Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 330

process requiring metabolic energy to generate the necessary positive pressure. Cavitation
avoidance is probably a much more efficient way to cope with reduced soil water, and stomatal
control of transpiration probably plays a major role in this respect.
3.8. Biological lag effects
Drought and salt stress can also produce chronic symptoms such as shoot die-back, crown and
root rot, tree decline and eventual death [14; 213]. In some seasons and in some field settings
too much water is the result of uncontrollable natural phenomena such as excessive rainfall,
high water table, and flooding. In other situations, too much water may be the result of water
management decisions such as starting the irrigation season too soon, applying too much water
per irrigation, irrigating too frequently, operating irrigation systems that apply water non-
uniformly, or exposing sensitive parts of the tree such as the root crown to excessive water [14;
213]. Conversely, too little water may result from starting the irrigation season too late,
applying too little water per irrigation, irrigating too infrequently, or operating irrigation
systems that apply water non-uniformly. 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; 213].
In 1986, Dreyer and Mauget [22] tested immediate and delayed effects of summer drought on
development of young walnut trees ( Juglans regia ). Two treatment periods were defined: in
spring, after the first shoot growth flush, and at the end of summer, following complete
cessation of shoot elongation. These treatments induced both immediate effects (halted
growth, reduction of leaf area) and significant delayed effects appearing at resumption of
watering. During summer, many normally quiescent buds resumed growth on trees submitted
to drought after rewatering. Winter dormancy of buds was reduced by late summer drought.
Unlike other trees, walnut trees showed no detectable residual effect on the timing of spring
bud burst the following growing season.
3.9. Gas exchange
Light-saturated net CO 2 assimilation rate (A max ) and stomatal conductance (g s ) are closely
related in many species [85; 107-108]. However it is not clear whether the reduction in carbon
fixation is due to closing of stomata or changes in leaf biochemistry. In walnut, A max decreases
at high temperatures [109-110], but it is not clear whether temperature has a direct effect on
photosynthesis, or just affects g s . Another hypothesis is that A max and g s are co-regulated under
water stress [111-112]. While g s is, at times, correlated with VPD l [113], an increasing body of
literature suggests that g s depends on leaf water status [72 -74; 84], possibly leaf or turgor
pressure potentials [85-86]. Thus, while both water status and VPD l affect g s , the mechanisms
of such responses are not clear.
In an attempt to answer this question, Rosati et al. [121] studied diurnal changes in the water
status and gas exchange of droughted [50% crop evapotranspiration (ETc)] and fully irrigated
(100% ETc) walnut trees, over 2 d. Stem water potentials (Ψs) ranged from -0.5 MPa in the
morning to -1.2 MPa in the afternoon under drought, and from -0.1 MPa to -0.4 MPa under