Leaf
Initiation
Leaf Water
Status
Leaf
Leaf
Expansion
ABA, Ck, SIDAnions,
Cations, pH, Amino Acids
and Other Signals
Soil
Surface
Root Growth &
Physiological
Activity
ABA
in
Soil
Root Turgor, Cell
Volume or Pressure
Nutrient
on Membranes
Status of
Signalling
Rhizosphere
From Roots
in Shallow
Drying Soil
Soil Water
Soil
Availability
Strength
Signalling from
Deep Roots
in Moist Soil
Figure 4. Factors influencing the generation of chemical information
(dotted lines), soil effects (circles), plant physiological and developmen-
tal processes (rectangles) in roots in drying soils (after Davies and
Zhang 1991).
has been strengthened by analogy with electrical
form properties (Taylor 1983). A shallow root sys-
engineering concepts such as the flow of electrici-
tem with a high root density in the surface soil
ty through a network of resistances (Fig. 5). Axial
can result in rapid depletion of available water
root resistance to water transport would increase
and nutrients, while a deep root system can make
in a deep root system (Meyer and Ritchie 1980);
available a large volume of soil for root extraction
however, it is an insignificant component com-
of water and nutrients (Chaudhery and Sandhu
pared with radial resistance (Taylor and Klepper
1983). A deeper root system is more efficient in
water uptake compared with a shallow root sys-
1971). Feddes (1981) summarized plant resistanc-
tem (Taylor and Klepper 1973).
es of various crops (Table 3). These data are empir-
In recent years, our understanding of the soil
ical and can be applied to conditions from which
they were derived. It is evident from the pub-
water flow through the soilplantair continuum
5
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