Effects of Nuclear Radiation on Green Plants
Nuclear or atomic radiation is a kind of ionizing radiation which consists of unstable atoms or radionuclides which decay spontaneously and change into different types of atoms. It contains different forms of energy particles such as alpha (helium nuclei), beta (electrons), gamma, protons (cosmic radiation), and neutrons (nuclear fission reaction). Of these, alpha radiation is nonpenetrating but dangerous if emitted inside the body. Beta radiation can penetrate human flesh and cause non healing sunburn like effect on skin. On the other hand gamma rays are highly penetrative high energy particles like X-rays and cosmic rays. Hazardous effects of nuclear radiation are generally caused by these gamma rays and green plants are extremely sensitive to gamma radiation.
Several in vitro irradiation experiments and data collected from accidental nuclear radiation sites have indicated that higher plants show wide range (5-500 rads/day) of radiosensitivity. It overlaps that of animals, microorganisms and plants like mosses, lichens and algae; microbes and lower plants being least sensitive to acute radiation doses. Nuclear radiation induces versatile changes in plant system at morphological, cellular, metabolic and genetic levels. Plant's response to irradiation varies according to age, growth stage, plant species, chromosome volume and high or low dose of radiation. Changes in concentrations of seed storage proteins, increased level of plant defence proteins followed by increased level of adaptability have been observed in plants from nuclear disaster sites. National Geographic Magazine (August, 1994) had reported in detail the effects of irradiation on plantations near Chernobyl accident site. Structural distortion or swelling in endoplasmic reticulum, thylakoid membranes, mitochondria, cell wall and plasma membrane of plant cells has been observed in vitro conditions. Alterations in plant vascular system, water transport and other cellular functions have also been found. These changes are usually irreversible and retained even after radiation has stopped. Foliage can get readily contaminated with radioactive substances and if ingested by herbivores; they are automatically exposed to radiation and suffer from its possible effects. Thus in a particular food chain, radiation dose of contaminated producers can reach upto tertiary consumers via primary and secondary consumers. Germination of seeds that have been exposed to radiation may show retarded growth, increased mortality or decreased percent germination.
Some plant species are found to become adaptive if exposed to radiation dose earlier in the life cycle so that in future such plants show survival even at higher dose of radiation. Radiation resistance in plants is acquired by triggering of defenses such as defensive proteins. Radiation hinders uptake and absorption of carbon dioxide and impairs the process of photosynthesis. Seeds and seedlings are less tolerant to even small dose of radiation and growth is greatly affected. It has been observed that plants grown from radiation exposed seeds or seedlings have differed morphological characteristics.
Alpha, beta and gamma rays of nuclear radiation are energetic enough to detach electrons from atoms or molecules thus ionizing them. Generation of free radicals which can damage or mutate DNA even at room temperature is the major property and the primary cause of danger by these ionizing radiations. Radiation induced genetic mutations can change plant vigor which in turn negatively affects plant productivity. Nuclear radiation induced mutations and DNA damage usually result in irreversible phenotypic and genotype changes in particular plant species. Degree of damage is directly proportional to intensity and time of exposure to the radiation dose as well as growth stage of plant. If plant in its embryonic state is exposed to radiation, mutational changes like infertility, increased disease susceptibility, reduced germination frequency, flowering, pod maturity and decreased grain/straw yield can be very harmful. However, we should not forget that some nuclear radiations as gamma particles at defined doses have been utilized in crop improvement protocols with interesting useful results such as increased disease resistance, vigor and high productivity. It also shows that negative or positive effects of radiation are chiefly dependent on dose of nuclear radiation. Radiation also affects production and function of plant growth hormones. Gigantism, dwarfism, abnormal rapid growth, underdeveloped root system or curly and misshaped leaves are some of the changes showed by growing plants after exposure to lower radiation dose; higher dose can kill plants of similar or different species. Radiation can be inhibitory to germination and responsible for DNA damage in embryonic state.
It is interesting to note that every component of the biosphere is continually exposed to natural nuclear radiations or it would be justifying to say that all living and nonliving components live in this atmosphere full of radiation and there is no place on the earth which is free from atomic radiation. Nuclear radiation is emitted from heavy and radioactive elements such as uranium, radium and radon found in earth's crust or rock strata. Apart from these, radiotherapy procedures, volcanic activity, nuclear weapons, and electric power plants using coal or nuclear energy are some of the major sources of nuclear radiation. We are well aware of hazardous effects of nuclear radiation which are evident from Hiroshima and Nagasaki atomic bombings (1945), Chernobyl nuclear disaster (1986) and very recently explosion and meltdown of nuclear power plant which was caused by destructive earthquake and tsunami at Fukushima (2011). Although humans are considered the most radiosensitive animals but detrimental effects of nuclear radiation on green plants should not be ignored and therefore demands intense research based investigations.
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