Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 329

similar. Stomata were completely closed at the incipience of xylem embolism in the leaf rachis.
Variations of E plant and leaf turgor pressure (P leaf ) were concurrent with bulk Ψ leaf . It is also clear
from this graph that stomata were completely closed at the incipience of leaf cell plasmolysis
(turgor loss point). The maintenance of xylem integrity and leaf turgor was closely associated
with stomatal closure during water stress in walnut [29]. Stomatal closure was rather pre‐
emptive in avoiding cavitation. This behavior might be explained by the potential for “cata‐
strophic xylem failure” [51]. There is a feedback between xylem conductance and xylem
pressure during cavitation. Cavitation decreases xylem conductance, which in turn decreases
xylem pressure and thus provokes more cavitation. Tyree and Sperry [51] and Jones and
Sutherland [63] have computed that catastrophic xylem failure occurs at the expense of some
xylem conductance and at a critical transpiration rate (E crit ) only slightly greater than the actual
maximum E. The hypothesis of a stomatal control of catastrophic xylem failure was evaluated
with a hydraulic model of a walnut tree explicitly taking into account the feedback between
xylem pressure and xylem conductance. Our simulations confirmed the results of Sperry et al
[64] and Comstock and Sperry [65]. Transpiration was maximized (E crit ) at the expense of all
conductance in the distal leaf rachis segment. E crit was therefore much higher than the actual
E plant . Using the same model, they have computed E plant provoking 1% (E 1PLC ) and 10% (E 10PLC )
loss of rachis conductance. The onset of tree water loss regulation occurred when E plant reached
E 1PLC and E plant tracked E 10PLC when plant conductance was further reduced. This model
suggests that the risk of catastrophic xylem failure was not associated with the stomatal
regulation in walnut. g s was not maximized at the expense of all xylem conductance. Rather,
xylem conductance was maximized at the expense of all g s . To experimentally validate these
computations, we have tried, without success, to feed stressed plants with fusiccocine, a drug
supposed to promote stomatal opening. The use of mutants lacking efficient stomatal regula‐
tion is probably a better way to test such hypotheses [66].
These experiments demonstrate that stomatal closure caused by soil drought or decreased air
humidity can be partially or wholly reversed by root pressurization [29].
3.7. Recovery of conductivity after drought-induced embolism
Recovery from drought-induced embolism is rarely reported in trees when the xylem has
experienced low water potentials. More often, the conductivity is restored only the following
year by the formation of a new ring of functional xylem. For tree species generating positive
xylem sap pressure in the roots during spring, like walnut, the recovery of conductivity is
partially achieved by flushing embolised vessels with pressurized sap and full recovery of the
transport ability occurs usually only after the new year ring has been developed [77]. Recovery
of xylem conductivity after embolism can occur during spring due to xylem pressure generated
by starch hydrolysis [78] or during transpiration, as has been reported for Laurus nobilis which
is able to recover despite predawn leaf water potential remaining as low as -1 MPa [81]. Similar
refilling events have been reported for a range of species [79-80]. Nevertheless, the reality of
such refilling of embolised vessels in transpiring trees is still a matter of debate and although
several models have been proposed to explain it, there is a clear need for further research in
this area [82]. Regardless of mechanism, embolism repair after drought remains a costly