Agriculture Reference
In-Depth Information
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Longevity (days)
Fig. 4.5
Longevity of individual organisms or leaves (
X
-axis) and metabolic rate per unit leaves.
For mammals, this gradient is nearly −1.0, but for photosynthesis by leaves, the gradient is only
about −0.66. The
lower line
parallel to photosynthesis is dark respiration. (From Reich 2001)
significant step in this direction by integrating leaf-level theory into his analysis of
canopy dynamics, but until recently (Hikosaka and Osone 2009) his emphasis has
been on the canopy. Although it is true that selection on foliar characteristics is
contingent on plant performance that is determined at the whole-canopy level, there
are constraints at the leaf level which may set limits on canopy design. For example,
Shipley et al. (2006) show that the spectrum of variation in foliar design is rooted in
trade-offs at the cellular and tissue levels within the leaf. There also may be some
fundamental linkages of this sort that extend to the scaling of metabolic activity for all
organisms (West et al. 1997; Brown et al. 2005), including plants (Reich 2001; Enquist
et al. 2007; Price and Enquist 2007). Reich (2001) points out that foliar metabolism
scales with leaf longevity much as animal metabolism scales with lifespan, although
with a different slope (Fig.
4.5
). What is uncertain is whether this scaling on leaf longev-
ity would converge to the slope for animals if whole-plant longevity were the scaling
factor. It is the give and take between functional constraints and opportunities at the
canopy versus foliar levels that will decide whole-plant leaf longevities and alternative
strategies for plant productivity. These interactions merit serious analysis. A fundamental
understanding of the different modes of leaf longevity that underlie the evergreen versus
deciduous habits and an explanation of which environments favor one or both habits
is likely to be found in the interplay of foliar- and canopy-level traits.
A second useful line of inquiry would be to seek a deeper understanding of the
roles of herbivory and disease as factors in the selection of leaf longevity. Chabot
and Hicks (1982) noted the significance of these factors, and they have been widely
acknowledged in subsequent work, but without ever being explicitly incorporated
into a theoretical analysis of variation in leaf longevity. We have considerable data
on the effects of both herbivores and disease on leaf function as well as on the
multitude of strategies for foliar defense, but no simple generalizations emerge
(Jones 2006; Nunez-Farfan et al. 2007; Howe and Jander 2008; Poland et al. 2009).
A more complete theoretical framework rooted in an assessment of foliar function
at the whole-plant level might help make sense of the voluminous but often con-
founding data on plant defense against herbivores and disease.
