CHAPTER 30.4How Do Stems and Leaves Conserve Water?

AP BiologySpring 2011

WATER LOSS

Most of water (>90%) absorbed by plant is lost due to evaporation out at the leaves

Only a small amount of water is available to maintain the plants metabolism

WATER-CONSERVING CUTICLE

Cuticle: translucent, water-impermeable layer of waxes, pectin, and cellulose fibers embedded in cutin Secreted from epidermal cells which cover the

outer surfaces of the plant

WATER-CONSERVING CUTICLE

Cuticle does not prevent sunlight from entering the leaf

Cuticle does restricts water loss, the inward diffusion of CO2, and outward diffusion of O2

CONTROLLED WATER LOSS OF STOMATA

Stomata: tiny, collapsible openings at the plant epidermis that regulate the passage of water, carbon dioxide, and oxygen

Guard Cells: a pair of these controls the opening and closing of the stomata

CONTROLLED WATER LOSS OF STOMATA

Stomata open and close in response to environmental cues Open stomata allow carbon dioxide to enter

BENEFIT Open stomata allow water to be lost

DETRIMENT

CONTROLLED WATER LOSS OF STOMATA

When plants are exposed to sunlight: Potassium and water move

into the guard cells Causes guard cells to

swell: Turgor Pressure Stomata of most plants

are open during the day and closed during the night

CONTROLLED WATER LOSS OF STOMATA

CAM Plants Cacti Adaptations allow

stomata to open at night when cells of these plants fix carbon dioxide

Stomata close during the day to conserve water in arid habitats

CONTROLLED WATER LOSS OF STOMATA

Water-stessed plants close their stomata in response to abscisic acid Absicsic acid: controls calcium ion entry into

guard cells Pollution:

Can clog stomata and damage them so that they remain permanently closed