Plant Physiology 2Photosynthesis
photosynthesis
• Photo means ‘light’ and synthesis means ‘to
make’
• Process in which plants convert carbon
dioxide and water into sugars using solar
energy
• Occurs in chloroplast
Photosynthesis:
6 CO2 + 6 H2O
C6 H12 O6 + 6 O2
carbon dioxide + water = sugar + oxygen
photosynthetic
products often
stored as starch
•Starch = glucose
polymer
Tracking atoms
STARCH
Fig. 10.1
Fig. 10.2a
Fig. 10.2b
Fig. 10.2c
Fig. 10.4
Chlorophyll
•Absorbs red & blue light
•Reflects green light
Fig. 10.6
Fig. 10.8
Fig. 10.20
Fig. 10.17
Rubisco
• Ribulose bisphosphate
carboxylase oxygenase
• (fixes CO2 & O2)
• Enzyme in Calvin
Cycle (1st step)
• Most abundant protein
on Earth
– Ca. 25% total leaf
protein
Photorespiration
•
•
•
•
•
When rubisco “fixes” O2, not CO2
Lose 1/2 C as CO2; costs 2.5 extra ATP
Take up O2
Only occurs in light
Occurs 1 out of 4 reactions under today’s
atmospheric [CO2]
• Rate increases with temperature
Types of photosynthesis
• C3
– The majority of plants
• C4
– CO2 temporarily stored as 4-C organic acids resulting in
more more efficient C exchange rate
– Advantage in high light, high temperature, low CO2
– Many grasses and crops (e.g., corn, sorghum, millet,
sugar cane)
• CAM
– Stomata open during night
– Advantage in arid climates
– Many succulents (e.g., cacti, euphorbs, bromeliades,
agaves)
Fig. 10.21
Fig. 10.22
Global Environmental Change &
Photosynthesis:
C3 vs. C4 vs. CAM
• Increasing CO2
• Increasing chronic and acute temperatures
• Increasing N (vs. decreasing C:N from
increasing CO2)
• Changes in water
CO2 effects on photosynthesis
• C4 > C3 at low CO2
• But, C3 > C4 at high CO2
*At high CO2, C3 more efficient than C4 at all temps.
(photosynthesis only, not other processes)
Photosynthetic N-use efficiency
• C4 plants need (have) less leaf N than C3
• Photosynthesis higher per unit N in C4
• Humans are increasing global N, which
benefits C3 more than C4
• Increasing CO2 decreases leaf N content,
more in C3 than C4
Photosynthetic water-use
efficiency
• C4 plants use less water than C3
• (cause stomates open less)
• Water availability may increase or decrease
in the future.
Predicting the future for plants
• How will increases in CO2, N, and chronic
and acute heat stress affect photosynthesis?
• Who will win or lose? C3? C4?
• How will pollution (eg, ozone) interact?
• Current research in my lab an example.
Elevated CO2
Hypothesis
Increased
leaf C:N
•High CO2 effects
greater in C3 than C4
and CAM species.
•High CO2 effects
greater on induced than
basal thermotolerance.
Decreased Heat-shock
proteins (Hsps)
Decreased
thermotolerance
no-pre-hs
pre-hs
40
30
corn
20
corn
10
0
40
0
1
2
3
4
5
0
1
2
sorghum
30
3
4
5
sorghum
20
Heat stress decreased
Pn in all species
(not the result of
stomatal closure).
Pn
10
0
Elevated CO2 had
negative effects on
Pn of C4 species, and
positive effects on C3
species.
40
30
barley
20
barley
10
0
40
wheat
30
wheat
20
10
0
0
1
2
3
4
5
0
1
Time (h)
2
3
Time (h)
700ppm CO2
370ppm CO2
4
5
Pre-heat shock has a
positive effect on Pn.
no-pre-hs
pre-hs
corn
0.6
corn
0.4
0.2
0.0
0.6 0
1
2
3
4
5
sorghum
6
0
1
2
3
4
sorghum
5
6
Heat shock
decreased Фet of all
C3 and C4 species
0.4
There was negative
CO2 effects on all
species, except for
wheat
0.2
Barley
0.4
Barley
0.2
0.0
0.6
0.6
wheat
There was positive
Pre-HS effects on all
species
wheat
0.4
Y Data
et
0.0
0.6
0.2
0.4
0.2
0.0
0.6
0.0
arabidopsis
arabidopsis
0.4
0.2
0.0
0
1
2
3
4
5
6 0
time (h)
1
2
3
4
5
6
SoyFACE: CO2 & ozone
photosynthetic electron transport
0.8
control
heat-stressed
0.6
0.4
0.2
0.0
_______
ambient
CO2 &
ozone
_______
elevated
CO2
_______
elevated
ozone
_______
elevated
CO2 &
ozone