Comparative seed dormancy and germination responses to drying and flooding in two Sri Lankan wild rice species

Sri Lankan wild rice, Oryza nivara and Oryza rhizomatis, are two threatened species occurring in habitats with contrasting dry and wet seasonal patterns. Both species produce dormant seeds at dispersal, yet their germination phenology remains poorly understood. This study aimed to elucidate their germination strategies and dormancy mechanisms to support conservation efforts. We hypothesized that dormancy alleviation in these species is niche-specific; specifically, that O. rhizomatis requires longer periods of dry storage (after-ripening) and shorter period of flooding than O. nivara. (i) Freshly harvested seeds of both species were subjected to nine dry-storage periods (0, 2, 4, 6, 8, 10, 12, 18, and 20 months) at 25°C and 60% relative humidity (RH). Following storage, seeds were incubated on moist tissue paper for 30 days at 25°C under a 12 h light/dark regime. Additionally, a second set of seeds stored for 6, 8, 18, and 20 months was exposed to flooding treatments (FTs) (1, 2, or 4 weeks) prior to incubation. (ii) In both species, germination percentage increased up to ∼80% with increasing dry-storage periods up to 12 months. Under prolonged dry storage, germination percentage declined in O. rhizomatis and none of the seeds germinated in O. nivara. In O. rhizomatis, a short FT (1 week) increased the germination percentage compared with dry storage alone, whereas longer flooding periods (2-4 weeks) reduced it. In contrast, flooding had no significant effect on seed germination in O. nivara. (iii) Overall, prolonged drying effectively breaks dormancy in both species, preventing germination immediately after dispersal when drought risk is high and seedling survival is low. The species exhibit distinct physiological dormancy breaking responses that likely contribute to their differing ecological and geographical distributions. Predicted increases in precipitation under climate change may constrain recruitment by limiting dry after-ripening or increasing flooding duration, posing a particularly high risk for the endemic O. rhizomatis.