الفهرس 
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Abstract
The present study was carried out at the Agronomy Department, Faculty of Agriculture, Menofya University and the Rice Research and Training Center (RRTC) experimental Farm, Sakha, Kafr EL-Sheikh, Egypt during 2010, 2011 and 2012 rice growing seasons.
The seeds of three rice varieties, namely , Sakha 105, Sakha 102 and Giza 178 were subjected to gamma rays doses (100-200-300 and 400 Gy). M2 generation was subjected to normal conditions to select the best genotype for high yield, short duration and semi dwarf. The M3 generations were subjected to two environments namely normal and drought conditions. The first, second and third generations were grown with the following objectives:
1- To study the response of different rice varieties to the different doses of gamma rays.
2- To induce some useful mutants having some desirable traits such as earliness, short stature and high yield under both normal and water stress conditions.
3- To identify the best genotypes which would be used in rice breeding program for drought tolerance.
The experiment was executed in a randomized complete block design (R.C.B.D) with three replications and the data were analyzed by using the ordinary analysis of variance to test the significance of differences among the genotypes.
The obtained results could be summarized as follows:
- Analysis of variance indicated highly significant mean squares among the genotypes for all the studied characters, which would
indicate overall wide differences among the genotypes under study. And also variation between plants treated with different doses of γ-rays.
- Environments mean squares were found to be highly significant for all the studied characters indicating that the performances of these genotypes differed from non-stress to stress conditions and therefore can be ranked as drought tolerant and sensitive genotypes.
A- The results of M2 generation:
- Increasing the dose of gamma rays decreases the mean values of plant height for the three rice varieties Sakha 105, Sakha 102 and Giza 178. The mean values of number of productive tillers/plant and hundred grain weight increased for the varieties Sakha 105, Sakha102 while decreased for Giza 178.
- The mean values of panicle length decreased by the dose of gamma rays for the three studied varieties. Sterility percentage increased and the highest increase was observed for Giza 178 and therefore, grain yield per plant was decreased. The mean values of number of days to heading was increased by increasing the dose of gamma rays for all the studied varieties.
- Wide ranges 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 range of variability for various characters increased in the irradiated populations, as compared to their respective controls. Further, the analysis of variances revealed that mean sum of squares for different characters were significantly higher than those of their
respective control. Thus the results of present study clearly indicated that mutagenic treatments were effective creating additional variability, which could significantly by utilized for the improvement of rice crop. 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, heritabilities and expected genetic advance.
- Heritability estimates, in general, exhibited the same trend as observed for the genetic variances with regard to different treatments. In general, heritability was moderate to high for most of the studied characters and ranged from 62-98% for days to heading, from 80-90% for plant height, from 89-97% for panicle length, from 87-95% for no. of productive tillers/plant, 58-82% for sterility%, 91-96% for 100 grain weight and 81-94% for grain yield/plant. The genetic advance was quite high for these traits in all the studied varieties. This indicated that for the improvement of such low heritable characters, selection in later generation would be more rewarding.
B - The results of the M3 generation:
Forty mutants were selected from M2 and grown in M3 generation from the different gamma rays treatments, as these mutants were grown and their parents in M3 during 2013 rice growing season under both normal and stress conditions. These lines were 13 mutants for Sakha 105, 10 mutants for Sakha 102 and 17 mutants for Giza 178.
These mutants were evaluated in the field under both normal and water stress conditions and the best desirable mutants were determined for some desired character specially drought tolerance.
A- Sakha 105 mutants:
- The best mutants for Sakha 105 under normal conditions were lines No. 2, 4, 5, 8, 9, 10, 11, 12 and 13 in days to heading, plant height, number of productive tillers per plant and grain yield per plant.
- The best mutants under water stress conditions were lines No. 1, 2, 4, 8, 9, 10, 11, 12 and 13 in days to heading, plant height, no. of productive tillers per plant and 100 grain weight.
B- Sakha 102 mutants:
- The best mutants for Sakha 102 under normal conditions were lines No. 2, 4, 5, 6, 9 and 10 in days to heading, no. of productive tillers per plant, grain yield per plant and 100 grain weight.
- The best mutants under water stress conditions were lines No. 2, 4, 5, 6 and 10 in sterility percentage, number of productive tillers per plant, grain yield per plant and days to heading.
C. Giza 178 mutants:
- The best mutants for Giza 178 under normal conditions were lines No. 1, 4, 6, 7 and 10 in days to heading, number of productive tillers per plant and grain yield per plant and sterility percentage.
- The best mutants under water stress conditions were lines No. 1, 3, 4, 5, 6, 7, and 10 in sterility percentage, days to heading, grain
yield per plant, no. of productive tillers per plant and 100 grain weight.
According to the results obtained the entire selected mutant plants (high tillers, early maturity and high yielding potentially) in M2 generation showed the same appearance in M3. Therefore, they could be considered a true breed.
The present findings clearly indicated that the useful morphological and physiological mutants isolated from the local varieties, could be suitable utilized directly or indirectly for the improvement of rice crop, furthy, from the present findings on the behaviour of mutagen-treated population with respective to quantitative characters, it appears that any good variety suffering from one or two drawbacks could be improved through the micro-mutational approach.