Agriculture Reference
In-Depth Information
Fig. 3.7
Relationship
between gross primary pro-
duction of forests and the
favorable period length. Data
were plotted from those of
Kira (1969)
Box 3.6
Leaf Construction Cost
The construction of leaves requires investments not only in materials but also
in the energy required to acquire those materials and assemble the leaf. The
constituent elements of the diverse chemicals in a leaf were acquired and
assembled into foliar tissues at some cost in respiratory energy, which in turn
was acquired through photosynthesis. Net primary productivity is essentially a
measure of the photosynthetic gains that accrue from investments in leaves, so
it only makes sense to measure the cost of those investments in a unit linked
directly to photosynthesis. Thus, leaf construction cost is usually quantified by
an estimate of the amount of glucose (the immediate product of photosynthe-
sis) required to construct a unit quantity (1 g or 1 m
2
) of leaf tissue.
Estimating the material cost of the carbon in a leaf is fairly straightforward
because leaf tissues typically are about 50% carbon. Because glucose is 40%
carbon, at least 1.2 g glucose can provide the carbon needed to construct each
gram of leaf tissue. The more difficult problem is estimating the additional
respiratory energy involved in acquiring other foliar constituents and actually
assembling the leaf. These energy components include, for example, the
respiratory costs of acquiring nitrogen, phosphorus, potassium, sulfur, and
other mineral elements contained in biochemicals critical to leaf function
such as chlorophyll and photosynthetic enzymes. There are two approaches to
this problem: one is based on measurements of respiration of growing leaves
and the other on analysis of the constituents of leaf tissue. Although it can be
technically difficult, one can measure the respiration associated with growing
leaves (Merino et al. 1982), which can be partitioned into components
(continued)
