Agriculture Reference
In-Depth Information
OtherStresses
Increased levels of UV radiation and ozone and herbicide are some of the factors
that impose stress in plants and they have developed various physiological and bio-
chemical adaptations to deal with these stress conditions. A number of components
in plants involved in the stress tolerance reactions are identified. PAs, CuAO/DAO
and PAO are among these components playing a role in these stress responses (re-
viewed in Groppa and Benavides 2008 ).
Increasing UV radiation in the atmosphere is a reality in Antarctic regions. A few
of the examples of association of PA in UV stress are: in response to UV-B radiation
total free PAs was decreased and conjugated PAs increased in Phaseolus vulgaris
plants (Smith et al. 2001 ), in cucumber leaves UV-B radiation resulted in decreased
total PA content concomitant with increased electrolyte leakage and weakening of
plant growth (An et al. 2004 ), in tobacco cultivars subjected to UV-B radiation, PAs
especially put was increased and after extended periods of UV-B exposure PA levels
declined (Lutz et al. 2005 ). The influence of UV-B on PA metabolism is an indicator
that plants indeed sense UV-B as a stress that also may affect crop yield.
Increasing ozone content in the biosphere is a great problem, which contributes
to global crop losses and forest decline. For the year 2000 global crop yield loss
due to ambient ozone was estimated to be worth $14-26 billion, and 40 % of this
damage occurred in China and India (Van Dingenen et al. 2009 ). Exposure to ozone
results in foliar injury, impaired photosynthesis, reduced growth and yield, and an
accelerated onset of senescence in plants (Langebartels et al. 1991 ). Plants have
evolved preventive mechanisms to minimize the damages caused by ozone. They
can limit entry of ozone to interior through stomatal closure or they have tolerance
mechanisms, which include the detoxification of ozone diffused into the leaf inte-
rior through chemical reactions with ascorbic acid or enzymatic conversion to H 2 O 2
(Chen et al. 2003 ). Since the toxicity of ozone results mainly from oxidative stress,
protecting plants through application of antioxidants is being investigated. PAs ex-
ert several functions, which counteract ozone effects (Langebartels et al. 1991 ).
Accumulation of PAs in response to ozone exposure and protection against ozone-
derived oxidative damage has been reported from different plant species for exam-
ple from barley (Rowland-Bamford et al. 1989 ), wheat (Raab and Weinstein 1990 )
and tobacco (Langebartels et al. 1991 ). Increased ADC activity and accumulation of
free and conjugated put was observed in tobacco cultivar Bel B (Langebartels et al.
1991 ). In accordance with this study, Van Buuren et al. ( 2002 ) showed that accu-
mulation of free put in both tobacco cultivars, ozone-resistant and ozone-sensitive
when exposed to ozone. In tissues undergoing cell death in ozone-sensitive cultivar,
accumulation of conjugated put and transient increase of ADC and ODC activity
was observed (Van Buuren et al. 2002 ). The protective function of PAs in ozone
damage may be due to the control of the cellular redox state, though the precise
mode of action remains unknown (Van Buuren et al. 2002 ).
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