Direct Observations of Stomatal Movements
243
Based on his measurements of the cuticular conductance of the leaf,
Meidner (1986) predicted transient phases in the stomatal movement induced by VPD changes. A stomatal response to an increase in VPD should
indicate a feedback mechanism through an induced imbalance that causes
turgor loss in the epidermal cells and an initial opening of the guard cells
before a new steady state is reached. Such a hydropassive response is
confirmed by the observations that the subsidiary cells of Tradescantia become visibly shriveled (Maier-Maerker 1979b; Kappen and Haeger 1991).
Nonami et al. (1990), although not referring directly to transient phases,
gave evidence by pressure probe neasurements that the stomatal opening is
mediated by cellular water transport from the mesophyll to the epidermis
and to the guard cells of Tradescantia virginiana. They also supported
Meidner's (1986) finding that the sensor for the humidity response is not
peristomatal transpiration as was postulated by Maier-Maerker (1979a).
Nonami et al. (1990) suggest that the hydraulic system is controlled by
the transpiratory water vapor loss from the mesophyll cells bordering the
stomatal cavity. Therefore one must conclude that such a feedback control
functions only when stomata are open; but if they close completely, they
cannot react to changed humidity. The transient phase as a part of stomatal
response was included by Assmann and Gershenson (1991) in their model
describing the water vapor responses of stomata.
The reduction or disappearance of the transient-phase oscillations in
response to changes of air humidity for a plant subjected to soil water stress
may be due to an accumulation of abscisic acid (Zhang and Davies 1989;
Gollan et al. 1989) and to a general loss of turgor.
Our observations show that the tendency of stomata to close is generally
strong in darkness, but a certain percentage of stomata remain open or are
only slightly closed (Tradescantia albifiora: 20%). An increase in VPD
forces these open stomata "to close in darkness. The pronounced stomatal
oscillation during the transient phase demonstrates that these movements
are only turgor-related and independent of metabolic processes. Once
stomata were closed in darkness, they could not be opened by air humidity
changes or even by a decrease in CO2 concentration (l77ppm CO 2 ) of the
ambient air. This observation and similar observations on stomata of white
regions of variegated Tradescantia leaves in light lead to the conclusion that
photosynthetic energy production is necessary to move the guard cells. This
energy may be provided also by the mesophyll.
The strong influence of CO2 deficiency in the air can be well demonstrated by our method. In light it overrides the humidity response of the
stomata and thus may cause a dangerous water loss while the CO2 uptake is
low. The pronounced transient phases with several oscillations show the
extent to which humidity and CO2 effects interact with each other, so the
humidity response is not absent, as was concluded by Raschke and Kiihl
(1969) from their experiment with maize. In darkness, stomata were not
forced to open by a decrease in the CO2 concentration to around 200 ppm
243
Based on his measurements of the cuticular conductance of the leaf,
Meidner (1986) predicted transient phases in the stomatal movement induced by VPD changes. A stomatal response to an increase in VPD should
indicate a feedback mechanism through an induced imbalance that causes
turgor loss in the epidermal cells and an initial opening of the guard cells
before a new steady state is reached. Such a hydropassive response is
confirmed by the observations that the subsidiary cells of Tradescantia become visibly shriveled (Maier-Maerker 1979b; Kappen and Haeger 1991).
Nonami et al. (1990), although not referring directly to transient phases,
gave evidence by pressure probe neasurements that the stomatal opening is
mediated by cellular water transport from the mesophyll to the epidermis
and to the guard cells of Tradescantia virginiana. They also supported
Meidner's (1986) finding that the sensor for the humidity response is not
peristomatal transpiration as was postulated by Maier-Maerker (1979a).
Nonami et al. (1990) suggest that the hydraulic system is controlled by
the transpiratory water vapor loss from the mesophyll cells bordering the
stomatal cavity. Therefore one must conclude that such a feedback control
functions only when stomata are open; but if they close completely, they
cannot react to changed humidity. The transient phase as a part of stomatal
response was included by Assmann and Gershenson (1991) in their model
describing the water vapor responses of stomata.
The reduction or disappearance of the transient-phase oscillations in
response to changes of air humidity for a plant subjected to soil water stress
may be due to an accumulation of abscisic acid (Zhang and Davies 1989;
Gollan et al. 1989) and to a general loss of turgor.
Our observations show that the tendency of stomata to close is generally
strong in darkness, but a certain percentage of stomata remain open or are
only slightly closed (Tradescantia albifiora: 20%). An increase in VPD
forces these open stomata "to close in darkness. The pronounced stomatal
oscillation during the transient phase demonstrates that these movements
are only turgor-related and independent of metabolic processes. Once
stomata were closed in darkness, they could not be opened by air humidity
changes or even by a decrease in CO2 concentration (l77ppm CO 2 ) of the
ambient air. This observation and similar observations on stomata of white
regions of variegated Tradescantia leaves in light lead to the conclusion that
photosynthetic energy production is necessary to move the guard cells. This
energy may be provided also by the mesophyll.
The strong influence of CO2 deficiency in the air can be well demonstrated by our method. In light it overrides the humidity response of the
stomata and thus may cause a dangerous water loss while the CO2 uptake is
low. The pronounced transient phases with several oscillations show the
extent to which humidity and CO2 effects interact with each other, so the
humidity response is not absent, as was concluded by Raschke and Kiihl
(1969) from their experiment with maize. In darkness, stomata were not
forced to open by a decrease in the CO2 concentration to around 200 ppm