Abstract
Using 3-year-old saplings of Kuma-sugi, a local cultivar being propagated. by cuttings, effects of seasons of gamma ray irradiation, pruning, and internal disbudding on induction of somatic mutation were studied.
Firstly in 1965, 10_??_11 saplings planted in pots with 30cm diameter were exposed to gamma rays of 580R at 96.7R/day (20h) for each combined treatment of the irradiation. season and pruning treatment (just after the end of the irradiation) as following:
1. Irradiation season (a) June (b) August (c) October
2. Pruning treatment (a) No pruning (b) Light pruning (c) Heavy pruning.
Detection of somatic mutation was continued until the winter of 1966. The somatic mutations are classified into the following five categories: (1) Chlorophyllous changes, (2) Waxless changes which seems to be reduced wax coating on green tissues, (3) Morphological changes in needles, (4) Changes in winter coloration of green tissues, (5) Morphological anomalies. In the fall of 1966, the number of the shoots which seem to emerge after the irradiation was counted to compute the mutation rate per shoot. As shown in Table 1, the highest somatic mutation rate per shoot as 2.13% was obtained in the August irradiation and it was low in the October irradiation. A heavy pruning made just after the end of the irradiation was found to be ineffective to induce somatic mutations. Kind and frequency of the somatic mutations are shown in Table 2. In the August irradiation, rate of the chlorophyll mutation was comparatively low, and changes in the wax coating and needle morphology were more frequent than those in the other irradiation seasons. Moreover, in the upper crown of the saplings, somatic mutation rate was always higher than those in the lower crown.
Concerning the sector size of the somatic mutations in Kuma-sugi is usually large and, in this experiment, nearly a half of the chlorophyll mutations were complete change. Meanwhile, most of the waxless changes and morphological changes were non-chimeric. The larger sector size of the somatic mutations may have resulted from a simple structure of shoot apical meristem of sugi and the internal disbudding with radiation as termed by YAMAKAWA and SEKIGUCHI8).
In the second experiment, 9 potted saplings were irradiated with gamma ray exposures of 300R, 600R and 900R at 100R/day (20h), respectively. The irradiation was terminated on the same day and ten shoot apices per exposure were sampled to make permanent preparates for microscopic observation every other day until 40 days after the end of the irradiation. The samples were fixed with CRAF III- or ALLEN'S fixatives and stained with safranin and fast green or feulgen reaction. The apical meristem of sugi is simple in comparison with that of angiosperms and seems to have one tunics layer and corpus. By the gamma ray irradiation of sugi saplings, even at 300R, some apical meristem begin to show their cell damage within the first week of the irradiation. In 7-10 days after irradiation some apical meristematic cones showed flat dome following sinking of the median part of the dome of apical initials. The damage was severer at the higher exposures, and the irradiated apical meristem was destroyed-an internal disbudding, and recovering date was also delayed with a positive correlation to gamma ray exposures. New apical meristem was expected to be regenerated from a comparatively smaller number of activated cells in the apical initial zone at lower exposure, and in axillary initial zone at higher exposure. It might thus result in a high frequency of complete somatic mutations or sports in sugi.
