After reading this article you will learn about:- 1. Introduction to UV-B Radiation 2. Effects of UV-B Radiation 3. Adaptations.
Introduction to UV-B Radiation:
Since the dramatic discovery of the ozone hole over the Antarctica in 1985, being caused by the releases of CFC’s and other greenhouse gases, there is great concern about this ozone depletion and the consequences of UV-B penetration on earth environment. It is well known that UV-B have detrimental impact on biota, it causes mutagenesis on DNA helix.
In the primitive earth, high levels of solar UV-B radiation at the earth’s surface reducing primeval atmosphere forced early organisms to live in submerged condition in water. Thereafter, photosynthesis of minute plank- tonic fresh water aquatic autotrophic organisms eventually lead to an increase of atmospheric oxygen and a decrease of CO2, which made possible the development of a stratospheric ozone layer (Fig. 17.1).
The development of stratospheric ozone might have been in proportion to the gradual atmospheric oxygen increase. In any case, stratospheric ozone, which absorbs solar UV-C completely and much of the UV-B radiation, reduced the flux of damaging solar UV on the earth’s surface and must have allowed the evolution of plant life.
There is no general agreement on the question of the timing of the occurrence of oxygen in the atmospheric and thus the development of the stratospheric ozone layer, since the reconstruction of historical levels of atmospheric oxygen remains indirect and uncertain.
There is evidence that, during the evolution of land plants, the thickness of the stratospheric ozone layer was less (and the flux of solar UV-B radiation higher) than at present. One implication of this is that the recent increase of solar UV-B radiation is not as high as during the early evolution of land plants.
It may be hypothesized that early land plants adapted to such high UV-B fluxes. This might imply that some old land plant groups e.g., terrestrial algae and lichens have become well adapted to such high UV-B radiation.
The development of stratospheric ozone, as a result of the rise of oxygen in the atmosphere, will have influenced the timing of the evolution of land plants. Without the ozone layer, shortwave UV-C and UV-B radiation likely prevented much evolution of terrestrial plants and animal life.
After the development of a stratospheric ozone shield much of the damaging solar UV-radiation was removed and this is thought to have allowed terrestrial plant life to evolve from aquatic environments. However, plants were also developing internal UV-filters, largely through secondary metabolism and production of (poly) phenolics.
Effects of UV-B Radiation:
1. Effects on Plants:
Well over 300 different plant species have been tested for responses to elevated level of UV-B and changes have been detected in just over half of them. Experimental results on the whole, indicate that broad leaved species are more vulnerable than narrow leaved species. Cucurbits and brassicas are especially sensitive.
A variety of symptoms viz., leaf margin curling, leaf necrosis, chlorosis and decolonation. The photosynthetic activities are severity affected. The membrane functions and enzymatic activities, plant’s phenology also influenced severity. The details of potential direct and indirect effects of enhanced UV-B radiation on higher plants are given in Table 17.1.
2. Effects on Microbes:
Most of the early understanding of the biological effects of UV-has come from bacteriological studies of the sterilising effects of UV-C. Latter it was established that both UV-C and UV-B may induce T = T dimer formation in DNA strand, while visible light and UV-A influence recovery from UV- B injury.
3. Effects on Marine Life:
UV radiation penetrates ocean water up to a certain depth. As such marine planktons and inter- tidal flora and fauna gets affect by UV-damage. But deep water animals do affect so much. UV- effects on coral beds are not much known till date.
Adaptation to UV-Radiation:
Though there are several reports that ambient levels of solar UV-B radiation induce environmental stress to higher plants, microbes and other terrestrial and aquatic life, yet there are many biota in both terrestrial and aquatic environment, those are capable of adapting UV-radiation exposures. The details of adaptation mechanism of higher plants against UV-B radiation damage are shown in Table 17.2.
Some microbes such as yeast or cyanobacteria, have additional mechanisms of UV-B protection. In presence of UV-B, they often release significant UV-B absorbing phenolics. However, the overall impacts of UV-B radiation on ecosystem structure and function is given in Fig. 17.2. 