Exchange and Transport 13.10 Limiting water loss in plants

Learning outcomes Students should be able to understand the following: How terrestrial plants are adapted to efficiently exchange gases and conserve water How Xerophytic plants are adapted to live where water loss may exceed their water uptake How Xerophytes are adapted to limit water loss through transpiration

Learning outcomes Candidates should be able to: Explain structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants

Balancing gas exchange with water loss Insects and plants compared Both have waxy / waterproof coverings over their gas exchange surfaces to limit water loss by evaporation Both can close the openings to their gas exchange systems (spiracles / stomata) to regulate water loss In plants stomata are predominantly on the underside of leaves to limit water loss by evaporation Insects have evolved small SA : Volume ratios to limit water loss from the body surface But the leaves of plants require a large surface area in order to absorb light and exchange gases for photosynthesis

Adaptations of plants Plants in different habitats are adapted to cope with different problems of water availability: 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

small leaf surface area stomata on lower surface of leaf only Some adaptations of xerophytes are: Adaptation How it works Example small leaf surface area less area for evaporation conifer needles, cactus spines low stomata density fewer gaps in leaves stomata on lower surface of leaf only more humid air on lower surface, so less evaporation most dicots sunken stomata maintains humid air around stomata marram grass, pine stomatal hairs marram grass, couch grass, heather

shedding leaves in dry/cold season succulent leaves and stem Some adaptations of xerophytes are: Adaptation How it works Example thick cuticle stops uncontrolled evaporation through leaf cells Most dicots Evergreens e.g. Holly shedding leaves in dry/cold season reduce water loss at certain times of year deciduous plants folded leaves maintains humid air around stomata marram grass, succulent leaves and stem stores water cacti extensive roots maximise water uptake

All Cacti are xerophytes Very dry habitat – not much rainfall

Left and right Epidermis of the cactus Rhipsalis dissimilis. Left: View of the epidermis surface. The crater-shaped depressions with a guard cell each at their base can be seen. Right: X-section through the epidermis & underlying tissues. The guard cells are countersunk, the cuticle is thickened. These are classic xerophyte adaptations. Cacti have sunken stomata, thick waxy cuticle, spines (reduced SA of leaves) and store water in stems

Transverse Section Through Leaf of Xerophytic Plant

Marram grass Habitat is different to cacti – plentiful rainfall but sand dunes with windy conditions so high water loss a problem

Marram grass Marram grass has sunken stomata, stomatal hairs and curl up leaves (decreases water potential from IN to OUT)

Marram grass Marram grass also has extended root system to help get to water which drains away rapidly through sand dunes

Written tasks AQA AS Biology textbook pg 201 Summary questions 1-4 2. Read ‘Not only desert plants have problems obtaining water’ on page 201 then answer application questions 1-4 3. Complete the exam style question about Xerophytic plants

Mark scheme - exam style question Answers – Xerophytes question   1.(a) E. superba; Largest proportion of shallow roots; 2 (b) Roots go to greatest depths; Able to get water when surface soil dried out; or Greatest root mass; Able to store more water; 2 (c) Curled leaves; Thick cuticle; Sunken stomata; Hairs; max 2 [6]

Homework Referring to the AQA AS Biology textbook and other (internet) sources: Use pictures and brief explanations to show how a variety of Xerophytic plants are adapted conserve water in dry habitats

Learning outcomes Students should be able to understand the following: How terrestrial plants are adapted to efficiently exchange gases and conserve water How Xerophytic plants are adapted to live where water loss may exceed their water uptake How Xerophytes are adapted to limit water loss through transpiration