9 Internal Coordination of Plant Responses
to Drought and Evaporational Demand
R. Losch and E.-D. Schulze
9.1 Introduction
Higher plants balance between desiccation by transpiration during periods
of CO2 uptake through open stomata and starvation during periods of
stomatal closure when preventing excessive water loss. Since fluxes of water
vapor and CO2 are linked by using the same pathway through the stomatal
aperture, the question of how plants regulate their water status is also of
significance to the ecophysiology of photosynthesis, the main theme of this
Volume.
Major advances in the understanding of stomatal regulation have been
made in the last decade. Initially, emphasis was given to the investigation of
the environmental response of gas exchange (Schulze and Hall 1982); however, it has become apparent meanwhile (Schulze 1986; Zhang and Davies
1990) that internal regulations are just as important for understanding
plant responses to drought in the soil and to evaporative demand in the
atmosphere. This internal regulation includes hormonal root-shoot signals,
as well as structural components in the leaf and in the stem.
In the following, we will briefly review stomatal responses to environmental stress, and we will then more explicitly explore physiological and
structural interactions between root and shoot as part of an internal coordination of regulating plant water relations.
9.2 Environmental and Plant-Internal Influences on Transpiration
Plants are the interface in the steep gradient of matter and energy exchange
within the soil-plant-atmosphere system. At lower levels of structural organization, poikilohydric plants depend passively upon their environment.
In contrast, higher plants are generally decoupled from the physically driven
fluxes of matter and energy so that permanent gradients of chemical substances and energy are established. By active metabolic processes, selectivity
in ion uptake can be achieved, resulting in quite different ion concentrations
in the soil and in plant compartments. By a highly water-impermeable
to Drought and Evaporational Demand
R. Losch and E.-D. Schulze
9.1 Introduction
Higher plants balance between desiccation by transpiration during periods
of CO2 uptake through open stomata and starvation during periods of
stomatal closure when preventing excessive water loss. Since fluxes of water
vapor and CO2 are linked by using the same pathway through the stomatal
aperture, the question of how plants regulate their water status is also of
significance to the ecophysiology of photosynthesis, the main theme of this
Volume.
Major advances in the understanding of stomatal regulation have been
made in the last decade. Initially, emphasis was given to the investigation of
the environmental response of gas exchange (Schulze and Hall 1982); however, it has become apparent meanwhile (Schulze 1986; Zhang and Davies
1990) that internal regulations are just as important for understanding
plant responses to drought in the soil and to evaporative demand in the
atmosphere. This internal regulation includes hormonal root-shoot signals,
as well as structural components in the leaf and in the stem.
In the following, we will briefly review stomatal responses to environmental stress, and we will then more explicitly explore physiological and
structural interactions between root and shoot as part of an internal coordination of regulating plant water relations.
9.2 Environmental and Plant-Internal Influences on Transpiration
Plants are the interface in the steep gradient of matter and energy exchange
within the soil-plant-atmosphere system. At lower levels of structural organization, poikilohydric plants depend passively upon their environment.
In contrast, higher plants are generally decoupled from the physically driven
fluxes of matter and energy so that permanent gradients of chemical substances and energy are established. By active metabolic processes, selectivity
in ion uptake can be achieved, resulting in quite different ion concentrations
in the soil and in plant compartments. By a highly water-impermeable