212
I.R. Cowan
how the guard cells could become "more flaccid than the rest of the leaf",
given some observations that suggested to him they must "share in the
general turgor of the leaf". He concluded his discussion of stomatal mechanism with the following remarks. "There is another possibility in which I
am inclined to believe, although I can give no positive evidence for it. The
guard cells may lose turgor spontaneously, i.e., not by simple evaporation,
but in response to a stimulus. And this may be the slight flaccidity of the rest
of the leaf."
Evidence for Darwin's idea was provided by Stalfelt (1929) when he
observed that stomata in Vicia faba began to close some 13 min after leaf
water content had been reduced by a particular small amount, and that the
movement, once begun, was not halted by resupplying the leaf with water.
He suggested that the movement was associated with loss of solute from
guard cells and called it "hydroactive" - as distinct from the more rapid, and
opposing movements caused by fluctuations of turgor in the neighbouring
epidermal cells which he called, somewhat inappropriately, "passive". Later
still, a possible stimulus of hydro active movements was identified. Little and
Eidt (1968) and Mittleheuser and van Stevenink (1969) demonstrated that
abscisic acid could cause stomata to close, and Wright and Hiron (1969)
showed that abscisic acid was formed in wilting leaves. The role of abscisic
acid as a messenger sensitizing guard cell metabolism to plant water stress
has been reviewed by Raschke (1987). More recently, the existence of yet
another signal, triggered by hydraulic conditions but not itself hydraulic,
causing stomatal movement has been established, although its character has
yet to be identified. It has been shown that the response of stomata to
declining soil water content is to some extent independent of the effect of
soil water content on the state of water in leaf tissue (Bates and Hall 1981;
Blackman and Davies 1985; Turner et al. 1985; Gollan et al. 1986).
The importance of these findings, in the context of this chapter, is that
stomata may respond to the water relations of plant tissues remote from the
guard cells without necessarily being in close hydraulic communication with
those tissues; and to the extent that the state of water in their own hydraulic
microenvironment is different from that in other parts of the plant, being
particularly affected by transpiration, they retain the capability of being
particularly sensitive to conditions promoting transpiration. This is rather
different from the suggestion by Darwin that a stimulus originating elsewhere in the leaf might account for the closure of stomata in dry air.
An extreme possibility is that the mechanism of the direct humidity
response is isolated from the hydraulic continuum within the plant altogether;
that the configuration of the guard cell wall is mechanically affected by the
relative humidity of the external microenvironment. It is an idea engendered
by observations (Ball et al. 1987) that the responses of stomata to variations
in vapor pressure and temperature can be more coherently treated as
though they were both manifestations of a sensitivity to relative humidity
rather than difference in absolute humidity between leaf and air. The prin-
I.R. Cowan
how the guard cells could become "more flaccid than the rest of the leaf",
given some observations that suggested to him they must "share in the
general turgor of the leaf". He concluded his discussion of stomatal mechanism with the following remarks. "There is another possibility in which I
am inclined to believe, although I can give no positive evidence for it. The
guard cells may lose turgor spontaneously, i.e., not by simple evaporation,
but in response to a stimulus. And this may be the slight flaccidity of the rest
of the leaf."
Evidence for Darwin's idea was provided by Stalfelt (1929) when he
observed that stomata in Vicia faba began to close some 13 min after leaf
water content had been reduced by a particular small amount, and that the
movement, once begun, was not halted by resupplying the leaf with water.
He suggested that the movement was associated with loss of solute from
guard cells and called it "hydroactive" - as distinct from the more rapid, and
opposing movements caused by fluctuations of turgor in the neighbouring
epidermal cells which he called, somewhat inappropriately, "passive". Later
still, a possible stimulus of hydro active movements was identified. Little and
Eidt (1968) and Mittleheuser and van Stevenink (1969) demonstrated that
abscisic acid could cause stomata to close, and Wright and Hiron (1969)
showed that abscisic acid was formed in wilting leaves. The role of abscisic
acid as a messenger sensitizing guard cell metabolism to plant water stress
has been reviewed by Raschke (1987). More recently, the existence of yet
another signal, triggered by hydraulic conditions but not itself hydraulic,
causing stomatal movement has been established, although its character has
yet to be identified. It has been shown that the response of stomata to
declining soil water content is to some extent independent of the effect of
soil water content on the state of water in leaf tissue (Bates and Hall 1981;
Blackman and Davies 1985; Turner et al. 1985; Gollan et al. 1986).
The importance of these findings, in the context of this chapter, is that
stomata may respond to the water relations of plant tissues remote from the
guard cells without necessarily being in close hydraulic communication with
those tissues; and to the extent that the state of water in their own hydraulic
microenvironment is different from that in other parts of the plant, being
particularly affected by transpiration, they retain the capability of being
particularly sensitive to conditions promoting transpiration. This is rather
different from the suggestion by Darwin that a stimulus originating elsewhere in the leaf might account for the closure of stomata in dry air.
An extreme possibility is that the mechanism of the direct humidity
response is isolated from the hydraulic continuum within the plant altogether;
that the configuration of the guard cell wall is mechanically affected by the
relative humidity of the external microenvironment. It is an idea engendered
by observations (Ball et al. 1987) that the responses of stomata to variations
in vapor pressure and temperature can be more coherently treated as
though they were both manifestations of a sensitivity to relative humidity
rather than difference in absolute humidity between leaf and air. The prin-