Question

Root pressure contributes to the (a) Ascent of sap in small herbaceous plants. (b) Re-establishment of continuous chains of water molecules in the xylem which often breaks down under the enormous tension created by transpiration. (c) Guttation (d) All the above

Short answer

The correct answer is (d) All the above.

Step-by-step solution

Understanding Root Pressure

Root pressure is a pressure developed in the roots of plants due to the active absorption of nutrients from the soil. This pressure helps to drive the movement of water and dissolved nutrients from the roots to the rest of the plant.

Analyzing Option (a)

Root pressure does contribute to the ascent of sap in small herbaceous plants. Nutrients are actively absorbed by the roots and this pressure helps push the water and nutrients up the stem of the plant. Therefore, option (a) is correct.

Analyzing Option (b)

Root pressure also helps in the re-establishment of continuous chains of water molecules in the xylem which often breaks down under the tension created by transpiration. So, option (b) is also correct.

Analyzing Option (c)

Guttation is the process of water droplet exudation at the leaf edges or leaf tips, which occurs due to root pressure. So, option (c) is also correct.

Final Decision

After analyzing options (a), (b) and (c), it's clear that root pressure contributes to all the scenarios listed. Thus, the correct answer is option (d) All the above.

Key concepts

Ascent of Sap

The ascent of sap refers to the upward movement of water and dissolved nutrients from the roots to the leaves of plants. This is an essential process for plant survival, as it transports vital resources to the areas of photosynthesis and growth.

Root pressure plays a pivotal role in the ascent of sap, particularly in small herbaceous plants. Root cells actively absorb minerals from the soil, creating a concentration gradient. Water follows the minerals into the roots by osmosis, generating root pressure. This pressure drives the water upwards in the xylem vessels against the force of gravity.

However, in larger plants and trees, root pressure alone cannot achieve the ascent of sap due to the tall stature and gravitational challenge. Instead, transpiration pull – the process by which water is pulled up the xylem as water vapor exits through the stomata in leaves – becomes the primary force.

Xylem Water Transport

Xylem water transport is a fundamental aspect of plant physiology, involving the movement of water from the roots, through the stem, and into the leaves. The xylem consists of tubular structures that facilitate this flow.

Water transport within the xylem can be influenced by several factors, including root pressure, capillary action, and transpiration pull. Root pressure provides the initial push, filling the xylem with water and nutrients. Capillary action, due to the cohesion and adhesion properties of water, allows the liquid to move through narrow vessels in the plant against gravity. But the most significant driver of water movement in tall plants is the transpiration pull, where water evaporates from the leaf surfaces, creating a negative pressure that draws water up from the roots. This mechanism allows for the efficient delivery of water to the highest leaves in the tallest trees.

Plant Transpiration

Plant transpiration is the process by which moisture is carried from the roots to small pores on the underside of leaves, where it changes to vapor and is released into the atmosphere. Transpiration serves several functions; it helps in temperature regulation, nutrient transport, and maintaining water movement through the plant.

Transpiration is closely linked to the photosynthesis process and the stomata – tiny openings on leaves – regulate this water vapor exchange. During transpiration, water loss creates a negative pressure within the xylem, which acts as a suction force, known as the transpiration pull. This pull is a dominant force in the ascent of sap, especially in larger trees. Additionally, the water escaping from the leaves during transpiration is critical in re-establishing the interrupted chains of water molecules, which is essential for continuous water flow through the xylem vessels and sustains the plant's water transport system.

Guttation

Guttation is the exudation of water droplets from the edges of leaves of some plant species, particularly noticeable during the night or early morning. This phenomenon occurs when root pressure builds up within the plant, and water is forced out through special structures called hydathodes at the leaf tips.

While guttation may resemble dew, it has a different origin. Dew is water that condenses from the atmosphere onto the leaf surface, while guttation comes from within the plant itself. Root pressure is responsible for guttation and you can often observe the process when the soil moisture level is high, and transpiration is low, like at night time. This buildup of root pressure and the subsequent release of water is another example of the essential roles that root pressure fills in plant life cycles and contributes to the maintenance of internal water balance in plants.