Agriculture Reference
In-Depth Information
Abiotic Stressors: Ozone and Natural Oxidants
Pollutants arising from anthropogenic sources fall outside the realm of specific
adaptive responses but nonetheless can elicit generalized stress response mecha-
nisms that have an evolutionary basis. Ozone provides a good example of this sort
of preadaptation. Foliar responses to tropospheric ozone from anthropogenic
sources are essentially the same as responses to UV radiation, drought, high tem-
peratures, or other natural sources of oxidative stress (Bussotti 2008). In general,
longer-lived leaves are more resistant to all oxidative stress whether natural or
anthropogenic in origin. In terms of leaf longevity, the impact of ozone-induced
oxidative stress, or probably most other anthropogenic pollutants as well, depends
on a dose-response relationship. At lower doses in which tissue-level repair mecha-
nisms confer sufficient resilience to maintain photosynthetic functions, leaf longev-
ity should be extended to recover the initial leaf construction costs as well as the
subsequent repair costs associated with the stress. At some higher dose, however,
we can expect the leaf to be abandoned and recovery of investments shifted to
shorter-lived leaves with higher production potential. Bussotti (2008) lends support
to these suppositions, which invite further study.
Abiotic Stressors: Salinity
The impact of salinity on leaf longevity has this same sort of dose-response depen-
dency, at least so long as the species have at least some degree of salinity tolerance
and do not simply die on exposure to saline conditions. Mangroves, a plant func-
tional group tolerant of levels of salinity in their tidewater habitats that would be
fatal for most plants, illustrate the interspecific variation and dose-response depen-
dence in salinity effects on leaf longevity. Leaf longevities increase from only 0.36
years for
Sonneratia alba
and 0.65 years for
Avicennia alba
at the seaside on up to
2.66 years for
Xylocarpus granatum
at the upper edge of a mangrove swamp in
Thailand (Imai et al. 2009). The leaf half-life of
Avicennia germinans
is 160 days
in a Venezuelan mangrove swamp (Suarez 2003), but under experimental condi-
tions in the absence of salt the half-life rises to 425 days, dropping to 195 days at
170 mol m
−3
NaCl and to 75 days at 940 mol m
−3
NaCl (Suarez and Medina 2005).
Although
Avicennia
tolerates salt, it is clear that increasing salinity decreases leaf
longevity. From a simplistic cost-benefit analysis, one might expect instead that
increasing salinity would impair photosynthetic activity and extend leaf longevity
to pay back the costs of leaf construction. This apparent contradiction is resolved
at the whole-plant level because the leaves of
Avicennia
function not only in pho-
tosynthesis but also in salt secretion (Suarez 2003; Suarez and Medina 2005).
Because the metabolic costs of salt excretion increase with leaf age and salinity,
there is a point at which carbon gains at the whole-plant level are better served by
shortening leaf longevity to take full advantage of the high foliar photosynthetic
