Effects of gamma-irradiation of seed potatoes on numbers of stems and tubers.

In field trials with the cultivars Bintje, Jaerla and Spunta, whose seed potatoes were treated with gamma-rays from a ^Co source with doses varying from 0.5 to 27 Gy, tuber yield, har­ vest index, and number of stems and tubers were determined. A dose of 3 Gy increased the number of tubers by 30 % in Spunta in two out of three trials and by 17 % in one trial in Jaer­ la, but it did not increase number of tubers in Bintje. Doses of 9 or 10 Gy did not influence the number of tubers nor stems, and decreased harvest index. A dose of 27 Gy yielded off-type plants with reduced yield and number of tubers. Gamma-radiation affected the growth of the apex of the sprout allowing lateral buds or divisions of the affected apex to develop into stems. To achieve larger numbers of tuber-bearing stems, tubers should preferably be irra­ diated at the start of sprout growth, about 5 months before planting.