Acquiring and retaining water Flashcards
Main water needs:
Photosynthesis
Transport
Structural support
Life in all forms started in the water and had to adapt to living on land.
The problem for plants
Water is required for photosynthesis and lost by
transpiration from the leaves. The leaves are also where sugars are produced
Sugars from leaves and water and minerals taken up by the roots are required for growth throughout plant so a complex transport system is required akin to plumbing
Plants loose water 50x faster than they take in carbon dioxide
The transpiration cohesion tension mechanism
Transpiration causes evaporation from the mesophyll cell walls, generating tension on the xylem. Cohesion among water molecules in the xylem transmits the tension from the leaf to the root, causing water to move through, from soil to atmosphere.
(see diagram for steps)
The process of transpiration pulls water up through the plants to the leaves. It requires high surface area in the roots
and high water potential (in soil) -> low ( in atmosphere)
See diagram for soil-plant water continuum
3 key water transfer processes for water
Osmosis – low energy
Mass flow – due to hydrostatic pressure water is pulled
Diffusion – into the atmosphere pulls water from leaves to atmosphere
This is measured in pressure units
Water potential is made of different components with variable values
(see slide in notes)
Most tissues have pos hydrostatic pressure
Sigma symbol before osmosis (s) is present because?
Gravitational potential is most significant in tall trees
In most cells osmotic and hydrostatic pressure are the main
water uptake by roots from soil interstices
Soils with large particles allow water to be taken up readily
Sand includes particles to 1mm
Clay is the opposite with tiny particles that pack close together with high capillary pressure
Osmotic potential in soil tends to be ~0
Hydrostatic pressure is also close to 0 in wet soil or increasingly neg in drier soils
Competition in root trying to take up water and water bonds to soil particles
Water uptake by roots
Less water close to the root as it is taken up by the plant
This creates a lower water gradient pulling water from further away
This depends on the hydraulic conductivity of the soil (higher in sand than clay etc.)
Hydraulic conductivity drops as water levels in the soil decrease
The role of subarin in roots
Subarin is a waterproof material that prevents water uptake the older areas of the roots become coated in this material. The purpose of root is to explore new soil to harvest minerals as well as water. Limiting water uptake to the tips is a more efficient way to access both at once
Types of root cell
see types 1-4 in diagram in notes
Aquaporins, inhibited or introduced acccording to water conditions, allow increased water transfer by creating channels between cells through cell walls
Water movement through the roots
Casparian strip is made of subarin material forcing water into the cell centre
+ has a role in ion regulation
Root pressure
Root pressure = when stomata close at night water is still pushed up from the root and water is observed to be pushed up out of leaf pores in the morning – but this has very minimal impact.
Water transport through the xylem
Xylem is specialised for rapid transport
Lignification: Strengthened by fibres (lignin) to withstand tension
Death of cytoplasm: No internal tissue – dead - Similar to a straw
Perforation plates
Bordered pits
Xylem vessels are not more efficient than tracheids.
This is because tracheids have torus in their ‘pit pairs’
Torus can block tracheid valves to cut off damaged areas and prevent transport of air bubbles
This torus makes conifers very resilient to short supply of water due to frozen or arid land conditions
If plants had to move water cell to cell it would be impossible
This is how we can be sure that there is a system like mass flow (see slide on Poiseuille equation)
water transport through the xylem: cohesion tension
Cohesion - water adheres to itself due to having areas of pos and neg charge
Adhesion – water adheres to the surface (of the tubes)
Tension – the pulling from the leaves
Hence water vapour can be maintained even at low pressure
Driving force is water evaporating from the leaves
Water reaches air spaces in leaves and evaporates through stomata
Water adhesion means that the smaller the curvature of the space the harder it is for the water to evap
water through the xylem: vulnerability curves
Vulnerability to reduction of water conductivity
Related to habitat adaptations
e.g. Cottonwood is a species that grows in damp conditions and not adapted to dry conditions
whereas the other species are more desert adapted – so can shift water at more neg tensions
Transpiration: boundary layer resistance
In still air water can condense around the stomata when the air is moving water is drawn away
So in still air the gradient is less steep
Some plants have rolled leaves such as marram grass stomata face in increasing humidity and reducing evaporation
Opposite being tree leaves designed to flex in wind to increase transpiration
