Problem 184 - Objective Biology for NEET

Question

Assertion: Stomata are absent in submerged hydrophytes. Reason: Respiration occurs by means of air chambers in submerged plants.

Step-by-Step Solution

Answer

Yes, the assertion and reason are correct. Stomata are absent in submerged hydrophytes because these plants, which are entirely under water, have air chambers that store necessary gases, thereby facilitating respiration and eliminating the need for stomata.

Step 1: Understanding the Role of Stomata in Plants

Stomata are tiny openings or pores that are mostly found on the underside leaf surface. They allow for the exchange of gases, such as the intake of carbon dioxide for photosynthesis and the release of oxygen. Moreover, water vapor is expelled during the process of transpiration.

Step 2: Understanding Why Stomata Are Absent In Submerged Hydrophytes

Submerged hydrophytes (water plants) are completely underwater. As such, they have no requirement for stomata. The underlying reason is that carbon dioxide, which is essential for photosynthesis, is readily available for them in the dissolved form in water. Furthermore, they do not need to conserve water, a critical function of stomata, because of their aquatic environment.

Step 3: Understanding the Role of Air Chambers

Respiration, which is a vital life process for plants, is facilitated in submerged hydrophytes via air chambers. These air chambers store gases that are necessary for photosynthesis and respiration. They provide buoyancy to the plant, and they allow for the transport of gases internally, even in the absence of stomata.

Key Concepts

Adaptations of Hydrophytes

Hydrophytes are plants that have evolved to live in aquatic environments, often presenting unique adaptations that enable them to thrive. A common feature in these plants is a drastically reduced or completely absent cuticle layer; since water loss through evaporation is not a concern, the need for this protective barrier is minimized. Additionally, the anatomy of hydrophytes often includes large air spaces in their tissue structure, which aid in buoyancy and facilitate the internal movement of gases.

For submerged hydrophytes, stomata are generally absent as gas exchange can directly occur with the surrounding water. Their leaves tend to be thin and feathery, allowing for an increased surface area that helps in the absorption of dissolved nutrients and gases. Moreover, the root system is typically reduced or transformed to take up solely an anchoring function since water and nutrients are absorbed directly through the leaves and stems. Understanding these adaptations aids students in comprehending why particular cellular structures like stomata are not needed in such an environment.

Plant Respiration

In terrestrial plants, respiration involves the exchange of gases through stomata; however, in hydrophytes, the mechanism is quite different. The presence of air chambers in hydrophytes helps these plants respire even when submerged. These specialized structures allow for the storage and movement of gases like oxygen for respiration and carbon dioxide for photosynthesis.

Even the roots, often in contact with anaerobic sediments, have adapted to absorb oxygen from the water column through aerenchyma tissues that contain large air spaces. These spaces not only store gases but also enable their diffusion throughout the plant's structure. It's intriguing to note that, while photosynthesis provides oxygen as a by-product, during nighttime or periods of low light, hydrophytes rely on this internal gas exchange to meet their respiratory needs. This adaptation underscores the key differences in respiratory mechanisms between hydrophytes and their land-based counterparts.

Photosynthesis in Aquatic Plants

Photosynthesis in aquatic plants, or hydrophytes, shares the basic principles with that of terrestrial plants but is adapted to an underwater existence. Despite living submerged, these plants must still perform photosynthesis to generate energy, using dissolved carbon dioxide (CO2) in water instead of gaseous CO2 from the air. Light intensity decreases with water depth, so hydrophytes are often found in shallow waters where sunlight can penetrate or have adapted to grow towards the water surface.

Submerged plants have evolved to utilize the available light efficiently by having more chloroplasts or pigments that can capture a broader spectrum of light. Furthermore, the water itself poses less limitation for CO2 and nutrient uptake since these are readily available and can be absorbed over the entire surface of the plant. Through these adaptations, hydrophytes overcome the challenges posed by their aquatic environment to successfully complete the process of photosynthesis.