Cytogenetic Impact of Gamma Irradiation and Its Effects on Growth and Yield of Three Soybean Cultivars

HEALTHY seeds of three soybean cultivars; Crawford, Giza 111 and Giza 35 were irradiated with gamma-irradiationat doses ranging from 100Gy to 600Gy with 100Gy interval. The cytogenetic impact of the applied doses was assessed in M-1 and M-2 plants, on mitotic activity and chromosomes in root tip meristems and vegetative growth, expressed as shoot and root length and fresh and dry weight of shoot and root as well as leaf area. The low doses of 100 and 200Gy increased mitotic activity, expressed as mitotic index MI and enhanced growth rate. In contrary, higher doses significantly retarded mitotic activity, increased the frequency of chromosomal abnormalities and reduced vegetative growth and yield, expressed as number of pods and seeds weight. The frequency of chromosomal abnormalities was dose dependent and its percentage varied among cultivars. Chromosomal stickiness and non-congression were the most common abnormalities at metaphase and chromosomal bridges and free chromosomes were the most common aberrations at ana-telophase whereas C-metaphase and c lagging chromosomes were only occasionally observed. No abnormalities were reported in the interphase of the M-1 plants, but in the M-2 plants, micronuclei and vacuolation of nuclei were frequently observed. The abnormalities reported here are mainly due to a clastogenic action that may have cause breakage and reunion of chromosomes. These findings emphasise the importance of assessing mitotic activity and chromosomes behaviour in plants used for mutation breeding to predict changes invegetative traits and yield of target genotypes.