12 Sep :Effects of Increased UV-B Radiation on Crops and ForestsMost plants have UV shielding, but not always sufficient for complete protection. Only a small proportion of the UV-B radiation striking a leaf penetrates into the inner tissues. When exposed to enhanced UV-B radiation, many species of plants can increase the UV-absorbing pigments in their outer leaf tissues.
Other adaptations may include increased thickness of leaves that reduces the proportion of inner tissues exposed to UV-B radiation and changes in the protecting waxy layer of the leaves. Several repair mechanisms exist in plants, including repair systems for DNA damage or oxidant injury. The net damage a plant experiences is the result of the balance between damage and protection and repair processes.
There are some UV-B-sensitive varieties of crops that experience reductions in yield. There are also UV-B-tolerant varieties, providing the opportunity to breed and genetically engineer for UV-B tolerant crops. For commercial forests, tree breeding and genetic engineering may be used to improve UV-B tolerance. While many forest tree species appear to be UV-B tolerant, there is limited evidence that detrimental UV-B effects accumulate slowly from year to year in sensitive species.
The biochemistry and physiology of plants are influenced by UV-B exposure such as in the accumulation of UV-B pigments. It is not possible to conclude whether or not the changes will have any appreciable impact on the quality of food. Plants and animals have, during their evolution, adapted to particular environments. They have acquired protection and repair mechanisms appropriate for their particular situations. However, the present rate of global change is so rapid that evolution may not keep up with it, particularly in long-lived plants like trees. Thus, plants adapted to low UV-B environments may suffer even from an increase that is smaller than the difference between natural levels at the equator and higher latitudes. For example, herbaceous plants native to the southern tip of South America (Tierra del Fuego, Argentina) and the Antarctic Peninsula have been shown to be affected by the current ambient UV-B levels. Over long times and many generations, there is the possibility that genetic adaptation can develop.
Effect of UV-B Exposure on Aquatic Life
Pure water is almost transparent to UV radiation. A beam of UV-B radiation can penetrate more than 500 meters through pure water before it is completely absorbed. Natural waters contain Uv absorbing substances, such as dissolved organic matter, that partly shields aquatic organisms from UV-B, although the degree of shielding varies widely from one water body to another.
In clear ocean and lake waters ecologically significant levels of UV-B can penetrate to tens of meters. In turbid rivers and wetlands, however, UV-B may be completely absorbed within the top few centimetres. Most organisms in aquatic ecosystems, such as phytoplankton, live in the illuminated euphotic zone close to the water surface where exposure to UV-B can occur. In particular, UV-B radiation may damage those organisms that live at the surface of the water during their early life stages. Most adult fish are well protected from excessive solar UV, since they inhabit deep waters. Some shallow water fish have been found to develop skin cancer and other Uvrelated diseases. The eggs and larvae of many fish are sensitive to UV-B exposure. Exposure to UV equivalent to 10 m depth resulted in a significant mortality of developing embryos and a significant decrease in length of larvae. These irradiances occur in many temperate latitudes where these ecologically and commercially important fish spawn. In contrast, lobster larvae seem to be tolerant to UV radiation even though they develop in the surface layers of the water column.
Global Warming -Alter the Effect of UV Radiation on Aquatic Ecosystems
Climate change may result in temperature and sea level changes, shifts in the timing and extent of sea ice cover, changes in wave climate, ocean circulation, salinity and altered stratification of the water column. These complex changes are likely to have significant impacts that will vary both spatially and temporally. These changes will affect biological systems (including human marine resources), the global hydrological cycle, vertical mixing and efficiency of carbon dioxide uptake by the ocean. Ozone-related increases in UV-B are an important additional ecological stress that will have both positive and negative impacts in association with the other factors.
Increasing carbon dioxide concentrations will result in warming of the troposphere and simultaneous cooling of the stratosphere, which favours further ozone destruction. One of the possible feedback mechanisms is change in cloud cover and increased rainfall, but this is not well understood. Global warming changes the amount of ice and snow cover in polar and sub-polar areas. Ice and snow strongly attenuate the penetration of solar radiation into the water column. Therefore, any substantial decrease in ice and snow cover will alter the exposure of aquatic ecosystems to solar UV radiation. Another aspect is the dependence of many physiological responses on temperature. Enzymatic repair mechanisms are inhibited by low temperature, while elevated temperatures may augment enzymatic repair mechanisms.
Effects of Stratospheric Ozone Depletion on Cycles in the Environment
Ozone depletion results in greater amounts of UV-B radiation that will have an impact on terrestrial and aquatic biogeochemical systems. Biogeochemical cycles are the complex interactions of physical, chemical, geological and biological processes that control the transport and transformation of substances in the natural environment and therefore the conditions that humans experience in the Earth’s system. The increased UV-B radiation impinging on terrestrial and aquatic systems, due to ozone depletion, results in changes in the trace gas exchange between the continents, oceans and the atmosphere. This results in complex alterations to atmospheric chemistry, the global elemental cycles, such as the carbon cycle, and may have an impact on the survival and health of all organisms on Earth, including humans.
Once in the atmosphere, trace gases such as CO2 interact with the physical climate system resulting in alterations to climate and feedbacks in the global biogeochemical system. Since atmospheric CO2 concentrations play a central role in determining the distribution of heat in the atmosphere, the multiple complex components of the physical climate system such as wind, air-sea momentum, heat exchange and precipitation are influenced. There are also similarly complex interactions between biogeochemical cycling on land and the integrated climate system that have important implications for organisms on Earth. At this stage it is not possible to predict the overall effects of these complex interactions.