Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 322

altered, because the level of xylem embolism remained low during these experiments.
However, if the ionic composition of the sap changed dramatically as a result of the drought,
then R shoot may have varied [37]. Stem pressurization provoked only an increase in R shoot when
the pressure exceeded the point of embolism induction (about 2.0 MPa) [89]. If the air was
propagated along the xylem flow path significantly beyond the injection point, R root and R leaf
(leaf hydraulic resistances) may also have being altered. Therefore, combining the results of
all these experiments, it is possible to determine whether g s and E plant (plant transpiration) were
responding specifically changes in Ψ soil , R soil , R root , and/or R shoot or not. Because air humidity,
air temperature, and light intensity were maintained constant in many such experiments, leaf
to air vapor deficits and leaf boundary layer conductance were also constant. Therefore, the
g s and E plant patterns corresponded in drought stress. The relationship between g s and hydraulic
parameters are likely to depend on these environmental conditions, contrary to the relation‐
ships with E plant [90].

The results showed that different experiments significantly reduced E plant and g s . Therefore,
the response of g s to Ψ soil , R root , R soil , and Rshoot was neither specific nor exclusive. An alter‐
native analysis of the problem is not to consider Ψ soil , R root , R soil , and R shoot individually but
rather to examine their combined effect on P rachis or Ψ leaf . The relationship between P, Ψ soil , R root ,
R soil , E plant , and g s under steady-state conditions is well described by the Ohm's law analogy [91]:

P rachis =Ψ soil - ( R soil +R root +R shoot ) .SFplant.g s .D

(1)

where SF plant is the plant leaf area and D the air vapor pressure deficit, two parameters that
remained constant during experiments. The gravity term and the xylem sap osmotic potential
are assumed negligible in equation 1. A similar relationship is obtained with Ψ leaf if we further
include the leaf blade hydraulic resistance. The dependency of g s or E plant on P rachis (water
pressure in the leaf rachis xylem) and Ψ leaf were similar whatever the experiments. This would
suggest that, g s were not correlated to changes in Ψ soil , R soil , R root , or R shoot per se but rather to
Prachis and/or Ψ leaf. An identical relationship was obtained between E plant and Cplant
(defined as [R soil + R root + R shoot ] -1). These results are in agreement with the finding of Saliendra
et al. [92], Sperry [93], and Hubbard et al. [94]. Many of the studies [29; 89; 92-94] concluded
that combining different experimental procedures, stomata were not responding to changes
in Ψ soil , R soil , R root , or R shoot per se but rather to their impact on P rachis or Ψ leaf [29].
Genetic variation in tissue water relations of black walnut under drought was studied in two
consecutive years by Parker and Pallardy [214]. Black walnut seedlings of some sources studied
in 1983 exhibited osmotic adjustment under drought in both leaves and roots. Significant
variation among sources in root tissue elasticity was also evident before drought, but was not
observed thereafter. Initial differences in osmotic potential at full saturation were not evident
at the point of turgor loss [214].

Walnuts close stomata under high leaf-to-air vapor pressure deficit (VPDl) or low leaf water
potential (Ψl) [61], preventing the stem water potential (Ψs) from becoming lower than -1.4
MPa, when cavitation occurs in the xylem [17; 62]. Hence walnut has been defined as a
“drought avoider” [61]. Daily course of Ψs and gas exchange was tested in previous studies