As to the Mode of Action of the Guard Cells in Dry Air
221
point A in Fig. 10.5. The dynamics of control also need consideration. If
one attempts to maintain the volume of a balloon constant, during the
process of inflating it in the normal way, the result is far from perfect; it is
almost impossible to prevent erratic fluctuations. The task becomes much
easier if the passageway into the balloon is constricted so that the influx or
efflux of air encounters an increased resistance. Herein lies an indication of
the second requirement for stability. The speed with which external pressure
is caused to vary with reference to observations of volume must exceed the
speed of volume change in response to change of external pressure.
As with air pressure and a balloon, so perhaps with water potential and
the guard cell. The source of water is in the guard cell wall, and control of
water potential is provided by the influence of stomatal aperture on rate
of evaporation in the parts of the wall bordering the substomatal cavity.
Stability requires that the steady-state change in potential of water in the
wall due to a given change in aperture should exceed the change in potential
of water in the guard cell associated with that change in aperture. This is
fulfilled by the data in Fig. lO.4a; the inverse slope of each of the straight
lines is greater than the inverse slope of the curve at the point of intersection.
The second requirement is that the speed with which potential of water in
the wall responds to variation in stomatal aperture should exceed the speed
with which aperture responds to variation in potential in the wall. Insofar as
stomatal aperture became steady at each level of ambient humidity (see Fig.
10.1), it would seem that this condition was met also.
Through these arguments it can be appreciated that the direct humidity
response might satisfactorily be explained by a feedback control system,
provided the guard cells have the property that internal water potential
tends to increase with decrease in guard cell volume and stomatal aperture.
I am inclined to put the matter more positively and say that the observations
of Schulze et al. (1990) are evidence that guard cells do have that property.
To carry the description of the control system envisaged further really
requires mathematical treatment, which I hope to present in another article.
However, certain aspects, particularly the roles of solute and solute regulation in the guard cell, demand at least a qualitative commentary here.
If solute were conserved in the guard cell, so that osmotic pressure varied
inversely with volume, it would gravitate against dg/d'l' being negative, as
an examination of the equation in the preceding section makes clear. Indeed,
if the guard cell were to have the characteristic of Fig. 10.5, it would be
made intrinsically stable for the same reason that a closed, air-filled balloon
is stable, and the g('I') relationship in Fig. 10.4 could not be explained in
terms of the mechanical properties of the guard cell. To explain the relationship it would be necessary that the relative decrease in pressure with
relative increase in volume exceed unity (whereas it is approximately 113 in
a balloon). That would be achieved by an air-filled system of a balloon
connected to a considerably larger rigid container. The image need not
offend our sensibility of the shape of guard cells for, after all, the balloon is
221
point A in Fig. 10.5. The dynamics of control also need consideration. If
one attempts to maintain the volume of a balloon constant, during the
process of inflating it in the normal way, the result is far from perfect; it is
almost impossible to prevent erratic fluctuations. The task becomes much
easier if the passageway into the balloon is constricted so that the influx or
efflux of air encounters an increased resistance. Herein lies an indication of
the second requirement for stability. The speed with which external pressure
is caused to vary with reference to observations of volume must exceed the
speed of volume change in response to change of external pressure.
As with air pressure and a balloon, so perhaps with water potential and
the guard cell. The source of water is in the guard cell wall, and control of
water potential is provided by the influence of stomatal aperture on rate
of evaporation in the parts of the wall bordering the substomatal cavity.
Stability requires that the steady-state change in potential of water in the
wall due to a given change in aperture should exceed the change in potential
of water in the guard cell associated with that change in aperture. This is
fulfilled by the data in Fig. lO.4a; the inverse slope of each of the straight
lines is greater than the inverse slope of the curve at the point of intersection.
The second requirement is that the speed with which potential of water in
the wall responds to variation in stomatal aperture should exceed the speed
with which aperture responds to variation in potential in the wall. Insofar as
stomatal aperture became steady at each level of ambient humidity (see Fig.
10.1), it would seem that this condition was met also.
Through these arguments it can be appreciated that the direct humidity
response might satisfactorily be explained by a feedback control system,
provided the guard cells have the property that internal water potential
tends to increase with decrease in guard cell volume and stomatal aperture.
I am inclined to put the matter more positively and say that the observations
of Schulze et al. (1990) are evidence that guard cells do have that property.
To carry the description of the control system envisaged further really
requires mathematical treatment, which I hope to present in another article.
However, certain aspects, particularly the roles of solute and solute regulation in the guard cell, demand at least a qualitative commentary here.
If solute were conserved in the guard cell, so that osmotic pressure varied
inversely with volume, it would gravitate against dg/d'l' being negative, as
an examination of the equation in the preceding section makes clear. Indeed,
if the guard cell were to have the characteristic of Fig. 10.5, it would be
made intrinsically stable for the same reason that a closed, air-filled balloon
is stable, and the g('I') relationship in Fig. 10.4 could not be explained in
terms of the mechanical properties of the guard cell. To explain the relationship it would be necessary that the relative decrease in pressure with
relative increase in volume exceed unity (whereas it is approximately 113 in
a balloon). That would be achieved by an air-filled system of a balloon
connected to a considerably larger rigid container. The image need not
offend our sensibility of the shape of guard cells for, after all, the balloon is