Determination of radiosensitivity of Coffea arabica var. 'Venecia' seeds to gamma-ray irradiation.

Abstract Coffee is one of the most commercially available raw materials, being the tropical product with the highest market value in the world. In Costa Rica it is the third most important product for agricultural exports and provides the main income for many families in the country. However, coffee is under threat due to coffee leaf rust disease (CLR). Mutation breeding in coffee is a promising approach to develop new varieties resistant to CLR. As a new technology for coffee, basic tests related to mutation induction need to be done. The plant material used was Coffea arabica var. 'Venecia' seeds, with a moisture content of 27.3%. The applied irradiation doses were 0, 80, 100, 120, 140, 160 and 180 Gy. For each treatment, three replicates of 200 g were used, with a seed number range of 765-808 units per replicate. The irradiated seeds were planted on the same day. Eighty days after treatment the number of seedlings was quantified, the hypocotyl height and radicle length were measured and the opening of cotyledons was determined for each dose. The effects of the radiation doses on seed germination frequency were recorded. At the dose of 80 Gy, germination was reduced over the control by 9.65%, at 100 Gy by 34.06%, at 120 Gy by 52.76%, at 140 Gy by 60.24%, at 160 Gy by 65.56% and at 180 Gy by 75.40%. Seedling growth was affected and a delay in opening of the cotyledons was observed at higher doses. This radiosensitivity test, based on seed germination as compared with unirradiated control, revealed that the LD50 for the variety tested is in the range 100-120 Gy experimentally, and according to the regression is 125 ± 30 Gy. This dose will be used for further bulk experiments and is of great importance, because the LD50 is considered as the range where the appearance of useful mutations in breeding programmes is favoured. The establishment of these parameters is a necessary advance to continue with measurements of genetic and phenotypical parameters to implement mutation breeding in coffee looking for new sources of resistance against CLR.