Agriculture Reference
In-Depth Information
the role of below-ground root structures of
J. regia
during the development of water stress
in field environments and variation in soil water uptake and its effect on plant water status
during dry and wet seasons have been published [17].
In recent years, hydrogen and oxygen isotopic application has contributed significantly to
tracing and understanding below-ground processes [23-24]. During water transport between
roots and shoots, the isotopic composition of xylem water remains unaltered from that of the
soil [23]. Therefore, it is reasonable to analyze the branch xylem water to determine the water
source [25]. Soil water is also a key factor in restoring forest ecosystems in arid and semi-arid
zones [26], while the efficiency of soil water uptake by trees could be the ultimate determining
factor in their productivity [26-27]. Therefore, knowledge of root distribution and mechanisms
of soil water extraction and transport by trees is indispensable for successfully restoring
ecosystems.
Walnut root growth differs in dry and wet seasons [17]. Mean root length in both the upper
(0-30 cm) and deep (30-80 cm) soil layers shortened when the soil water content and relative
humidity of the air were lower [17]. After rain events, re-watering, or irrigation, the total root
length increased compared with dry periods [17]. The abundance of new roots significantly
increased in both the upper and deep soil layers in response to the rain and rewatering events.
The growth of new roots was greater in the upper soil profile than in the deep soil profile.
Dead root length in the upper soil layer was significantly greater in the wet season than in the
dry season, while no difference in dead roots has detected in the deep soil layer between the
seasons and diameter of the roots did not significantly change by season [17]. Water supply to
trees involves two major steps: absorption and transport of water (i.e. ascent of sap), both
driven by transpiration. The efficiency of soil water absorption in trees depends on both spatial
extension and density of their root system [18].
There is significant variation in the vertical distribution of roots among different walnut
varieties [17; 212]. Roots are the most abundant at 10-30 cm depth, followed by 0-10 cm depth.
Root biomass decreases with depth below 30 cm. Generally, most of the root surface area, root
length density and root biomass were confined to the upper soil layers (0-30 cm), accounting
for 61, 62.5 and 79% of the total root measurements from the 0-80 cm soil layers, respectively
[17]. Walnut roots were mainly distributed in the upper soil layers at our study sites and likely
in the whole region. Soil moisture was a key factor regulating root growth and water uptake
efficiency of the roots [17; 212]. 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
were characterized by increased pre-dawn branch xylem PLC, reduced pre-dawn leaf water
potential and transpiration with soil drying [17]. In addition to serving as an indicator of water
sources, changes in the stable-hydrogen isotope (δD) values in walnut branch xylem water
reflected plant water status and the severity of soil drought.
2.3. Excess water supply
On soils subject to flooding or with shallow restrictive layers, excess soil moisture can also be
a problem. Excess soil moisture during the growing season leads to decreased oxygen in the
Search WWH ::
Custom Search