PLANT ADAPTATION TO ANTHROPOGENIC FACTORS - 2017

4. EFFECT OF IONIZING RADIATION ON PLANT ORGANISMS

4.4. Components of the Radiation Syndrome in Plants

It is difficult to identify any biological process or morphological Structure that remains entirely unaffected following exposure to a sufficiently high dose of radiation. Even a Brief Overview of how higher plants react to irradiation reveals just how many aspects of plant METABOLISM and functioning are altered. Chromosomal and chromatid aberrations, nuclear pyknosis, disruptions in Cell/39.html">Mitochondrial and Chloroplast ultrastructure, and Changes in membrane permeability are all hallmark cellular damages that inevitably lead to impaired cell Functions.

At the level of Organs and the whole plant, alterations in overall architectonics become apparent. Roots may exhibit enhanced branching driven by the inhibition of axial growth, increased activity of secondary Meristems, and The formation of roots originating from pericycle Cells. The Development of ROOT hairs is likewise impaired. Stem Morphology reflects radiation-induced suppression of apical meristems combined with the activation of dormant buds. Irradiation can alter phyllotaxis and leaf arrangement patterns, as well as trigger fasciations and tumor-like growths. Occasionally, the branching pattern itself is modified. Radiobiological processes give rise to aberrant leaf blade shapes, the fusion of multiple leaf primordia, and altered venation patterns. Sometimes, leaves become twisted due to uneven blade growth, which also results in a crinkled appearance. Deviations in the formation of reproductive organs are frequently observed, ultimately leading to various malformations.

Irradiation impacts a wide array of physiological processes: the course of ontogenesis is either accelerated or delayed, which manifests as altered morphogenetic rates. Severe damage to the meristems of a seed embryo can yield seedlings completely devoid of formative Tissues, commonly referred to as "y-sprouts." Irradiation also alters the levels of various intracellular compounds—sugar phosphate esters, organic acids, Amino Acids, NUCLEOTIDES, as well as more complex molecules such as pigments, phytohormones, and secondary metabolites. Altogether, these changes culminate in a dose-dependent, generalized shift in metabolism, affecting Respiration, Photosynthesis, The Biosynthesis of numerous compounds, and structural biogenesis.

All of these reactions in irradiated plants can be viewed as manifestations of radiation syndrome—a complex constellation of disease symptoms constituting radiation Pathogenesis.

Amidst the wide array of symptoms associated with radiation damage in plants, a limited set of underlying causes can be identified. At the core of plant injury is the death of cells within formative tissues (meristems). This leads to widespread damage across numerous organs and tissues, given that all plant organs originate from meristems. Consequently, the full picture of radiation syndrome in plants encompasses a broad spectrum of changes that grows increasingly diverse when irradiation occurs during periods of peak meristematic activity.

It is worth noting that the radiation syndrome in plants strongly depends on exposure conditions and the physiological state of the plants at the moment of radiation exposure (Fig. 15).

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Fig. 15 - Components of the radiation syndrome

Induction of Organogenesis by irradiation. Radiation exposure frequently alters the normal pathway of plant organogenesis. When a vegetative plant is irradiated, organogenesis is disrupted as a direct consequence of meristematic cell inactivation.

Factors originating in irradiated calli can diffuse through the nutrient medium to non-irradiated explants, thereby inducing organogenesis in the latter. Consequently, it is logical to assume that organogenesis is induced either by the products of radiation-Chemical Reactions among tissue-contained substances, or because irradiated callus cells produce cytokinin-like compounds. Indoleacetic acid, kinetin, and myo-Inositol, previously irradiated with γ-rays at doses up to 250 Gy, were tested for their effects on organogenesis; all of these irradiated substances—myo-inositol in particular—were found to trigger organogenesis in tobacco callus. Earlier studies demonstrated that irradiated sucrose likewise acquires The ability to induce organogenesis through its radiation-chemical degradation products.

In the radiation-induced organogenesis of plants, one should distinguish between Direct and Indirect radiation effects, as well as remote mediated effects driven by regulatory mechanisms—specifically, the release of apical dominance and the action of physiologically active substances generated by The impact of radiation on tissues.

Cell gigantism. Plant irradiation is frequently followed by the appearance of abnormally large, giant cells. Such cells have been documented in peanut cell cultures (Arachis hypogaea). Exposing this culture to a dose of 500 Gy completely halted Cell Division, yet cell expansion continued unchecked, allowing 50–60% of the cells to reach colossal proportions. These giant cells are characterized by dense Cytoplasm, an increased number of starch grains, and active cyclosis. Evidently, the formation of giant cells reflects a loss of control over cell elongation. Normally, cells capable of division initiate the process only upon reaching a specific size. However, when irradiation prevents cell division, the limits on maximum cell size are lifted.

This radiation-induced cell gigantism has also been studied in the alga *Oedogonium cardiacum*. In this species, giant cells arise not only directly from irradiated cells but also from daughter cells formed through the division of irradiated cells that initially showed no division anomalies. Cell gigantism is closely linked to mitotic arrest. Ultrastructural analysis of these giant algal cells revealed disruptions in the packing of protein molecules within intracellular ultrastructures, which may play a role in morphogenesis control. Furthermore, it is possible that ionizing radiation damages the Cytoskeleton, leading to altered Cell size and shape.

