BIOL 205: Plant water relations and transpiration (Lecture 6)

What is the main factor of productivity and global plant distribution and why?

Water availability: structures biomes - temperature is secondary as temperature just determines the form of water and the amount

What are the roles of water in plants? Water is >70% of biomass

• Cellular reactions (substrate for light reactions but amount is very small)

• Uptake of mineral nutrients

• Phloem transport of sugars, hormones, etc

• Turgor: plant structure

• Evaporative cooling

What kind of factors do plants manipulate to make water move?

Pressure and osmosis because they can’t make water move via active transport

What are some characteristics of water?

• Polar - charge varies across the molecules (high dielectric constant)

• Strong force of cohesion between H2O molecules

What is water potential?

The potential energy in water - its tendency to move from one area to another relative to pure water at atmospheric pressure and ambient temperature - measured in megapascals

What is the general equation for water potential?

Total= solute +pressure +gravity +matric

All measured in potentials

What is solute potential?

How much stuff is dissolved in water = osmotic potential

Can water potential values be negative?

Yes, they are relative to pure water

Why could a salt solution have a potential of -10 mPa?

Aby solute makes the potential negative because water flows from where there is a lot of solute dissolved to places where there is little solute dissolved. There would be more potential energy in pure water because it would flow into the salt solution if separated by a semi-permeable membrane. Water flows to lower potential.

How can cells metabolically manipulate solute potential and thus total water potential?

Adding or removinf solutes in cells like by phloem loading

What is pressure potential?

Water can be moved by tension (pulling) or pressure (pushing). Ex: a syringe - pulling up causes the plunger inside the barrel to be lower than the atmosphere and water moves into the region of lower pressure and pushing down on the plunger causes the pressure inside the barrel to be higher than atmospheric. Water moves out, towards the region of lower pressure

What are the implications of solute potential for RBCs in isotonic and hypotonic solutions?

Water bursts cells when in hypotonic solution because the water is flowing to lower potential in the “saltier” cell

How does turgor pressure work?

Positive pressure inside cells is contained by the cell wall, producing turgor pressure in a plant. Turgor is a physical support for non woody plants it ensures that it can maintain its shape

What is the usual pressure potential for plant cells?

Typically around 0.6-0.8 MPa but can reach as high as 1.5 MPa in a well-watered plant.

Why does water move from soil to root to xylem to leaf?

It moves from higher to lower water potential

What happens to leaf water potential when plants wilt?

Solute (osmotic )potential is more negative, because less water is in the cells - cells are flaccid so pressure potential is zero and the total water potential is very negative

What causes a stronger drive for water movement?

The greater the difference in water potential between two compartments - the stronger the drive for water movement

What is transpiration?

process of water movement through the plant and its evaporation to the atmosphere

What is transpiration for?

• Nutrient transport

• Evaporative cooling

• Water needed for growth and cellular reactions

• Turgor (plant structure)

Why does the amazon need vegetation for its microclimate?

To make clouds for the water cycle as the vegetation evaporates water

What path does water take through a plant?

roots→ xylem → leaves → Atmosphere

What are characteristics of the xylem?

• Xylem cells are dead, and fit together via perforations = pipes

• Allow rapid bulk flow with low resistance

• Up to 45 m/hr in trees

• In the leaves, water moves from vessels to cellular matric, and then evaporates into spaces within the leaf

What allows water to move against gravity?

Continuous water column from the roots to the leaves

• Cohesion: (H bonds) between water molecules

• Adhesion: water molecules are attracted to cell walls

• Tension: As molecules are removed from the column by evaporation in the leaf, more are drawn up (under tension)

What happens if adhesion fails in the xylem? What causes this?

• Formation of bubbles causes air-vapour blockage that breaks the water column

• Causes: tissue damage, freezing and drought issues

How does root pressure cause water movement?

Hydrostatic pressure resulting from high ion concentration in the roots

• Soil waterL dilute ionic solution

• Roots absorb ions from soil water; ions buildup in xylem sap. Decrease in xylem water potential, triggers increased uptake of soil water

• Builds up a small positive hydrostatic pressure in the xylem

How does gas exchange work at the leaf surface?

H2O evaporates cell surfaces→ intercellular space → diffuses out the stomata

CO2 diffuses through the stomata into the lead, dissolves into water on cell surfaces, used for photosynthesis

Why is the stomata opening conflicting?

• Concentration gradients go in opposite directions

• Gradient is stronger for H2O

• H2) is smaller than CO2 and diffuses faster

Why is photosynthesis paradoxical?

Plants must lose water to photosynthesize but dehydration=death/

A large surface area for exchanging gases offers a large surface area for desiccation. C4 and CAM are adaptations for this

What drives the rate of transpiration?

Force and resistance

What is force?

• The difference in water potential between the air and soil

• Greater difference = greater force = more transpiration

What is resistance?

Greater resistance = less evaporation. Ex: the cuticle - wax on the leaf surface often thick in desert plants

What is the boundary layer of plants?

Layer of still air, thicker - less evaporation which is affected by trichomes and leaf size

Is there more or less boundary layer when there is higher windspeed?

less

What is the relationship between wind speed and transpiration?

Transpiration = (water potential of leaf - atmosphere water potential )/ resistance

Ex: Transpiration could increase if wind reduces boundary later and water potential gradient increased

How does the stomata open and close?

Open and close via turgor. Cells held open when turgid and collapse when flaccid

What is the path of how the stomata opens and closes?

Environmental/physiological cues → plant hormones → Move solutes out of guard cells → water follows solutes, impacting turgor → Stomata close

When solute concentration in guard cell is high, water moves out and what increases?

Turgor increases.

What is stomatal aperture?

How open the stomata is. It is responsive to environmental cues

How do you maximize the trade off between water and CO2?

• With tightly closed stomata, CO2 might limit photosynthetic carbon assimilation

• There is an ideal point at which the exchange of CO2 and H2O is optimal

• With a wide-open stomata, the rate of transpiration may be more than needed for max photosynthetic rate

What are ways to adapt or tolerate when water is low?

• Bryophytes tolerate desiccation

• Most tracheophytes cannot tolerate desiccation and die

• SOme desert plants evade drought, they survive the dry season as seeds, sprouting and flowering in a brief period of rain

• Some desert plants tolerate dry conditions through adaptations such as deep roots, C4 photosynthesis and tiny or absent leaves