DISCUSSION
Effect of phosphorus deficiency on root
architecture
The changes in root architecture observed under phosphorus
deficiency suggest that resources are redistributed to
the roots to allow continued exploration for phosphorus.
Phosphorus deficiency resulted in the inhibition of both
affected by phosphorus deficiency (Table 1). The maintenance
of growth of these roots would be expected to maximize
the soil volume colonized by roots, and, therefore,
access to phosphorus resources.
In real soils, phosphorus availability is non-uniform.
From an economic perspective (Bloom, Chapin & Mooney
1985; Fitter 1991), an efficient root system would explore
the soil extensively in order to locate phosphorus-rich
patches or microsites, but once such a patch is found,
should intensively exploit that patch. This strategy could
be implemented if roots responded differently to the phosphorus
availability in their immediate soil environment,
such that branching was more intense where phosphorus
was most concentrated, whereas extension of main root
axes was accentuated when phosphorus was limiting. In
previous research on plants grown with heterogeneous
phosphorus distribution, roots growing in a low phosphorus
patch elongate but do not proliferate (develop quantities
of fine roots), while in a high phosphorus patch, the
reverse occurs: main root elongation is reduced, but proliferation
is enhanced (Drew & Saker 1978; Snapp, Koide &
Lynch 1995). In this study, we found a similar response to
phosphorus availability in a homogeneous substrate, as has
also been reported for Chrysanthemum (Hansen & Lynch
1998) and some families of Pinus radiata (Theodorou &
Bowen 1993). The ability of plants to sense and respond to
local phosphorus availability suggests the presence of a
signalling mechanism to regulate changes in plant growth.