Plant Ecology 5460 "Water Relations"

Water Potential

The difference in potential energy between pure water and water in a system.

Water Potential equation

Ψ = Ψp+Ψπ+Ψm+Ψg

Ψp= pressure/turgor pressure

Ψπ= osmotic potential

Ψm= matric potential (adhesion to objects)

Ψg= gravitational potential

Transpiration causes ________ pressure in xylem, _________ water potential

negative, reducing

ClickerQ: What would happen to water potential if the concentration of solutes increases with no changes in pressure potential in a leaf?

Water potential would decrease.

A plant stem has _____ water potential, while a hose has _______ water potential.

Negative, positive

Units of water potential?

mPa, mega pascals, pressure.

How does H2O move between areas of different water potentials?

high potential to low potential

SPAC

Soil Plant Atmosphere Continuum

Key points of cohesion-tension theory of sap ascent?

1) Transpiration causes potential differential between leaves and air. This is the main driving factor in long distance water transport in plants. 2) Adhesion and cohesion of water.

Stomatal conductance of H2O and CO2

Rate at which H2O (gsw) leaves stomata and how much CO2 (gco2) goes in based on the concentration differences between intracellular and ambient areas and how open stomata area. Units of flux.

Describe the changes in water potential of the plant through the day

Predawn: highest water potential (closest to 0). Plant water potential ~ soil water potential. Midday: lowest water potential (most negative). Evening: water potential starts to increase.

Embolism

Air bubbles in xylem

Cavitation

Rapid breakage of water in an entire xylem conduit.

What is a boundary layer and what does it do?

A layer of very still air covering the leaves. Boundary layers limit transpiration. Transpirational flux is greater in moving air for any stomatal aperature than it is for still air.

How does water pressure at predawn change in relation to the day of the year?

Water pressure in the predawn is highest (closest to 0) in may. It decreases into the summer and is lowest in July/August. It begins to rise in September.

How does water pressure at mid day change in relation to the day of the year?

Water pressure in the mid day is highest in May and gradually decreases into September. It does not begin to increase in September.

Describe the advantages/disadvantages of two strategies during drought?

Hydraulic Safety: This reduces risk of embolism during drought but reduces efficiency of water uptake during periods of plenty.

Hydraulic Efficiency: This increases efficiency of water uptake during times of plenty but increases risk of embolism during drought.

Nocturnal Transpiration: A study in Red Butte Gardens show that plants from drier areas have greater stomatal conductance at night, compared to species from wetter areas that are planted in the same garden. List the three theories for why this occurs.

1) Competitive advantage over other plants through increasing their own soil-root water potential gradient.

2) Repairing embolisms at night for full functionality during the day.

3) To maintain cooler night time temperatures.

Where does water enter the root?

Through root hairs. There is a waxy coating over the main root surface and only root hairs are permeable to water. This increases water pressure in the root, assisting in long distance water transport.

What happens physiologically to plants in drought?

1) Stomata close

2) Photosynthesis declines

3) Hydraulic dysfunction

ClickerQ: What plant is likely to have the lowest (most negative) leaf water potential?

A plant under drought in midday full sun.

What is the function of stomata?

Stomata regulate transpiration and CO2 uptake.

How do stomata open?

K+ and Cl- are pumped into guard cells, which increases the osmotic potential, which results in water movement into the cell. The water expands the guard cell, opening the stomata. The stomata closes by releasing the water and electrolytes out of the guard cell.

What type of plants have vessles? What type of plants have tracheids?

Vessles are only found in angiosperms; all vascular plants have tracheids.

What is the difference between vessels and tracheids?

Vessels are thicker with perforated plates in between each unit. Only angiosperms have vessels. Tracheids are long and thin. Both are dead cells.

Xylem transports___ and phloem transports____.

water; sugar