TARGAS-1 Operation Manual V. 1.02
153
(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 (
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
), and this is compensated
for in the calculation:
𝑨𝑨
= (
𝑪𝑪
𝒊𝒊𝒊𝒊
×
𝑾𝑾
)
−
[
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
× (
𝑾𝑾
+
𝑬𝑬
)]
(A.14)
To calculate net CO
2
assimilation we rearrange equation (
A.13
) to:
𝑨𝑨
=
−��
(
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
− 𝑪𝑪
𝒊𝒊𝒊𝒊
) ×
𝑾𝑾�
+ (
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
×
𝑬𝑬
)
�
TARGAS-1 calculates and displays the CO
2
difference (
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
− 𝑪𝑪
𝒊𝒊𝒊𝒊
). As related to the calculated values
in the TARGAS-1 display:
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
=
𝑪𝑪𝑯𝑯𝟐𝟐𝒂𝒂
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
− 𝑪𝑪
𝒊𝒊𝒊𝒊
=
𝑪𝑪𝑯𝑯𝟐𝟐𝑪𝑪
Intercellular CO
2
Concentration
Calculate CO
2
concentration in the sub-stomatal cavity (
𝑪𝑪
𝒊𝒊
) using the equation derived by von
Caemmerer & Farquhar, 1981
(A.15)
The
sub-stomatal CO2 concentration,
𝑪𝑪
𝒊𝒊
, is given by:
𝑪𝑪
𝒊𝒊
�𝝁𝝁𝒎𝒎𝒎𝒎𝒎𝒎
𝒎𝒎𝒎𝒎𝒎𝒎
−𝟏𝟏
�
=
��𝒈𝒈
𝒄𝒄
−
𝑬𝑬
𝟐𝟐
�
×
𝑪𝑪
𝒎𝒎𝒐𝒐𝒐𝒐
� − 𝑨𝑨
�𝒈𝒈
𝒄𝒄
+
𝑬𝑬
𝟐𝟐
�
(A.16)
Where
𝒈𝒈
𝒄𝒄
is the total conductance to CO
2
transfer:
𝒈𝒈
𝒄𝒄
�𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎
𝒎𝒎
−𝟐𝟐
𝒔𝒔
−𝟏𝟏
�
=
�
𝟏𝟏
(
𝟏𝟏
.
𝟓𝟓𝟓𝟓𝟓𝟓
×
𝑯𝑯
𝒔𝒔
) + (
𝟏𝟏
.
𝟑𝟑𝟐𝟐
×
𝑯𝑯
𝒃𝒃
)
�
×
𝟏𝟏𝟎𝟎
𝟑𝟑
(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.