Ultraviolet radiation stress: molecular and physiological adaptations in trees

The ozone layer acts like a giant sunshade, protecting forests and other life forms on the Earth's surface from much of the Sun's harmful ultraviolet radiation. The depletion in stratospheric ozone layer due to anthropogenically released pollutants such as CFCs during the last few decades has resulted in increased UV radiation at ground level. UV radiation (100-400 nm) consists of UV-C (100-280 nm), UV-B (280-315 nm) and UV-A (315-400 nm) radiations. Out of these three, UV-B radiation has a disproportionately large photobiological effect on forest tree species. Enhanced UV radiation affects the tree species by modifying both their biological and biochemical environment. Damage may occur in a number of ways including the direct destruction of genetic material DNA at molecular level, disruption of membranes and other cell structure and generation of highly reactive chemicals known as "free radicals". It also inhibits various physiological processes including photosynthesis, nutrient assimilation, chlorophyll and protein synthesis, which results into reduced growth and development of the tree as a whole. Enhanced UV radiation especially UV-B radiation leads to several biochemical changes in the plants and synthesize secondary metabolites (condensed tannins, phenolics, flavonoids etc.) in the leaves and other parts of the plant. These secondary metabolites are synthesized as an adaptive mechanism in trees against enhanced UV radiation and have UV-absorbing properties. It reduces the insect herbivory, several viral and fungal diseases of a tree. Elevated CO2 concentration ameliorates the damaging effects of UV radiation, whereas, drought in combination to enhanced UV radiation has inhibitory effect on plant growth and development. The inhibitory effects of enhanced UV radiation on the tree growth and development are more pronounced in the trees grown at the higher altitudes and at the equator as compared to the trees grown at lower altitude and at higher latitude. Trees undergo several changes to overcome the damaging effects of increased UV radiation by expressing new genes, synthesizing UV-absorbing compounds (secondary metabolites) and reactive oxygen trees are able to tolerate UV radiation stress.