As to the Mode of Action of the Guard Cells in Dry Air
217
stomata be associated with water loss from the inner wall of the guard cell
when closure causes the total rate of transpiration and, therefore, rate of
water loss from the inner wall of the guard cell to decrease? That is the
problem to be addressed if the observations of Lange and his associates
are to be explained without recourse to the hypothesis of peristomatal
transpiration.
10.4 Hypothesis
10.4.1 Feedback
If it is to be rate of transpiration from the leaf, or a proportion of that
rate, that influences stomatal function when stomatal aperture responds to
ambient humidity, then it is appropriate to replot the data of Schulze et al.
in the form of Fig. lO.4a. As rate of transpiration is the product of leaf
conductance and humidity difference, humidity differences are represented
in the figure as the inverse slopes of straight lines from origin to data points.
When humidity difference is increased, conductance decreases. At first rate
of transpiration increases but eventually, when humidity difference is large,
rate of transpiration decreases also.
That the potential of water in the leaf declines with increase in rate of
transpiration may be inferred from the variation in leaf water content (Fig.
lO.4b). Presumably the potential of water in the guard cells also declines -
although, for reasons that have been discussed, not necessarily to the same
extent. That is why the transpiration axis in Fig. 10.4 is oriented as it is; it is
taken to represent decreasing water potential in the guard cells, 'P, as
indicated at the base of the figure. The result is what may seem a very
strange variation of stomatal aperture with potential, one in which there is a
phase in which aperture increases while potential decreases. Of course,
increase in the rate of transpiration will cause decrease in potential of water
in the subsidiary cells also, and that will tend to open the stomatal pores.
However, while it may be that this enhances the peculiar characteristic of
Fig. lO.4a, I do not think it can be responsible for it.
Let it be supposed, instead, that what underlies the relationship of conductance to potential in Fig. lO.4a is a relationship between guard cell
volume, V, and turgor pressure, P. To deduce the second from the first
would require more information than is available but, provided conductance
is a monotonically increasing function, g = g(V), of volume, the general
characteristics of its shape are readily deduced. The transformation of the
potential axis in Fig. 10.4 to one representing pressure is given by
P = 'P + 11,
where 11 is the osmotic pressure within the guard cell. We do not know to
what extent solute content of the guard cells may have readjusted to the
217
stomata be associated with water loss from the inner wall of the guard cell
when closure causes the total rate of transpiration and, therefore, rate of
water loss from the inner wall of the guard cell to decrease? That is the
problem to be addressed if the observations of Lange and his associates
are to be explained without recourse to the hypothesis of peristomatal
transpiration.
10.4 Hypothesis
10.4.1 Feedback
If it is to be rate of transpiration from the leaf, or a proportion of that
rate, that influences stomatal function when stomatal aperture responds to
ambient humidity, then it is appropriate to replot the data of Schulze et al.
in the form of Fig. lO.4a. As rate of transpiration is the product of leaf
conductance and humidity difference, humidity differences are represented
in the figure as the inverse slopes of straight lines from origin to data points.
When humidity difference is increased, conductance decreases. At first rate
of transpiration increases but eventually, when humidity difference is large,
rate of transpiration decreases also.
That the potential of water in the leaf declines with increase in rate of
transpiration may be inferred from the variation in leaf water content (Fig.
lO.4b). Presumably the potential of water in the guard cells also declines -
although, for reasons that have been discussed, not necessarily to the same
extent. That is why the transpiration axis in Fig. 10.4 is oriented as it is; it is
taken to represent decreasing water potential in the guard cells, 'P, as
indicated at the base of the figure. The result is what may seem a very
strange variation of stomatal aperture with potential, one in which there is a
phase in which aperture increases while potential decreases. Of course,
increase in the rate of transpiration will cause decrease in potential of water
in the subsidiary cells also, and that will tend to open the stomatal pores.
However, while it may be that this enhances the peculiar characteristic of
Fig. lO.4a, I do not think it can be responsible for it.
Let it be supposed, instead, that what underlies the relationship of conductance to potential in Fig. lO.4a is a relationship between guard cell
volume, V, and turgor pressure, P. To deduce the second from the first
would require more information than is available but, provided conductance
is a monotonically increasing function, g = g(V), of volume, the general
characteristics of its shape are readily deduced. The transformation of the
potential axis in Fig. 10.4 to one representing pressure is given by
P = 'P + 11,
where 11 is the osmotic pressure within the guard cell. We do not know to
what extent solute content of the guard cells may have readjusted to the