Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 320

elongation in species that exhibit continuous growth or multiple flushing. Drought may not
inhibit the first growth flush that usually occurs before peak drought intensity, but may
decrease the number of nodes formed in the new bud that will then expand during the second
(or third, etc.) flush of growth. If drought continues, all growth flushes will be affected [212].
As a consequence, severe drought limits leaf area production by reducing the number and
viability of leaf buds and thus the tree's ability to recover an efficient crown development after
resuming normal water availability [212]. At the stand level, leaf area index may be reduced
by as much as 2-3 the year following a severe drought [212-213], without any tree mortality,
and the recovery of LAI to pre-drought levels may require several years. Leaf area index of
walnut stands may also decreases after severe drought, due to an abnormal shedding of older
leaves. Such a reduction in tree leaf area has also been reported from crown transparency
observations, as used for tree vitality assessment in European forest condition monitoring and
in walnut stands of Iran [169; 213]. When too much or too little water is applied repeatedly
over the life of the orchard, it may be at the expense of overall productivity and orchard
longevity [14].
3. Physiological responses to abiotic stresses:
3.1. Plant water status
During stress by water deficit, the water status of the plants plays a key role in the activation
of defense mechanisms. Contrasting results under the same experimental conditions can be
related to difference in species, growth conditions, and stage of the plants [221]. Decline in
relative water content in the walnut seedlings at different osmotic potentials was paralleled
by a substantial decrease in water potential (Ψw), especially in tolerant genotypes (Figure 2).
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 (Figure 2). So these genotypes have
mechanisms (like ion homeostasis, osmotic regulation) to keep an osmotic potential gradient
in leaf and stem tissues and are tolerant to osmotic stress [213].
The water status of a plant is a function of uptake (by roots) and loss (via stomata and cuticle)
of water. Water status in walnut under stress conditions was investigated in several previous
studies. Parker and Pallardy [214] demonstrated genetic variation in the drought response of
leaf and root tissue water relations of seedlings of eight sources of black walnut ( Juglans
nigra L.) using the pressure-volume technique. Tissue water relations were characterized at
three stages of a drying cycle during which well-watered plants were allowed to desiccate and
then were re-irrigated. Sources varied both in the capacity for, and degree of, leaf and root
osmotic adjustment, and in the mechanism by which it was achieved. A decrease in osmotic
potential at the turgor loss point (Ψπp) of 0.4 MPa was attributable to increased leaf tissue
elasticity in seedlings of four sources, while seedlings of an Ontario source exhibited a 0.7-0.8