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resistance increases through embolism in the xylem. The plant water content recovers at night,
equalizing to the soil water potential and allowing the plant to reach its highest water potential
just before dawn. Trees are even more sensitive to changes in atmospheric humidity [58-59],
however, and stomates close as the vapor pressure deficit between the leaf and the air increases
[57]. Hydraulic conductivity of the soil and root-soil contact is potentially important in limiting
water flux to roots in drying soil [60]. The xylem water potential necessary to induce this
cavitation varies widely among plants [48-49] and has been shown to correlate with the lowest
xylem water potentials normally experienced under natural conditions [50]. Plants tend to
control stomata such that the xylem water potential does not fall below cavitation inducing
pressures [51-52]. As soil moisture or humidity declines, either transpiration is reduced or leaf-
specific hydraulic conductivity is increased. In this way, plants balance the demand for
transpirational water loss and carbon uptake by leaves with allocation to root absorption or
stem-conducting tissue [53-54; 209-210]. There is only a modest negative relationship or trade-
off between the hydraulic conductivity and the susceptibility to drought cavitation for the
wild-land species that have been examined to date [55]. This may be because susceptibility to
cavitation is more a function of vessel and tracheid pit anatomy than conduit size [56].
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, the point at which cavitation occurs in the xylem [29]. Many species have been found to
operate very close to the point of embolism. Stomata controls both plant water losses and sap
pressure and thus may actively control the risk of xylem embolism [63].
Many hypotheses have been raised about xylem embolism and cavitation in walnut. Rsoil,
Rroot, Rshoot, and Ψsoil have been used to identify hydraulic parameters associated with
stomatal regulation during water stress and test the hypothesis that stomata control embolism
during water stress [29]. Clear hydraulic segmentation was reported in a few species like
walnut trees (
Juglans regia
) [212-213]. In these species, petioles disconnect the leaves from the
stem through massive cavitation during drought and avoid irreversible damage to perennial
parts of the tree. Nevertheless, this is not a general trend; some species showing more vulner‐
able twigs than petioles. Fewer data are available for root vulnerability than for branches but
roots were found to be less vulnerable. [47-48].
At elevated CO
2
, the decreased osmotic potential, symplasmic water fraction and rate of water
transport, increased the modulus of elasticity and no changes in the formation of xylem
embolism were found in tolerant walnut varieties [83; 213]. We postulate here that embolism
and cavitation are important factors which influence the tracheid volume in stressed environ‐
ments in walnut species [17].
3.4. Leaf water potential and branch xylem embolism at pre-dawn
Predawn leaf water potential varies by season with a significant difference in pre-dawn
embolism of walnut between dry and wet seasons. The pre-dawn embolism of walnut branches
was found to be 23.20% and 26.60% on 2 July and 15 August, respectively, higher than the
17.56% and 16.25% observed on 27 August and 6 October, respectively (Figure 3b) [17]. As
drought progressed, the water potential reached a minimum of −1.51 MPa on 15 August. After
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