ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaienko 2005

Chapter 9. RADIORESISTANCE OF ORGANISMS

9.4. Radioresistance of plants

Plants vary greatly in their Structure and Life cycle, which directly affects their radiosensitivity.

Algae are the most ancient plants and represent the largest group among lower plants. In the course of phylogenesis, they evolved from microscopic unicellular organisms into complex multicellular forms.

Like microscopic algae (see Section 9.3), multicellular algae differ in their Radioresistance. For instance, filamentous green algae are relatively radioresistant. In Spirogyra crassa, the lethal dose exceeds 150 Gy. One of the reasons for this enhanced radioresistance is presumably the polycentric nature of their Chromosomes, which helps preserve genetic material within chromosome fragments induced by ionizing radiation at doses of 30 Gy and above. High radioresistance is also typical of colonial algae, particularly the order Volvocales. Generally, algae exhibit a dose fractionation effect and are capable of repairing sublethal and potentially lethal damage.

Bryophytes are ancient in origin and relatively simple in structure among higher plants, yet they exhibit fairly high radioresistance. For example, growth arrest in the gametophyte of Drepanocladus aduncus occurs upon exposure to doses of 250–800 Gy. However, even under these doses, The formation of additional lateral branches was observed due to the inhibition of apical Cells.

Ferns are also relatively radioresistant plants. When exposed to ionizing radiation, they exhibit responses similar to those of mosses.

Gymnosperms and angiosperms are characterized by a wide Variability in radioresistance.

Plant radioresistance depends on numerous factors, including Genome Organization, DNA Repair capacity, Cell Cycle stage, and the presence of radioprotective compounds that prevent or mitigate radiation-induced damage. An inverse relationship has been found between Chromatin content and the radioresistance of plant cells. Depending on the nuclear DNA content, the D₀ value for these plant cells ranges from a few grays to hundreds of grays. However, quantitatively assessing the radioresistance of higher plants based solely on the effects of ionizing radiation on individual cells is incorrect, as there is no direct correlation between the radioresistance of isolated cells and that of a multicellular Organism. The death of sprouts, let alone adult plants, occurs at Doses of ionizing radiation tens and hundreds of times higher than those lethal to individual cells of these plants.

The radioresistance of higher plants depends on their physiological state at the time of irradiation. For instance, the radioresistance of seeds is typically 10–100 times higher than that of plants during the growing season. A correlation exists between plant radioresistance across different growth stages: high seed radioresistance generally corresponds to increased radioresistance in the growing plant.

The most radiosensitive structures in plants are actively proliferating Meristems. Upon irradiation, growth processes are the first to be altered. Depending on the dose, these effects can be either stimulatory or inhibitory. The respective doses for certain plant seeds are listed in Table 9.5.

Class="center">Table 9.5

STIMULATORY AND INHIBITORY DOSES OF β-RADIATION FOR SEEDS OF CERTAIN PLANTS

(according to Gudkov, 1991)

Under METABOLISM/18.html">The Influence of stimulatory doses, plants experience an increase in the level of phytohormones, which act as growth promoters. The enhanced activity of the phytohormonal system results from the expression and activation of specific genes triggered by ionizing radiation.

Growth-stimulating doses are applied in crop production to increase the yield of agricultural crops by 10–12 %.

The main biological manifestations of damaging doses of irradiation in plants include:

1. Morphogenetic effects of chronic irradiation, manifested as growth inhibition, abnormal SHOOT development, ectopic bud formation, altered needle size and orientation in conifers, and disruption of leaf macrostructure.

2. Alterations in Anatomical Structure, specifically the Structure of Chloroplasts (enlargement, shape distortion, degradation of thylakoid and lamellar membranes); accumulation of large plastoglobules in Plastids; and an increase in cell number and size during the formation of giant leaves and needles.

3. Changes in physiological processes, namely a decrease in chlorophyll content and its ratios, reduced Transpiration rates; increased Lipid Peroxidation; and elevated levels of Phenolic Compounds due to adaptive metabolic adjustments.

4. Impairment of protein biogenesis, manifested as increased variability in protein composition; altered levels of Proteins and free Amino Acids, which serve as precursors for Protein Synthesis; synthesis of novel proteins; and altered Gene Expression of certain biosynthetic pathways.

5. Induction of genome instability, including an increased frequency of Chromosomal aberrations, chlorophyll Mutations, impaired gene expression, and other cytogenetic effects.

6. Tumor formation, specifically the induction of bacterial plant tumors and the loss of growth control.

There is data concerning the radiosensitivity (the ability of an organism to respond to minimal doses) of over 2,000 plant species. Lilies are among the most radiosensitive (with an LD₅₀ of up to 10 Gy), followed by conifers (LD₅₀ of 10–20 Gy for pine, and 20–60 Gy for spruce) and grapes (LD₅₀ of 10–90 Gy). Among herbaceous plants, legumes are quite sensitive (broad bean LD₅₀ is 100 Gy). Cereal crops exhibit low radiosensitivity, while cruciferous plants show the lowest, apparently, with radishes having an LD₅₀ of 1000–1500 Gy.

Experimental studies on the acute and chronic effects of ionizing radiation indicate that 10 Gy is the threshold dose for damaging effects, beyond which radiation stimulation is no longer observed.

When converted to chronic annual exposure, this dose corresponds to a surface soil activity of 3.7 • 10¹⁴ Bq/km².

When examining the effects of γ-radiation on an oak-pine forest, it was established that at a dose rate of 0.02–0.05 Gy/day (approximately 7.3–18.3 Gy/year), no changes were observed in trees of this age. At a dose rate of 0.1–0.2 Gy/day, pine trees begin to show a response, subsequently displaying browning followed by needle drop. Increasing the dose rate to 1 Gy/day leads to a reduction in forest species diversity. Radiation dose rates exceeding 1 Gy/day result in the death of higher plants, which are almost entirely wiped out at a dose of 5 Gy/day.

The most resilient organisms in forest biocenoses are small shrubs and grasses (such as heather and sedge), a significant portion of which are located within the soil and are capable of producing ROOT sprouts that quickly regrow.

It should be noted that major radiation accidents—most notably the 1957 Kyshtym accident at the radiochemical plant in the Southern Urals and the 1986 Chernobyl NPP accident in Ukraine—have established that pine is the most radiosensitive higher plant to ionizing radiation. The browning and subsequent shedding of needles in pine trees resulted primarily from external exposure to radioisotopes that fell out from the accident cloud, which was rich in short-lived radioisotopes. The root systems of the trees, which sorbed radioisotopes from the soil, suffered relatively little damage.