Problem 217 - Objective Biology for NEET

Question

Assertion: Bulliform cells in grasses use to minimise water loss under water stem condition. Reason: They are flaccid due to water stress and make the leaf curl inside thus minimise surface area for transpiration.

Step-by-Step Solution

Answer

The assertion: 'Bulliform cells in grasses use to minimise water loss under water-stem condition' is correct. Similarly, the reason: 'They are flaccid due to water stress and make the leaf curl inside thus minimise surface area for transpiration' is also correct, and both assertion and reason are interconnected. Bulliform cells aid grasses to conserve water under water stress by curling leaves inward to reduce exposed surface area for transpiration.

Step 1: Understand Bulliform Cells

Bulliform cells are large, bubble-like cells found in the epidermis of leaves, particularly in grasses. These cells play a critical role in plant's adaptation to varying water conditions.

Step 2: Role of Bulliform Cells under Water Stress

During periods of water scarcity, bulliform cells lose their turgor pressure - this means they become flaccid or lose their rigidity due to lack of water.

Step 3: Effect of Flaccid Bulliform Cells

When the bulliform cells become flaccid, it results in the leaf curling inward. This is due to the change in cell structure as a result of water loss.

Step 4: Relating Flaccid Bulliform Cells to Minimization of Water Loss

The curling of leaves inward due to flaccid bulliform cells minimizes the leaf surface area exposed, thus reducing the surface area available for transpiration. Transpiration is the process where water is lost from the surfaces of leaves in form of vapour. So, by reducing the exposed surface area, it minizes water loss through transpiration.

Step 5: Verifying the Assertion and Reason

Given the role of bulliform cells in adapting to water stress by becoming flaccid, which in turn causes leaf curling and reduces exposure surface area for transpiration, the assertion and the reason are correct and linked, thus making the whole statement true.

Key Concepts

Water Stress Adaptation

Water stress is a condition that plants experience when there is an insufficient amount of water available to them. This can result from drought, poor soil water-holding capacity, or intense sunlight that increases evaporation. To adapt to these conditions, plants undergo several changes to conserve water.

In grasses, bulliform cells play a crucial role in water stress adaptation. These are specialized cells located on the upper surface of a leaf, and their primary function is to regulate the opening and closing of the leaf in response to water availability.

• When water is plentiful, bulliform cells are turgid, meaning they are swollen with water, and the leaf remains open.
• When water is scarce, these cells lose their turgidity, becoming flaccid.

This transition from turgid to flaccid leads to a mechanism that helps the plant reduce water loss. Through this process, plants effectively manage their water resources during droughts or periods of limited water availability.

Leaf Curling

Leaf curling is an adaptive response to water stress wherein the leaf rolls or folds into itself. This phenomenon, primarily mediated by bulliform cells, plays a crucial role in the plant's survival during dry periods.

When bulliform cells lose water and become flaccid, they initiate the leaf curling process. This response can be likened to how a tightly inflated beach ball collapses when air is let out. By curling, the leaf minimizes its exposure to sunlight, effectively reducing heat absorption and further water loss.

Benefits of leaf curling include:

• Reduced leaf surface area exposed to the sun, limiting the amount of heat and light reaching the leaf.
• Decreased transpiration rates by protecting internal leaf surfaces from direct exposure to environmental conditions.

This natural mechanism ensures plants conserve precious water, enabling them to survive in environments with fluctuating water availability.

Transpiration Reduction

Transpiration is the process through which water is carried through plants from roots to leaves, where it changes to vapor and is released into the atmosphere. While necessary for nutrient transport within the plant, transpiration can also lead to significant water loss.

In conditions of water scarcity, reducing transpiration becomes vital. The curling of the leaf due to flaccid bulliform cells directly contributes to transpiration reduction. By minimizing the leaf surface area exposed to the external environment, the plant reduces the potential surface area through which water can be lost as vapor.

• Less exposed surface area means less water is able to escape.
• Maintaining even minimal water reserves can be crucial for plant function and survival.

This reduction in transpiration is an essential survival mechanism for many plants, allowing them to endure harsh conditions until more favorable environments arise.