As to the Mode of Action of the Guard Cells in Dry Air
215
in the content of potassium, thought to be the cation mainly responsible for
osmotic pressure, did take place, but followed, rather than accompanied,
stomatal movement. Of course, it seems inevitable that the processes affecting control of solute content in guard cells will be influenced by a change
in solute concentration consequent on change in guard cell volume (see
MacRobbie 1987), and they might well cause a readjustment enhancing and
prolonging the initial movements. Responses to humidity sometimes, and
perhaps usually, extend over longer periods of time than those observed
by Fanjul and Jones. In particular, reopening at high humidity following
closure at low humidity has been shown to be slow, as in Fig. 10.3 (see also
Kappen et al. 1987; Kappen and Haeger 1991), and it is difficult to imagine
that metabolism is not somehow involved.
10.3.3 Hydrology of the Epidermis
The evidence due to Stahl that the transpiration from the guard cells is
especially active was obtrained by using, as a tracer of water movement in
leaves, thallium supplied in solution to cut petioles as sulfate, and subsequently precipitated as thallium chloride following the supply of sodium
chloride. After short periods of uptake, the black crystals of thallium chloride
could be seen only in the guard cells. With successively longer periods they
became apparent in the epidermal cells neighboring the guard cells, and
then throughout the epidermis. The technique was a neat modification of
one employed by Schimper (1890) - the use of thallium sulfate as a reagent
to locate sodium chloride taken up by leaf tissue.
To explain the direct humidity response on the basis of evaporation from
the region of the guard cell, it is necessary to assume that the distributions
of the sites of evaporation and conductivities to water in the epidermis are
such that the draw-down in water potential is sufficiently greater in the
guard cells than the subsidiary cells in order that the "passive" action of
the subsidiary cells be overcome. There would seem to be no conceptual
impediment to that; nor to a difference in time constants for release or
uptake of water by the cells such that the initial response of the stomata to
increase in rate of transpiration is to open. All of this can be mimicked by
an electrical analog consisting of resistors and two capacitors representing
the pressure:volume relations of the cells (e.g., Cowan 1972). The critical
question is whether evaporation from the neighborhood of the guard cells is
under stomatal control or not.
Seybold (1962) supposed that transpiration in the vicinity of the stomatal
complex, peristomatal transpiration as he called it, takes place through the
external epidermal cuticle. Maercker (1965a,b, and as Maier-Maercker
1979) has provided evidence, and summarized that of others showing that
rate of water loss near the stomata is greater than that from other regions of
the epidermis, and has argued that it plays a decisive role in the stomatal
215
in the content of potassium, thought to be the cation mainly responsible for
osmotic pressure, did take place, but followed, rather than accompanied,
stomatal movement. Of course, it seems inevitable that the processes affecting control of solute content in guard cells will be influenced by a change
in solute concentration consequent on change in guard cell volume (see
MacRobbie 1987), and they might well cause a readjustment enhancing and
prolonging the initial movements. Responses to humidity sometimes, and
perhaps usually, extend over longer periods of time than those observed
by Fanjul and Jones. In particular, reopening at high humidity following
closure at low humidity has been shown to be slow, as in Fig. 10.3 (see also
Kappen et al. 1987; Kappen and Haeger 1991), and it is difficult to imagine
that metabolism is not somehow involved.
10.3.3 Hydrology of the Epidermis
The evidence due to Stahl that the transpiration from the guard cells is
especially active was obtrained by using, as a tracer of water movement in
leaves, thallium supplied in solution to cut petioles as sulfate, and subsequently precipitated as thallium chloride following the supply of sodium
chloride. After short periods of uptake, the black crystals of thallium chloride
could be seen only in the guard cells. With successively longer periods they
became apparent in the epidermal cells neighboring the guard cells, and
then throughout the epidermis. The technique was a neat modification of
one employed by Schimper (1890) - the use of thallium sulfate as a reagent
to locate sodium chloride taken up by leaf tissue.
To explain the direct humidity response on the basis of evaporation from
the region of the guard cell, it is necessary to assume that the distributions
of the sites of evaporation and conductivities to water in the epidermis are
such that the draw-down in water potential is sufficiently greater in the
guard cells than the subsidiary cells in order that the "passive" action of
the subsidiary cells be overcome. There would seem to be no conceptual
impediment to that; nor to a difference in time constants for release or
uptake of water by the cells such that the initial response of the stomata to
increase in rate of transpiration is to open. All of this can be mimicked by
an electrical analog consisting of resistors and two capacitors representing
the pressure:volume relations of the cells (e.g., Cowan 1972). The critical
question is whether evaporation from the neighborhood of the guard cells is
under stomatal control or not.
Seybold (1962) supposed that transpiration in the vicinity of the stomatal
complex, peristomatal transpiration as he called it, takes place through the
external epidermal cuticle. Maercker (1965a,b, and as Maier-Maercker
1979) has provided evidence, and summarized that of others showing that
rate of water loss near the stomata is greater than that from other regions of
the epidermis, and has argued that it plays a decisive role in the stomatal