Agriculture Reference
In-Depth Information
Leaf Longevity to Maximize Whole-Plant Carbon Gain
Kikuzawa (1991) adopted and elaborated the idea that leaf longevity should be set
not simply by the magnitude of the construction cost, but also by considering the
influence of leaf production potential on the time required to recoup the cost of leaf
construction. More specifically, Kikuzawa reasoned that leaf longevity should be
selected to maximize lifetime net carbon gain, not for the leaf alone but more gener-
ally for the individual plant that bears the leaf (Kikuzawa 1991).
In this context, consider the carbon gain by a single leaf. It has long been recog-
nized (Šesták 1981) that, at the time of leaf maturation, the instantaneous photosyn-
thetic rate of the leaf is at its maximum and then declines with leaf age. Let this
maximum daily photosynthetic rate be a and express the daily photosynthetic rate
at time t after leaf maturation as
pt a
( )
=
· (1
t b
/
)
(4.2)
where a / b is the rate of decline in photosynthetic rate with time and b is the time
when the rate becomes zero. Thus, b defines the potential leaf longevity (Ackerly
1999). The cumulative net carbon gain per unit area of leaf ( G ) arises in the sum-
mation of photosynthetic gain per unit time ( p ) over the leaf lifetime minus the
carbon cost of leaf construction:
t
Gt
()
=
pt t C
()d
(4.3)
0
where C is the cost to produce the leaf expressed as g[glucose] · m[leaf] −2 . The
construction cost ( C ) is estimated as the product of leaf mass per unit leaf area
(LMA, g m −2 ) and a factor ( c ) to convert a unit weight of glucose to a unit weight
of leaf tissue. This conversion factor, which is itself referred to as a construction
cost in the literature, falls in the range 1.1-1.9 g[glucose] · g[leaf] −1 and can be taken
as a constant value of 1.5 g[glucose] · g[leaf] −1 for most purposes (Griffin 1994;
Diemer and Korner 1996; Villar and Merino 2001; Villar et al. 2006).
Box 4.1 Marginal Gain
Microeconomic models used to maximize economic gain in commercial
enterprises can be adapted to analyses optimizing resource gain in plants.
Plants acquire, store, and allocate different kinds of resources such as carbon
and nitrogen through investments in resource gain capacity such as leaf and
root production (Bloom et al. 1985). In this modeling framework, plants are
predicted to obtain resources at the lowest possible cost and utilize them to
gain the highest possible return. Marginal gain essentially expresses the effi-
ciency of resource gain, not simply the total amount of gain. For example, a
plant should continue to acquire and invest the resources required to produce
leaves and roots until the marginal gain on the investment becomes equivalent
to the marginal costs of acquiring the resources. Additionally, we can expect
that the plant should adjust the allocation of resources so that growth is equally
limited by all required resources.
 
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