EXPERIMENTALLY INDUCED CHROMOSOME ABERRATIONS IN PLANTS

The finding of Lilly and Thoday that potassium cyanide produces structural chromosome changes in root tips of Vicia faba was confirmed. Like mustards, diepoxides, and maleic hydrazide, potassium cyanide seems to act on cells at early interphase. A tendency of cyanide breaks to be concentrated in heterochromatic segments of the chromosomes was evident. The production of chromosome aberrations by cyanide proved to be practically unaffected by the temperature during treatment. In agreement with Lilly and Thoday, the effect of potassium cyanide was found to be dependent on oxygen tension during treatment. The effect of potassium cyanide increases with increasing oxygen concentration up to 100 per cent oxygen. In the absence of oxygen, potassium cyanide was not completely inactive, but produced a low, though significant frequency of aberrations. Pretreatments with 2.4-dinitrophenol did not influence the effect of potassium cyanide. When bean roots were treated with potassium cyanide before a treatment with 8-ethoxycaffeine, or at the same time as they were treated with 8-ethoxycaffeine, the effect of 8-ethoxycaffeine was almost completely suppressed. The effects of a number of other heavy metal complexing agents were also tested. Sodium fluoride, potassium thiocyanate, carbon monoxide, o-phenanthroline, 2.2-bipyridine, and sodium azide were without radiomimetic effect under the conditions employed, and so was a mixture of sodium azide and sodium fluoride. A low, but quite significant, radiomimetic effect was obtained after treatments with sodium diethyldithiocarbamate, cupferron, and 8-hydroxyquinoline. Under anaerobic conditions, the effects of cyanide and cupferron were both quantitatively and qualitatively indistinguishable. Unlike the effect of cyanide, the effect of cupferron was not enhanced by the presence of oxygen. The effects of the same heavy metal complexing agents were tested on the activities of the enzymes catalase and peroxidase. The activities of both of these enzymes were found to be totally inhibited only by potassium cyanide. In the other cases, little correlation was found between ability to inhibit the activities of these enzymes and ability to produce chromosome aberrations. In a number of experiments, hydrogen peroxide was found to be without radiomimetic effect, whether alone or in combination with potassium cyanide. t-Butyl hydroperoxide proved to be active. The effect of t-butyl hydroperoxide was substantially increased by pretreatments with 2.4.-dinitrophenol. The results are discussed, and it is concluded that the observations made do not support the hypothesis that hydrogen peroxide is involved in the production of chromosome aberrations by potassium cyanide. The possibility that organic peroxides are involved cannot be excluded on the bases of the experimental results. As an alternative hypothesis, it is suggested that iron or other heavy metals are present in the chromosomes and that cyanide and other heavy metal complexing agents produce chromosome aberrations by reacting with these metals.