Agriculture Reference
In-Depth Information
incorporating these effects into a comprehensive model of leaf longevity is not
straightforward. First, there are two elements to foliar defense against herbivores
and disease: constitutive and induced defenses (Karban and Baldwin 1997).
Chabot and Hicks (1982), as well as subsequent cost-benefit models for leaf
longevity in this vein (Kikuzawa 1991, 1995a,b; Kikuzawa and Ackerly 1999),
have assessed a constitutive cost at the time of leaf construction, which cannot
account for the cost of induced defensive responses to herbivore or pathogen
attack. Second, the efficacy of an induced plant response is highly contingent
on the ecology of the interaction between plant and attacker. For example, early
abscission of gall-infested leaves can act as the density-dependent mortality factor
for the gall-forming insects (Sunose and Yukawa 1979; Yukawa and Tsuda
1986), thus reducing the risk of attack for uninfested or future leaves. This sort
of selective shortening of leaf longevity is illustrated by the response of
Populus
attacked by the gall-forming aphid (
Pemphigus betae
); nearly 90% of freshly
fallen green leaves were gall infested, compared to less than 10% of the leaves
still attached to the trees (Williams and Whitham 1986). On the other hand,
infection of
Populus
by a rust fungus such as
Melampsora medusae
can result
in anything from complete to only slight leaf loss (Newcombe and Chastagner
1993). Third, accounting the marginal value of a leaf at the time of attack
requires assessing the return on initial investments to that point in time, the
potential future return from the leaf in light of the cost and potential efficacy of
any induced defenses, and integrating these costs and benefits at the whole-
plant level. An effective model for the response of leaf longevity to herbivore
or pathogen attack thus must scale up from the leaf to whole-plant level to
address the underlying question of tolerance versus defense (Nunez-Farfan
et al. 2007) as strategies for plant response to herbivory and disease.
Box 8.4
Mangroves
Many tree species in five different plant families have evolved the capacity to
grow in intertidal swamps along the ocean shoreline in tropical and subtropi-
cal regions. These trees, which are commonly referred to as mangroves, have
converged to distinctive morphological and physiological adaptations to sur-
vive the stress associated with the twice-daily tidal alternation of saltwater
versus freshwater around their roots. Mangroves are usually evergreen
because their leaves are important for maintaining the metabolic and physical
processes involved in salt exclusion and maintenance of stable tissue water
potentials. Although mangroves have the evergreen leaf habit, the longevities
of their individual leaves in fact are not very long, usually only 6-12 months
(Gill and Tomlinson 1971), or sometimes up to 24 months (Tomlinson
1986).
