Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

8. Adaptation and Resistance of Plants to Adverse Environmental Factors
8.8 Radioresistance

The BIOLOGICAL EFFECTS OF Ionizing Radiation result from its impact at multiple Levels of Organization, ranging from the molecular to the organismal and population levels. A common feature of these effects is that significant biological damage is triggered by a minuscule amount of energy and a small number of primary radiochemical reactions. For instance, gamma irradiation at a dose of about 10 Gy (1,000 rad), which is lethal to mammals, absorbs an energy amount capable of raising body Temperature by merely 0.001 °C.

A distinction is made between Direct and Indirect effects of radiation on living organisms. The direct effect involves radiochemical transformations of molecules at the site where radiation energy is absorbed. A direct hit on a molecule puts it into an excited or ionized state, with the damaging effect being directly linked to molecular ionization. The indirect effect of radiation involves damage to molecules, membranes, Organelles, and Cells caused by Water radiolysis products, which are generated in vast quantities within the irradiated Cell. When a charged radiation particle interacts with a water molecule, it induces its ionization.

In the presence of dissolved oxygen, water radiolysis also yields the potent oxidizing agent HO₂ (H⁺ + O₂ → HO₂) and novel peroxides (HO₂ + H → H₂O₂). Over their brief lifespan of 10^-6 – 10^-5 s, these strong oxidants can damage or alter numerous biologically vital molecules, including Nucleic Acids, Enzymes, and Membrane Lipids. Furthermore, when water radicals interact with Organic compounds in the presence of oxygen, organic peroxides are formed, which further contributes to radiation-induced damage to cellular molecules and structures.

Reducing the oxygen concentration in the medium or tissue mitigates the extent of radiation injury. This "oxygen effect" manifests across all Levels of biological organization, from the molecular to THE TISSUE LEVEL.

Prior injuries can become amplified or progressively worse due to radiation-induced radiotoxins, the accumulation of errors during METABOLISM/36.html">DNA Replication, RNA and Protein Synthesis, and the impairment of enzymes responsible for synthesizing crucial biological compounds.

For a cell, the most hazardous consequence of irradiation is disruption of the unique Introduction/20.html">DNA Structure. Other alterations involve radiation impacts on the nuclear membrane and Chromatin. Irradiation can also inactivate enzymes involved in repairing DNA molecule damage. Ultimately, these and other lesions at both the DNA and chromatin levels manifest as altered protein synthesis, disrupted Cell Cycle phases, Chromosomal aberrations, increased mutation frequencies, impaired regulatory systems, and cell death.

Among plant Tissues, Meristems exhibit the highest radiosensitivity. They are referred to as critical plant tissues because radiation damage to meristems dictates the onset of radiation sickness and the death of the entire Organism.

A typical PLANT RESPONSE TO irradiation is the alteration of growth processes, with both stimulatory and inhibitory effects observed depending on the dose. For instance, low doses of radiation (0.35–0.5 Gy, or 0.035–0.05 krad) stimulate the growth of pea and corn seedlings for 4–6 days post-irradiation, after which the growth enhancement ceases. The stimulatory effect of low doses (5 Gy, or 500 rad) is utilized commercially for pre-sowing irradiation of corn seeds, increasing their yield by 10–12%. Vegetative plants show the lowest radioresistance: lethal radiation doses for seedlings of highly radiosensitive field beans (6–8 Gy, or 0.6–0.8 krad) and peas (10–15 Gy, or 1.0–1.5 krad) are comparable to lethal doses for many mammals (approximately 10 Gy, or 1 krad). Irradiation induces diverse morphological anomalies in plants, such as size alterations, leaf curling and wrinkling, organ hypertrophy, and the appearance of tumor-like growths on all Organs.

Plant radioresistance varies significantly throughout ontogeny. Plants are most sensitive to radiation during seed germination and the periods of spore and gametogenesis. As seeds mature, their radioresistance increases to a maximum.

Unicellular plants exhibit the highest resistance to irradiation immediately after Cell Division is completed and at the end of the DNA Synthesis phase.

Both the phylogeny and ontogeny of plants have evolved under conditions of natural radioactivity. In the Cytology/cytology/16.html">Early stages of life on Earth, the Background radiation was much higher than it is today and gradually decreased due to the decay of radioactive elements in the Earth's crust; this may explain why evolutionarily older organisms possess enhanced radioresistance. High doses of radiation can be tolerated by cyanobacteria, Fungi, and Lichens.