By exposing plant root tips to aqueous solutions containing 0.75 ‐ 1 X 10‐2M 8‐ethoxycaffeine (EOC), a high frequency of “subchromatid” and chromatid exchanges are produced. The exchange reaction is induced during prophase and late interphase and can be completely suppressed by anoxia and by inhibitors of oxidative phosphorylation. We have studied the relationship between the frequency of EOC‐induced exchanges and the ATP level in root tips of Vicia faba. Treatments were performed on intact lateral roots (cytological studies) and on excised main root tips (ATP determinations). The concentration of ATP in the roots was increased by pre‐treatments with adenine or adenosine and decreased by exposing the roots to sodium azide. No exchanges were produced by EOC when the concentration of ATP was reduced to 40 % of the normal. An adenine‐induced 70 % increase of the concentration of ATP resulted in a frequency of exchanges which was three times higher than the frequency obtained at a normal level of ATP. Pre‐treatments with adenine counteracted the reduction of ATP concentration and EOC‐induced exchange frequency which is obtained in the presence of sodium azide.

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The effects ofcaffeine, 8-ethoxycaffeine(EOC), and 6-methylcoumarin (6-MC) on chromosome structure were studied with root tips of Alliumproliferum (Egyptian or tree onion) and cell cultures of the Chinese hamster as experimental materials. The results indicate that the three substances produce chromosomal aberrations by similar mechanisms.Root tips were treated at 20°C. The majority of the aberrations were obtained in cells exposed during late G, and prophase, and "subchromatid" and chromatid exchanges predominated among the aberrations produced. The effect was dependent on the ATP concentration in the roots. Very few aberrations were produced when the generation of ATP was suppressed by anoxia or by sodium azide. On a molar basis, EOC was approximately four times more active than caffeine.When treated at 37"C, Chinese hamster cells were affected during the interphase period of DNA synthesis (the S-phase). The effect consisted of a pronounced fragmentation of chromatids. The aberrations were produced independently of the concentration of ATP in the cells. At 37"C, caffeine and EOC were about equally effective as chromosome-breaking agents.When treated at 17"C, Chinese hamster cells responded to caffeine and EOC in very much the same way as did plant cells. The majority of the aberrations were produced in cells treated during prophase or G,. About half of these aberrations (gaps excluded) were "subchromatid" and chromatid exchanges. However, the frequencies of chromatid breaks and the degree of incompleteness of the exchanges were considerably higher in hamster cells than in Allium root tips. EOC was considerably more effective than caffeine. The effect was dependent on ATP.

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