Lecture 5: Conserving Water

Question 1

What does CAM stand for?

Answer 1

Crassulacean Acid Metabolism

Question 2

CAM is a

Answer 2

photosynthetic adaption to a fluctuating or periodic water supply

Question 3

What % of flowering plant species are CAM

Answer 3

5-10%

Question 4

Where does CAM in plants occur (what environments)

Answer 4

in plants from arid regions (eg. cacti) and in tropical epiphytes (e.g. orchids and bromeliads)

Question 5

CAM is always associated with..

Answer 5

Succulence, at least at a cellular level

Question 6

E.g. of typical CAM plants:

Answer 6

• pineapple (bromeliad)
• spanish moss
• epiphytic orchids
• prickly pear
• agave tequilana

Question 7

Desert climates are characterised by

Answer 7

scarce and variable precipitation

Question 8

the canopy in a tropical rainforest can sometimes be

Answer 8

very dry.
High temp (30) coupled with low humidity (60%) is characteristic of arid steppe regions

Question 9

At night CAM pathway:

Answer 9

• Stomata open so CO2 enters cell
• CO2 enters the cell converted to bicarbonate anion (HCO3-)
• CHO (carbohydrate) converted to C3
• CHO + HCO3- fixed by PEP carboxylase to a C4 acid
• C4 acid stored in vacuole

Question 10

In the day CAM pathway:

Answer 10

• Stomata shut
• C4 acid removed from vacuole
• decarboxylases convert C4 acid to CO2 + C3
• CO2 enters calvin cycle (rubisco used) results in carbohydrate (CHO)
• C3 also converts to CHO

Question 11

Phases of CO2 fixation in CAM plant

Answer 11

• Stomata open at night and CO2 is fixed.
• At the beginning of the day, stomata close.
• CO2 fixed by the Calvin Cycle behind closed stomata

Question 12

In CAM plants carboxylation and decarboxylation are separated in ___

Answer 12

time. not space (as in C4)

Question 13

Malate is formed when and decarboxylated when

Answer 13

Malate formed by PEP carboxylase at night and decarboxylated in day.

Question 14

Carbohydrates broken down when and CHO reformed when

Answer 14

Carbohydrates broken down at night to provide the PEP for PEP carboxylase. CHO reformed during the day following malate decarboxylation and CO2 fixation.

Question 15

Gas-exchange at night results in

Answer 15

water conservation as cooler at night so less H2O is lost

Question 16

CAM plants penalty:

Answer 16

• poor productivity
• lower photosynthesis
• lower growth

Question 17

CAM gown under well H2O conditions act like

Answer 17

C3 plants

Question 18

As leaves assimilate CO2 they must also

Answer 18

restrict the amount of water lost from the leaf

Question 19

ways in which water is lost from the leaf

Answer 19

• boundary (unstirred) layer [ga]
• cuticular [gc]
• stomatal [gs]
• mesophyll [gm] conductance

Question 20

model used to show water loss from leaf known as

Answer 20

conductance model
ga
gc / gs
gm

Question 21

increasing the boundary layer:

Answer 21

leaf hais and sunken stomata

e.g. marram grass

Question 22

Stomatal conductance usually ____ factor regulating gas‐exchange.

Answer 22

major

An order of magnitude less than the boundary layer conductance.

Question 23

cuticular conductance is always

Answer 23

small

Question 24

mesophyll conductance depends on

Answer 24

structural factors (cell walls and membranes) and biochemical factors but can be comparable to stomatal conductance

Question 25

stomatal action:

Answer 25

balloon and tape
• When the guard cells are flaccid, the stomatal pore is closed.
• Upon illumination, H+‐ATPase mediates efflux of H+ from the guard cells and leads to K+ uptake into the guard cells via activation of K+ channels
• The uptake of ions raises the internal osmotic pressure and lowers the water potential so that water flows from the epidermal cells into the guard cells via aquaporins
• The wall is thickened on the inside so that the guard cells bow outward and an elliptical pore 5‐10 μm develops.

Question 26

stomatal opening can be affected by:

Answer 26

• light
• CO2
• Abscisic acid (ABA)

Question 27

how does light affect stomatal opening

Answer 27

• low light promotes opening
• blue light more effective than red light (activates H+-ATPase)

-CAM plants are different (nocturnal opening)

Question 28

how does CO2 affect stomatal opening

Answer 28

CO2-free air promotes opening

Question 29

Abscisic acid (ABA)

Answer 29

ABA synthesised in the roots of plants in drying soil

-ABA promotes closure (inhibited H+-ATPase)

Question 30

C3, C4 & CAM compared:

typical species of economic importance

Answer 30

C3- wheat, rice, potatoes
C4- Maize, millet, sugarcane
CAM - Agave, vanilla pineapple

Question 31

C3, C4 & CAM compared:

% angiosperms

Answer 31

C3- ca. 90
C4- 3
CAM - 7

Question 32

C3, C4 & CAM compared:

typical habitat

Answer 32

C3- wide distribution
C4 - savannah grasslands
CAM- xeric sites (including epiphytes)

Question 33

C3, C4 & CAM compared:

First product of CO2 fixation

Answer 33

C3 - glycerate 3-P
C4 - malate (two fixation processes separated in space)
CAM - malate (two fixation processes separated in time)

Question 34

C3, C4 & CAM compared: anatomy

Answer 34

C3 - bundle -dheath not green
C4 - Kranz anatomy
CAM - cellular or tissue succulence

Question 35

C3, C4 & CAM compared: photorespiration

Answer 35

C3 - up to 40% of photosynthesis
C4 - not detectable
CAM - low