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R. Losch and E.-D. Schulze
cutinized epidermis the homoiohydric plants keep their water relations at
more or less stable levels even in dry surroundings. By simultaneous root
water uptake and controlled stomatal transpiration they keep a persistent
but regulated water flow through the plant, driven by the water potential
gradient between soil and atmosphere. The evaporation of this water in the
leaves permits a partitioning of the energy absorbed by shoots and leaves
from the incoming radiation into sensible and latent heat. As a consequence,
shoot temperatures are at least partly decoupled from air temperatures and
remain in a range which is advantageous for overall metabolism and water
status (Lange 1959; Gates 1976). Ion distribution within the plant and
energy consumption by evaporation are linked intimately to the water
transport through the plant within the soil-plant-atmosphere pathway.
Apparently, a control of this transpiration stream through the plant is more
efficient at the interface where water molecules escape from the leaf, i.e., at
the stomata, than a regulation of root water uptake (Meinzer and Grantz
1990). Therefore, a complicated system of stomatal responses to the environmental parameters has developed during the evolution of cormophytic
plants which allows them to adapt their transpiration to the balance of
supply and demand (Schulze and Hall 1982) and to grow in different environments, from humid to extremely arid sites.
Transpirational water diffusion through the stomatal pores is driven
by the leaf-to-air vapor pressure difference and restricted by the variable
diffusion resistance of the stomatal pores. The cuticle acts as a parallel
resistor of high, constant resistance. The boundary layer resistance is connected in series with both the cuticular and the stomatal resistance. It is
dependent on plant and leaf size, but not on stomatal responses to the
environment. A variable stomatal resistance is mediated by the guard cell
responses to factors such as visible and thermal radiation, ambient humidity,
CO 2 content of the air, and sometimes various pollutants. Their effects have
been studied in great detail (Hall et al. 1976; Schulze and Hall 1982), and
successful attempts have been made to mathematically model stomatal
conductances and transpiration as function of these external factors (e.g.,
Losch and Tenhunen 1981; Kiippers and Schulze 1985; Losch et al. 1992).
Bulk plant water potential has been taken most often in such studies as a
term integrating the effects of internal plant factors, particularly water
shortage. Several findings, however, indicate that this parameter is a rather
incidental one, not linked directly with the stomatal responses to drought.
For example, use of this parameter alone often does not yield satisfactory
results in simulation approaches (Rosa et al. 1991; Losch et al. 1992). Longterm and short-term water stress effects were distinguished (Schulze and
Kiippers 1979), and with time it became evident that stomatal closure upon
water shortage is a completely metabolically governed process. Therefore,
the causal interpretation of drought effects upon stomatal apertures shifted
from the assumption of simple hydraulic effects (StaIfelt 1956) to the detection of a sophisticated interaction between altered plant functions by water
R. Losch and E.-D. Schulze
cutinized epidermis the homoiohydric plants keep their water relations at
more or less stable levels even in dry surroundings. By simultaneous root
water uptake and controlled stomatal transpiration they keep a persistent
but regulated water flow through the plant, driven by the water potential
gradient between soil and atmosphere. The evaporation of this water in the
leaves permits a partitioning of the energy absorbed by shoots and leaves
from the incoming radiation into sensible and latent heat. As a consequence,
shoot temperatures are at least partly decoupled from air temperatures and
remain in a range which is advantageous for overall metabolism and water
status (Lange 1959; Gates 1976). Ion distribution within the plant and
energy consumption by evaporation are linked intimately to the water
transport through the plant within the soil-plant-atmosphere pathway.
Apparently, a control of this transpiration stream through the plant is more
efficient at the interface where water molecules escape from the leaf, i.e., at
the stomata, than a regulation of root water uptake (Meinzer and Grantz
1990). Therefore, a complicated system of stomatal responses to the environmental parameters has developed during the evolution of cormophytic
plants which allows them to adapt their transpiration to the balance of
supply and demand (Schulze and Hall 1982) and to grow in different environments, from humid to extremely arid sites.
Transpirational water diffusion through the stomatal pores is driven
by the leaf-to-air vapor pressure difference and restricted by the variable
diffusion resistance of the stomatal pores. The cuticle acts as a parallel
resistor of high, constant resistance. The boundary layer resistance is connected in series with both the cuticular and the stomatal resistance. It is
dependent on plant and leaf size, but not on stomatal responses to the
environment. A variable stomatal resistance is mediated by the guard cell
responses to factors such as visible and thermal radiation, ambient humidity,
CO 2 content of the air, and sometimes various pollutants. Their effects have
been studied in great detail (Hall et al. 1976; Schulze and Hall 1982), and
successful attempts have been made to mathematically model stomatal
conductances and transpiration as function of these external factors (e.g.,
Losch and Tenhunen 1981; Kiippers and Schulze 1985; Losch et al. 1992).
Bulk plant water potential has been taken most often in such studies as a
term integrating the effects of internal plant factors, particularly water
shortage. Several findings, however, indicate that this parameter is a rather
incidental one, not linked directly with the stomatal responses to drought.
For example, use of this parameter alone often does not yield satisfactory
results in simulation approaches (Rosa et al. 1991; Losch et al. 1992). Longterm and short-term water stress effects were distinguished (Schulze and
Kiippers 1979), and with time it became evident that stomatal closure upon
water shortage is a completely metabolically governed process. Therefore,
the causal interpretation of drought effects upon stomatal apertures shifted
from the assumption of simple hydraulic effects (StaIfelt 1956) to the detection of a sophisticated interaction between altered plant functions by water