Direct Observations of Stomatal Movements
241
creasing CO2 supply, although stomatal aperture increased. In darkness
the respiratory rate was not affected by the decrease in ambient CO 2
concentration. Transpiration and stomatal conductivity, however, were
positively correlated with stomatal aperture (Fig. 11. 7).
Figure 11.8 illustrates to what extent CO2 deficiency overrules other
influences such as low air humidity. At normal CO2 concentration, stomata
closed significantly if the water vapor pressure difference between ambient
and leaf was increased (Fig. 11.6). At a CO2 concentration of 177 ppm,
stomata were closed only very slightly and showed a distinct transient phase
with at least two oscillations (Fig. 11.8). These were also apparent after the
CO2 concentration returned to normal. The stomatal response had little
effect on the CO2 uptake rate of the leaf (Fig. 11.8) but transpiratory water
loss may have been maintained at a high level (Fig. 11.7). Thus, CO 2
deficiency causes an unfavorable PIT ratio.
11.4.3 Response to Heat
Stomatal aperture and evaporation conditions can independently cause
variations in transpiration rates (Schulze et al. 1972). Another factor is
temperature. Our direct method confirms that stomata have a tendency to
open wider at high leaf temperatures. At superoptimal temperatures (in
Vicia [aba above 35°q, stomatal aperture was significantly increased, as
was transpiration rate. In Fig. 11.9, stomatal conductance is calculated using
the formula of Parlange and Waggoner (1970) based on the geometric
changes of the stomatal aperture (length, width, and deepness of the stomatal pore). The values correspond well to the measured transpiration rate,
but not to leaf conductance calculated from water relations in the chamber
by means of the formula given by Ball (1987). The discrepancy between
these different ways of determining stomatal conductance was also demonstrated in an experiment with 50 stomata (Vanselow 1990). These observations may stimulate further investigation of the parameters used in the Ball
formula.
11.4.4 The Transient Phase and Other Pecularities of the Stomatal
Response
The transient phase as a stomatal response "in a direction opposite to the
final response" (Raschke 1970) was early recognized in experiments with
leaves or leaf sections in osmotic solutions. Mohl (1856) already interpreted
it as an antagonism between guard cells and epidermal cells, and Stalfelt
(1929) termed it a hydropassive response. Hydropassive movements in
response to air humidity changes were considered to be induced by extremely rapid humidity changes (Maier-Maerker 1979b) and thus to be
241
creasing CO2 supply, although stomatal aperture increased. In darkness
the respiratory rate was not affected by the decrease in ambient CO 2
concentration. Transpiration and stomatal conductivity, however, were
positively correlated with stomatal aperture (Fig. 11. 7).
Figure 11.8 illustrates to what extent CO2 deficiency overrules other
influences such as low air humidity. At normal CO2 concentration, stomata
closed significantly if the water vapor pressure difference between ambient
and leaf was increased (Fig. 11.6). At a CO2 concentration of 177 ppm,
stomata were closed only very slightly and showed a distinct transient phase
with at least two oscillations (Fig. 11.8). These were also apparent after the
CO2 concentration returned to normal. The stomatal response had little
effect on the CO2 uptake rate of the leaf (Fig. 11.8) but transpiratory water
loss may have been maintained at a high level (Fig. 11.7). Thus, CO 2
deficiency causes an unfavorable PIT ratio.
11.4.3 Response to Heat
Stomatal aperture and evaporation conditions can independently cause
variations in transpiration rates (Schulze et al. 1972). Another factor is
temperature. Our direct method confirms that stomata have a tendency to
open wider at high leaf temperatures. At superoptimal temperatures (in
Vicia [aba above 35°q, stomatal aperture was significantly increased, as
was transpiration rate. In Fig. 11.9, stomatal conductance is calculated using
the formula of Parlange and Waggoner (1970) based on the geometric
changes of the stomatal aperture (length, width, and deepness of the stomatal pore). The values correspond well to the measured transpiration rate,
but not to leaf conductance calculated from water relations in the chamber
by means of the formula given by Ball (1987). The discrepancy between
these different ways of determining stomatal conductance was also demonstrated in an experiment with 50 stomata (Vanselow 1990). These observations may stimulate further investigation of the parameters used in the Ball
formula.
11.4.4 The Transient Phase and Other Pecularities of the Stomatal
Response
The transient phase as a stomatal response "in a direction opposite to the
final response" (Raschke 1970) was early recognized in experiments with
leaves or leaf sections in osmotic solutions. Mohl (1856) already interpreted
it as an antagonism between guard cells and epidermal cells, and Stalfelt
(1929) termed it a hydropassive response. Hydropassive movements in
response to air humidity changes were considered to be induced by extremely rapid humidity changes (Maier-Maerker 1979b) and thus to be