Some factors influencing radioresistance and tumor induction in plants.

Abstract

A comprehensive review is presented of the effects of acute and chronic irradiation on plants. Major emphasis is placed on the cytological, histological, morphological, and developmental responses of plants to ionizing radiation The tolerance of plants to ionizing radiation varies over a wide range. Original data and a summary of some published information concerning tolerance to both acute and chronic exposure will bc presented. Various correlations between known characteristics and plant tolerances can be made. For instance, chromosome size and chromosome number are both of great importance in determining the tolerance of a given species. In addition to the inherent factors controlling radiosensitivity many environmental treatments can alter the response of plants to radiation treatment. The importance of these factors, whether laboratory controlled or a part of a natural environmnent, are considered. The possible significance of radioactive fall-out in relation to plant growth and the nature and extent of -its uptake by plants will be mentioned. Practical uses of radiation other than in plant breeding are so far rather restricted. Many reports of stimulation of plant growth have appeared but not all are reproducible. Recently a dramatic case has been reported for peach seedlings. Such findings may offer important cluesmore » to factors controlling growth and dormancy in plants. (auth) This presentation seeks to relate in time and magnitude the biological events that are irferred from paired dose lethality studies on experimental animals and the histopathologic observations following a single large x-ray exposure. Tbe paired dose studies lead to the assumnption that recovery from susceptibility to the lethal consequenes of the second dose occurs in general at a rate such that within from a few days to a few weeks half- recovery is attained. The question is asked whether this recovery can be represented as a single exponential function Results obtained with partial body irradiation of rats suggests that differert tissues repair at different rates and that departures from the exponen-tial relationship at very short testing intervals as found in selected data from the literature may perhaps be tentatively assigned to this cause. Other reasons are cited for doubting that recovery at all times may be precisely described as a single exponential process. The existence of an irreparable injury component is based on the life span shortening found for animal survivors of a large single dose. The amount of life shortening per roentgen depends on dose size. Less established is the proposition that the irreparable damage is constant throughout life and can be measured at all times by the reduction in the magnitude of a second dose necessary to produce an acute lethal result. Histopathologic mdiation effects, reversible and irreversible, the temporal phases in the sequence of effects, and the tissue repair phenomema are related to phases of injury measured by paired- dose techniques or life span and are found compatible with certain hypotheses based on such measurements. Irradiated animals dying acutely or subacutely die largely of reparable but inadequately repaired injuries. Those dying later but prematureiy die largely of diseases of the species advanced in onset as a result of advanced aging processes. The progressive arteriolocapillary fibrosis of aging, advanced by initinl radiation injury of small blood vessels and fibrotic repair processes, precedes in development and later accompanies parenchyrmal senescence, which seems to be secondary to vascular senescence. (auth)« less

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