CO2 moves from the air to the leaf to the chloroplast by diffusion (but really CO2 moves both directions)

At the same time, H2O vapor moves out of the leaf by diffusion (but really H2O vapor moves both directions)

Ca = external CO2 concentration

(note that this leaf has stomata only on the “abaxial” or bottom side. Some leaves also have stomata on the adaxial, or upper surface. Leaves with stomata on both sides are called “amphistomatous”)

Ci = internal CO2 concentration. This value can be measured (indirectly) with common gas exchange instruments

Some definitions ….

CO2 diffuses into leaves, moving “down” a concentration gradient

Ca = 370-400 ppm?

Typical CO2 concentration of a C3 plant at midday is about 270-300 ppm

The CO2 concentration at the site of fixation approaches “zero”

The diffusive movement of CO2 into and out of a leaf can be described by Fick’s Law:

Net flux = concentration * conductance

(a membrane or barrier with a “conductance” to substance “x” = gx)

Net flux of “x” = Fx

[xo] = concentration of “x” on the “outside” of “barrier”

[xi] = concentration of “x” on the “inside” of the “barrier”

Fx = ([xo] – [xi]) * gx

• Conductance is a PROPERTY of leaf, kind of analogous to its “porosity” to CO2 or H2O vapor. It is NOT a “rate”!!!

Conductance is the inverse of resistance. Both quantities are commonly used. The symbol “g” is commonly used for conductance, “r” for resistance

gH2O = conductance to water vapor

gCO2 = conductance to CO2

gs = stomatal conductance (usually to water vapor)

gl = total leaf conductance (usually to water vapor)

The units used for conductance and resistance can be very confusing -

Applying Fick’s Law to carbon assimilation :

Net C assimilation = (ca-ci) * gleaf

Or: Aleaf = ca (1- ci/ca) * gleaf

(Norman 1982; Franks & Farquhar 1999)

Factors affecting net assimilation (A) and stomatal conductance (gleaf):

• Vapor pressure deficit, D (that is related to the humidity of the air)

• Soil Moisture, • Temperature, T

Aleaf = ca (1- ci/ca) * gleaf

f(D, )f(T)

Factors affecting net assimilation (A) and stomatal conductance (gleaf):

• Vapor pressure deficit, D (that is related to the humidity of the air)

• Soil Moisture, • Temperature, Temperature, TT

Aleaf = ccaa (1- (1- ccii/c/caa)) * gleaf

f(D, )f(T)

Humidity and vapor pressure deficit

The portion of total air pressurethat is due to water vapor is

water vapor pressure (ea)

measured in kPa

When air has no extra capacityfor holding water, the vapor pressure

is termed:saturation vapor pressure

(es, units kPa)

Saturation vapor pressure is mostly a function of air temperature

When air temperature falls withouta change in water content, the

point of condensation is called thedew point temperature

Relative Humidityis the ratio between actual

vapor pressure (ea)and saturation vapor pressure (es)

RH = ea/es

Vapor Pressure Deficit (D)is the difference between saturation

vapor pressure (es)and actual vapor pressure (ea)

D = es - ea

Stomata respond to the vapor pressure deficit between leaf and air (D). Stomata

generally close as D increases and the response is often depicted as a nonlinear

decline in gs with increasing D.

(Breda et al. 2006) (Oren et al. 1999)

Sto

mat

a (c

anop

y) c

ondu

ctan

ce

D (kPa)

Rel

ativ

e co

nduc

tanc

eg l

eaf/g

leaf

-max

imum

D (kPa)

LnD (Vapor pressure deficit)

Vapor pressure deficit, D (kPa)

0

1

0

1

Relative conductancegleaf/gleaf-maximum

1

0

532 4

Relative conductancegleaf/gleaf-maximum

gleaf/gleaf-maximum= -0.6 LnD +1

0.6

gleaf/gleaf-maximum= 1

(Oren et al. 1999)

Stomata respond to the vapor pressure deficit between leaf and air (D). Stomata generally close as D increases and the

response is often depicted as a nonlinear decline in gs with increasing D.

