206
I.R. Cowan
water loss is influenced by atmospheric humidity deficit. It was a stimulus
for theory of optimal stomatal action in which the responses to light and
dryness of the atmosphere are equally important - the first in promoting
photosynthesis! and the second in prolonging the conditions in which photosynthesis can occur. However, this chapter is not so much to do with the
effect of the response to humidity on plant carbon and water economy
as with the mechanism; a mechanism which so far has defied conclusive
explanation. In this matter, also, the work of the Wiirzburg group is of the
utmost importance.
10.2 Two Seminal Experiments
Lange et al. (1971) caused stomata to close and open by decreasing and
increasing the humidity of air flowing over the outer surface of strips of
the lower epidermis of Polypodium vulgare and Valerianella locusta. They
were able to induce movements in a single stoma or a group of stomata by
the use of small jets of air of varying humidity. They found also that the
reaction of the stomata was influenced by the humidity of the air to which
the inner surface of the epidermis was exposed. When the inner surface of
the epidermis was put in contact with liquid water over its whole length, the
stomata did not respond to variation in ambient humidity. When a small
subepidermal space was created by introducing an air bubble only 2 mm in
diameter, then the stomata above reacted to change in ambient humidity.
The species that were used in this experiment are unusual in that, in intact
leaves, the lower epidermis is attached to the remainder of the leaf only at
the margin and main veins; the stomatal apparatus is particularly well suited
to act as a humidity sensor because there is minimal hydraulic contact with
the mesophyll.
The second experiment (Schulze et al. 1972) was one of a series with
cultivated and wild plants in the Negev Desert (Lange et al. 1975). The data
in Fig. 10.1 relate to single attached twigs of Prunus armeniaca enclosed in a
naturally illuminated cuvette in which ambient temperature was controlled
so as to keep leaf temperature constant. As the difference in humidity
between leaves and air was successively increased by decreasing ambient
humidity, the conductance of the stomata to vapor diffusion was caused to
decrease and the net rate of assimilation to decrease. In this respect, the
observations are unremarkable. What is remarkable is that each decrease
in conductance was so great that it caused the rate of transpiration to
decrease despite the increase in humidity difference between leaf and air.
1 If the sensitivity to light is described as a tendency for stomata to close when light
intensity is inadequate to sustain rapid photosynthesis, it becomes evident that it, also, is
an adaptation that conserves water.
I.R. Cowan
water loss is influenced by atmospheric humidity deficit. It was a stimulus
for theory of optimal stomatal action in which the responses to light and
dryness of the atmosphere are equally important - the first in promoting
photosynthesis! and the second in prolonging the conditions in which photosynthesis can occur. However, this chapter is not so much to do with the
effect of the response to humidity on plant carbon and water economy
as with the mechanism; a mechanism which so far has defied conclusive
explanation. In this matter, also, the work of the Wiirzburg group is of the
utmost importance.
10.2 Two Seminal Experiments
Lange et al. (1971) caused stomata to close and open by decreasing and
increasing the humidity of air flowing over the outer surface of strips of
the lower epidermis of Polypodium vulgare and Valerianella locusta. They
were able to induce movements in a single stoma or a group of stomata by
the use of small jets of air of varying humidity. They found also that the
reaction of the stomata was influenced by the humidity of the air to which
the inner surface of the epidermis was exposed. When the inner surface of
the epidermis was put in contact with liquid water over its whole length, the
stomata did not respond to variation in ambient humidity. When a small
subepidermal space was created by introducing an air bubble only 2 mm in
diameter, then the stomata above reacted to change in ambient humidity.
The species that were used in this experiment are unusual in that, in intact
leaves, the lower epidermis is attached to the remainder of the leaf only at
the margin and main veins; the stomatal apparatus is particularly well suited
to act as a humidity sensor because there is minimal hydraulic contact with
the mesophyll.
The second experiment (Schulze et al. 1972) was one of a series with
cultivated and wild plants in the Negev Desert (Lange et al. 1975). The data
in Fig. 10.1 relate to single attached twigs of Prunus armeniaca enclosed in a
naturally illuminated cuvette in which ambient temperature was controlled
so as to keep leaf temperature constant. As the difference in humidity
between leaves and air was successively increased by decreasing ambient
humidity, the conductance of the stomata to vapor diffusion was caused to
decrease and the net rate of assimilation to decrease. In this respect, the
observations are unremarkable. What is remarkable is that each decrease
in conductance was so great that it caused the rate of transpiration to
decrease despite the increase in humidity difference between leaf and air.
1 If the sensitivity to light is described as a tendency for stomata to close when light
intensity is inadequate to sustain rapid photosynthesis, it becomes evident that it, also, is
an adaptation that conserves water.