Agriculture Reference
In-Depth Information
the relatively high costs of leaf construction in sclerophylls. Mesophyllic leaves,
which cost less to construct and have higher photosynthetic capacity, were associ-
ated conversely with the deciduous habit. The Orians and Solbrig (1977) model
embodies ideas about trade-offs in foliar design still prevalent today and stands as
the first theoretical model associating leaf photosynthetic function and leaf longevity
with the distinction between evergreen and deciduous plants.
A seminal review a few years later by Chabot and Hicks (1982) marks a turning
point in consideration of the nature and causes for the evergreen versus deciduous
habits. Their review consolidated ideas emerging in the previous decade, decisively
shifting the discussion from questions of resource availability and resource
management at the whole-plant level to leaf longevity as a central trait in foliar
function that determined whether a plant was evergreen or deciduous. Picking up
on the perspective of Orians and Solbrig (1977), they presented a cost-benefit
analysis of leaf carbon economy based on the premise that leaves are fundamentally
photosynthetic organs, which over their lifetime must repay to the plant the carbon
cost of their construction. More formally, they introduced the following equation
for the carbon economy of a single leaf:
∑∑
G P PCWHS
= − −−−−
(1.1)
f
i
u
i
where
G
is the net carbon gain by a single leaf that is exported to other parts of the
plant over a year,
P
f
i
is the carbon gain by a leaf at age
i
during any favorable period
for photosynthesis over the year, and
P
u
i
is the net carbon exchange of the leaf
during any periods unfavorable for photosynthesis. Because the photosynthetic gain
during an unfavorable period is by definition zero or nearly so, the net gain during
an unfavorable period typically will be negative consequent to respiratory carbon
losses associated with maintenance and defense of the leaf. The term
C
is the
construction cost to produce the leaf. Although the actual construction of a leaf
occurs over some finite period of time, Chabot and Hicks (1982) imposed the
cumulative construction cost at the time of leaf expansion when the leaf becomes
photosynthetically active; the leaf construction cost therefore is independent of
time. Similarly, any damage by wind (
W
) or herbivores and pathogens (
H
) is also
accumulated and considered independent of time during the leaf lifespan. Finally,
Chabot and Hicks (1982) recognized that some part (
S
) of the photosynthate
produced by the leaf might be stored or utilized in foliar tissues rather than trans-
located to another part of the plant, and hence would not contribute to the net gain
of the plant from that individual leaf. Reasoning in this conceptual framework and
reviewing available data, Chabot and Hicks (1982) argued that leaf longevity thus
should be determined by the balance between costs represented in the negative
terms of (1.1) and benefits represented in the positive terms. Their conceptual
framework and the literature they reviewed firmly placed the leaf in the context of
the plant as a whole, inviting subsequent analyses of how variation in leaf demog-
raphy contributes to the distinction between evergreen and the deciduous habits.
