Air Pollutant Interactions with Pests and Pathogens
The
exponential economic growth in developed countries since the mid-nineteenth
century has been accompanied by an increase in the concentrations of various
tropospheric trace gases. Among the gaseous pollutants most studied during the
past decades are carbon dioxide (CO2) with a global distribution, ozone and
peroxyacety lnitrate (PAN) with a more regional dimension, and others such as sulfur
dioxide (SO2), nitrogen oxide (NO2), and hydrogen fluoride (HF), which are
associated with emissions from local sources. Besides their potential to
influence the global climate and their direct effects on plant growth and yield,
their effects on plant-pathogen or plant–insect relations may already be, or
could become, an important factor affecting plant health. However, evaluation
of these effects is difficult because of the diverse temporal/spatial distribution
and differences in the chemical behavior and because of the complex
interactions between the effects of the trace gases and agronomic factors
relevant for plant resistance to diseases and insect pests, including fertilizer
and pesticide use, crop variety, soil management, water supply, etc. Also, direct
effects of gaseous pollutants on the pathogen or insect cannot easily be
separated from indirect effects on the host plants through changes in
physiological processes. Experiments under controlled conditions can only
reveal a limited picture of the multitude of possible effects that may occur in
the field. Despite these limitations, a number of studies mainly carried out
between 1970 and 2000 have resulted in a significant amount of data describing
specific effects of the main trace gases on the incidence of plant diseases and
insect pests. These can be compiled and used to formulate some generalizations.
EFFECTS OF AIR POLLUTANTS AND ELEVATED CO2 ON PLANT DISEASES
Among the
photooxidants, ozone has been the prevalent compound studied for its effects on
both plants and plant diseases. Because its photochemical production is favored
under conditions of high irradiance, direct effects of ozone on fungal or
bacterial pathogens are less likely, as these only grow actively on plant
surfaces during wet and cloudy periods. The same applies to soilborne pathogens.
Therefore, the main pathway for ozone effects on diseases occurs indirectly via
changes in the physiology of the plant. A number of physiological changes
induced by ozone are important with respect to plant disease resistance.
Numerous physiological effects of ozone may impair the conditions for growth of
pathogens, particularly of biotrophs, such as accelerated ageing/premature
senescence, degradation of membrane lipids accompanied with increased cellular
leakage, reduced net photosynthesis, increased protein degradation, or enhanced
ethylene production, in combination with changes in factors directly affecting
resistance. These include elevated antioxidant levels, reinforcement of cell
walls (lignin, callose, extensins), induction of phytoalexins, or expression of
PR proteins.
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