Factors controlling seed germinability of rat's tail radish (Raphanus sativus L.) cv. pakki-hood at a low temperature

Seed germ inability of rats tail radish, cv. Pakki-hood (Raphanus sativus L.) introduced from northern Thailand occurs at a low temperature. We examined several factors that are considered to be the main barrier to germination of seeds under low temperature conditions. A temperature of 5 PC during incubation slightly delayed the decrease of the seed coat resistance, compared with 20 deg C. However, there was no difference in the decrease of the pressure required to penetrate the seed coat of either cultivars after a 4-day exposure to 5 deg C., indicating that the changes in the seed coat resistance cannot explain the difference in cold sensitivity. Water uptake occurred rapidly, within 6h of imbibition in both cultivars, regardless of whether the temperature was optimal or suboptimal, indicating that water uptake through the seed coat in both cultivars was not affected by the temperature. At 5 deg C, no further water uptake took place in the seeds of Shogoin within 6 days, while the uptake of water in the seeds of Pakki-hood increased slightly after 4 days of imbibition, along with radicle elongation. Polypeptide patterns from the germinating seeds were analyzed by two-dimensional gel electrophoresis at different germination stages. At 5 deg. C, the original difference in the protein content between both cultivars remained more or less the same during imbibition. The accumulation of osmotically active substances at 5 deg C as a result of protein degradation was not higher in Pakki-hood than in Shogoin, and the differences in cold sensitivity could not be fully explained based on the differences in protein degradation. However, the difference between the extent of protein degradation at 5 deg C and 20 deg C was larger in Shogoin than in Pakki-hood. Therefore, the proteinases in the seeds of Shogoin were more affected by the low temperature than those in the seeds of Pakki-hood.