What is the primary substance excreted from the plant via guttation? (A) Glucose, (B) nitrogenous waste, (C) urea, (D) xylem sap, or (E) excess salts.
This question asks us about guttation, which is sometimes called droplet exudation. Let’s review this process and address some relevant key terms along the way in order to answer our question correctly.
You may recall that transpiration is an important process by which water is lost through evaporation from the aerial, or upper, parts of a plant, like its leaves and stem, into the atmosphere. This might sound like a bad thing as water is a vital reactant for photosynthesis, which is how most plants make their own food. However, in moderation, transpiration is very useful, as it provides an upward pull that moves water through a plant from its roots to its leaves, where most photosynthesis takes place.
This water, and some dissolved minerals it carries, moves through tubelike structures called xylem vessels in the form of xylem sap. The evaporation of water, mainly from the leaves, helps to pull this xylem sap upward from elsewhere in the plant. The majority of transpiration occurs through small pores found on leaves called stomata, which also allow the exchange of other gases between the leaf and the atmosphere. For example, carbon dioxide is the other key reactant in photosynthesis and needs to be absorbed through the stomata.
To prevent the excess loss of water, transpiration mostly occurs during the day, as this is when the light that is essential for photosynthesis is available. At night or in the early hours of the morning, the stomata tend to close. This is because without light, photosynthesis cannot occur. So the water loss that results from transpiration would not provide any real benefit to the plant.
However, plant roots may still absorb excess amounts of water, which provides an upward root pressure, moving xylem sap up through the xylem vessels, albeit at a slower rate than if the stomata were open. Since this water cannot be evaporated from the closed stomata or used in photosynthesis during these periods of low light intensity, it needs to be eliminated in another way.
Guttation is a process that usually occurs during the nighttime or in the early morning when a plant’s stomata are closed. It involves xylem sap being released from pores called hydathodes found in the margins of leaves. This forms droplets of water and often some dissolved mineral salts, which are then lost from the plant.
Now that we have reviewed the process of guttation, we can eliminate some of our answer choices. Options (A), (B), and (C) involve molecules that are not primarily excreted by the plant during guttation. Answer option (E), excess salt, might seem like a logical answer at first, since guttation will eliminate excess water and dissolved mineral salts. However, we know that not just salts are eliminated through guttation, but mineral salts dissolved in water in the form of xylem sap.
Therefore, the primary substance excreted from the plant via guttation is (D), xylem sap.