Water Relations

Question 1

State Fick’s law

Answer 1

Diffusion rate = diffusivity x concentration gradient

Question 2

What are positive/negatives of plants moving to land?

Answer 2

+ Air has 10000x greater diffusivity thatn water, easier to get CO2
- Problem of dehydration

Question 3

What are the two solution to dehydration on land?

Answer 3

• Solution 1: tolerate dessication e.g. bryophytes, some lycophytes. Enter dormant dehydrated state, rejuvenate in rain
• Solution 2: transport water. This evolved in mosses, liverworts, lycophytes, just not xylem yet. Culminated in evolution of xylem in “vascular plants”.

Question 4

What is the accepted theory for how plants move water to their leaves?

Answer 4

Cohesion-tension theory, based on idea that plants use capillarity as a source of tension

Question 5

Where does capillarity work in plants?

Answer 5

At the air water interface in the leaves, evaporation drives a pull.

Question 6

What stops the column of water breaking?

Answer 6

Hydrogen bonds between water molecules

Question 7

What is water potential?

Answer 7

Potential of water to move by osmosis, gravity, capillarity, pressure/tension

Question 8

What is the capillary pore size at the air-water interface

Answer 8

10nm, can drag water 2800m

Question 9

What forces must the plant overcome to pull water from the ground?

Answer 9

Gravity, friction, capillarity of soil

Question 10

Try to remember megapascal values for different things (see book)

Question 11

What is the inward pressure on xylem vessels?

Answer 11

2 MPa

Question 12

How does cavitation occur?

Answer 12

• Cell walls have pores to allow water to flow between, pit membrane prevents air from entering
• Pores are very small and can withstand a lot of presure- capillary force prevents water entering as capillary radius > size of pores
• As tension increases, capillary radius gets smaller, air invades further
• Tension becomes too great and air gets sucked through and explosively expands, effectively boiling the xylem

Question 13

How do conifers isolate embolism bubbles?

Answer 13

Torus + margo, act as valve

Question 14

Where are “weak xylem” species found?

Answer 14

Rainforest

Question 15

Where are “tough xylem” species found?

Answer 15

Arid zones

Question 16

How are stomata opening controlled?

Answer 16

• Turgor pressure, controlled by creating an osmodic gradient created using ATP
• Decrease in leaf water potential causes increase in ABA
• ABA opens Cl- specific channels open in guard cell membrane when stomata needs to close, Cl- flows out followed by water

Question 17

What happens to stomata at night?

Answer 17

They close

Question 18

What usually happens before cavitation occurs?

Answer 18

Process of stomatal closure is usually complete