Morphological anomalies. Following the seed irradiation of plants, seedlings frequently develop morphological anomalies, which manifest as damage to specific organs such as leaves and stems. These anomalies primarily affect the first true leaves, whose primordia had already developed within the seed embryo and were thus directly damaged. The Nature of this phenomenon is fairly straightforward: a fraction of the meristematic cells loses its capacity to divide, failing to generate the cellular streams necessary for normal leaf blade formation, and resulting in constrictions where cell generation halted. Subsequent leaves of newer developmental orders generally show no signs of radiation injury.

When vegetative plants are irradiated, morphological anomalies can emerge in any organ that was at the primordial bud stage during exposure. All such anomalies can be classified as meristemogenic. Much less frequently, changes arise As a result of radiation-induced somatic Mutations in loci that control morphogenesis. If cells carrying such mutations are not eliminated during subsequent cell divisions, organs with altered morphological structures may develop. Changes of this type are genetic in nature and can be either heritable or morphogenetic.

Radiation chimeras. Due to the stochastic distribution of absorbed energy among embryonic cells during the irradiation of seeds, seedlings, or vegetative plants, the genomic damage inflicted across different cells varies considerably. In other words, irradiation ensures that formative cells, including initials, comprise a heterogeneous population of damaged cells harboring various genetic defects. If these injuries are severe enough to preclude cell division, the damaged cells fail to generate cell lineages or form corresponding sectors within tissues or organs during histogenesis. Conversely, if the radiation-induced genomic alterations do not prevent cell division, the progeny of these cells become incorporated into developing organs and tissues in accordance with the reproductive Functions of the respective cell lines. As a result, a plant grown from an irradiated seed, or a SHOOT developed from an irradiated bud, consists of genetically distinct tissues, effectively forming a chimera. Within the seed embryo, cells are potentially capable of forming various plant tissues. Because radiation-induced damage is stochastic, plants grown from seeds exposed to the exact same dose of ionizing radiation may develop into chimeras of entirely different natures. For a chimera to arise, somatic mutations must occur within the initial Cells of the embryo.

Lifespan of the irradiated plant. Irradiated plants may exhibit shifts in the duration of their vegetative period, even though the regulatory systems controlling the timing of developmental phases are known to be highly conservative and largely dependent on factors responsible for tracking "biological time." However, beyond responding to external geophysical cues, plant development is also governed by an endogenous timekeeper. The superimposition of external signals and internal developmental pacing typically results in a strictly regulated temporal unfolding of ontogenesis. Irradiation disrupts the duration of the vegetative period, extending it at higher doses. One reason for delayed plant development following irradiation is the elongation of Cell Cycle durations. Given that plant development, viewed as a sequential series of morphological events, requires a specific number of cells generated by apical meristems, any deceleration in cell division naturally leads to a slowdown in overall plant development. In seed irradiation experiments, this is visibly apparent as a developmental lag in seedlings compared to controls. At exceptionally high doses, development is profoundly suppressed, the vegetative period is severely prolonged, and plants occasionally fail to transition into the generative phase entirely. Notably, the reaction of plants to photoperiodic influences displays a remarkably high Radioresistance.

Changes in ploidy induced by irradiation. Ionizing radiation can alter the ploidy level of plant cells. The Emergence of tetraploid cells has been observed when irradiating the buds of vegetatively propagated plants. Genomic mutation—specifically the doubling of the chromosome set—does not hinder cell proliferation, and can give rise to a shoot in which a portion of the cells (sometimes quite substantial) is tetraploid. The frequency of genomic mutations can exceed that of other mutation types. For instance, in the muscadine grape (*Vitis rotundifolia*), tetraploids were obtained in which all cells of the internal tissues were tetraploid, whereas the epidermis remained diploid. In the mulberry (*Morus nigra*), acute γ-ray irradiation of shoots induced tetraploid cells that established their own cell lineages during shoot growth, resulting in a chimera composed of both diploid and tetraploid cells. Radiation-induced conversion of diploid cells into tetraploids has also been documented in other plant species.

Under The Influence of radiation, haploid plants can likewise develop. This phenomenon occurs when flowers are pollinated with radiation-killed pollen, which induces parthenogenesis as a result of pseudogamy. Obtaining haploid plants via this method holds practical significance for plant breeding.

Radiostimulation of plants. When examining the dose-response relationship of plant growth functions—measured via height or biomass—by testing increasing radiation doses ranging from minuscule fractions of a gray up to higher levels, many plant species reveal a specific dose interval wherein irradiated plants exhibit more vigorous growth than non-irradiated controls. Such experiments necessitate a detailed exploration of dose ranges spanning from fractions of a gray to several tens of grays. The enhancement of plant growth functions at low radiation doses is termed radiostimulation, while the dose range within which this stimulatory effect occurs is referred to as the stimulating dose interval, and sometimes simply as "low doses"—although from the perspective of radiation-chemical reactions, these doses are not all that small.

Radiostimulation is observed not only following seed irradiation, but also when ionizing radiation is applied to the bulbs of various plants, other organs of vegetative propagation (such as cuttings), and seedlings.

Because they are linked to the acceleration of growth processes, the manifestations of radiostimulation can vary: accelerated seed germination, enhanced field emergence, increased branching, accelerated root formation in cuttings, elevated seed productivity, and the accumulation of greater vegetative biomass. Improvements in product quality are likewise noted.