PLANT ADAPTATION TO ANTHROPOGENIC FACTORS - 2017

4. THE EFFECT OF IONIZING RADIATION ON PLANT ORGANISMS

At the present stage, radioactive radiation has emerged as a novel anthropogenic factor in environmental pollution. The appearance of this type of pollution is associated with the mining of radioactive ores (such as uranium) and The Use of radioactive decay energy for both military and peaceful purposes. Catastrophes at facilities utilizing nuclear energy, such as nuclear power plants, pose a significant threat to the existence of living organisms. One such catastrophe was the Chornobyl Nuclear Power Plant accident in 1986. The consequences of this accident prompted large-scale studies of the radiobiological effects of irradiation.

Radiobiological reactions in plants largely depend on the initial distribution of ionizing radiation energy absorbed by the Organism's Cells and Tissues during irradiation. Therefore, in radiobiological research, irradiation conditions determine the experimental outcome. Irradiation conditions include the type of ionizing radiation, radiation dose, dose rate, duration of the irradiation period, the state of the irradiated object at the time of exposure, the combination of irradiation with other physical or Chemical factors (such as exposure to visible light, elevated or reduced Temperature, an oxygen-enriched or oxygen-depleted atmosphere, etc.). The entire plant or its individual parts can be irradiated, which also characterizes the irradiation conditions.

Studying plant radiobiological reactions under varying irradiation conditions provides information that helps elucidate the mechanisms underlying radiation pathology in the organism.

In radiobiological experiments with plants, it is possible to significantly diversify irradiation conditions by selecting appropriate radiation sources and specific irradiation programs. In nature, where plants, like all other organisms, are exposed to ionizing radiation from natural radioactive substances, irradiation conditions differ in that radiation penetrates the plant continuously throughout its life. To determine how significant Background radiation is to the vital activity of a plant organism, researchers attenuate the intensity of this radiation to very low values by shielding plants with Materials that do not contain radioactive substances.

4.1. Types of Ionizing Radiations

Radiobiological reactions in plants are determined by factors that can be divided into two groups: those related to The Nature of the plant itself, and those related to the CHARACTERISTICS OF THE ionizing radiation and the Methods of exposure. Ionizing radiation encompasses various types of radiation. Common to all types of radiation is its ability, upon passing through matter, to induce acts of discrete energy transfer—namely, the ionization and excitation of atoms and molecules. The energy transfer for The ionization of atoms and molecules is driven by the interaction of radiation with the electron shells of the atoms in the substance. During ionization, charged ions of both signs (positive and negative) are formed from neutral atoms or molecules. Ionization generally occurs through the ejection of an electron from outer orbitals; therefore, the interaction of radiation with matter must provide such an electron with sufficient energy for its complete detachment from the atom. The amount of energy corresponding to this condition is called the ionization potential.

Excitation of atoms or molecules involves their transition to a higher energy state, known as the excited state. The formation of the excited state of atoms and molecules requires The transfer of a portion of energy that ensures the corresponding electronic transitions.

Excited atoms and molecules exhibit increased reactivity due to the appearance of unpaired electrons; in this case, the substance is said to be in a free-radical state. Thus, As a result of the interaction of ionizing radiation with matter, ions of both signs and free-radical states of atoms and molecules are produced.

A distinction is made between corpuscular and non-corpuscular (electromagnetic) radiations. Corpuscular radiations are characterized by the fact that their particles possess rest mass, with the main characteristics being particle mass, electric charge, and initial energy. Electromagnetic radiations are characterized by frequency or wavelength, which determine the energy of individual quanta. Both corpuscular and non-corpuscular radiations are characterized by energy spectra—the distribution of radiation intensity According to the energy of quanta or particles.

While corpuscular radiations are known in which all particles have the same energy level, There are also those in which the initial energy of the particles is unequal: there may be several energy values or the energy may be described by a continuous distribution. Electromagnetic radiations can likewise consist of a stream of quanta of equal energy or be characterized by a continuous energy distribution.

