Tages- und jahresperiodik der strahlenresistenz pflanzlicher zellen

Abstract

The outer and inner epidermises of red onion scales of Allium cepa and leaflets of the moss Mnium cuspidatum show a marked diurnal rhythm of their resistance toward α- and u.v.-rays (Figs. 1 and 3). In the onion epidermises a-radiation resistance reaches its maximum at 12 noon, its minimum at 8 p.m. Mnium cuspidatum show the maximum of the α-radiation resistance at midnight and the minimum between noon and 4 p.m. In addition to it the diurnal changes of nuclear sizes of the inner epidermises of onion scales were measured (maximum: 12 noon, minimum: 8 p.m.). The curves parallel each other (Fig. 2). Both the cells of the onion scale and those of the leaflets of various mosses and liverworts also show a marked annual rhythm of resistance to α- and u.v.-radiation (Figs. 4-9). The bryophytes reach their maxima of radiation resistance in June and their minima in December, the onion epidermises are most sensitive to radiation in June and most resistant in December. The nuclear size of the inner epidermises of onion scales show also an annual rhythm beside the diurnal rhythm (Fig. 11). The nuclei reach maximum size in summer, minimum size in winter. While in the diurnal rhythm of the inner epidermises of onion scales the maxima of nuclear size coincide with the maximum radiation resistance of the cells, in the annual rhythm the cells show their greatest radiation resistance when the nuclei are smallest and their greatest sensitivity when the nuclei are largest (Fig. 10). The survival curves of α-irradiated outer epidermises of onion scales show a rapid drop within 24 hr after intensive irradiation, while in epidermises exposed to lower doses of radiation death occurs only after some days. Cell death within 24 hr is believed to be caused by direct damages of the cytoplasm, the slow death after some days, on the other hand, by injuries of the nuclei, which are caused by much lower doses and which have a gradually increasing harmful effect upon the metabolism of the cell. (6,24,25) Our observations refer, unless stated otherwise, to the radiation resistance of cytoplasm within 24 hr. The coincidence in the diurnal rhythm of the maxima of nuclear size and radiation resistance of the cytoplasm seem to contradict the observations of Sparrow and co-workers, (26, 27, 28, 29) according to which plants with smaller nuclear volumes show greater radiation resistance than plants with a bigger nuclear volume. Sparrow's determination of radiation resistance was, however, based on the observation of injury in development and growth made after a long period of time. The irradiation doses used by him were much lower, and he did not determine the radiation resistance of the cytoplasm itself, but the damage indirectly caused by injuries to the much more radiosensitive cell nuclei. Results similar to those of Sparrow could be found in long-term experiments with outer epidermises of red onion scales: at the times when the maximum of nuclear size (9 a.m., 418 μ 2) and the minimum (6 p.m., 339 μ 2) were observed, epidermises of onion scales were exposed to an irradiation series each covering a very wide range of doses (Table 1, Fig. 12, 13). Again, in the case of doses having a lethal effect within 24 hr, the maxima of nuclear size and radiation resistance coincided. After one to three weeks slices with nuclei which were smaller at the time of irradiation (6 p.m. series) now were more resistant than cells with bigger nuclei at the time of irradiation. The larger nucleus in the diurnal rhythm, which is probably more active, seems to have supplied the cytoplasm with substances (enzymes?) which, in spite of heavy damage to the nucleus itself, make it possible for the cytoplasm to survive higher doses within 24 hr. Lower doses, however, which no longer affect the less radiosensitive cytoplasm, damage larger nuclei—in accordance with Sparrow's findings—more than the smaller ones. Experiments concerning the causative factor of the annual resistance rhythm yielded the following results: ( a) On the other hand, the radiation resistance of bryophytes seems to be directly dependent on day length. Experimental shortening of the daylight period in summer with all other conditions remaining constant brought about a marked lowering of radiation resistance (Figs. 16-18). This would correspond to the fact that the maximum of radiation resistance observed in the annual rhythm of bryophytes in June coincides with the longest day of the year and the minimum in December with the shortest day (vice versa in the case of onions).