TARGAS-1 Operation Manual V. 1.02 77 support@ppsystems.com
g
(
mmol m
‐
s
‐
)
=
1
r
× 10
mmol
mol
Net Photosynthesis
Determine the rate of net photosynthesis (A) from the difference between CO
2
concentrations entering
(C
in
) and exiting (C
out
) the cuvette
(A.13) IRGA CO
2
readings are corrected for water vapor, temperature, and atmospheric pressure. The
addition of transpirational water vapor dilutes the air leaving the cuvette (
C
), and this is compensated
for in the calculation:
A =
(
C
× W
)
‐
[
C
×
(
W + E
)
]
(A.14) To calculate net CO
2
assimilation we rearrange equation (A.13) to:
A = ‐
(
C
‐C
)
× W
+
(
C
× E
)
TARGAS-1 calculates and displays the CO
2
difference (C
‐C
). As related to the calculated values in
the TARGAS-1 display:
C
= CO2a
C
‐C
= CO2d
Intercellular CO
2
Concentration
Calculate CO
2
concentration in the sub-stomatal cavity (C
) using the equation derived by von
Caemmerer & Farquhar, 1981
(A.15) The sub-stomatal CO2 concentration,
C
, is given by:
C
(
mol mol
‐
)
=
g
‐
× C
‐A
g
+
(A.16) Where g
is the total conductance to CO
2
transfer:
g
(
mmol m
‐
s
‐
)
=
1
(
1.585 × r
)
+
(
1.37 × r
)
× 10
(1.585 is the diffusion ratio of CO
2
and water in air, and 1.37 is the diffusion ratio of CO
2
and water in the
boundary layer).
Note: These calculations are based on the following assumptions:
• the leaf is exposed on both upper and lower leaf surfaces
• the upper and lower boundary layer resistances are similar
• stomata are evenly distributed on both upper and lower leaf surfaces.
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