Radiation-induced variation in blast disease resistance in rice

Abstract

A rice variety widely grown in Taiwan, Taichung No. 65, belonging to the “Horai” or Japonica type and suseptible to the blast desease(caused by Piricularia oryzae), was used for this irradiation experiment. The kinds and dosages of radiation used were gamma-ray (form Co60 source, 11 Kr), X-ray (20Kr and 25Kr), and thermal neutron (15hrs. and 20hrs. ). Thc dormant seeds were treated. The process of irradiation was carried out either at the National Institute of Genetics of Japan (Co60 treatment) or at the Brookhaven National Laboratory of U.S.A. (the other treatmcnts), through the kindness of the authorities concerncd. The treated plants were grown in Taichung Agricultural Improvement Station for two to four gencrations without artificial selection. 572 lines belonging to the R5 generation of gamma-ray treatment and bulk populations of thc R3 generation of the other treatments were tested, together with the non-treated contros, in the experimental field at Tonshu, Taichung Prefecture, Taiwan, where the disease was an epidemic both in the first and second crop seasons. Data were collected on an individual plant basis with a series of index-numbers showing the degree of susceptibility to leaf blast. As shwn in Table 2 and Fig.1, irradiated populations generally showed wider ranges of variation in the susceptibility than controls. The variation. caused by irradiation appeared to be continuous and tended toward high resistaucc. Plants in the control plots were seriously damaged and almost killed by the disease, while a part of plants or lines in the irradiated plots showed resistance and survived. The plants selected from a similar experiment in the first crop season for high resistance were found to be resistant in the second crop scason, indicating that the induced variation was heritable (Table 3). The heritability values in this case were estimated to be 13% to 24%. A resistant line was established from this experiment. Based on these experimental results, suggestions were made that the genetic variation in disease resistance found in ilrradiated plots might be due to induced polygenic mutations, and that irradiation might be useful tool for an improvement of blast disease resistance in rice.