Adaptations to arid environments Biology ATAR Unit 2 Heinemann Biology 1 Chapter 13; pages 254-256 Adaptations to arid environments

Keywords Transpiration Evaporation Potometer Stomata Guard cells Flaccid Turgid Phyllode C4 & CAM photosynthesis plants Succulents Photosynthetic stems Acquifer

Learning outcomes Understand, describe and explain the adaptations plants have to be able to survive in hot, dry conditions

Photosynthesis & respiration During the day, the rate of photosynthesis in plants is greater than the rate of respiration More CO2 is used up in photosynthesis than can be produced by respiration The net result is that during the day plants take up CO2 and release O2 At night plants take in O2 and release CO2

Transpiration Transpiration is the movement of water up through the xylem from the roots to the leaves Transpiration is driven by evaporation at the leaves Evaporation creates a pulling force that continually draws more water up from the roots

Plant adaptations Plants in different habitats possess different adaptations: Mesophytes plants adapted to a habitat with adequate water Xerophytes plants adapted to a dry habitat Halophytes plants adapted to a salty habitat Hydrophytes plants adapted to a freshwater habitat

Effect of temperature If water loss ˃ water gain: High temperatures increase the rate of evaporation from the leaves This increases the rate of transpiration Roots need to absorb increasing amounts of water to keep pace with the rate of transpiration If water loss ˃ water gain: Cells become flaccid (lose water) Stomata close No CO2 diffuses into the plant Photosynthesis slows down & stops Plant wilts Plant dies unless situation is reversed

Leaf adaptations Leaf adaptations aim to: Reduce rate of evaporation Reduce leaf temperature – reflect heat Reduce exposure to the sun’s heat – surface area Create humid micro- environments – slows evaporation

Leaf adaptations Long narrow leaves Fewer stomata Stomata underneath the leaf Sunken stomata Hairs on leaf Leaves that hang vertically Waxy cuticle Rolled up leaf Modified leaf structure (phyllodes in Acacia) Open stomata at night, close during day (C4 & CAM photosynthesis plants)

Sagebrush grows in drier climates and areas with high exposure to sunlight.  The leaves are small and tightly clustered.  When it gets dry enough, sagebrush will often look dead. Sometimes the bush is blown off its roots by the wind.  This is one way its seeds are distributed.

The Utah juniper is a medium-sized tree The Utah juniper is a medium-sized tree.  It has extremely small leaves that are often referred to as needles.  The leaves grow in tight clusters and have a dry texture. 

Root hairs All plants have root hairs Root hairs are extensions of root epidermal cells These extensions greatly increase the surface area for water absorption and maximise water uptake Root hairs are found near the root tip

Root adaptations Extensive, large root systems that cover a large volume of soil Roots that form a network along the surface to absorb dew Tap roots that penetrate to the acquifer (underground water)

Succulents Succulents have a few specialised adaptations: Their stem and/or leaf cells can store water Some have photosynthetic stems (eg. cacti) This reduces evaporation while increasing energy production

Structural adaptations Plants lose a lot of water through their leaves. The mulga tree leaves are reduced to spines, which reduces water loss through the smaller leaf surface. The boab tree has its own water storage area in a swollen trunk. .

Structural adaptations The mulga tree's root system lies close to the base of the tree. The tree's spiny leaves grow upward. When it rains the leaves capture the water and funnel it down along the branches to the center of the tree. The water then falls to the ground near the trunk of the tree where tree roots are concentrated.

Annual desert plants Annual desert plants also have special adaptations: Short life cycle Rapid growth and flowering period Produce seeds quickly When significant rain falls over a short period their seeds germinate and grow quickly while there is enough water available. The seeds produced must be heat and drought tolerant

Xerophyte adaptations summary How it works Example thick cuticle stops uncontrolled evaporation through leaf cells   small leaf surface area less surface area for evaporation conifer needles, cactus spines low stomata density smaller surface area for diffusion sunken stomata maintains humid air around stomata marram grass, cacti stomatal hairs (trichores) marram grass, couch grass rolled leaves marram grass extensive roots maximise water uptake cacti 18