Annual Report 2003

gamma ray from 44 TBq 60 Co was used asthe low LET ionizing radiation control. Thesurvival capability was determined by theexistence of a growing point. The lethal dose50 (LD 50 ) and mean lethal dose (D 37 ) of eachbuckwheat variety were obtained from adose-response curve.We found that a penetrating depth of 1.7mm was necessary to reach the target cells,and when the penetrating depth was lessthan 2.2 mm, the effect of the hulls wasremarkable in buckwheat seeds. The relativelethal effect per dose increased with theincreasing LET in both C and Ne ions. Theeffects of Ar and Fe ions were higher thanthose of the He, C, and Ne ions. The LD 50 ofthese ions ranged from 30-500 Gy (Rot) and10-300 Gy (Bot). Bot was more sensitivethan Rot to these ions.The RBE was estimated as the D 37 ofgamma ray (control) divided by the obtainedD 37 value in Rot and Bot. The maximum valuesof RBE in D 37 were 17.7 and 22.5 in Botand Rot, respectively. Those were observedat 305 keV/µm (Fig. 3).The mutation rates of 20 Ne 8+ (350 MeV)were the same as those for the gamma raysand lower than those for any other ion beamsin the M 2 generation of tartary buckwheat.It seemed that mutation rates of 20 Ne 8+ (350MeV) were not exact because of the shallowpenetrating depth (Fig. 4). The mutationrates increased with the dose rose, whereasmortality and sterility rates in the M 1 generationalso increased. The optimal dose formutation induction by carbon ion ( 12 C 5+ (220MeV)) was estimated to be about 20-40 Gy.Some mutants were obtained by ionbeams. Most of mutants were morphologicalones, while a few leaf color mutants wereobtained (Fig. 5).would be disease-resistant and afford highqualitytea, however, there are no trichomelesstea plants suitable for green teabreeding among the existing cultivars orgene resources.Thus, we made two varieties of trichomelsstea mutants were with -rays .RBE00.1 1 10 100 1000LET (keV/m)Fig. 3The relationship between the RBE (D 37 ) and the LET (surface) in Bot (a)and Rot (b). Gamma ray (), He ions (), C ions (), Ne ions (), Arions () and Fe ions () irradiated to the intact seeds.Mutation rate (%)25201510500.1 1 10 100 1000LET (keV/m)302520151050(a)Fig. 4Mutation rate in the M 2 generation of tartary buckwheat.RBE25201510550 100 200 300 400 20 40 60 80 100 20 40 60 80 100 20 40 100Gamma ray 4 He 2+ (100MeV) 20 Ne 8+ (350MeV) 12 C 5+ (220MeV)Dose (Gy)(b)Inducing mutation of tea trichomelessIn the tea (Camellia sinensis (L.)O.Kuntze), trichomes are seen on the surface ofyoung leaves (except for a wild variety), withhigh density in the midrib, and scarcely inmature leaves. It would be desirable tobreed a trichomeless tea cultivar, whichFig. 5Yellow leaf mutant.Annual Report 2003 117