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

• 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 photosynthesisNon-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 photoinhibitionLimitation of photosynthesis by accumulation of photosynthates under decreased

transportGene 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

• 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

• 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

• 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

• 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

Erice et al. 2006

Effects of elevated CO2 concentration

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