As to the Mode of Action of the Guard Cells in Dry Air
213
ciple involved needs no discussion partly because it must be evident to
those in a discipline that has made practical use, extending from the yucca
hygrometer to the hair hygrograph, of the influence of absorbed water
on strain in biological materials; but more particularly because a recent
experiment has disproved the idea. Mott and Parkhurst (1991) broke the
nexus between humidity and transpiration rate by measuring stomatal conductance at different ambient vapor pressures in air and in helox (21 % O2
and 79% He, in which water vapor diffuses 2.3 times more redily than in
air). They showed that stomata were not at all affected by ambient vapor
pressure per se, but were very much affected by the evaporative potential -
this being the product of the relevant diffusion coefficient and the difference
in humidity across the leaf epidermis and boundary layer. The experiments
are made the more significant by the fact that one of the species used,
Phaseolus vulgaris, exhibited the direct humidity response; that is to say,
rate of transpiration declined with increase in evaporation potential.
It seems, then, that the mechanism of the direct humidity response must
share in the general turgor of the leaf to some extent, and indeed the
sequence of events that takes place when conditions affecting transpiration
in a leaf are varied is similar to that occurring after a sudden disturbance of
the water supply to the leaf, as Darwin observed. When rate of transpiration
is stimulated, there is often a transient opening - a "passive" movement due
to decrease in the "antagonism" of the epidermal cells. There is then a
closing movement, sometimes followed by fluctuations. The pattern shown
in Fig. 10.3 is very much like that seen by Raschke (1970) when the pressure
of water supplied to a detached leaf of Zea mays was suddenly reduced.
It is the nature of the closing movement that is little understood. When
the potential of water in the epidermis is caused to vary, there must undoubtedly result some efflux or influx of water in the guard cells. Are
the consequent volume changes sufficient, in normal circumstances, to
cause substantial stomatal movement? And if additional water movement is
brought about as a result of solute transport and change in the content of
osmotica in the guard cell, is that solute transport instigated directly by the
change in the state of water, as with a turgor pressure sensing mechanism
for example (Gutknecht 1968; Coster et al. 1970)? Or is it a response to the
changes in concentrations of ions in the guard cell, following the initial
hydromechanical volume change? It is perhaps worth noting that Stalfelt
(1955), in reexamining "hydroactive" closure of stomata, was uncertain
whether loss of solute preceded or followed the initial volume changes in
the guard cells, and, indeed, remarked on evidence that closure might
sometimes take place without any loss of solute at all. To the extent that
phytohormones initiate closure, effects on metabolism or membrane permeability come first. But when closure is initiated by change in the local
state of water we remain as uncertain today as was StaIfelt.
There are two reasons for thinking that the direct humidity response is
hydromechanical in origin. First, the response can be fast. Fanjul and Jones
213
ciple involved needs no discussion partly because it must be evident to
those in a discipline that has made practical use, extending from the yucca
hygrometer to the hair hygrograph, of the influence of absorbed water
on strain in biological materials; but more particularly because a recent
experiment has disproved the idea. Mott and Parkhurst (1991) broke the
nexus between humidity and transpiration rate by measuring stomatal conductance at different ambient vapor pressures in air and in helox (21 % O2
and 79% He, in which water vapor diffuses 2.3 times more redily than in
air). They showed that stomata were not at all affected by ambient vapor
pressure per se, but were very much affected by the evaporative potential -
this being the product of the relevant diffusion coefficient and the difference
in humidity across the leaf epidermis and boundary layer. The experiments
are made the more significant by the fact that one of the species used,
Phaseolus vulgaris, exhibited the direct humidity response; that is to say,
rate of transpiration declined with increase in evaporation potential.
It seems, then, that the mechanism of the direct humidity response must
share in the general turgor of the leaf to some extent, and indeed the
sequence of events that takes place when conditions affecting transpiration
in a leaf are varied is similar to that occurring after a sudden disturbance of
the water supply to the leaf, as Darwin observed. When rate of transpiration
is stimulated, there is often a transient opening - a "passive" movement due
to decrease in the "antagonism" of the epidermal cells. There is then a
closing movement, sometimes followed by fluctuations. The pattern shown
in Fig. 10.3 is very much like that seen by Raschke (1970) when the pressure
of water supplied to a detached leaf of Zea mays was suddenly reduced.
It is the nature of the closing movement that is little understood. When
the potential of water in the epidermis is caused to vary, there must undoubtedly result some efflux or influx of water in the guard cells. Are
the consequent volume changes sufficient, in normal circumstances, to
cause substantial stomatal movement? And if additional water movement is
brought about as a result of solute transport and change in the content of
osmotica in the guard cell, is that solute transport instigated directly by the
change in the state of water, as with a turgor pressure sensing mechanism
for example (Gutknecht 1968; Coster et al. 1970)? Or is it a response to the
changes in concentrations of ions in the guard cell, following the initial
hydromechanical volume change? It is perhaps worth noting that Stalfelt
(1955), in reexamining "hydroactive" closure of stomata, was uncertain
whether loss of solute preceded or followed the initial volume changes in
the guard cells, and, indeed, remarked on evidence that closure might
sometimes take place without any loss of solute at all. To the extent that
phytohormones initiate closure, effects on metabolism or membrane permeability come first. But when closure is initiated by change in the local
state of water we remain as uncertain today as was StaIfelt.
There are two reasons for thinking that the direct humidity response is
hydromechanical in origin. First, the response can be fast. Fanjul and Jones