478
Ch. Korner
between predawn water potentials of -0.2 and -2.0MPa. At the latter
value gpO! reached 25% of gmax (still substantially higher than gmin)'
Seasonal courses of gpO! in mediterranean ecosystems are well documented
(e.g., Miller and Poole 1979; Roberts et al. 1981; Tenhunen et al. 1987a)
and mainly follow water availability. According to Tenhunen et al. (1987a),
gpO! declines fast between predawn water potentials of -0.3 and -0.7MPa
and thereafter remains at about 25% of gmax until very low values are
reached «3.0 MPa).
In woody plants from semi-arid or arid regions, gpO! is predominantly
controlled by water availability, but predawn water potential does not seem
to be a good indicator for gpO!' Schulze et al. (1980) found accumulative
predawn water potentials to be of better predictive value. It should be reemphasized that most plants in dry regions (except succulents) reduce their
leaf area in response to water stress. Leaf she ding and leaf withdrawal in
graminoids are thus by far the most important means of controlling plant
transpiration on a land area basis in these types of land coverage. As long as
leaves are active in drought-deciduous species, gpO! does not vary a lot (yet
diurnal courses may change). Also for continental dry grassland (steppe,
prairie) presence or absence of green leaf area is the main means of control
of water loss, and Monson et al. (1986) found very little variation in gpO!
during the active growing season. Relatively stable predawn water potentials
in the course of the season in a desert environment have been reported by
Nilsen et al. (1983). Accordingly, they found little seasonal variation in gpO!
in active leaves. In the humid tropics, seasonal variation in gpO! may not be
synchronized among species and may largely reflect flushing rhythms of
species or even individuals.
22.8.2 Short-Term and Diurnal Changes
A large number of diurnal courses of leaf diffusive conductance in the field
have been published from which response functions were distilled. Some
examples (rather than an exhaustive review) will be presented below. These
should serve as a source of information on typical response phenomena
likely to occur in the field.
22.8.2.1 Responses to Photon Flux Density (PFD)
Responses of gael to PFD generally follow saturation functions comparable
to the hyperbolic functions known for net photosynthesis. The two key
determinants of such functions are the dark minimum of g (glow) and the
PFD required to achieve gpO!' Leaf conductance in the dark varies considerably, and values may range from 5 to 30% of gpO!, with values between
Ch. Korner
between predawn water potentials of -0.2 and -2.0MPa. At the latter
value gpO! reached 25% of gmax (still substantially higher than gmin)'
Seasonal courses of gpO! in mediterranean ecosystems are well documented
(e.g., Miller and Poole 1979; Roberts et al. 1981; Tenhunen et al. 1987a)
and mainly follow water availability. According to Tenhunen et al. (1987a),
gpO! declines fast between predawn water potentials of -0.3 and -0.7MPa
and thereafter remains at about 25% of gmax until very low values are
reached «3.0 MPa).
In woody plants from semi-arid or arid regions, gpO! is predominantly
controlled by water availability, but predawn water potential does not seem
to be a good indicator for gpO!' Schulze et al. (1980) found accumulative
predawn water potentials to be of better predictive value. It should be reemphasized that most plants in dry regions (except succulents) reduce their
leaf area in response to water stress. Leaf she ding and leaf withdrawal in
graminoids are thus by far the most important means of controlling plant
transpiration on a land area basis in these types of land coverage. As long as
leaves are active in drought-deciduous species, gpO! does not vary a lot (yet
diurnal courses may change). Also for continental dry grassland (steppe,
prairie) presence or absence of green leaf area is the main means of control
of water loss, and Monson et al. (1986) found very little variation in gpO!
during the active growing season. Relatively stable predawn water potentials
in the course of the season in a desert environment have been reported by
Nilsen et al. (1983). Accordingly, they found little seasonal variation in gpO!
in active leaves. In the humid tropics, seasonal variation in gpO! may not be
synchronized among species and may largely reflect flushing rhythms of
species or even individuals.
22.8.2 Short-Term and Diurnal Changes
A large number of diurnal courses of leaf diffusive conductance in the field
have been published from which response functions were distilled. Some
examples (rather than an exhaustive review) will be presented below. These
should serve as a source of information on typical response phenomena
likely to occur in the field.
22.8.2.1 Responses to Photon Flux Density (PFD)
Responses of gael to PFD generally follow saturation functions comparable
to the hyperbolic functions known for net photosynthesis. The two key
determinants of such functions are the dark minimum of g (glow) and the
PFD required to achieve gpO!' Leaf conductance in the dark varies considerably, and values may range from 5 to 30% of gpO!, with values between