Lecture 5: Conserving Water Flashcards
What does CAM stand for?
Crassulacean Acid Metabolism
CAM is a
photosynthetic adaption to a fluctuating or periodic water supply
What % of flowering plant species are CAM
5-10%
Where does CAM in plants occur (what environments)
in plants from arid regions (eg. cacti) and in tropical epiphytes (e.g. orchids and bromeliads)
CAM is always associated with..
Succulence, at least at a cellular level
E.g. of typical CAM plants:
- pineapple (bromeliad)
- spanish moss
- epiphytic orchids
- prickly pear
- agave tequilana
Desert climates are characterised by
scarce and variable precipitation
the canopy in a tropical rainforest can sometimes be
very dry. High temp (30) coupled with low humidity (60%) is characteristic of arid steppe regions
At night CAM pathway:
- 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
In the day CAM pathway:
- 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
Phases of CO2 fixation in CAM plant
- 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
In CAM plants carboxylation and decarboxylation are separated in ___
time. not space (as in C4)
Malate is formed when and decarboxylated when
Malate formed by PEP carboxylase at night and decarboxylated in day.
Carbohydrates broken down when and CHO reformed when
Carbohydrates broken down at night to provide the PEP for PEP carboxylase. CHO reformed during the day following malate decarboxylation and CO2 fixation.
Gas-exchange at night results in
water conservation as cooler at night so less H2O is lost
CAM plants penalty:
- poor productivity
- lower photosynthesis
- lower growth
CAM gown under well H2O conditions act like
C3 plants
As leaves assimilate CO2 they must also
restrict the amount of water lost from the leaf
ways in which water is lost from the leaf
- boundary (unstirred) layer [ga]
- cuticular [gc]
- stomatal [gs]
- mesophyll [gm] conductance
model used to show water loss from leaf known as
conductance model
ga
gc / gs
gm
increasing the boundary layer:
leaf hais and sunken stomata
e.g. marram grass
Stomatal conductance usually ____ factor regulating gas‐exchange.
major
An order of magnitude less than the boundary layer conductance.
cuticular conductance is always
small
mesophyll conductance depends on
structural factors (cell walls and membranes) and biochemical factors but can be comparable to stomatal conductance
stomatal action:
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.
stomatal opening can be affected by:
- light
- CO2
- Abscisic acid (ABA)
how does light affect stomatal opening
- low light promotes opening
- blue light more effective than red light (activates H+-ATPase)
-CAM plants are different (nocturnal opening)
how does CO2 affect stomatal opening
CO2-free air promotes opening
Abscisic acid (ABA)
ABA synthesised in the roots of plants in drying soil
-ABA promotes closure (inhibited H+-ATPase)
C3, C4 & CAM compared:
typical species of economic importance
C3- wheat, rice, potatoes
C4- Maize, millet, sugarcane
CAM - Agave, vanilla pineapple
C3, C4 & CAM compared:
% angiosperms
C3- ca. 90
C4- 3
CAM - 7
C3, C4 & CAM compared:
typical habitat
C3- wide distribution
C4 - savannah grasslands
CAM- xeric sites (including epiphytes)
C3, C4 & CAM compared:
First product of CO2 fixation
C3 - glycerate 3-P
C4 - malate (two fixation processes separated in space)
CAM - malate (two fixation processes separated in time)
C3, C4 & CAM compared: anatomy
C3 - bundle -dheath not green
C4 - Kranz anatomy
CAM - cellular or tissue succulence
C3, C4 & CAM compared: photorespiration
C3 - up to 40% of photosynthesis
C4 - not detectable
CAM - low
