The present investigation was carried out at the Farm of the Rice Research and Training Center, Sakha, Kafr El Sheikh Egypt during rice growing seasons 2006 and 2007. Dry seeds of the four rice varieties namely; Sakha 101, Sakha 102, Sakha 104, and Egyptian Yasmine were irradiated with 100, 200 and 400 Gy gamma rays. The seeds of the different treatments in addition to the parental varieties (as control) were grown and transplanted individually in a randomized complete block design with three replications for each treatments along with the control to study the response of different rice varieties to different doses of Gamma rays for some agronomic characters, grain yield and its components and to select some useful mutants carrying desirable characters from different populations in M2 generation such as earliness, short stature, resistance to blast, and high yielding ability. The results of the M1 generation indicated that in general, germination percentage decreased and the reduction was significant for the Sakha 101cv, while number of days to heading, plant height and panicle length were decreased by increasing the doses of Gamma rays for most of the studied varieties. On the other hand, the mean values of number of panicles per plant were increased by increasing the dose of gamma rays for the Sakha 101, Sakha 102 cvs. The spikelet sterility increased with the increase in the dose of gamma rays for all studied varieties and therefore, grain yield per plant was decreased. In the M2 generation, increasing the dose of gamma rays decreased germination %, and plant height for the two rice varieties; Sakha 102, and Egyptian Jasmine. The mean values of number of panicles per plant were increased for the Sakha 101, 102 and Sakha 104 cvs, while it decreased with Egyptian Yasmine. One hundred grain weight was decreased for Sakha 102, Sakha 104 and Egyptian Yasmine, while the Sakha101variety was not affected by gamma rays for this trait. Panicle length and chlorophyll content was increased by the dose of gamma rays for the three varieties; Sakha 101, Sakha 104, and Egyptian Yasmine. Sterility % was increased and the highest increase was observed for the indica rice variety Egyptian Jasmine and therefore, grain yield per plant was decreased for these varieties. Wide ranges of variability were detected in M2 irradiated populations of all varieties for most of the characters studied comparing with the controls, indicating the possibility of selecting mutants with desirable traits to be tested in subsequent generations. The genetic parameters of studied characters for both control and M2 irradiated populations of all varieties indicated that, in most cases, there were considerable increases by irradiation for both phenotypic and genotypic variances, genetic coefficient of variation, heritability as well as expected genetic advance.
