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I.R. Cowan
an analogy and not an explanation of guard cell mechanics. It serves the
purpose of emphasizing that there are some rather demanding requirements
relating to mechanics and structure if solute is conserved in the guard cell
and the direct humidity response is to be explained along the lines I postulate.
I am led to think, partly for that reason but also because of the protracted
nature of responses to humidity, that the guard cell movement is associated
with (though not driven by) active control of osmotic pressure. A simple
model is represented by the relationship d(IIV)/dt - II* -II, expressing a
continuing tendency for II to return to a magnitude II* which is dependent
on light and other factors affecting active solute transport and/or membrane
permeabilities but independent of guard cell volume. With control of this
nature, water potential would tend to vary as turgor pressure and therefore
solute would have no influence whatever on the shape of the steady-state
relationship between g and 'I' such as that in Fig. 10.4. However, osmotic
pressure would determine the position of the curve with respect to the 'I'
axis; the greater II*, the farther the curve would be placed to the left, and
therefore the larger the stomatal conductance corresponding to any given
humidity difference. And the dynamics of solute control would influence the
dynamics of change from one steady state to another. Following a step
change, a decrease say, in humidity difference, there would be a rapid loss
of water from the cells of the stomatal complex culminating in a quasi-steady
state in which stomatal aperture might be increased due to reduction in the
"antagonism" of the subsidiary cells. There would then be a slower loss of
water from the guard cells, limited in speed but not in magnitude, by the
rate at which the solute control system operates. The associated closure of
the stomata in this phase would be assisted by increased turgor in the
subsidiary cells.
The assumption that osmotic pressure is maintained more nearly constant
than it would be if solute were conserved relieves us of some difficulties
in interpreting Fig. lO.4a in terms of guard cell mechanics; but the necessity
of explaining how dP/dV might be negative remains. To invoke elastic
properties such as those of rubber will not suffice for several reasons, one of
which has to do with stability of guard cell shape. Although it might be
possible to construct a balloon which has the shape of a deflated guard cell
when deflated, and that of an inflated guard cell when fully inflated, it would
not retain the end-to-end symmetry of a guard cell during the phase when
pressure decreases with volume. Even a spherical balloon does not stretch
uniformly during inflation, as may readily be shown by drawing a grid on its
surface.
10.4.3 Piers and Vaults
The evolution, in the 12th and 13th centuries, of Gothic cathedrals of
successively lighter construction but wider vaults involved a development of
I.R. Cowan
an analogy and not an explanation of guard cell mechanics. It serves the
purpose of emphasizing that there are some rather demanding requirements
relating to mechanics and structure if solute is conserved in the guard cell
and the direct humidity response is to be explained along the lines I postulate.
I am led to think, partly for that reason but also because of the protracted
nature of responses to humidity, that the guard cell movement is associated
with (though not driven by) active control of osmotic pressure. A simple
model is represented by the relationship d(IIV)/dt - II* -II, expressing a
continuing tendency for II to return to a magnitude II* which is dependent
on light and other factors affecting active solute transport and/or membrane
permeabilities but independent of guard cell volume. With control of this
nature, water potential would tend to vary as turgor pressure and therefore
solute would have no influence whatever on the shape of the steady-state
relationship between g and 'I' such as that in Fig. 10.4. However, osmotic
pressure would determine the position of the curve with respect to the 'I'
axis; the greater II*, the farther the curve would be placed to the left, and
therefore the larger the stomatal conductance corresponding to any given
humidity difference. And the dynamics of solute control would influence the
dynamics of change from one steady state to another. Following a step
change, a decrease say, in humidity difference, there would be a rapid loss
of water from the cells of the stomatal complex culminating in a quasi-steady
state in which stomatal aperture might be increased due to reduction in the
"antagonism" of the subsidiary cells. There would then be a slower loss of
water from the guard cells, limited in speed but not in magnitude, by the
rate at which the solute control system operates. The associated closure of
the stomata in this phase would be assisted by increased turgor in the
subsidiary cells.
The assumption that osmotic pressure is maintained more nearly constant
than it would be if solute were conserved relieves us of some difficulties
in interpreting Fig. lO.4a in terms of guard cell mechanics; but the necessity
of explaining how dP/dV might be negative remains. To invoke elastic
properties such as those of rubber will not suffice for several reasons, one of
which has to do with stability of guard cell shape. Although it might be
possible to construct a balloon which has the shape of a deflated guard cell
when deflated, and that of an inflated guard cell when fully inflated, it would
not retain the end-to-end symmetry of a guard cell during the phase when
pressure decreases with volume. Even a spherical balloon does not stretch
uniformly during inflation, as may readily be shown by drawing a grid on its
surface.
10.4.3 Piers and Vaults
The evolution, in the 12th and 13th centuries, of Gothic cathedrals of
successively lighter construction but wider vaults involved a development of