Studies on the Mechanism of the Oxygen Effect on the Radiosensitivity of Tradescantia Chromosomes

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Abstract
Irradiation of inflorescences of 2 clones of T. paludosa was made in an exposure chamber with attachments for evacuation and introduction of gases. A single X-ray dose of 400 r at 50 r/min. was used with 7 different percentages of O2 in the exposure chamber. Prepns. of the first post-meiotic mitosis in micro-spores were examined for chromosomal interchanges and interstitial deletions. Pre-exposure evacuations to remove air from the anthers had no effect on the frequency of aberrations. Except for irradiation in air (21% O2) there was close agreement between the 2 clones, with a rapid increase in frequency of aberrations between 2 and 20% O2, followed by a gradual incline towards 100% O2. The mechanism of the O2 effect on initial breakage or on recovery was detd. by using a single X-ray dose of 300 r at 300 r/min. Pre-treatments with and without oxygen were given, and exposures were made in a vacuum or in O2, with the addition or removal of O2 during or immediately after irradiation. The results showed the pre- and post-treatments had no effect on aberration frequency but that it was increased by the presence or addition of O2 during irradiation and decreased by the removal of O2 during irradiation. O2 seems to affect chromosome breakage rather than restitution, though it is probable that the effect is indirect, through the production of a substance such as H2O2, which in turn may modify the restitution process.