When you see drops of liquid at the tips or along the edges of leaves you are witnessing the effects of something called guttation. Guttation is caused when root pressure pushes water up from the roots into leaves and stems. This typically occurs during the night when stomata are closed and transpiration is over for the day. Guttation should not be confused with morning dew. Dew will cover the entire surface area of the leaf. Guttation occurs only at the tip or along the edges.
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What Plants Experience Guttation?
Guttation occurs only in vascular plants. That is, plants that have xylem, a specialized tissue, to circulate water and nutrients throughout the plant. Almost all of the plants in your garden and home are vascular plants. Vegetable plants, perennials, all flowering plants, trees, conifers, grasses and ferns are vascular plants.
Why Does Guttation Take Place at Night?
The conditions must be right for guttation to occur. Transpiration stops at night when the stomata on leaves close. Water in the soil is absorbed by the roots of vascular plants by osmosis when the water potential in the roots is lower than the water potential in the soil. Root pressure forces the water up through the roots into the stems and leaves of plants.
How Does the Liquid Get Out of the Leaf?
Specialized structures called hydathodes are located on the tips and edges of leaves. When excess water accumulates in the leaf at night, the hydathodes open allowing water droplets to escape. When these droplets evaporate in the morning, you might see a white residue or crust on the leaves. This crust is caused when the sugars and minerals in the droplets of water are left behind after evaporation.
How Do Plants Release Excess Water During the Day?
During the day, plants transpire, that is, release water vapor through the stomata. Stomata are pores found on the epidermis (on the leaves and stems) of vascular plants. Stomata open in response to light and close in the absence of light. Stomata also exchange oxygen for carbon dioxide. Transpiration is the action of releasing water from the leaves and stems. Transpiration keeps plants cool, draws water up from the roots into the leaves and stems and allows the movement of nutrients and water through the xylem.
Why Doesn't Guttation Occur Every Night?
Guttation can only take place when there is more water in the soil than in the roots of the plant. The water potential in the roots has to be lower than the water potential in the soil (the amount of water in the roots is lower than the amount of water in the soil). This contributes to root pressure. Root pressure supplies the means by which guttation takes place. Root pressure occurs at night when stomata are closed and transpiration has stopped or during the day if transpiration is slow. If water potential is balanced between roots and soil and there is no transpiration, there will be no guttation.