Photosynthetic downregulation over longterm CO2 enrichment in leaves of sour orange Citrus aurantium

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Photosynthetic down-regulation over long-term CO2
enrichment in leaves of sour orange (Citrus
aurantium) trees

• Adam N.R., Wall G.W., Kimball B.A., Idso S.B.,
  Webber A.N., (2004). New Phytologist 163 341-347

Presented by Kourtnee Marr
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Trees and Rising CO2

• Forest trees are a major sink for Carbon
• Contribute up to 70 of the CO2 fixed on land
• Are the natural C sinks large enough to sequester
  anthropogenically released CO2 ?

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Trees and Rising CO2

• CO2 fertilization effect
• Increasing rate of carboxylation by rubisco,
  stimulating photosynthesis
• Can be offset by long-term decline in
  photosynthetic enzymes
• Extent of down-regulation will depend on
  environmental conditions
• N availability

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Trees and Rising CO2

• Past research either conducted on seedlings or
  have been short-term (few months to a few years)
• Wanted to investigate the long-term effects of
  CO2 enrichment on trees

www.ipcc.ch/
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Research Questions

• Does 14 years of CO2 enrichment cause
  photosynthetic down-regulation in trees that are
  not nutrient limited?
• Is the equilibrium level of wood biomass
  production a result of down-regulation?
• How do these responses vary throughout the year?

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Methods Plant Culture

• Trees grown for 14 years in open-top chambers
• Ambient and ambient 300 umol mol-1 of CO2
• Optimal soil moisture and nutrient levels
• Above ground wood biomass growth
• Trunk circumference ? biovolume ? mass

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Methods Gas Exchange

• Licor 6400
• At midday in spring, summer, fall, and winter
• Generated ACi curves
• Cis ranging from 50-900 umol CO2 mol-1
• Temperature 25C
• Radiation 1000 umol m-2m-1
• Relative Humidity 50

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Methods Biochemical Assays

• Leaves allowed to reach steady state Pn at growth
  their growth CO2
• Removed and frozen in liquid N
• Full activity and in situ activity of Rubisco
  determined
• Chl concentrations determined
• SDS-PAGE

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Results Gas Exchange
Year long averages of PSN, Gs, E, Ci, and CiCa
were higher in the enriched chambers
Surprise!! Elevated CO2 usually lowers Gs and E,
due to stomatal closure
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Results Gas Exchange
Ci values were increased 150 (spring), 100
(summer), and 141 (fall) in the elevated chambers
Vcmax values in the elevated chambers were 78
(spring), 82 (summer), and 65 (fall) of the
ambient
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Results Gas Exchange
PSN values were increased in the spring, summer
and fall
E values were increased in the spring and fall
Gs values were increased in the spring
CiCa values were increased in the spring,
summer, and fall
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Results Biochemical
Full activity and initial activity of Rubisco in
the elevated chambers were 75 and 81 of the
ambient
Concentrations of Total Chl and chl a showed
significant CO2 x date interactions
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Results Biochemical
Significant reductions of chl and chl a values in
the elevated chambers were seen in the fall
Full activity of Rubisco in the elevated chambers
was 71 of the the ambient
Densities of RBcL in the elevated chambers were
75 (spring), 73 (fall), and 80 (winter) of the
ambient
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Discussion

• 2nd year of experiment - PSN rates in the
  enriched plots were 2.84 times that of the
  ambient
• 6th year - PSN rates had decreased to 1.75 times
  that of the ambient
• 14th year PSN rates were 1.45 times that of the
  ambient

• Indicates that the enhancement of PSN due to CO2
  enrichment is declining!
• Suggests that down-regulation of the
  photosynthetic apparatus has occurred

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Discussion - Down-regulation

• The 228 increase in the Ci levels in enriched
  leaves and the greater CiCa ratios could
  indicate
• A reduced capacity of leaves to use CO2
• or
• Could be a result of greater Gs in the enriched
  leaves

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Discussion - Down-regulation

• The year-long avg. for content of RBcL was
  significantly reduced by CO2 enrichment
• Strong evidence that down-regulation has occurred

• The increases in Ci and CiCa could be due to
  increased in Gs as well as down-regulation of
  Rubisco content

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Discussion - Down-regulation

• Shows relationship between the CO2-induced
  enhancement of wood biomass production and the
  enhancement of PSN

• Idso Kimball (2001) concluded that these trees
  had reached equilibrium with respect to the
  CO2-induced enhancement of above-ground wood
  biomass

• Determined that their was a relationship between
  the CO2-induced enhancement of wood biomass
  production and enhanced PSN (Pearsons
  Coefficient of r.9986)

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Discussion - Down-regulation

• Because these two factors are closely related,
  and since wood biomass ratio has reached
  equilibrium, they assume that the PSN ratio has
  also reached equilibrium.
• Conclude that the decline in both wood biomass
  enhancement and PSN enhancement are at least
  partly due to down-regulation of the
  photosynthetic apparatus.

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Discussion - Chl and N data

• In the 4th-8th year, Idso et al. (1996) found a
  4.8 reduction in Chl a and total N
• 9th-10th year there was an 11 reduction in N in
  leaves
• In the 14th year, Chl a was reduced by 9 and
  RBcL was reduced 22

• Reduction in Rubisco protein likely due to
  down-regulation and N dilution

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Conclusions

• CO2-induced enhancement of PSN has declined after
  14 yrs of exposure
• Decline strongly correlated with decline in
  above-ground wood biomass enhancement ratio
• The strong correlation between wood biomass
  enhancement ratio and PSN enhancement ratio
  indicates that PSN has declined to an equilibrium
  level

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Conclusions

• Down-regulation has resulted in reduced Rubisco
  content
• However, reduction in N content (11) does not
  account for all of the reduction observed in
  Rubisco
• Showed that enhancement of PSN declined as a
  result of down-regulation of Rubisco, despite
  availability of N
• Likely that trees in N-limited conditions will
  show an even greater PSN acclimation response in
  the future!!