Changes in the activity of antioxidant enzyme superoxide dismutase ...

(UV + O 3
; O 3
+ UV) SOD activity remains similar (Fig.). The research showed that
adaptation by low UVB and ozone doses towards one of the other factor influences
the increase of SOD activity in plants.
Discussion. Abiotic stress condition weakens plants; they become more susceptible
to pathogens, and this leads to extensive losses to of agricultural crop worldwide.
Usually the impact of environmental factors is not individual but combined. Therefore,
under conditions of climate changes plants have to adapt to simultaneous impact of
several factors: drought and heat; cold stress and drought combined with high light
conditions. For example, during heat and high ozone stresses plants open leaf stomata
for transpiration to avoid overheating, but thus the way for more abundant passing of
ozone into leaf intercellular ducts is opened, otherwise stomata would close. Therefore
in case of complex stress plants might require conflicting or antagonistic responses
(Pasqualini et al., 2003; Mittler, 2006).
The response type depends upon the plant genotype. Cultivated plants are more
susceptible to stresses than wild plants, which are better adapted to climate changes.
Even the slightest changes of climate factors could be relevant for agricultural plants.
Previous investigations have shown that even the lowest UVB (2 kJ m -1 ) and ozone
doses reduce the leaf size of a model plant Crepis capillaris; it is particularly true in
case of ozone (Rančelienė et al., 2006). Ozone, as a strong oxidant, or its secondary
derivatives, such as ROS (Reactive Oxygen Species) frequently cause leaf necroses.
It is known that viruses also cause necroses of plant leaves. In case of hypersensitivity
reaction necrotic spots, localizing the virus spread, form on the injured areas of leaves.
There is an opinion that ozone also triggers a hypersensitive response (Koch et al.,
2000). Therefore, necroses indicate the death of some cells irrespective of the stress
factor causing them: viruses, bacteria, UVB, ozone, cold and drought. So the damages
indicate the so-called programmed cell death (Pasqualini et al., 2002). But response
and defense genes are simultaneously induced, and they determine triggering of plant
defence mechanism to antioxidative response (Yun-Hee Kim et al., 2007). Antioxidative
enzyme systems, especially SOD enzyme, actively participates in this process. Our
research showed that after preadaptation by low doses the repeated treatment by other
factor doubly increases the activity of antioxidative SOD enzyme. It demonstrates that
plants adapted to one factor are frequently more tolerant to other factors. It confirms
the results of our earlier researches performed with cold-resistant interspecific potato
hybrids treated with UVB (Vyšniauskienė et al., 2006). However, results of other 7-year
long researches performed with plants growing in tundra showed that under conditions
of stratospheric ozone depletion and by enhanced UVB, the supplementation of UVB
in field produced no negative effect upon growth parameters (Rozema et al., 2006). It
demonstrates that in the course of time plants become UVB-tolerant.
Conclusions. 1. Increased activity of plant enzymes, such as SOD, after the UVB
and ozone irradiation was assessed as adaptational response of plant towards oxidative
stress caused by harmful factors. 2. Plants adapted to one factor are frequently more
tolerant to the impact of other factors.
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