xylem part 3

When water is moving through the plant, which component does water spend the most time in?

The xylem

Casparian strip

Waterproof band in the cell walls of root endodermal cells that act as a selective barrier to water and solute uptake, forcing them to take the symplastic route of the endodermis before reaching the vascular tissues (xylem)

Xylem cell types need to have

Strong lignifies cell walls

Types of xylem cells

Trachieds and vessel elements

Trachieds

Found in all vascular plants, long spindle shaped cells in overlapping vertical files to conduct water and provide support, radius less than 50 um

Vessel elements

Found in angiosperms, gnetophytes, some ferns, radius up to 500 um, shorter and wider than Trachieds with perforated end walls that conduct water more efficiently due to the continuous and wider tubes

Pits

Allow for passage of water laterally between water conducting cells

Pit membranes

Made up of primary cell walls and middle lamella, lying in the center of each pit allowing water to pass between xylem conduits and limits embolisms

Perforation plates

Connects vessel elements to form vessels

What’s longer: vessels or Trachieds

Vessels

What restricts the movement of water through the xylem?

Gravitational potential and waters viscosity

Waters viscosity

The resistance to deformation

Increasing the radius of the xylem will do what to the flow rate

Exponentially increase it

Why are longer cells more advantageous when

They require less movement between pits (since they hinder water movement) resulting in uninterrupted movement for long distance transport

Why are conifers the tallest trees?

They have specialized pit membranes called Tori which are surrounded by porous flexible structures called margos.

There is resistance when water goes through the xylem, is it more efficient the cell to cell transfer?

Yes far more (x 10^10)

What pressure differential is needed to get water up the tallest trees?

Pressure difference required to overcome waters viscosity are 0.01 MPa/m. Over 100m the total pressure difference is 1Mpa. At 100m gravitational potential is 1MPa higher compared with bottom of the tree. Total pressure gradient of 2 MPa is required to overcome viscosity and gravity

Why can’t we increase root pressure to move water up a tree?

Increased +ve pressure means more water needs to move into the cells. Water can only move in if there is a higher solute concentration. Once water moves up to the leaves and is transpired out, solute will be left in the leaves leading to damage and wilting

If pressure at the base can’t be increased, what else can change?

-ve pressure to have more H bonds for enhanced water and nutrient uptake by changing solute concentration which in turn changes water potential

Driving force of xylem transport

Water adheres to hydrophilic components such as cellulose microfibrils. As water evaporates, surfaces of remaining water are drawn into interstices of cell walls forming curved air water interfaces.

What does the curvature of water air do when driving force on xylem is occurring

It induces tension owing to high surface tension of water. As more water is removed curvature increases (radius decreases) and pressure potential becomes more negative.

What increase rate of transpiration

Ultimate energy source of the sun

What are the physical stressors of extreme xylem tension

Intense force on cell walls and low pressure of water

How do plants prevent gas bubbles from forming

Cohesion and adhesion, preventing the formation of nucleation sites (casparian strip filters out bubbles when forcing water into xylem)

What kind of pressure causes increased gas bubbles in xylem

Negative pressure

Embolism

Air entering the xylem causing bubbles to grow until the entire water conducting cell is filled

How can air enter the plant

Injury, leaf abcission, adjacent damaged conducts, and freezing forces dissolved gases out of solution leading to cavitation

Why is cavitation a problem in the xylem

Water cannot flow if there are breaks

What helps reduce cavitation

Pit membranes, interconnectivity, finite lengths of tracheary elements, reduced tension at night, new growth of xylem tissues

How do leaves pose a resistance to water movement

Water potential gradient regulates movement of water. Concentration gradient of water vapour controls transpiration (high [ ] inside leaves and low outside). High hydraulic resistance. Waxy cuticle, internal air spaces, and stomata control water movement.

Main factors affecting transpiration rate

Leaf temperature, stomatal resistance (number and diameter), boundary layer resistance (wind speed and leaf size). Opening and closing of stomata balance water retention and co2 for photosynthesis

Boundary layer resistance

Layer of air surrounding lead which impedes the transfer of heat and water vapour between leaf and atmosphere. More wind in envt will disrupt layer and create a bigger [ ] gradient of water vapour for driving force to act on

Guard cells

Surround stomata to control pore size. Increase in turgor pressure causes stomata opening

Types of guard cells

Dumbbell shaped in grasses and kidney shaped in other plants

How do guard cells open

Specific alignment of cellulose microfibrils responsible for opening stomata.

How are microfibrils orientated in guard cells

In normal cylindrical cells, they’re orientated transverse to the long axis but in guard cells they fan out rapidly from the pore

Transpiration ratio

Up to 400 molecules of water lost for every molecule of co2 fixed due to [ ] of water 50 times greater than co2, co2 diffuses 1.6 times slower in air than water, assimilation of co2 requires transport across plasma membrane, cytoplasm, and chloroplast envelope

Subsidiary cells

Release split into guard cells decreasing water potential. Water movies into guard cell making it turgid and opening stomata