Chemistry Reference
In-Depth Information
(iii)
solubilisation effects - differences in P solubility close to the root surface
arising from changed soil chemical conditions.
This is a complex set of properties that produces inter-related effects on
internal, physiological efficiency and the efficiency of recovery (for definitions
see ref. 120). The interactions arise mainly because any additional nutrients
provided by externally efficient roots may also stimulate root growth. For
example, model simulations of rice showed that small changes (22%) in root
diameter or internal efficiency had large effects (three-fold) on P uptake. The
same result could be achieved by a 33% increase in root external efficiency, but
only 10% of the three-fold increase in P uptake was directly attributable to the
direct effect of increased external root efficiency, with 90% due to enhanced
root growth as a consequence of higher P uptake per unit of root.
121
These
studies concluded that large genotypic differences in P uptake from P-deficient
soils can result from small differences in tolerance mechanisms and that these
small changes will be difficult to detect as changes in recovery efficiency
because they are likely to be overshadowed by the effects on root growth.
The importance of different root architecture in response to soil conditions,
or as a consequence of genotypic differences in root growth, for nutrient
uptake is starting to emerge.
122
For example, four genotypes of common bean
representing distinct shoot growth habits (erect determinate, erect indetermi-
nate, prostrate indeterminate, and climbing) were grown in containers of an
oxisol and a range of root parameters measured up to 14 days after planting.
123
Table 5 shows that there were significant differences between the genotypes
after 14 days in root length and mass, number of roots arising from the base of
the hypocotyls (basal roots), and root growth and root elongation rates. The P-
efficient genotype Tostado, which grows well in highly acidic, infertile soils in
Rwanda,
d
n
1
r
2
n
g
|
1
d
n
6
h
3
had the
most vigorous seedling
root
system
which was
highly
branched and
with
numerous
basal
roots,
whereas
the
landrace Porrillo
Table 5
Root growth parameters for four genotypes of common bean grown
in containers of an oxisol for 14 days. Values are the mean of four
replicates,
with
the
standard
error
shown
in
brackets.
(Source:
adapted from Lynch).
123
Genotype
Tostado
Porrillo sintetico
Carioca
HAB 229
Total root dry weight
(g plant
21
)
0.38 (0.03)
0.23 (0.05)
0.27 (0.04)
0.28 (0.01)
Total root length
(m plant
21
)
65.9 (23.9)
23.9 (2.6)
35.1 (5.0)
49.6 (15.6)
Number of basal roots
252 (14)
171 (22)
271 (39)
216 (44)
Relative total root
growth rate (d
21
)
0.20 (0.01)
0.20 (0.04)
0.18 (0.01)
0.25 (0.01)
Relative total root
elongation rate (d
21
)
0.48 (0.03)
0.38 (0.03)
0.40 (0.03)
0.42 (0.02)
