Question

Exudation of xylem is due to (a) Passive absorption (b) Root pressure (c) Guttation (d) Presence of transpiration

Short answer

The correct answer is (b) Root pressure.

Step-by-step solution

Define the terms

Xylem exudation refers to the secretion of water and nutrients from the xylem, a tissue in plants that transports water and minerals from the roots to the rest of the plant. Passive absorption is the process by which water and solutes are absorbed by plant roots due to differences in concentration, without the need for energy input. Root pressure is a force that helps to drive fluids upwards into the above-ground parts of plants. Guttation is the exudation of drops of sap on the tips or edges of leaves of some vascular plants. Transpiration is the process where plants lose water into the atmosphere, which creates a negative pressure that pulls water up in the plant.

Compare definitions to the process of xylem exudation

By comparing the definitions, it can be identified that root pressure is related to the upward movement of water and nutrients, while passive absorption, guttation and transpiration do not specifically relate to xylem exudation.

Confirm the answer

The correct answer should be Root pressure as it is the force that contributes to the movement of fluids in the xylem tissue, thus leading to exudation.

Key concepts

Root Pressure

Root pressure is a critical physiological process in plants, which aids in the upward movement of water and dissolved nutrients from the roots to the plant's aerial parts. Think of it as a push from below, helping to distribute vital resources throughout the plant.

This pressure is generated when the plant actively transports minerals into the xylem vessels, a part of the plant's vascular system. As the solute concentration increases in the xylem, water follows by osmosis, creating a positive pressure that forces fluid upwards. This process occurs primarily at night or when transpiration is low, filling up the xylem with water and thereby, creating the necessary push.

• Essential for nutrient and water transport
• Occurs predominantly at night
• Helps maintain water and nutrient flow within plants

Understanding root pressure is valuable for explaining phenomena such as the ascent of sap or the occasional droplets on leaf margins, even when soil moisture levels are high and transpiration rates are low.

Transpiration

Transpiration is akin to plant sweating. It's the process by which moisture is carried through the plant from the roots to small pores on the underside of the leaves, where it changes to vapor and is released into the atmosphere. Transpiration serves multiple functions: it helps with the transport of water and nutrients from the roots to the leaves, cools the plant, and also enables the movement of water from the soil into the plant's root system.

As water vapor exits the plant leaves, it creates a negative pressure within the xylem channels. This vacuum-like effect pulls more water upwards from the roots, similar to sipping on a straw. This is crucial on hot, dry days when plants could quickly dehydrate without this clever system.

• Facilitates cooling of plant
• Drives upward water movement
• Major driver of water circulation in the plant

Effective transpiration is the sign of a healthy plant and a crucial part of the water cycle within the ecosystem.

Guttation

Guttation is the process in which water, along with dissolved minerals and nutrients, is exuded from the pores, known as hydathodes, at the edges or tips of leaves. This typically occurs during the night or early morning hours when soil moisture is high, but transpiration rates are low due to cooler temperatures and higher humidity.

Guttation should not be confused with dew, which condenses from the atmosphere. Instead, guttation is an internal process where root pressure forces the liquid out. Although less common than transpiration, guttation is especially evident in certain grass species and small herbaceous plants.

• Visible as droplets on leaf tips or edges
• Occurs during low transpiration periods
• Resulting droplets contain dissolved minerals

While it can be a sign of overwatering in domestic plants, guttation is a normal physiological operation in many plants that also serves to release excess water and minerals.