Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 328

P rachis leveled at approximately -1.4Mpa. At this point, the degree of xylem embolism in the leaf
rachis was still less than 10 PLC [29].
The relationship between cavitation vulnerability and climate has been investigated in several
tree species. Conifer seedlings originating from the most mesic populations were found to be
the most susceptible to water-stress-induced cavitation [76]. Walnuts, native to dry zones, are
less susceptible to drought-induced cavitation than species native to well-watered areas [75].
3.6. Evidence for a stomatal control of xylem embolism in walnut
Effect of stomatal closure is to maintain P rachis above a threshold value around -1.4 MPa and
Ψ leaf above approximately -1.6 MPa. To further understand this behavior, it is necessary to
identify a major physiological trait associated with a stomatal closure that would threaten plant
integrity at lower P rachis and/or Ψ leaf values [74]. The answer to this question is obviously very
complex, because many traits are probably involved and correlations between them probably
exist [29; 213]. Cochard et al argues that, xylem cavitation is correlated with the stomatal
closure [29; 70-73]. A physiological trait associated with a stomatal closure during water stress
should meet at least the following three main conditions. First, its impairment should represent
a serious threat to plant functioning. This results from the consideration that the reduced
carbon gain, reduced growth, reduced reproductive success, etc. So the gain associated with
the regulation should overcome the loss. Cavitation is a serious threat for plants because it
impairs the xylem conductive capacity and may eventually lead to leaf desiccation and branch
mortality [95]. Indeed, leaf desiccation was not observed in some studies as long as the xylem
integrity was maintained. Leaf desiccation was noticed only when high levels of embolism
were measured in the leaf petioles [29]. The gain associated with stomatal closure was thus
the maintenance of leaf vitality, which largely overcomes the drawbacks cited above.
The second condition is that the impairment of the trait should be water deficit dependent
because the effect of stomatal closure is precisely to prevent excessive leaf dehydration. The
mechanism of water stress-induced cavitation has been well documented [138]. Air is sucked
into the xylem lumens through pores in the pit wall when pressures in the sap exceed the
maximum capillary pressures that can sustain the pores. Therefore, the likelihood of cavitation
occurrence is directly determined by the degree of water deficit in the xylem, more precisely
by P rachis . The maintenance of leaf turgor above cell plasmolysis is another physiological trait
that might also satisfy these first two conditions.
The third condition is that the impairment of the trait should have the same water deficit
dependence as stomata. Stomata were completely closed in walnut trees when P rachis reached
about approximately -1.4 MPa and Ψ leaf about approximately -1.6 MPa. The impairment of the
trait associated with stomatal closure should therefore occur at comparable P rachis or Ψ leaf values.
The leaf rachis was the most vulnerable organ along the sap pathway in the xylem and was
also exposed to the lowest xylem pressure values. Leaf rachis is therefore the Achilles' heel of
the walnut tree sap pathway. Segmentation in xylem vulnerability to cavitation has been
demonstrated for several other species [29; 94]. A lot of variation exists between species, and
occasionally the roots appear to be the most cavitation sensitive organs in the plant [96]. The
dependencies of leaf rachis xylem embolism and transpiration on water deficit were very