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17.1.2.1: Adaptations to Reduce Transpiration - Biology

17.1.2.1: Adaptations to Reduce Transpiration - Biology


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Learning Objective

Describe the various adaptations that help plants reduce transpiration rate.

Plants have evolved over time to adapt to their local environment and reduce transpiration. Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Plants that grow in dry environments and plants that grow on other plants (epiphytes) have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (Figure (PageIndex{1})). Additionally, they often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface (Figure (PageIndex{2})). These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants .

The size and shape of photosynthetic structures also influences transpiration rate. Succulent plants, common in deserts, have thick, fleshy leaves or stems (Figure (PageIndex{1}), left). Other plants, such as the evergreen shrubs of the chaparral, have small, thick, tough leaves (Figure (PageIndex{3})). Compared to thin, broad leaves, these shapes reduce surface area-to-volume ratio and decreases the opportunity for water loss. Plants with thin, broad leaves that live in climates with hot, dry seasons (such as chaparral or tropical forests that have a wet and dry season) may be deciduous, losing their leaves during these seasons to limit transpiration (Figure (PageIndex{4})).

As discussed in the Photosynthesis chapter, CAM plants close their stomata during the day when light and high temperatures would otherwise increase transpiration rate. C4 plants reduce the need to frequently open stomata by creating a high carbon dioxide concentration in the bundle sheath cells, which conduct the Calvin cycle. Regardless of photosynthetic pathway, plants can open and close stomata to regulate transpiration rate based on environmental conditions.


Examples of Adaptations | Organisms

a. The kangaroo rat in North American deserts is capable of meeting all its water requirement by internal oxidation of fat (water is a byproduct) in absence of water.

b. It can concentrate its urine, so that minimal volume of water is used to expel excretory products.

2. Adaptations in Desert Plants:

a. Roots grow very deeply to explore any possibility of available underground water. Proline is a typical osmolyte, synthesised by plants under different environmental stress conditions in xerophytes.

b. Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimise water loss through transpiration. They have special photosynthetic pathway (CAM) that enables their stomata to remain closed during day time and to minimise the transpiration.

c. Some desert plants like Opuntia, have no leaves. They are reduced to spines and photosynthesis occurs in flattened stems.

3. Adaptations in Mammals:

a. Mammals from colder climates generally have shorter ears and limbs to minimise heat loss. This is called Allen’s rule.

b. In polar regions, aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces the loss of body heat.

4. Adaptations at High Altitudes in Humans:

a. At high altitude places like Rohtang Pass near Manali (> 3500 m) and Mansarovar (in China occupied Tibet) people suffer from altitude sickness.

b. Its symptoms are nausea, fatigue and heart palpitations. This is because at low atmospheric pressure of high altitudes, body does not get enough oxygen. The relief occurs gradually due to acclimatization.

The body cope up with this low oxygen stress by:

(a) Increasing red blood cells production.

(b) Decreasing the binding affinity of haemoglobin.

(c) Increasing the breathing rate.

5. Adaptations in Desert Lizards:

a. They absorb heat from the sun when the body temperature drops below the comfort zone, e.g., desert lizards. Therefore, they move into shade when the ambient temperature starts increasing.

b. Some species burrow into the soil and escape from the above ground heat.

6. Adaptations in Aquatic Habitats (Hydrophytic Plants):

a. Aquatic plants Hydrophytes have evolved aerenchyma for buoyancy and floating. They have covering of wax to avoid damage of water. Roots are generally absent in plants like Hydrilla and Nymphaea.

b. A large variety of marine fishes and in vertebrates live at great depths in the ocean where the pressure could be > 100 times the normal atmospheric pressure.

7. Adaptations to Saline Environments (Halophytic Plants):

a. The plants of saline habitats which have not only the ability to tolerate high concentrations of salts in their rooting medium but are able to obtain their water supply from the same are called halophytes.

b. These are found in tidal marshes coastal dunes, mangroves and saline soils. Certain green algae are also found in these areas, e.g., Dunaliella.

c. Mangroves are the areas that have not only excess salt but also excess water or anaerobic conditions besides difficulty in anchoring and seed germinations.


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