Seed dormancy in red rice. XIII: Interaction of dry-afterripening and hydration temperature

While red rice (Oryza sativa L.) can remain dormant and viable for many years when fully imbibed, the environmental factors that stimulate germination or induce secondary dormancy in the field have not been characterized. In this study, the interactions between the extent of dry-afterripening and germination temperature have been evaluated as possible factors. Red rice dispersal units (florets) were afterripened for 0-10 weeks at 30 degrees C and incubated in water at 1, 5, 15, 20, 25, 30 and 35 degrees C for 2 weeks; then, all the ungerminated florets were transferred to 30 degrees C for two additional weeks. Germination at the end of each of the two sequential treatments was compared to define both the effect of differing temperatures on germination (first treatment), and the effect of these temperatures on subsequent germination at the optimum temperature (30 degrees C, second treatment). In afterripening red rice, the opening of the temperature window for germination begins at 25-35 degrees C. Fully dormant florets acquired the ability to germinate >= 90% at 30 degrees C after 4 weeks of dry-afterripening. However, imbibing florets for 2 weeks at 15 degrees C followed by 2 weeks at 30 degrees C, yielded suboptimal germination and induced a degree of secondary dormancy, dependent upon the extent of previous dry-afterripening. Cold stratification (1 degrees C) had a consistent promotive effect on the subsequent germination, particularly when preceded by 1 -2 weeks of dry-afterripening at 30 degrees C. To monitor the effects of germination temperature, median afterripening time was utilized as a relative dormancy index, and changes in this index have been interpreted as an overlapping of germination and the temperature-induced changes in the dormancy status. Field weather data suggested that low-temperature stratification may be a germination trigger in the field, even in southern Louisiana, and this merits further investigation in studies of soil-buried seeds.