| Genetic effects of thermal neutrons on wheat S. MATSUMURA National Institute of Genetics, Misima, Japan The thermal neutron irradiations were conducted in the thermal colum of the Japan Atomic Energy Research Institute's Nuclear Reactor, JRR-1. Hole No. 7 (North 6"phi Thermal Column Access Port) was selected to keep gamma contamination in the thermal column as small as possible. The thermal neutron flux was calculated to be 2.6 x 108 n/cm2 -sec when the reactor was operated at 25 kilowatts. In a preliminary experiment dormant seeds of Triticum monococcum flavescens were kept in this hole for 1-5 weeks (actually 131.5-1068.7 kWh: 4.9-40.0x1012 n/cm2). The data are shown in Table 1. The higher was the dosage of the thermal neutrons, the more delayed were germination and growth of seedlings. The seeds were almost uniformly injured. There was no germination at 4-5 week treatment (31.4-40.0x1012 n/cm2, IV-V) and the seedlings did not grow and died in early stage. In the root tips just after germination the frequency of chromosome aberrations increased with the increase of dosage and in IV and V more than 95 % of dividing cells had chromosome aberrations. In PMC's a ring of 4 chromosomes was observed. In X2 chlorophyll mutations, such as albina, xantha, viridis etc. were found. The frequency of chromosome aberrations and chlorophyll mutations increased in proportion to the dosage. In the next experiment the reactor was operated at 40 kW and the seeds of T. monococcum (2x), T. durum (4x) and T. vulgare (6x) were subjected to thermal neutrons at 5 different distances in the reactor hole No. 7 for 2 weeks (actually 990.8 kWh). The thermal neutron flux ranged from 0.49-4.2x108 n/cm2 -sec (total flux: 4.4-37.5x1012 n/cm2). At 37.5x102 n/cm2 T. monococcum did not germinate, while the seeds of T. durum and T. vulgare germinated but the seedlings died in on early stage without further growth. At 20.6x1012 n/cm2 about 2/3 of seeds germinated in T. monococcum but the seedlings soon died, while in T. durum and T. vulgare slow growth of seedlings was observed. Therefore, T. monococcum was most sensitive to thermal neutrons and T. durum was unexpectedly most resistant. There was no significant difference between T. durum and T. vulgare. |