9.5 Plant adaptations to water available

What do land plants exist in a state of constant compromise between?

getting the carbon dioxide they need for photosynthesis and losing the water they need fir turgor pressure and transport

Adaptations that most plants have to conserve water (3)?

waxy cuticle to reduce transpiration from the leaf surfaces, stomata found mainly on the underside of the leaf that can be closed to prevent the loss of water vapour and roots that grow down to the water in the soil

What will happen to plants in very hot, dry and breezy conditions?

water will evaporate from the leaf surfaces very rapidly

Mesophytes?

plants able to take up sufficient water to replace transpiration (most plants).

Xerophytes?

plants with structural and physiological adaptations that enable them to survive and reproduce in hot dry conditions - areas with very low water availability

What is greater than the water taken up by roots in these very hot areas?

water lost via transpiration is greater than taken up by roots

Apart from in very dry and hot conditions, where else are xerophytes found?

in very cold and icy conditions - water in the ground is not freely available to the plants because it’s frozen

Xerophytes: what is their rate of transpiration the same as when water is abundant?

the same as other plants

Xerophytes conserving water: thick waxy cuticle - in most plants what percentage of the water loss by transpiration is through the cuticle?

10% - so a particularly thick waxy cuticle helps to minimise water loss

Xerophytes conserving water: sunken stomata - what does stomata sunken in pits (found on xerophytes) reduce?

exposure to air currents

Xerophytes conserving water: sunken stomata - how does this limiting the exposure to moving air decrease water loss?

creates a microclimate of still, humid air that reduces the water vapour potential gradient so reduces transpiration

Xerophytes conserving water: reduced numbers of stomata - what does xerophytes having reduced numbers of stomata decrease (2)?

decreases their water loss by transpiration and decreases their gas exchange capabilities

Xerophytes conserving water: reduced leaves - why can water loss be greatly reduced by reducing the leaf area?

decreases the SA:V ratio

Xerophytes conserving water: hairy leaves - how do hairs minimise the loss of water by transpiration from the surface of the leaf?

traps a layer of still air between the hairs which reduces the water vapour potential gradient

Xerophytes conserving water: curled leaves - how does it reduce water loss by transpiration?

confines all the stomata within a microenvironment of still, humid air to reduce the water vapour potential gradient (decreases diffusion of water vapour from the stomata)

Xerophytes conserving water: succulents - where do succulent plants store water in their stems and roots?

specialised parenchyma tissue in their stems and roots

Xerophytes conserving water: succulents - how did they get their name?

swollen, fleshy appearance

Xerophytes conserving water: succulents - water is stored in plentiful amounts to be used when?

in times of drought

Xerophytes conserving water: leaf loss - how do some plants prevent water loss through their leaves?

simply loss their leaves when water is not available

Xerophytes conserving water: root adaptations - what do the root adaptations that many xerophytes have help them to do?

help them to get as much water as possible from the soil

Xerophytes conserving water: root adaptations - what can long tap roots growing deep into the ground do?

can penetrate several metres, so they can access water that is a long way below the surface

Xerophytes conserving water: root adaptations - an adaptation that is able to absorb anu available water before a rain shower evaporates?

a mass of widespread, shallow roots with a large surface area

Xerophytes conserving water: avoiding the problems - how are some plants adapted to cope with the problems of low water availability?

by avoiding the situation entirely

Xerophytes conserving water: avoiding the problems - what do plants that lose their leaves and become dormant, or die completely do?

leave seeds behind to germinate and grow rapidly when rain falls again

Xerophytes conserving water: avoiding the problems - what do others survive as(3)?

as storage organs such a bulbs, corms or tubers

Xerophytes conserving water: avoiding the problems - what do those who can withstand complete dehydration do?

appear dead but when it rains the cells recover, the plant becomes turgid and green again and begins to photosynthesise

Xerophytes conserving water: avoiding the problems - what enables these plants to survive this way unharmed?

disaccharide trehalose

Hydrophytes?

plants that live either partially or completely submerged in water

Hydrophytes: what do they need special adaptations to cope with?

growing in water - problems with oxygen uptake

Hydrophytes: 2 examples?

water lilies and water cress

Hydrophytes: why is it important in surface water plants that the leaves float?

so that they are near the surface of the water to get the light needed for photosynthesis

Hydrophytes: what is a major problem for them?

water-logging - the air spaces in the plant need to be full of air, not water, for the plant to survive

Hydrophytes adaptations: very thin or no waxy cuticle - why is water loss by transpiration not an issue?

hydrophytes don’t need to conserve water as there is always plenty available

Hydrophytes adaptations: many always-open stomata on the upper surfaces - what does maximising the number of stomata maximise?

maximises gaseous exchange

Hydrophytes adaptations: many always-open stomata on the upper surfaces - why are the stomata usually open all the time for gaseous exchange and the guard cells are inactive?

unlike other plants, there is no risk to the plant of loss of turgor as there is always an abundance of water available

Hydrophytes adaptations: many always-open stomata on the upper surfaces - in plants with floating leaves why do the stomata need to be on the upper surface of the leaf?

so they are in contact with the air

Hydrophytes adaptations: reduced structure to the plant - what supports the leaves and flowers (so there’s no need for strong supporting structures)?

the water

Hydrophytes adaptations: wide, flat leaves - why do some hydrophytes have wide, flat leaves that spread across the surface of the water?

to capture as much light as possible

Hydrophytes adaptations: small roots - why is there less need for uptake by the root?

water can diffuse directly into stem and leaf tissue so there’s less need for uptake by roots

Hydrophytes adaptations: large surface areas of stems and roots under water - what does this maximise?

the area for photosynthesis and for oxygen to diffuse into submerged plants

Hydrophytes adaptations: air sacs - what do some hydrophytes have air sacs to enable them to do?

to enable the leaves and/or flowers to float on the surface of the water

Hydrophytes adaptations: aerenchyma - what is it?

specialised parenchyma tissue that forms in the leaves, stems and roots of hydrophytes

Hydrophytes adaptations: aerenchyma - what does it have that is formed by apoptosis (programmed cell death) in normal parenchyma?

has many large air spaces

Hydrophytes adaptations: aerenchyma function - what does it make the leaves and stems?

more buoyant

Hydrophytes adaptations: aerenchyma function - what does it form for the movement of substances, e.g. oxygen, to tissues below the water?

forms a low-resistance internal pathway

Hydrophytes adaptations: aerenchyma function - what does it creating this low-resistance pathways help the plant to cope with?

anoxic conditions (extreme low oxygen conditions) in the mud by transporting oxygen to the tissues

Aerenchyma: what is it found in?

crop species that grow in water e.g. rice

Aerenchyma: how does it contribute to rice crop’s impact on global warming through the production of atmospheric methane?

provides a low resistance pathways by which methane produced by the rice plants can be vented into the atmosphere

When can water become waterlogged?

in situations where there is plenty of water

When the roots become waterlogged what is in short supply?

air not water

Pneumatophores?

special aerial roots that grow upwards into the air and have many lenticels, which allow the entry of air into the woody tissue (response to waterlogging)