Specific-locus experiments show that female mice exposed near the time of birth to low-LET ionizing radiation exhibit both a low mutational response and a dose-rate effect

Female mice were exposed to 300 R of 73-93 R/min X-radiation either as fetuses at 18.5 d post conception (p.c.) or within 9 h after birth. Combining the similar results from these two groups yielded a specific-locus mutation frequency of 9.4 X 10(-8) mutation/locus/R, which is statistically significantly higher than the historical-control mutation frequency, but much lower than the rate obtained by irradiating mature and maturing oocytes in adults. Other females, exposed at 18.5 days p.c. to 300 R of 0.79 R/min gamma-radiation, yielded a mutation frequency that was statistically significantly lower than the frequency at high dose rates. The low-dose-rate group also had markedly higher fertility. It appears that the dose-rate effect for mutations induced near the time of birth may be more pronounced than that reported for mature and maturing oocytes of adults. A hypothesis sometimes advanced to explain low mutation frequencies recovered from cell populations that experience considerable radiation-induced cell killing is that there is selection against mutant cells. The reason for the relatively low mutational response following acute irradiation in our experiments is unknown; however, the finding of a dose-rate effect in these oocytes in the presence of only minor radiation-induced cell killing (as judged from fertility) makes it seem unlikely that selection was responsible for the low mutational response following acute exposure. Had selection been an important factor, the mutation frequency should have increased when oocyte killing was markedly reduced.     

The use of growth retardants to improve microtuber formation by potato (Solanum tuberosum)

The growth retardant chlormequat stimulated microtuber formation by a recalcitrant cultivar of potato (Solanum tuberosum), but reduced microtuber fresh weight in a cultivar that tuberised readily in its absence. Inhibition of microtuber growth by high concentrations of chlormequat was confirmed using a different in vitro system where all cultivars tuberised in the absence of growth retardants.Alternative growth retardants were tested. Daminozide also had a detrimental effect on microtuber fresh weight, but ancymidol and paclobutrazol did not inhibit microtuber growth at the concentrations required for stimulation of tuberisation by recalcitrant cultivars. In addition, 10^-5 M ancymidol and paclobutrazol inhibited premature sprouting of microtubers in vitro.Abbreviations BA benzyladenine - DMSO dimethyl sulphoxide - FW fresh weight - PAR photosynthetically active radiation - SE standard error