INTRODUCTION Radio-sensitivity of selected cowpea (Vigna unguiculata) genotypes to varying gamma irradiation doses L. Horn1,2 and H. Shimelis2 1. Directorate of Research and Training, Plant Production Research, Ministry of Agriculture, Water and Forestry, Private Bag 13184, Windhoek, Namibia Email: Lnhorn@yahoo.com 2.School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa Cowpea (Vigna unguiculata L. Walp., 2n=2x=22) is an important component of the traditional cropping systems in the semi-arid tropics About 95% of farmers in northern Namibia grow cowpea Cowpea productivity is generally low at 250 to 350 kg/ha in Namibia Mutation breeding is useful in crop improvement and functional genomics A collaborative research between the International Atomic Energy Agency (IAEA) and the Government of the Republic of Namibia was initiated during 2009 to improve food security crops using mutation breeding Exposure of plant propagules including seeds, tissues and organs to an appropriate dose of gamma radiation induces genetic variation for breeding (Mba et al., 2010) The objective of this study was to determine an ideal dose of gamma radiation to induce genetic variation in three cowpea genotypes widely grown in Nambia. INDRODUCTION Fig. 2 A CO60 source SHINDIMBA BIRA NAKARE Cowpea genotypes (Fig. 1) Nakare (IT81D-985), Shindimba (IT89KD-245-1) and Bira (IT87D-453-2) were used for the study Thirty seeds per genotype were gamma irradiated in three replications at seven irradiation doses (0, 100, 200, 300, 400, 500 and 600 Gy) were applied using a CO60 source Gamma cell (Fig. 2) at the IAEA as illustrated in Fig. 3 Data collected included germination %, hypocotyl and epicotyl lengths measured in cm Fig. 1 Cowpea genotypes used in the study MATERIALS & METHODS RESULTS AND DISCUSSION A significant (P<0.01) interaction was detected between genotypes and irradiation doses (Table 1) Germination % decreased drastically in all the three varieties with increased radiation doses (Fig. 4, Table 2) There was no germination observed for Nakare and Shindimba above 300 and 400 Gy, respectively Genotype Bira could withstand the radiation doses of up to 600 Gy and displayed 47% germination at this dose (Table 2, Fig. 5) Fig. 3 Gamma irradiation and mutation generations for selection. Irradiate to produce M1 seeds M1 plants M2 plants M2 seeds M3 seeds Planting and Selection continues until desired traits are attained Table 1. Analysis of variance on percent germination, epicotyl and hypocotyl lengths between cowpea genotypes, radiation dose and their interaction Varied doses of gamma radiation applied on three different cowpea genotypes influenced germination, arly growth and development differently LD50 for genotypes Nakare, Shindimba and Bira were achieved at 168.54, 194.89 and 671.38 Gy, respectively (Fig. 4) The selected dose of the gamma radiation may help as a generic treatment dose to induce large scale mutagenesis in cowpea e Irradiation doses leading to an average of 50% damage (LD50) to seed germination in genotypes Nakare, Shindimba and Bira varieties were determined. The result demonstrated that cowpea genotypes required specific irradiation dose before large-scale mutagenesis is undertaken Induced mutations are random events and as such published radiation conditions might not result in the same mutation events for different genotypes CONCLUSIONS Fig. 4 Germination responses of Nakare, Shindimba and Bira against gamma radiation doses Fig. 5 Cowpea genotype Bira Irradiated at 600 Gy and M1 seedlings. Table 2. Mean and standard deviation on percent germination, epicotyl and hypocoyl lengths among three cowpea genotypes tested using seven irradiation doses. The IAEA is acknowledged for funding the study through the TC project in collaboration with the Government of the Republic of Namibia The IAEA research team at the Joint FAO/IAEA Agriculture and Biotechnology Laboratory Seibersdorf, Vienna, Austria are thanked for research support AKNOWLEDGEMENTS REFERENCES Mba C, Afza R, Jain SM (2010). Induced mutagenesis in plants using physical and chemical agents. Joint FAO/IAEA. Programme, Vienna, Austria, pp. 91-94