Internal Coordination of Plant Responses to Drought and Evaporational Demand
191
transported to the leaves (Zhang and Davies, 1987). In isolated root tips of
Pisum and Commeiina, root tip water contents were reduced from 100 to
60% and the corresponding solute potential fell from -0.8 to -1.4 MPa.
Turgor was reduced from 0.3 to 0.1 MPa. The root tip ABA content rose
from 0.05 to more than 0.5 ng ABA mg- 1 dw. ABA increase with water
stress was even more pronounced in Commelina (from 0.2 to 1.2 ng ABA
mg-1dw). In both cases there was a threshold (0.2MPa for pea and 0.3MPa
for Commelina roots) when the ABA content increased drastically with
further tissue drought. In another study using maize as the experimental
plant, Zhang and Davies (1989b) demonstrated an increase of root ABA
contents progressing deeper into the soil as the profile dried down from the
surface. Gollan et al. (1989) showed that with a drought-induced increase of
root ABA content, a corresponding increase in xylem sap ABA concentration occurred, and Zhang and Davies (1989a) showed a log-linear relationship between the reduction of sunflower leaf conductance and the increase
of ABA concentration in the xylem sap. Despite all this evidence of ABA
effects on stomata, it must be made clear that the underlying mechanism is
still more complicated. Wartinger et al. (1990) found no relation between
ABA transport in the xylem and stomatal conductance during the course of
a day, but a threshold response of maximum conductance (Fig. 9.2).
There is sufficient evidence to show that with soil drought a greater
amount of ABA is produced by the roots and becomes transported in
the transpiration stream to the leaf epidermis. There it may cause some
degree of stomatal closure without necessarily involving the bulk leaf water
potential. However, several factors may interact with a direct and instantaneous response. ABA may be (1) sequestered into the mesophyll, (2)
metabolized in the mesophyll and in the epidermis (Heckenberger 1993),
and (3) loaded into the phloem and thus transported back to the root
(Schurr 1992a). (4) In addition, other substances being produced or taken
up by the roots under drought might be transported by the transpiration
stream (e.g., calcium ions, other phytohormones like kinetins) and act
synergistically with ABA in regulating plant water loss in a coordinated
fashion. Gollan et al. (1992) demonstrated that, in addition to Ca 2 + and
pol-, N0 3 - transport rate is reduced with soil water shortage (Fig. 9.3).
This might increase the acropetal transport of ABA by changing the xylem
pH (Schurr 1992a,b).
In addition, one cannot exclude the fact that turgor reduction of the
leaves will affect stomatal apertures hydraulically and that ABA synthesis or
release from storage compartments of the aerial parts of the plant occurs.
Zhang and Davies (1989a) observed that old leaves lost turgor first in a
drying cycle and that upon their wilting, xylem sap ABA concentrations rose
sharply. Thus, ABA coming from both roots and older leaves could intensify stomatal closure under drought. Finally, even a local feedback
accumulation of active ABA must be considered, since the guard cell acidification by external ABA acting upon the plasmalemma could effect a
191
transported to the leaves (Zhang and Davies, 1987). In isolated root tips of
Pisum and Commeiina, root tip water contents were reduced from 100 to
60% and the corresponding solute potential fell from -0.8 to -1.4 MPa.
Turgor was reduced from 0.3 to 0.1 MPa. The root tip ABA content rose
from 0.05 to more than 0.5 ng ABA mg- 1 dw. ABA increase with water
stress was even more pronounced in Commelina (from 0.2 to 1.2 ng ABA
mg-1dw). In both cases there was a threshold (0.2MPa for pea and 0.3MPa
for Commelina roots) when the ABA content increased drastically with
further tissue drought. In another study using maize as the experimental
plant, Zhang and Davies (1989b) demonstrated an increase of root ABA
contents progressing deeper into the soil as the profile dried down from the
surface. Gollan et al. (1989) showed that with a drought-induced increase of
root ABA content, a corresponding increase in xylem sap ABA concentration occurred, and Zhang and Davies (1989a) showed a log-linear relationship between the reduction of sunflower leaf conductance and the increase
of ABA concentration in the xylem sap. Despite all this evidence of ABA
effects on stomata, it must be made clear that the underlying mechanism is
still more complicated. Wartinger et al. (1990) found no relation between
ABA transport in the xylem and stomatal conductance during the course of
a day, but a threshold response of maximum conductance (Fig. 9.2).
There is sufficient evidence to show that with soil drought a greater
amount of ABA is produced by the roots and becomes transported in
the transpiration stream to the leaf epidermis. There it may cause some
degree of stomatal closure without necessarily involving the bulk leaf water
potential. However, several factors may interact with a direct and instantaneous response. ABA may be (1) sequestered into the mesophyll, (2)
metabolized in the mesophyll and in the epidermis (Heckenberger 1993),
and (3) loaded into the phloem and thus transported back to the root
(Schurr 1992a). (4) In addition, other substances being produced or taken
up by the roots under drought might be transported by the transpiration
stream (e.g., calcium ions, other phytohormones like kinetins) and act
synergistically with ABA in regulating plant water loss in a coordinated
fashion. Gollan et al. (1992) demonstrated that, in addition to Ca 2 + and
pol-, N0 3 - transport rate is reduced with soil water shortage (Fig. 9.3).
This might increase the acropetal transport of ABA by changing the xylem
pH (Schurr 1992a,b).
In addition, one cannot exclude the fact that turgor reduction of the
leaves will affect stomatal apertures hydraulically and that ABA synthesis or
release from storage compartments of the aerial parts of the plant occurs.
Zhang and Davies (1989a) observed that old leaves lost turgor first in a
drying cycle and that upon their wilting, xylem sap ABA concentrations rose
sharply. Thus, ABA coming from both roots and older leaves could intensify stomatal closure under drought. Finally, even a local feedback
accumulation of active ABA must be considered, since the guard cell acidification by external ABA acting upon the plasmalemma could effect a