Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 332

soil water supplementation could maintain the hydraulic conductivity of roots in the nutrient-
rich upper soil throughout the dry season [46], keeping roots ready to extract water when
moisture becomes available in the upper soil. Otieno et al [105] found that Quercus suber , with
a deep root system, took up most of its required water from the deep soil layers during drought
to maintain good water status, but no growth was recorded during this time. Water in the
upper soil layers, however, seemed to play a more important role in tree productivity. Values
of δD trace the ratio of water sources, but not the absolute amount of water. Lower δD values
suggest that xylem water has a higher ratio of water from the deep soil layers, but cannot be
automatically translated into greater water uptake from the deep soil layers. Such a finding
could also indicate reduced water uptake from the upper soil layers or a mixture of reduced
water uptake from the upper layers and increased water uptake from the deep layers. Thermal
dissipation probe transpiration measurements indicated that the daily sap flow decreased by
around 30% on the driest day in comparison with 2 July and 27 August, suggesting that the
highest xylem δD on 15 August would be mainly attributed to reduced water uptake from the
upper soil layers. Additionally, the δD values in xylem water were significantly correlated
with the shallow soil layers (0-20 and 20-40 cm depths), but not so significantly correlated
with the deeper soil layers (40-60 cm depths) [17], suggesting that water uptake by walnut
would tend to be mainly determined by water supply of the upper soil. During water transport
between roots and shoots, the isotopic composition of water remains unaltered; therefore, it is
reasonable to believe that water in sap flow was also mainly provided by the upper soil [17].
Many studies with stable isotopic hydrogen and oxygen on seasonal changes in water sources
investigated the water source shift from upper to deep soil layers with decreasing precipitation,
and the results sometimes imply that water uptake from deep soil, where water is available,
could solve the drought problem. Deep water can help but not always enogh to avoid serious
stress.
Walnut roots were mainly distributed in the upper soil layers [212]. Soil moisture was a key
factor regulating root growth and water uptake efficiency of the roots [17]. The shallow roots
had reduced efficiency in water uptake in the dry season, and therefore J. regia was compelled
to extract a greater ratio of water from the deep soil layers. However, the shift was not able to
prevent water stress on the plants, which was characterized by increased pre-dawn branch
xylem PLC, reduced pre-dawn leaf water potential and transpiration with soil drying. In
addition to serving as an indicator of water sources, changes in the δD values in walnut branch
xylem water reflected plant water status and the severity of soil drought [17].
In previous studies, comparison of the δD values of plant stem water and soil water at different
depths revealed the existence of different water source partitioning patterns between different
soil moisture conditions in a planted walnut forest for example in northern China [17]. The
δD values showed that plants mainly used water from the upper soil in the wet season, while
upper and deep soil water more or less equally contributed to plant xylem water in the dry
season. The result is consistent with that of previous studies. McCole and Stern [102] reported
a change in juniper water use from a predominantly deep water source during summer, when
it was hot and dry, to a predominantly upper soil source during winter, when it was cool and
wet. Pinus edulis and Juniperus osteosperma largely use monsoon precipitation during the