Plant Physiology 2-Photosynthesis

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

Tracking atoms STARCH

photosynthetic products often stored as starch

•Starch = glucose polymer

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

Increased leaf C:N

Decreased Heat-shock proteins (Hsps)

Decreased thermotolerance

•High CO2 effects greater in C3 than C4 and CAM species.

•High CO2 effects greater on induced than basal thermotolerance.

Hypothesis

corn

0 1 2 3 4 5

0

10

20

30

40

corn

0 1 2 3 4 5

wheat

0 1 2 3 4 5

0

10

20

30

40

wheat

0 1 2 3 4 5

700ppm CO2

370ppm CO2

no-pre-hs pre-hs

Pn 0

10

20

30

40

sorghum sorghum

0

10

20

30

40

barley barley

Time (h) Time (h)

Heat stress decreased Pn in all species(not the result of stomatal closure).

Elevated CO2 had negative effects on Pn of C4 species, and positive effects on C3 species.

Pre-heat shock has a positive effect on Pn.

corn

0 1 2 3 4 5 6

corn

0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

wheat

Y D

ata

0.0

0.2

0.4

0.6

700ppm CO2370ppm CO2

wheat

time (h)

0.0

0.2

0.4

0.6

no-pre-hs pre-hs

arabidopsis

0 1 2 3 4 5 60.0

0.2

0.4

0.6 arabidopsis

0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

Barley Barley

0.0

0.2

0.4

0.6 sorghum sorghum

φ et

Heat shock decreased Фet of all

C3 and C4 species

There was negative CO2 effects on all species, except for wheat

There was positive Pre-HS effects on all species

SoyFACE: CO2 & ozone

phot

osyn

thet

ic e

lect

ron

tran

spor

t

0.0

0.2

0.4

0.6

0.8

controlheat-stressed

_______ambientCO2 &

ozone

_______elevatedCO2

_______elevated ozone

_______elevatedCO2 &

ozone