Internal Coordination of Plant Responses to Drought and Evaporational Demand
195
metabolized in the leaf. Feeding ABA into the xylem of sunflower leaves at
a known dose caused a transient depression of conductance (Fig. 9.4) and a
rapid recovery which was not caused by ABA export (as shown by phloem
blockage). Recovery was related to the light-dependent metabolization of
ABA in the leaf (Heckenberger 1993). The light-dependent metabolization
of ABA may also explain the significant relation between maximum conductance in the morning and xylem ABA and the contrasting lack of such
correlation during the course of a day (see Fig. 9.2; Wartinger et al. 1990).
9.4 Leaf Anatomy, Canopy Structure, and Stomatal Function
Water stress reduces cell and tissue growth by immediate hydraulic effects as
well as by phytohormones such as ABA (Bradford and Hsiao 1982). It has
been speculated that such nonhydraulic effects of soil drying reduce leaf
expansion. An obvious reduction in leaf growth of cereal species without
apparent change of leaf water potential was observed when roots were
exposed to soil compaction (Masle and Passioura 1987) or soil drying (Passioura 1988; Saab and Sharp 1989) due to possible nonhydraulic root signals.
Possible differences in water potential between the mature leaf tissue and
the basal meristematic region of the gramineoid leaves of these species were
not considered. However, Michelena and Boyer (1982) already showed that
osmotic adaptation occurs preferentially in the elongating region of maize
leaves, so that turgor is maintained there even at low water potentials.
However, also in these cases a decrease of leaf elongation during a drying
cycle was observed which apparently was due to a nonhydraulic effect.
Zhang and Davies (1990) found a negative log-linear relationship between
the rate of leaf growth and the concentration of ABA in the xylem of maize
and sunflower. They emphasized that the higher ABA concentrations under
drought treatments did not result simply from a reduced amount of water as
solvent for the acropetally transported ABA. Instead, extra ABA resulting
from soil drying is present in the xylem sap and is probably responsible for
the reduced leaf elongation.
Additional experiments are needed with dicots in which the leaf development differs from that in gramineoid leaves. However, sufficient evidence
emerges of morphogenetic effects of root messengers produced during soil
drought stress and possible influences of ABA on cell wall elasticity and
membrane conductivity (Glinka and Reinhold 1971; Kutschera and Schopfer
1986a,b). It is suggested that cell growth will be modified by phytohormonal
effects without changes in turgor resulting in greater or lesser rates of cell
and tissue enlargement. The mechanisms of these processes were reviewed,
e.g., by Aspinall (1986) and Barlow (1986).
Apparently, these growth processes result in plant foliage with a surface
area and a structure that correspond with the water availability and supply
capacity of the plant's root and shoot system. An example for such a
195
metabolized in the leaf. Feeding ABA into the xylem of sunflower leaves at
a known dose caused a transient depression of conductance (Fig. 9.4) and a
rapid recovery which was not caused by ABA export (as shown by phloem
blockage). Recovery was related to the light-dependent metabolization of
ABA in the leaf (Heckenberger 1993). The light-dependent metabolization
of ABA may also explain the significant relation between maximum conductance in the morning and xylem ABA and the contrasting lack of such
correlation during the course of a day (see Fig. 9.2; Wartinger et al. 1990).
9.4 Leaf Anatomy, Canopy Structure, and Stomatal Function
Water stress reduces cell and tissue growth by immediate hydraulic effects as
well as by phytohormones such as ABA (Bradford and Hsiao 1982). It has
been speculated that such nonhydraulic effects of soil drying reduce leaf
expansion. An obvious reduction in leaf growth of cereal species without
apparent change of leaf water potential was observed when roots were
exposed to soil compaction (Masle and Passioura 1987) or soil drying (Passioura 1988; Saab and Sharp 1989) due to possible nonhydraulic root signals.
Possible differences in water potential between the mature leaf tissue and
the basal meristematic region of the gramineoid leaves of these species were
not considered. However, Michelena and Boyer (1982) already showed that
osmotic adaptation occurs preferentially in the elongating region of maize
leaves, so that turgor is maintained there even at low water potentials.
However, also in these cases a decrease of leaf elongation during a drying
cycle was observed which apparently was due to a nonhydraulic effect.
Zhang and Davies (1990) found a negative log-linear relationship between
the rate of leaf growth and the concentration of ABA in the xylem of maize
and sunflower. They emphasized that the higher ABA concentrations under
drought treatments did not result simply from a reduced amount of water as
solvent for the acropetally transported ABA. Instead, extra ABA resulting
from soil drying is present in the xylem sap and is probably responsible for
the reduced leaf elongation.
Additional experiments are needed with dicots in which the leaf development differs from that in gramineoid leaves. However, sufficient evidence
emerges of morphogenetic effects of root messengers produced during soil
drought stress and possible influences of ABA on cell wall elasticity and
membrane conductivity (Glinka and Reinhold 1971; Kutschera and Schopfer
1986a,b). It is suggested that cell growth will be modified by phytohormonal
effects without changes in turgor resulting in greater or lesser rates of cell
and tissue enlargement. The mechanisms of these processes were reviewed,
e.g., by Aspinall (1986) and Barlow (1986).
Apparently, these growth processes result in plant foliage with a surface
area and a structure that correspond with the water availability and supply
capacity of the plant's root and shoot system. An example for such a