Plant water regime
• Regulation of gas exchange by stomatal
opening
– Stomatal limitation of transpiration rate and
photosynthetic rate
– Water use efficiency
– Global climate change
– Antitranspirants
Regulation of transpiration
• Limitation of transpiration rate (E) by stomatal conductance (gs):
• lsE = (E/E) / (gs/gs)
• lsE = rs / (ra + rs + ri)
• dependent mostly on ra
• transpiration of canopy is dependent on leaf transpiration and LAI
• stomatal limitation of transpiration is generally higher than stomatal
limitation of photosynthetic rate
Comparison of transport of water vapour and carbon dioxide in a leaf
Regulation of photosynthetic rate
• Stomatal limitation of photosynthetic rate
• lsPN = (PN/PN) / (gs/gs)
• lsF = rs / (ra + rs + ri + rm)
• stomatal and nonstomatal limitation of photosynthetic rate
• determination of photosynthetic rate under high CO2 concentration,
simultaneous measurements of gas exchange and chlorophyll
fluorescence, calculation by models
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Cowan, Farquhar (1977): optimum stomatal regulation = maximum carbon
gain at minimum water loss
E/PN = E/gs/PN/gs = const.
Grassi and Magnani 2005
Calculation of stomatal and nonstomatal limitation of photosynthesis
Homobaric and heterobaric leaves, effect of “stomatal patchiness“
Photosynthesis under stress
Stomatal limitation of photosynthesis
Non-stomatal limitation of photosynthesis
• decrease in gm
• decrease in carbonic anhydrase activity
• decrease in ATP formation
• decrease in carboxylation, decrease in amount and activity of RuBPC,
decrease in RuBP regeneration
• decrease in pigment content due to decrease in their synthesis and increase in
their degradation. Car are more stable that Chl. Importance of Car and
xanthophylls as defence against photoinhibition
• decrease in activities of photosystem 1 and 2 often in consequence of damage
of chloroplast ultrastructure. PS 2 usually more sensitive than PS 1 (PS 2 degradation and slow recovery of D1 protein). Indicators are changes in Chl a
fluorescence
Photoinhibition, leaf movements against photoinhibition
Limitation of photosynthesis by accumulation of photosynthates under decreased
transport
Gene expression, rbcS, rbcL
Effects of transient and permanent water stress (Monti et al. 2006)
Effect of water stress on parameters of Chl a fluorescence
WUE
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WUE = PN/E
1 mol CO2 per 300 - 500 (C3), 250 (C4) or 100 (CAM) mol of water
(DH2O/DCO2 = 1.7)
WUEm = M/E
WUE i („intrinsic“ WUE) = PN/gs
• Under mild stress WUE is usually increased, but under severe stress
WUE is often decreased
Relationship between transpiration rate and photosynthetic rate as affected
by irradiance
Methods for WUE determination
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Measurements of gas exchange
Carbon isotope discrimination
13C ‰ = (Rsample/Rstandard - 1)  1000, R = 13C/12C
13C for CO2 diffusion in air -7.8 ‰, for CO2 transport in cytoplasm -9.5 to
-17.7 ‰, Rubisco – 23.8 ‰, PEPC – 2.03 ‰
• Rubisco  PEPC, diffusion
• 13C for C3 plants -23 až -36 ‰, C4 -9 až -18 ‰, CAM -9 až -36 ‰
• 13C =  13Cair -  13Cleaf
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Farquhar et al. 1989: 13CP ‰ = 13Ca - a - (b - a) ci/ca,
where 13Ca - 13C in air, 13CP - 13C in plant, a - 13C connected with CO2 diffusion,
b - 13C during carbon fixation by RuBPC, ci/ca - internal and ambient CO2 concentration ratio
higher WUE  lower ci/ca  lower 13C
Carbon discrimination and WUE (Monti et al. 2006)
Methods for WUE determination
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Oxygen isotope discrimination (Barbour et al. 2002)
18O ‰ = (Rsample/Rstandard - 1)  1000, R = 18O/16O
18Oe = 18Os + * + k + (18Ov - 18Os - k) ea/ei
where 18Oe - 18O at site of evaporation, 18Os - 18O water source,
18Ov - 18O in air, * - decrease in water vapour tension due to heavier
isotope, k - fractionation during diffusion through air boundary layer
and stomata, ea/ei - ratio of ambient and internal water vapour
concentration
Sheshshayee et al. 2005
Global climate change
Elevation of CO2 concentration
Increased temperature
More often occurrence of drought
Different possible effects of predicted climate change
Effects of increased CO2 concentration
1) Short-term increase induces increase of PN
2) Long-term increase induces increase or decrease of PN
3) E and gs remain unchanged or decrease
4) WUE increases
5) Water consumption decreases or increases
Effects of elevated CO2 concentration
Effects of elevated CO2 concentration
Effects of elevated CO2 concentration
Erice et al. 2006
Antitranspirants
• 1) film-forming antitranspirants (e.g. polyvinyliden chloride, polyvinyl
chloride, polystyrene, polyethylene, polypropylene, silicon) – no
compound is more permeable for CO2 than for water
• 2) inhibitors of stomata opening (e.g. CO2, ABA, phenylmercuric
acetate) – often expensive or poisonous
• 3) compound increasing reflectance (e.g. kaolin)
• in all cases not only decrease in transpiration rate but also in
photosynthetic rate and growth, therefore practical use only in special
cases
Time and space scale of processes connected with stomata