9.5 Plant adaptations to water availability

Xerophytes

Plants that live in dry conditions

• Where there’s low water availability, so low amounts of water for transpiration

  • E.g. in dry, salty conditions

  • E.g. v. cold & icy conditions

Examples of xerophytes

Cacti and marram grass

What are the strategies xerophytes use for conserving water? (8)

1. A thick waxy cuticle

2. Sunken stomata

3. Reduced numbers of stomata

4. Reduced leaves

5. Hairy leaves

6. Curled leaves

7. Succulents

8. Leaf loss

How having a thick way cuticle helps conserve water

• Helps minimise water loss by reducing rate of transpiration

• Stops transpiration from top of leaf

How having sunken stomata helps conserve water

• Stomata located in pits

• Reduces air movement, producing a microclimate of still, humid (moist) air

  • Reduces water vapour potential gradient & so reduces transpiration.

How having reduced stomata helps conserve water

• Reduces their water loss by transpiration

• Reduces their gas exchange capabilities

How having reduced leaf S.A. helps conserve water

• Greatly reduced SA : V ratio, minimising the amount of water lost in transpiration.

• Fewer stomata. less water loss

• Less space for transpiration to occur

How having hairy leaves helps conserve water

• Creates a microclimate of still, humid air around stomata

  • Traps air, increases humidity

• Reduces water vapour potential gradient & minimising loss of water by transpiration from the surface of the leaf

How having curled leaves helps conserve water

• Confines all of the stomata within a microenvironment of still (less wind), humid air to reduce diffusion of water vapour from the stomata

  • Decreased S.A. exposed to sun decreases rate of transpiration, less loss of water

  • e.g. marram grass

How having specialised parenchyma tissue helps conserve water

• Succulent plants store water in specialised parenchyma tissue in their stems and roots.

  • Parenchyma tissues w. gaps between cells that store water, (gaps in spongy mesophyll)

• Water is stored when it is in plentiful supply and then used in times of drought

How leaf loss helps conserve water

• Little to no transpiration occurs

  • Retains water

  • Don’t waste resources

Root adaptations of xerophytes

• Long tap roots grow deep into ground (can penetrate several metres), so can access water that is a long way below the surface.

• A mass of widespread, shallow roots with a large S.A. able to absorb any available water before a rain shower evaporates is another adaptation.

Hydrophytes

Plants that live in water, or around edges of water

• Face low levels of oxygen at night, CO2 during day

• Too much water (soil permanently saturated w. water)

• Face threat of waterlogging

  • Air spaces need to be full of air for plant to survive

Examples of hydrophytes

Water lilies

Adaptations of hydrophytes (8)

1. Very thin or no waxy cuticle

2. Many always open stomata on upper surfaces

3. Reduced structure to plant

4. Wide, flat leaves

5. Small roots

6. Large S.A. of stems & roots under water

7. Air sacs

8. Aerenchyma

Reason for thin to no waxy cuticle on upper surface of plant

No need to conserve water

• Transpiration not issue

Reason for many always open stomata on upper surface?

• Maximises/maintains gaseous exchange that occur in air instead of in the water

Reasons for reduced structure in hydrophytes

Less supporting tissue needed as water supports plant

• Increases buoyancy, leaves lighter

• xylem is significantly reduced as there is no need to transport water throughout the plant

Reasons for wide, flat leaves

• Spread across surface of water to maximise capture of light for photosynthesis

Reasons for small roots in hydrophytes

• Water diffuses directly into stem & leaf tissue, less need for uptake by roots

Reasons for large S.A. of stems & roots under water

Maximises area for photosynthesis & for O2 to diffuse into submerged plants

Reasons for air sacs in hydrophytes

• Large air spaces inside give them buoyancy

  • keeps them close to water surface where there’s more light for photosynthesis

Reasons for aerenchyma in hydrophytes

• Has many large air spaces

  • Makes leaves & stems more buoyant

  • Forms low-resistance internal pathway for movement of substances e.g. oxygen to the tissues below the water.

  • Helps plant cope w. anoxic conditions in the mud, by transporting oxygen to the tissues.

Special aerial roots benefits

They grow upwards towards the air

• Have many lenticels, allowing entry of air into woody tissue