Radiation-induced changes in the export and distribution of photoassimilated carbon in soybean plants

Abstract

Soybean plants 18–21 days post initiation of germination received acute doses ranging from 0.25 to 4.9 kR of gamma radiation from a 60Co source. At 15–240 min following irradiation and at light intensities of either 1600 or 2800 ft-c, the plants were allowed to photoassimilate 14CO 2 for 15 min. After an additional 45 min the plants were sectioned and the 14C content assayed in order to determine the magnitude of translocation and distribution pattern of exported 14C-photoassimilate. A 33% reduction was observed in the export of 14C to the region above the node of the source leaf after a radiation dose as low as 1.0 kR. In comparison, a radiation-induced reduction in the extent of photoassimilate export was not observed until an absorbed dose of 3 kR. Despite the difference in radiosensitivity, the distribution pattern 30 min following irradiation and the magnitude of export 2hr after irradiation were observed to return to normal. Furthermore, the recovery of the export process was only observed when the plants were illuminated at 2800 ft-c. The data are interpreted to suggest that the radiosensitive sites affecting changes in export and distribution are different and that these two aspects of translocation can be modified independently of each other. Since no differences were observed between control and irradiated plants in regard to the partitioning of 14C between the 80% ethanol-soluble and insoluble fractions, or in the relative amounts of 14C-compounds present in the ethanol-soluble fraction of the source leaf, it is not likely that the radiation-induced reduction in the magnitude of photoassimilate export was due to a reduced availability of substrate for translocation.