The effect of ionizing radiation on the survival of free plant cells cultivated in suspension cultures

A study has been made of the effects of X-rays and fission neutrons on the survival of free cells of the annual composite Haplopappus gracilis. Free cells (80% unicellular, 20% bicellular), obtained by filtration of a suspension culture, were 80% diploid with the remaining 20% mostly tetraploid. A high efficiency (> 80%) plating technique was used, and the radiosensitivities of stationary and exponentially growing cells were compared. The survival curves showed that fission neutrons were more effective than X-rays in killing cells from both stationary and exponentially growing cultures. The r.b.e. of neutrons at survival lower than 0.5 was 3 to 5. Stationary cells were more sensitive than exponential cells to both X-rays and neutrons. At doses of X-rays up to about 4 Gy, the plating efficiency of irradiated cells was higher than that of the unirradiated control cells. It is proposed that radiation-induced endogenous cytokinine synthesis is probably responsible for this stimulation. (U.K.)