Internal Coordination of Plant Responses to Drought and Evaporational Demand
189
floated on kinetin or zeatin solutions. It was concluded, therefore, that a
continuous supply of cytokinins from the roots counteracted any stomatal
closing tendencies. This is the case of a negative message from the roots in
order to sustain maximal stomatal opening in well-watered plants. But
Davies et al. (1986) emphasized how little is known, even currently, about
the effects of phytohormones other than ABA on plant water relations.
Only few data exist about the involvement of kinetin in plant water stress
responses (e.g., Itai and Vaadia 1965, 1971; Bradford and Hsiao 1982), and
also the findings of Blackman and Davies (1984, 1985) on the effects of rootproduced cytokinins on stomatal opening have not been repeated in subsequent years. FuBeder et al. (1992) found no clear evidence of antagonistic
effects of cytokinins and ABA in field-grown almond trees. Only when
ABA was not limiting stomatal aperture, may cytokinins have affected
stomatal opening in the early morning. Munns and King (1988) postulated
that still other messenger substances, apart from ABA, are transported from
roots under soil water shortage to the leaves and cause stomatal closure.
They collected xylem sap from detopped pressurized wheat plants growing
in dry soil and fed this xylem sap into detached wheat leaves through their
cut edges. Stomatal closure occurred in these leaves even if ABA was
removed from the xylem sap prior to application. However, it was shown
later that this may have been due to specific experimental conditions, and
that ABA fed into the xylem sap caused a dose response on stomatal
aperture (Heckenberger 1993).
It has been further suggested (Gollan et al. 1992) that in addition to the
amount of phytohormones, the concentrations of mineral ions such as Ca 2 +
and nitrate will be affected by a reduced water uptake and transport through
the plant. Ions like Ca 2 + are known to be involved in the regulation of
stomatal apertures (e.g., De Silva et al. 1985; Atkinson et al. 1989). Such an
effect would belong to the third category suggested by Gowing et al. (1990),
the modification by drought of the rate of substances flowing in the transpiration stream.
From several investigations it has become clear meanwhile that higher
amounts of ABA are produced in roots experiencing water stress (e.g.,
Rivier et al. 1983; Robertson et al. 1985; Lachno and Baker 1986; Wartinger
et al. 1990). A long-distance transport of ABA from roots to shoots also has
been documented in several studies (e.g., Hartung 1977; Prochazka 1982),
and concentrations of ABA in the xylem sap have been measured (Munns
and King 1988; Zhang and Davies 1989a; Schurr et al. 1992). However,
attempts to correlate bulk leaf ABA levels with stomatal closure failed when
the roots were exposed to drought (Beardsell and Cohen 1975; Blackman
and Davies 1985; Zhang et al. 1987). This might be due to the fact that bulk
leaf ABA contents were determined rather than epidermal contents. Large
amounts of physiologically inactive ABA sequestered in the mesophyll
(Heilmann et al. 1980) will mask the detection of local epidermal ABA
changes which affect the guard cells.
189
floated on kinetin or zeatin solutions. It was concluded, therefore, that a
continuous supply of cytokinins from the roots counteracted any stomatal
closing tendencies. This is the case of a negative message from the roots in
order to sustain maximal stomatal opening in well-watered plants. But
Davies et al. (1986) emphasized how little is known, even currently, about
the effects of phytohormones other than ABA on plant water relations.
Only few data exist about the involvement of kinetin in plant water stress
responses (e.g., Itai and Vaadia 1965, 1971; Bradford and Hsiao 1982), and
also the findings of Blackman and Davies (1984, 1985) on the effects of rootproduced cytokinins on stomatal opening have not been repeated in subsequent years. FuBeder et al. (1992) found no clear evidence of antagonistic
effects of cytokinins and ABA in field-grown almond trees. Only when
ABA was not limiting stomatal aperture, may cytokinins have affected
stomatal opening in the early morning. Munns and King (1988) postulated
that still other messenger substances, apart from ABA, are transported from
roots under soil water shortage to the leaves and cause stomatal closure.
They collected xylem sap from detopped pressurized wheat plants growing
in dry soil and fed this xylem sap into detached wheat leaves through their
cut edges. Stomatal closure occurred in these leaves even if ABA was
removed from the xylem sap prior to application. However, it was shown
later that this may have been due to specific experimental conditions, and
that ABA fed into the xylem sap caused a dose response on stomatal
aperture (Heckenberger 1993).
It has been further suggested (Gollan et al. 1992) that in addition to the
amount of phytohormones, the concentrations of mineral ions such as Ca 2 +
and nitrate will be affected by a reduced water uptake and transport through
the plant. Ions like Ca 2 + are known to be involved in the regulation of
stomatal apertures (e.g., De Silva et al. 1985; Atkinson et al. 1989). Such an
effect would belong to the third category suggested by Gowing et al. (1990),
the modification by drought of the rate of substances flowing in the transpiration stream.
From several investigations it has become clear meanwhile that higher
amounts of ABA are produced in roots experiencing water stress (e.g.,
Rivier et al. 1983; Robertson et al. 1985; Lachno and Baker 1986; Wartinger
et al. 1990). A long-distance transport of ABA from roots to shoots also has
been documented in several studies (e.g., Hartung 1977; Prochazka 1982),
and concentrations of ABA in the xylem sap have been measured (Munns
and King 1988; Zhang and Davies 1989a; Schurr et al. 1992). However,
attempts to correlate bulk leaf ABA levels with stomatal closure failed when
the roots were exposed to drought (Beardsell and Cohen 1975; Blackman
and Davies 1985; Zhang et al. 1987). This might be due to the fact that bulk
leaf ABA contents were determined rather than epidermal contents. Large
amounts of physiologically inactive ABA sequestered in the mesophyll
(Heilmann et al. 1980) will mask the detection of local epidermal ABA
changes which affect the guard cells.