Instantaneous discrimination and some problems

1
Instantaneous discrimination and some problems

• Gary Lanigan, Nick Betson, Joel Dunn Howard
  Griffiths
• Dept. of Plant Sciences
• University of Cambridge

2
Photosynthetic discrimination

• Occurs during physical or chemical processes
• During photosynthesis, fractionation occurs
  during diffusion to carboxylation sites during
  fixation by RUBISCO
• Diffusion in air 4.4
• RUBISCO 29
• Further fractionation due to (photo)respiration?

3
Relationship with A, E WUE

• Can use d13C as a proxy for WUE via the
  relationship with pi/pa
• Changes in d18O are mainly dependent on VPD with
  d18O (air) depleted in proportion to gs

4
Mesophyll Conductance, gi

• Relationship between 13C, 18O and water use
  efficiency holds as long as no further drawdown
  from ci to cc

5
Measurement of gi

• From gas exchange, can calculate Di

C
i


D
6
.
22
4
.
4
i
C
a

• From on-lining, obtain measured Dobs
• Generally lower than Di since Rubisco not
  presented
• with ci but cc which may be 20-100 ppm lower.

• Therefore, difference between Di and Dobs shows
• us what impact gi has on the drawdown from ci
  to cc

6
Since leaf discriminates against 13C, air leaving
the cuvette is enriched relative to that entering
and the difference is a measurement of this
discrimination
7
Experiment

• Vicia faba Phaseolus vulgaris subjected to 10mM
  and 1mM N
• Subjected both species to a range of vapour
  pressures
• Look at relationships between A, E and WUE

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40
40
30
30
20
20
obs
obs
D
D
10
10
0
0
0.05
0.55
1.05
0.05
0.55
1.05
p
/p
p
/p
i
a
i
a
Phaseolus
Vicia
10
Phaseolus vulgaris
12
10
8
Di-Dobs
6
High N
Low N
4
2
-0
-2
0
0.01
0.02
0.03
0.04
A/Pa
Vicia faba
5
4
Di-Dobs
3
High N
Low N
2
1
0
0
0.01
0.02
0.03
0.04
0.05
A/Pa
11

• Dark and photorespiratory fractionations may
  alter the online signal
• Dark CO2 produced via pyruvate dehydrogenase f
  0-6
• Photorespiration Measurable isotopic effects f
  7-12

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• Not only must (photo)respiratory fractionation be
  taken into account
• Refixation of respired CO2 will reduce potential
  (photo)respiratory effects
• Expressed by r where
• r Vc .
• Vc gcpi

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INSTEAD OF
Dobs z (do-de)
.1000 1000 do- z A/Vc (do-de)
WE HAVE TO USE
Dps z A/Vc (do-de)-(1-r) Rd/Vc (drs-e-do)-(1-
r)G/pc(dfs-f-do). 1000 1000 do- z
A/Vc (do-de) Rd/Vc (drs-e-do) G/pc(dfs-f-do)
AND RUIN OUR WEEKEND!
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D13C corrected for (photorespiration and
refixation) f8
Phaseolus vulgaris
12
10
8
Di-Dps
6
High N
Low N
4
2
0
-2
0
0.01
0.02
0.03
0.04
A/Pa
Vicia faba
5
4
3
Di-Dps
High N
Low N
2
1
0
0
0.01
0.02
0.03
0.04
0.05
A/Pa
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Future work

• Investigating (photo)resp. effects on spp. with
  different growth stategies (spenders savers)
• Investigate the effects of differing leaf
  morphologies on discrimination