Developmental changes induced by gamma irradiation in Ipomoea batatas L. Lam (sweet potato)

Storage roots of Ipomoea batatas L. Lam were exposed to gamma irradiation at 90, 180, and 240 Gy. The two highest doses caused a delay in the initiation of rooting and especially of shooting; they also caused formation of rosette-like, stemless shoots without apical meristems. The storage roots irradiated with 90 Gy did form stems, but 50% of them had no apical meristem. The irradiated storage roots produced absorbent roots which developed shoots after 70 days of growth. Nonirradiated storage roots did not differentiate shoots from the absorbent roots. When grown in vitro, phytomers, stem segments with leaves and an axillary meristem, separated from the shoots of irradiated storage roots and exhibited growth aberrations, very intense rooting, and a delay in shooting. Phytomers from nonirradiated normal plants were irradiated with 10, 20, 30, 40, and 90 Gy and grown in a hormone-free medium. The 40- and 90-Gy doses delayed shooting as well as rooting. Only phytomers exposed to 40 and 90 Gy differentiated shoots from the absorbent roots. A stimulation of growth revealed in the accumulation of dry mass was found in the shoots of phytomers irradiated with 10 to 30 Gy. The long after-effects of irradiation as well as possible causes of growth stimulation are discussed.