Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 229

r s

Leaf water
potential
Figure 6.15 Schematic representation of the response of stomatal resistance to exter-
nal and internal factors (after various graphs in Willmer and Ficker, ( 1996 , and Oke,
1987 ). Note that the leaf water potential is negative, that is, larger potential means a
less negative value (less water stress).

T leaf

PA R

VPD CO 2

and the environment. The responses of stomata to internal and external factors 4 are
summarized as follows (the responses of the stomatal resistance are given schemat-
ically in Figure 6.15 ):

• Radiation . Stomata need to be open only if photosynthesis can occur. Hence, stomata
react on PAR. The stomatal reaction to light appears to be direct: the guard cells are sen-
sitive to (even low levels of) light. CAM plants form an exception with respect to this
response: they have their stomata closed during daylight.
• Temperature. There is a direct effect of temperature on the metabolism of the guard
cells: with increasing temperature the metabolic activity increases until an optimum tem-
perature is reached above which the activity decreases to prevent damage to the cells.
The increased metabolism causes the stomata to open. An indirect effect of temperature
is that temperature will increase respiration, which in turn will increase the internal CO 2
concentration in the leaf. This will cause the stomata to close. The optimum tempera-
ture (where the minimum resistance occurs) differs between plants but is of the order
of 20 °C to 40 °C. At the same time, the photosynthesis process (that affects A n in Eq.
( 6.29 )) is also temperature dependent.
• Atmospheric vapour deicit . The vapour pressure deicit is an expression of the dry-
ness of the air. If the air outside the leaves is very dry, the stomata are closed. Because
a high vapour pressure deicit causes a high transpiration rate, the response to vapour
pressure deicit could be interpreted as a response to high transpiration rates (Monteith,
1995 ). One proposed mechanism is that water evaporates from the cells in the leaves.
The resulting decrease in turgor then sets up a feedback loop that reduces stomatal aper-
ture to the extent that the turgor is restored. However, there are also indications that the
guard cells or cells in their vicinity react directly to the vapour pressure deicit (Willmer
and Fricker, 1996 ).
• Internal CO 2 concentration. The internal CO 2 concentration is a compromise between
the need for a high concentration to supply the photosynthesis process with sufi-
cient building material and the need for a low concentration (relative to the external

4 Here, these factors are presented independently, but in practice it is dificult to separate them, as some factors will
change simultaneously (Willmer and Fricker, 1996 ).