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a) Determine the temperature sensitivity
E
0
for this data set (taking the data from the
irst night as the reference).
b) Determine the soil respiration when the soil temperature is 17 °C.
6.5 Dry Matter Production
There are two fundamental reasons for a close relation between dry matter production
and water use by crop stands. First, both the processes of CO
2
assimilation and of H
2
O
transpiration strongly depend on radiant energy. Second, during water shortage both
processes are in the same way reduced by stomatal control.
At the beginning of the 20th century agricultural scientists started to search for the
relationship between water use and dry matter production. It turned out that experi-
ments with plants growing in deep pots or containers were much easier to control than
experiments in the ield. In the ield transpiration and evaporation could not be sepa-
rated and the soil water balance was dificult to determine. In containers, however,
evaporation could be prevented by covering the soil and an accurate soil water balance
could be derived. As an example,
Figure 6.16
shows the relationship between cumula-
tive transpiration and accumulated dry matter for Kubanka wheat. In the experiments
on which
Figure 6.16
is based, different amounts of water were applied that produced
the depicted different levels of biomass (Ehlers and Goss,
2003
). Other examples with
a proportional relation between transpiration and dry matter production are given by
Kirkham (
2005
).
In ield experiments soil evaporation is inevitably included in measured water use.
Figure 6.17
shows data on water use and biomass from northeast Germany. Forage
maize was grown in lysimeters with varying supplies of water, allowing determination
of the evapotranspiration (Mundel,
1992
). The intercept with the
x
-axis is 194 mm,
and might be viewed as an estimate of the cumulative soil evaporation in the presence
of plants.
We may deine the
transpiration eficiency
or
water productivity, WP
T
(kg m
-3
) as:
DM
WP
T
=
a
(6.34)
T
with
DM
a
the actual cumulative dry matter (kg m
-2
) and
T
a
the cumulative actual tran-
spiration (m). In the case of Kubanka wheat in the Great Plains (
Figure 6.16
)
WP
T
= 2.07 kg m
-3
, while in case of forage maize in Germany (
Figure 6.17
)
WP
T
= 8.93
kg m
-3
. There are two reasons for this striking difference of more than a factor 4: (1)
the atmospheric evaporative demand, which was much higher in the experiments with
Kubanka wheat; and (2) wheat is a C
3
crop and maize is a C
4
crop. At the leaf surface,
stomates control the gas exchange of CO
2
and H
2
O. The exchange of the two gases
through the stomatal openings is a diffusion process. In the case of CO
2
, the con-
centration in the atmosphere is fairly constant. However, in the case of H
2
O, the air
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