The Movement of Water in Plants

Why do plants need water?

Plants need water to maintain turgor pressure. 
Turgor pressure helps to keep the plant erect and is accomplished
when the plasma membrane pushes against the cell wall.

Plants combine water with carbon dioxide to make glucose.
Water is also used to cool down the plant through evaporation.

Where does the water come from that a plant uses?

Most of the water comes from the soil and must be moved up the plant.

How is water transported into and up through a plant?

The overall movement of water can be determined
by the sum of osmotic potential and hydrostatic pressure.
Water potential is a parameter that indicates the
tendency of water to go from one place to the next. 

Water moves from a high osmotic potential to a low osmotic potential
(or a high water potential to a low water potential).

There are two phases in which water moves:
(1) from soil to root and (2) from root to leaf. 

Solutes are moved through plants in three ways: 
(1) Active transport (carriers and pumps)  
(2) convection (solutes can move along with water as bulk flow)
 (3) diffusion. 

Water movement from soil to root is largely due to
osmotic potential where the solute concentration in the root
is higher than the solute concentration in the soil. 

In order to create and maintain this concentration
difference, active pumping of the solute is required. 
ATP will be needed for this maintenance.    
In most plant cells, the H⁺ ATPase is primary active transport. 

The hydrogen ion gradient is used by co-transporters and exchangers. 
Active pumping leads to an increased solute concentration. 
Because of this, there is a greater tendency for water to flow to the vascular bundle. 

Water movement from the root to the leaf is largely
due to the evaporation-tension-cohesion mechanism. 

Water evaporates at the leaves, thereby decreasing pressure.  
Away from the leaves there is a higher pressure gradient.
This drives the water up, to the lower pressure, thus equalizing things.
ATP is not needed for this process. 

On a smaller scale, root pressure due to osmotic potential drives water from the root to the leaf. 
In the root there is a high concentration of solute. 
When water comes into the root, the pressure builds. 
Water is driven up the plant as a result. 
Evidence of this can be found in guttation. 
Guttation is when water is extruded out through the leaf due to root pressure.

Water is transported by either apoplastic transportation or symplastic transportation. 

    Apoplastic transport is when water is transported between cells up to the endodermis.

    Symplastic transport is when water is transported
through the plasma membrane and cytoplasm. 
Water following this route may take longer to transport and require ATP. 
Symplastic transport can pass through the endodermis whereas apoplastic transport cannot.

Material and water must be transported through the endodermis symplastically. 
Both of these pathways are needed to transport
water and dissolved substances to the vascular bundle.   
To transport water and material across the endodermis,
symplastic transport is needed (because the endodermis is relatively impermeable).
Once transported across the endodermis and to the stele,
materials must then move from the symplastic area to the apoplastic area (the xylem). 
These two pathways work in conjunction with one another.

Cross-Section of a Root:

After water is brought into the vascular bundle, where does it go?

Transfer cells take the material from the cell cytoplasm and transport it to the xylem
(movement is from the symplastic cytoplasm to the apoplastic xylem).

Plants in Dry Environments

In dry environments, plants keep their stomata closed to limit water loss
yet they must open their stomata to exchange gases with the environment.

In dry environments, there is a also a physical increase in evaporation. 
Plants heat up faster and need to open their stomata to cool off.

Plants deal with a dry environment in the following ways:
·Time their growth season to match the rainy season.
·Accumulate solutes in the cell (by changing their osmotic potential). 
This way a plant improves its ability to pull water from the ground/soil.
·Develop a waxy cuticle
·Regulate stomata openings so that they open at night
(These plants are called CAM plants).
·Temporarily reduce the surface area of their leaves
Corn plants do this.  This reduces the rate of evaporation.
·Only produce leaves during periods of heavy rain.
·Change leaf orientation to reduce evaporation.
·Store water succulents.

*Also, different root systems enable a plant to adapt to dry environments. 
Deep tap roots are useful for collecting lots of water deep in the earth. 
A dense, shallow root system is useful when there is heavy competition among plants.
 

Plants in Wet Environments

A wet environment is disadvantageous for
a plant because moisture can lead to rotting. 
Wet environments can also prevent transpiration and alter
important processes such as photosynthesis and gas exchange.

In very wet conditions, plants have a can have a difficult time getting oxygen.
The concentration of oxygen in air far exceeds concentration of
oxygen in water, yet plants need oxygen to break down
glucose and perform cellular respiration.  

Plants adapt to wet environments in several ways:
·The development of pneumatophores (roots that come out of the water)
·A shallow root system
·Aerenchyma cells provide large open spaces that can
be filled with air and allow for its easy transport.
·Some plants actually metabolize anaerobically
·By better supporting themselves plants can prevent being completely submerged in water.

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