Agriculture Reference
In-Depth Information
Box 3.3
Allocation and Partitioning of Resources
Both the products of photosynthesis and the mineral resources available
for plant growth are in finite supply. Hence, there inevitably are limits
and trade-offs imposed on plant function. Carbohydrates and mineral
resources used in growth are not available for reproduction. Plants parti-
tion resources differentially to satisfy competing demands, with the result
that cumulative allocations to plant parts differ. For example, biomass
allocated to leaves, stems, roots, flowers, and fruits arise in the parti-
tioning of net primary production (NPP) and reflects trade-offs imposed
by the requirements for survival and reproduction in a given environ-
mental regime.
Box 3.4
Allometry and Isometry
The form and function of organisms can vary with their size. For example,
the allocations to root, stem, and leaves can shift with total plant size. Such
size-dependent changes can be expressed by a power function of the form
A
=
aW
b
where
A
is a measure of some aspect of form or function,
W
is an
appropriate measure of size, and
a
and
b
are allometric constants. This equa-
tion is mathematically equivalent to log (
A
) = log (
a
) +
b
log (
W
), which
graphs as a straight line. If
b
is exactly unity, then
A
is directly proportional
to
W
and the relationship is said to be isometric. In an isometric relation-
ship, a twofold increase in size results in a twofold increase in form or func-
tion. In many biological cases, however, form and function change
disproportionately with size:
b
is not unity and the relationship is said to be
allometric. For example, allometric relationships are used in forest science
to estimate the biomass of standing trees. The biomass of entire trees (
W
) or
their parts such as leaves (
W
L
), branches (
W
B
), stems (
W
S
), or roots (
W
R
) all
are correlated to measures of body size such as trunk diameter at breast
height (
D
) and tree height (
H
). Using the equation
W
=
aD
b
, we can estimate
the total biomass of a tree simply by measuring its diameter at breast height.
Species differ in the degree to which their total biomass changes dispropor-
tionately with size, but in all cases the relationship is allometric and
b
is less
than unity.
