- Say no to water leaks Water use efficiency and C3, C4 and CAM plants.
J . Ar t é s , S . B e r n a l , J . C . C a r m o n a , C . C a r r a s c o s o
INTRODUCTION
First living organisms on Earth, which were able to uptake atmospheric carbon dioxide, were common ancestor of existent Cyanobacteria. These organisms used an ancient metabolic process
incorporating CO2 into cells with a 3-carbon pathway (called C3 carbon pathway). Nowadays, there are three carbon fixation pathways: C3, with a 3-carbon molecule, C4, with a 4-carbon molecule, and
CAM, which have different advantages in different environments. The aim of C 4 and CAM pathways is improve water use efficiency (WUE). We will take it a little big further in following lines.
C3
Comparative anatomy
Advantages and
disadvantages of C3
C3 photosynthesis is the most common carbon fixation
pathway. It’s called C3 because CO2 is first incorporated
into a 3-carbon compound, where Rubisco is implicated
in the uptake of CO2.
C3 plants have higher WUE than C4 plants
under mild and moist condition and under
shade.
These plants have the stomata open during the day
and their photosynthesis takes place throughout the leaf.
Nevertheless, C3 plants have lower
performance under dry and hot conditions,
they can be saturated by sunlight.
Kranz anatomy
Figure 1. C4 plants have got different mesophyll cells
and their bundle sheath cells are the most developed.
There are some species which
have an intermediate metabolism
between C3 and C4. The origin of
this metabolic pathway is
unknown (Flaveria sp.).
C3 (Hordeum vulgare), C4 (Sorghum sp.) and CAM (Agave americana) plants.
CAM
C4
CAM plants uptake CO2 at night because their stomata
are opened then. In addition, they stored carbon as acids
which will be used in the day. Calvin cycle and
photosynthetic carbon reduction (PCR cycle) take place
during light periods (Figure 3). In conclusion, metabolic
pathways are separated in time. These plants have a low
capacity to assimilate CO2. Some examples of CAM
plants are Cactaceae, Agavaceae, some Orchidaceae and
Bromeliaceae (succulent plants).
C4 plants incorporate CO2 with a 4-carbon compound and
their stomata open during the day. These plants use PEP
carboxylase such as enzyme involved in the uptake of CO2. This
photosynthesis takes place in mesophyll cells (Figure 1) and it
protects Rubisco from photorespiration. These metabolic
pathways are localized in different places: in mesophyll cell and
bundle sheath cell (Figure 2). These processes improve WUE
because the crop yield is increased. C4 pathway can be found in
Cyperaceae, some Poaceae and Brassicaceae and Asteraceae or
Caryophyllales with modified mesophyll.
Table 1. Differences between C3, C4 and CAM plants.
Comparison
Photosynthesis rate, growth and productivity are low
in CAM plants, middle in C3 plants and high in C4
plants. However, the plants which have the higher
WUE are CAM (Table 1).
C4 plants increase the crop yield and CAM plants
reduce water losses to improve their WUE.
Differences
Enzyme
Anatomy
Photosynthesis rate
Photorespiration
WUE (kg/m3)
Growth
Productivity
Optimal temperature
Examples
C3 metabolic pathway can be found in different living
organisms but C4 and CAM can be found only in
vascular plants.
C3
Rubisco
Normal
Middle
Yes
C4
Rubisco and PEP
Kranz
High
No
Low (1-3)
Middle
Middle
20-30 ºC
Wheat, barley, rice, pepper,
tomato
Middle (2-5)
High
High
30-45 ºC
Corn, sorghum,
sugarcane
CAM
Rubisco and PEP
Succulent
Low
Yes (during the day)
No/Low (during the night)
High (10-40)
Low
Low
High temperatures
Pineapple, cactus, agaves
Advantages and
disadvantages of
CAM plants have higher WUE than C3
plants under arid conditions because
they open stomata at night, when
transpiration rates are lower. They grow
slower than C3 and C4 plants.
Advantages and
disadvantages of C4
CAM
C4 plants fix carbon dioxide faster
than C3 plants under high light and
temperature conditions. This change
is only an advantage in hot
conditions. In fact, this is a
disadvantage in cold conditions.
Figure 2. Metabolic pathways take place in mesophyll cells and
bundle sheath cells. These differences improve their
photosynthesis rate because they cancel photorespiration
effects.
These plants have higher WUE
because PEP Carboxylase fixes CO2
faster and they don’t need to open
stomata so much.
Figure 3. CAM plants synthesize their organic
acids during the night. Next day, these plants
metabolise the organic acids.
CAM plants can survive dry spells, O2
given in the photosynthesis is used for
respiration and vice versa.
Conclusion
The changes, which have experimented some plants
to improve WUE, are potential adaptations to their
environments. C4 and CAM metabolisms are efficient in dry
and arid environments, where having a high WUE is
essential. Despite of having a low WUE, C3 plants are the
most extended .
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