Root pressure

• Root pressure occurs when the roots continue to take in mineral and the rate of transpiration is low.  

• This process is produced by osmotic pressure in the cells of the root.
•  Root pressure common during spring seasons before the leaves develop and rate of transpiration is rapid. 
• Thus, water is forced from the high pressure (in roots) to low pressure (in leaves). 

The upward movement of water in plants

• Water transport occurs in a plant's xylem tissue & moves upward through a plant through xylem tissue.
• The fluid mixture of materials flowing through the xylem is called sap.  

• Diffusion, transpiration and capillary action must occur together in order to move the upward through plant.

1. Diffusion 

• Plant roots absorb water from the soil through diffusion or osmosis.
• The roots (higher salt concentration). The water flows from the low salt side to the high salt side to the concentration equal.
• The membrane is semi-permeable but if the salt concentration inside the root becomes too low, it will actively transport salt into roots. 

2. Capillary action

• Water molecules have a strong attraction to each other causing the molecules to stick together.
• They also adhere to the surface of the xylem.
• As the plant transpires, water molecule evaporates, pulling another water molecule up through the xylem.
• The water sticks to the side of the xylem and not fall back down.

Cohesion force: The ability of molecules of the same kind to stick together. Water molecules are polar, having positive and negative sides, which causes their cohesion.

Adhesion force: The tendency of molecules different kinds to stick together. Water sticks to the cellulose molecule in the walls of xylem, the force gravity give rise of water within xylem.

3. Transpiration 

• Plants lose water through the leaf surface and stomata involves transpiration.
• Plants lose water through the leaf surface from the stomata or pores in the leaf surface used for respiration and photosynthesis. 

Water transport in plants

 Transport systems are found in the vascular plants. Two types of vascular tissue:

• Xylem = transports water and minerals in the plants
• Phloem = transport food materials; synthesized by the leaves during photosynthesis and hormones.

Transport of water and minerals in plant 

The path taken is : soil - roots - stems - leaves 

Most plants secure the water & minerals from their roots. The minerals (e.g., K+ , Ca+) travel dissolved in the water. 

Water & dissolve mineral nutrients enter the plant via 2 routes 

• water & mineral nutrients enter the plant via 2 routes

1. Symplast: it crosses the plasma membrane and then passes from cell to cell through plasmodesmata.
2. Apolast: in the spaces between the cells and in the cells walls themselves. This water has not crossed a plasma membrane. 
   

Pathway of water into roots 

• Access water to the apoplast: Uptake of soil solution (water + minerals) of root hairs.
• Access water to the symplast: Water + minerals across the plasma membrane of root hairs.
• Water and minerals move along the apoplast and transported into the protoplast of epidermis cells, then cortex cells and move inward via the symplast.

 

Plant Water Relation

What is Plant Water Relation?

Plant water relations involve the absorption of water, ascent of sap, loss of water by transpiration, and the internal water balance of the tree. 

Water Properties

1. Polar molecule

2. Cohesion and adhesion

3. High specific heat

4. Density-greatest at 40C

5. Universal solvent of life

1. Polarity of Water

In water molecule, 2 hydrogen atoms form single polar covalent bonds with an oxygen atoms. because oxygen is more electronegative.

- the region around oxygen has a partial negative charge.

- the region near the two hydrogen atoms has a partial positive charge.

A water molecule is a polar molecule with opposite ends of the molecule with opposite charge.

Water has a variety of unusual properties because of attractions between these polar molecules.

- The slightly negative regions of one molecule are attracted to the slightly positive regions, forming a hydrogen bond. 

The oxygen end "acts" negative while the hydrogen end "acts" positive. It causes the water to be POLAR. 

2. Cohesion and Adhesion

Hydrogen bonding makes water cohesive and adhesive. Water is sticky because of H bonding. 

Cohesion = water molecules are attracted to other water molecules. 

Adhesion = water molecules are attracted to other substances. 

Water molecules sticking to each other forming a rope that is pulled out as water vapor "exhaled" through the leaves (cohesion).

Water molecules stick the walls of the xylem on the inside of the plant. 

3. Specific Heat 

Water has high specific heat. The amount of heat that must be absorbed or lost for 1.0 gram of a substance to change its temperature  by 1 Celsius. 

This is because water molecules are constrained by hydrogen bonding. 

Moderates temperatures on Earth

- Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.

- Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. 

4. Density of Water

Water is less dense as a solid than as a liquid. 

- At temperature above 4 Celsius, water expanding when it warm and contracting as it cools. 

- At temperature 0 Celsius, water freezes and it forms a crystalline lattice because each water molecules has H bonding locked to four neighbors. 

- The H bonds keep the molecules for enough apart. 

5. Universal solvent of life

Water potential 

Water potential is a useful measurements to determine water-deficit stress in plants. Water potential can affects plants in many ways.

a) Atmospheric water potential

- one of the factors that influences the rate of transpiration or water loss in plants.

b) Soil water potential

- influences the water available for uptake by plant roots.