Agriculture Reference
In-Depth Information
Box 4.3
(continued)
triggering their senescence. When a new leaf appears at the top of the canopy,
an older, shaded leaf should fall at the bottom of the canopy in the steady state.
Although leaves are fixed in their absolute position on the branch where they
originated, their relative position in the canopy becomes progressively deeper
as leaves develop on growing shoots at the upper and outer peripheries of the
canopy. As the canopy grows over time, absolute leaf positions that once were
at the growing periphery and well lighted inevitably become deeply shaded and
unable to sustain a viable leaf. The change in relative position through the life-
time of an individual leaf is analogous to the change in the real position of
leaves from the exterior to the interior of the canopy over time. Thus, we can
speak of a
canopy ergodic hypothesis
that predicts the average light regime, and
photosynthetic rates of leaves across positions at a moment in time are equiva-
lent to those of a single leaf through time, at least so long as the canopy is rea-
sonably close to a condition of steady-state growth (Kikuzawa et al. 2009).
Westoby et al. (2000) also considered the topic of the “time value of a leaf,” but
went beyond Harper (1989) to formally incorporate the concept into a theory
predicting leaf longevity. They recognized that the functional value of a leaf as a
carbon-gaining organ decreases over time for a variety of reasons: intrinsic loss of
function with age, shading in the course of canopy growth, the effects of damage
by pathogens or herbivores, and similar considerations. In this context they assessed
the trade-off between investments that could slow losses of leaf function over
time and those that involved transport to create new leaves. Taking the rate of the
age-dependent reduction in foliar function to be
k
and the organic matter trans-
ported from the leaf to other parts of the plant body as
E
, they then expressed the
amount of transport from a unit amount of leaf over its lifetime (
R
) as
L
(
)
kt
RE t
−
=××
∫
SLA e
d
(4.19)
0
Integrating this relationship as
E
×
SLA
1e
kL
( )
−
(4.20)
R
=
−
k
leaf longevity (
L
) then can be expressed as
1
kR
(4.21)
L
=− −
n
1
k
E
×
SLA
This analysis suggests that leaf longevity is a function of the lifetime amount of
transported photosynthate (
R
), the maximum rate of transport at the time of full leaf
expansion (
E
), the rate of decline in transport rate with leaf age (
k
), and specific leaf
area (SLA). The analysis lets us visualize the relationship between leaf longevity
