Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 323

of Rosati et al. [121]. Stem water potential (Ψs) decreased during the day and was lower in
droughted than in control trees [121]. The lowest average Ψs values were -1.2 MPa in drought‐
ed trees and -0.4 MPa in control trees.
The decline in relative water content in Persian walnut seedlings at different osmotic potential
was paralleled by a substantial decrease in water potential (Ψw), especially in tolerant
genotypes [213]. Values of Ψw decreased during the day and subsequently recovered and re-
equilibrated at night, showing a pattern of progressive decline during the drought treatment.
During the last day (29 th day) of the drought treatment, Ψw decreased in all plants subjected
to drought stress. But in 'Panegine20' and 'Chandler' progeny, there was a quick reduction in
Ψw from -1.8 MPa in control plants to -4.9 at -2.0 MPa of osmotic treatments [213].
3.2. Stomata responses to water stress
Foliar conductance to water vapor of mesophytes and crop plants often lie in the range of 10-20
mm s -1 under conditions in which stomata are largely open, and these figures fall to values
near 0.1 mm s -1 or lower-equivalent to the cuticular conductance—when stomata close
[164-165]. In xerophytes and many trees, conductance under water stress can fall still lower to
values approaching 0.01 mm s -1 . Clearly, understanding the factors that control stomatal
aperture will be crucial to future developments toward improving vegetative yields in the face
of increasing pressure on water resources and arable land usage.
At the same time, the guard cells that surround the stomatal pore have become a focus of
attention in fundamental research. The ability of these cells to integrate both environmental
and internal signals and their unique situation within the leaf tissue has provided a wealth of
experimental access points to signal cascades that link membrane transport to stomatal control.
Stomata have a fundamental role in controlling two of the most important processes in
vegetative plant physiology, photosynthesis and ranspiration: they open to allow sufficient
CO 2 to enter the leaf for photosynthetic carbon fixation, and they close to reduce transpiration
under conditions of water stress [192]. The mechanics of stomatal function are intimately
connected with their morphology. On the other hand, as may be expected, estimates of the
change in guard cell volume between the closed and open states of stomata vary between
species because, even in one species, guard cell size can vary dependent on growth conditions
and the age of the plant [192].
A study about stomatal density of leaf samples in different walnut varieties revealed that the
shape and volume of stomata significantly differ among varieties [212]. Tolerant and semi-
tolerant varieties had a small volume of guard cells and high stomatal density especially in the
abaxial epidermis of leaves [212]. So these varieties have a high potential to maximize CO 2
entry to the leaf for photosynthetic carbon fixation and they close quickly to reduce transpi‐
ration under conditions of abiotic stress [212].
3.3. Xylem embolism under abiotic stresses
A certain degree of water stress is generally experienced by plants irrespective of life cycle and
habitat [57]. Particularly in trees, the decrease in water potential may be greater, since hydraulic