HOMEOSTASIS IN PLANTS: TEMPERATURE Biology Stage 3 Text: Chapter 9 Page 192 Chapter 11; pages Text: Chapter 14 Pages

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

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

Photosynthesis & respiration During the day, the rate of photosynthesis in plants is greater than the rate of respiration More CO 2 is used up in photosynthesis than can be produced by respiration The net result is that during the day plants take up CO 2 and release O 2 At night plants take in O 2 and release CO 2

Stomata CO 2 diffuses into the plant through tiny pores called stomata (stoma singular) Each stoma is surrounded by 2 guard cells These guard cells control the opening and closing of the stoma In most plants, the stomata are located on the underside of the leaf and open during the day

Guard cells Guard cells: Are joined at the ends Have a thicker cell wall on the side of the stoma Have bands of inelastic fibre around the cell wall Contain chloroplasts to make glucose for the energy needed to drive the H + /Cl - and K + /Na + pumps

Guard cells Guard cells pump in K + ions to increase the osmotic gradient across the cell membrane The chloroplasts produce glucose through photosynthesis, also increasing the osmotic gradient Water moves into the cell The guard cells swell (become turgid) and the stoma opens

Animation Sodium potassium pump sites/ /student_view0 /chapter38/how_the_sodium_pot assium_pump_works.html

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

Transpiration A potometer is a device that can measure the rate of respiration of a plant. The rate at which the water level moves down is dependent on the rate at which water evaporates from the leaves

Effect of temperature 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 CO 2 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 (C 4 & CAM photosynthesis plants)

C 4 & CAM photosynthesis plants Can store CO 2 as a C 4 or CAM compound for later use Are able to close their stomata during the day or for part of the day and use the stored CO 2 for photosynthesis More efficient method in hot, dry climates

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

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