other hand, under the high-absolute-humidity condition, the rate
of respiration decreased linearly to almost zero at 0.5 MPa with
increasing total pressure reviewed the effects of a
high partial pressure of CO2 on respiration. In that review, it was
indicated that the CO2 concentration on the internal side of a leaf
(Ci) is one of the factors that affect respiration, as expressed by the
following equation;
dCl
dt
= R
z
+ k
z
(Ca −
Cl), (1)
where Ca is the atmospheric CO2 concentration, R is the apparent
respiration rate, which is assumed to be constant, z is the leaf thickness,
and k is the leaf surface and boundary layer conductance for
CO2 diffusion. In this study, the atmospheric CO2 concentration (Ca)
and leaf thickness (z) are constant for the experimental time. The
value of k depends on the diffusion coefficient of CO2 (D). The value
of D is expressed as
D
=
D0
T
T0
2.0 P0
P
, (2)
where D0 is 0.138, T is the temperature, T0 is 273 K, P0 is 0.1 MPa
and P is the total pressure. This equation indicates that the diffusion
coefficient of CO2 (D) increases in proportion to the square of
temperature (T) and decreases with increasing total pressure (P).
As a result, the left-hand side of Eq. (1) decreases under elevated
total pressure, and the rate of respiration also decreases. However,
the rate of respiration increased with increasing pressure from
0.3 to 0.5 MPa under the low-absolute-humidity condition.