In radiobiological experiments, the most commonly used radiation types include X-rays, gamma radiation, and synchrotron radiation among non-corpuscular radiations, and electrons, protons (hydrogen atom nuclei), α-particles, π-mesons, and accelerated nuclei of various elements up to ⁹²U among corpuscular radiations. Neutrons—elementary particles lacking an electric charge—are also classified as ionizing radiation. During their interaction with matter, charged particles are produced, which cause the ionization and excitation of atoms and molecules as they pass through the substance. Neutrons thus act as ionizing radiation as a result of secondary, rather than primary, interaction processes with matter.

The charge of a proton, positron, or positive π-meson (muon) is characterized as a single elementary positive electric charge, whereas their respective antiparticles correspond to a single elementary negative electron charge, which equals 1.6021892 · 10^-19 C. In individual acts of interaction with matter, electric charge manifests as an integer multiple of the electron charge.

X-rays represent electromagnetic radiation with a wavelength of 10–0.001 nm, which corresponds to a quantum energy of 0.12–1237 keV. For comparison, It is worth noting that the wavelengths of ultraviolet rays are 200–300 nm.

X-rays are formed during the deceleration of fast electrons (generated in a vacuum) within matter. In X-ray tubes created for this purpose, targets made of tungsten or molybdenum are most frequently used to decelerate electrons. The sharp deceleration of electrons in these metals is accompanied by the generation of X-ray radiation with a complex energy spectrum. The higher the voltage across the X-ray tube used to accelerate electrons, the shorter the wavelength of the resulting X-rays. Shorter-wavelength rays are termed hard X-rays, while longer-wavelength rays are termed soft X-rays.

Gamma rays are electromagnetic radiation emitted by atomic nuclei during their radioactive decay. The emission of γ-quanta accompanies β-decay, K-capture, and α-decay. Additionally, γ-quanta are generated during electron-positron pair annihilation and the decay of certain particles, such as π-mesons.

Unlike X-rays, which possess a continuous energy spectrum, γ-rays emitted by radioactive element atoms are represented by one or several discrete energy levels—monoenergetic quanta. Typically, a single γ-ray quantum carries substantially greater energy than in the case of X-rays; in other words, γ-rays are "harder" than X-rays. Radioactive isotopes ⁶⁰Co and ¹³⁷Cs are most commonly used as sources of γ-rays.

Synchrotron radiation represents electromagnetic waves emitted by accelerated relativistic electrons in a synchrotron. It is characterized by a continuous energy spectrum ranging from the deep ultraviolet to X-rays. Irradiations are performed using a beam of synchrotron radiation extracted from the synchrotron. Studies on the effects of this type of radiation on plants have only recently begun, yet a high efficiency of interaction between this type of radiation and matter has been established.

β-particles are accelerated electrons produced during the decay of numerous radioactive isotopes undergoing β-decay. This radiation is characterized by a continuous energy spectrum. The penetrating power of β-particles is significantly weaker than that of γ-quanta of the same energy. Most frequently, researchers study the biological action of β-radiation from isotopes incorporated within cells and tissues that decay with the emission of β-particles, such as ³²P, ³⁵S, and ³H. For this purpose, compounds with molecules incorporating the respective β-emitting isotope are introduced into plants via roots, foliar feeding, or other methods.

Neutrons are uncharged particles that can possess kinetic energy ranging from hundredths of a fraction to many millions of electron-volts. These particles constitute a component of the penetrating radiation of a nuclear explosion. For experimental purposes, neutron fluxes are obtained in nuclear reactors and specialized neutron generators, where neutrons are generated via nuclear reactions. The majority of neutrons produced in the ²³⁵U fission chain reaction possess energies between 0.5 and 2.0 MeV. These neutrons are referred to as fast neutrons. As fast neutrons pass through matter, their energy is expended in interaction processes with the atoms of that substance, and those neutrons that are not absorbed in nuclear reactions lose kinetic energy and become slow particles whose energy corresponds to thermal equilibrium with surrounding atoms.

The Specificity of the action of a particular type of radiation is determined primarily by the spatial distribution density of ionized and excited molecular states arising from the interaction of radiation particles or quanta with the molecules of matter, as well as the pattern of distribution of these states within The Cell. Since the characteristics of the initial state of an object at the time of its irradiation may be similar under the action of Different types of radiation, the subsequent radiobiological reactions will likewise be similar.