water movement is a powerful, solar-driven suction, whereas the nocturnal process is a slower, internal push. This shift in mechanism allows the plant to manage its water resources efficiently whether the sun is shining or not.
The Daytime Water Cycle Driven by Transpiration
The rapid movement of water during daylight hours is powered by the sun through a process called transpiration. Transpiration is the evaporation of water vapor from the plant’s leaves, primarily through tiny pores known as stomata. When the stomata open to allow carbon dioxide uptake for photosynthesis, water is inevitably lost to the atmosphere.
This water loss from the leaf surface creates a strong negative pressure, or tension, which pulls the entire column of water up from the roots through the xylem tissue. This process, known as the Cohesion-Tension Theory, relies on two physical properties of water. Water molecules exhibit cohesion, meaning they stick strongly to each other, and adhesion, meaning they stick to the walls of the narrow xylem vessels, creating a continuous, unbroken chain stretching from the root hairs to the leaves. This pulling force, which can be equivalent to a negative pressure of around -2 megapascals (MPa), is powerful enough to draw water to the top of the tallest trees.
The rate of water absorption during the day is directly proportional to the rate of water loss through transpiration. Because the pull originates in the leaves and is transmitted down, water uptake is very fast when the sun is out and the air is dry. This daytime cycle is a high-volume process, delivering the water required for photosynthesis and helping to cool the plant through evaporative cooling.
How Plants Absorb Water When Stomata Close
As light fades, most plants close their stomata to conserve water, which eliminates the driving force of transpiration pull. Water absorption at the roots continues through a different mechanism known as root pressure. This uptake relies on the fact that the root cells actively transport mineral ions and other solutes into the xylem.
This active accumulation of solutes creates a lower water concentration inside the root’s vascular cylinder compared to the surrounding soil water. Following the principle of osmosis, water naturally moves from the area of higher concentration (the moist soil) into the area of lower concentration (the root cells). The continuous influx of water into the roots, without the compensating loss through transpiration, generates a positive hydrostatic pressure within the xylem.
This positive root pressure pushes the column of water up the xylem, ensuring a slow but steady supply of water and dissolved minerals throughout the night. Although this push is far weaker than the daytime transpiration pull—often only capable of raising water a few meters—it maintains a continuous flow. This steady, low-volume movement prevents the water column in the xylem from breaking and allows the roots to continue mining the soil for nutrients.
The Phenomenon of Guttation
The most visible physical evidence that plants absorb and move water at night is guttation, which refers to the appearance of small droplets of liquid water on the tips or edges of leaves, often mistaken for dew. This phenomenon typically occurs in the early morning when the soil is very moist and the air is highly humid, preventing residual evaporation.
The positive root pressure generated during the night forces the excess water out of the leaf tissue. This liquid is exuded through specialized pores located at the ends of leaf veins called hydathodes. Unlike stomata, which open and close, hydathodes are small, permanent openings.
The fluid released during guttation is not pure water, but rather a dilute solution of the xylem sap, containing various dissolved minerals and sugars. Guttation is most common in small, herbaceous plants and grasses, where the root pressure generated can be sufficient to push the water column all the way to the leaf margins.