9.1 (Adaptation of Plants in deserts (Vertical stems to absorb sunlight…
Adaptation of Plants in deserts
Vertical stems to absorb sunlight early and late in the day but not in the midday when light is most intense
Very thick waxy cuticle
Spines instead of leaves to reduce SA:VOL ratio of transpiration
CAM physiology which involves opening stomata during the cool nights instead of on the intense heat of the day.
Root hair cells trap water
Water loss by Transpiration
Transpiration is the loss of water vapor from the stems and leaves of plants
The inevitable consequence of gas exchange in a leaf
Leaves must absorb CO2 for use in photosynthesis and excrete O2 as a waste
Gas exchange requires a large area of moist surface which is provided by the mesophyll.
In many leaves there is a spongy mesophyll in the lower part of the leaf with a network of air spaces that increase the surface area of moist cell walls exposed to air
Unless the air spaces are fully saturated, water evaporates from the moist cell walls which ensures that the air spaces have a high relative humidity so water vapor tends to diffuse from them to the air outside the leaf
The epidermis of most plant leaves secretes wax to form a waterproof coating to the leaf called the waxy cuticle
This helps prevent excessive transpiration, but also blocks gas exchange. Pores are therefore needed in the epidermins for CO2 to enter and O2 to leave.
These pores are called stomata and if open allow gas exchange and water vapor to escape.
Structure and function of Xylem
Xylem is a tissue in plants that provides support and transports water only upwards
Tension causes the water to move up to the leaves. This tension forces are generated in the leaves by transpiration and are due to the adhesive property of water. Since water adheres strongly to cellulose in plant cell walls
Tension is transported from one water molecule to the next due to the cohesive properties of water because of hydrogen bonding.
Lignin helps the cell walls withstand the extremely negative pressure of the xylem vessels.
Measuring Transpiration Rates
As the plant transpires it draws water out of the capillary tube to replace losses
Because the capillary tube is narrow, small losses of water from the plant give measurable movements of the air bubble
Factors affecting Transpiration rates
Heat is need for evaporation of water from the surface of mesophyll cells.
As temperature increases the rate of transpiration rises and higher temperatures increase the rate of diffusion through air spaces. However in very high temperatures stomata may close
Water diffuses out of the leaf when there is a concentration gradient between the humid air spaces inside the leaf and the air outside
As atmospheric humidity is reduced, the concentration gradient gets steeper and transpiration occurs faster
Increases until it peaks, and then drops.
In still air, humidity builds up around the leaf which lowers the concentration gradient and transpiration occurs slower.
High wind velocities cause stomata to close
Medium velocities of wind are optimum.
Water uptake in roots
Plants absorb water and mineral ions from the soil using their roots.
Branching roots and root hairs increase the SA:VOL ratio and the rate of exchange
Concentration of ions is lower in roots than in soil, so ions are absorbed by active transport
Root hair cells have mitochondria which produce ATP
Then water moves from low solute to high solute concentration via osmosis