If D <1, then gleaf/gleaf-max = 1 Aleaf/Aleaf-max = 1 / max = 1

If D > 1, then gleaf/gleaf-max= -0.6 LnD +1 Aleaf/Aleaf-max < 1 / max < 1

GPP = {f(D)f(f(TT)f()f() ) f(COf(CO22))}*APARAPAR = Aleaf/PAR

Aleaf = ca (1- ci/ca) * gleaf

Stomata respond to changes in soil moisture ( ). During water

shortage, when drops below ca. 0.2, gleaf declines gradually

down to very low values

Soil moisture, (m3 m-3)

0.1 0.30.2 0.4

Modified after Breda et al. (2006)

Soil moisture, (m3 m-3)

0

1

0.1 0.50.30.2 0.4

gleaf/gleaf-maximum = s +bRelative conductancegleaf/gleaf-maximum

gleaf/gleaf-maximum = 1

s

0

1

0.1 0.50.30.2 0.4

0.2

0.08

Soil moisture, (m3 m-3)

Relative conductancegleaf/gleaf-maximum

If > 0.2, then gleaf/gleaf-max = ? Aleaf/Aleaf-max = ? / max = ?

If < 0.2, then gleaf/gleaf-max= ? Aleaf/Aleaf-max < ? / max < ?

GPP = {f(f(DD)f()f(TT)f(CO)f(CO22))f()}*APARAPAR

= Aleaf/PARAleaf = ca (1- ci/ca) * gleaf

Stomata respond to changes in soil moisture ( ). During water shortage, when drops below ca. 0.2, gleaf declines gradually down

to very low values

Factors affecting net assimilation (A) and stomatal conductance (gleaf):

• Vapor pressure deficit, Vapor pressure deficit, DD (that is related to (that is related to the humidity of the air)the humidity of the air)

• Soil Moisture, Soil Moisture, • Temperature, T

Aleaf = ccaa (1- ci/ca) * * ggleafleaf

f(D, )f(T)

Temperature effect on Ci/Ca and on net assimilation

Ci : Typical CO2 concentration is about 270-300 ppm

Ca = external CO2 concentration (Ca = 380-400 ppm?)

Temperature (C)

0

A/Amax

Ci/Ca

5 3020 40

Temperature (C)

0

0.6

5 3020 40

1

Warren and Dreyer (2006)

If T <20C or T> 30 C, then ci/ca = ? Aleaf/Aleaf-max = ? / max = ?

If 20 C<T <30C, then ci/ca = ? Aleaf/Aleaf-max = ? / max = ?

GPP = {f(f(DD))f(T)f(CO2)f(f())}*APARAPAR

= Aleaf/PARAleaf = ca (1- ci/ca) * gleaf

ci/ca respond to changes in temperature (T). Under low or high T, ci/ca increases gradually

to high values

Next week’s assignment:1) Using clumping indexes, LAI and values for a conifer stand (Loblolly pine forest, Duke Univ.) and for a Eucalyptus plantation (New Zealand), calculate their Monthly GPP (potential GPP).

- Loblolly pine: = 2.37 gC MJ-1 APAR

- Eucalyptus plantation: = 3.85 gC MJ-1 APAR

2) Assuming that all of the above parameters vary by plus or minus 20%, calculate how Annual GPP would be affected for each forest type.

GP

P

-20% +20%LAI

, Clumping =constant

GP

P

-20% +20%Clumping

, LAI =constant

GP

P

-20% +20%

Clumping, LAI =constantG

PP

-20% +20%LAI

, Clumping =constant

GP

P

-20% +20%Clumping

, LAI =constant

GP

P

-20% +20%

Clumping, LAI =constant

Lobl

olly

pin

e

Euc

alyp

tus

ReferencesBreda N. et al. 2006. Temperate forest trees and stands under severe drought: a review. Annals of Forest Science. 63:625-644.

Dye, P.J. et al. 2004. Verification of 3-PG growth and water-use predictions in twelve Eucalyptus plantation stands in Zululand, South Africa. For. Ecol. Management. 193:197–218

Franks PJ, Farquhar GD. 1999. A relationship between humidity response, growth form and photosynthetic operating point in C3 plants. Plant, Cell Environment 22:1337–1349.

Norman J. M. 1982. Simulation of microclimates, in Biometeorology in integrated pest management, edited by J. L. Hatfield and I. J. Thomason, p. 65-99, Academic, New York.

Oren R. et al. 1999. Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant, Cell and Environment 22: 1515-1526

Waring W.H. and S.W. Running 1998. Forest ecosystem analysis at multiple scales. 2nd Ed. Academic press. San Diego, CA 370p.

Warren C.R. and E. Dreyer. 2006. Temperature response of photosynthesis and internal conductance to CO2: results from two independent approaches. Journal of Experimental Botany 57:3057-3067.