Seed ecology and conservation status of Sri Lankan wild rice (Oryza) species

Sri Lanka is home to five wild rice (Oryza) species, namely Oryza eichengeri Peter, O. rufipogon Griff, O. nivara Sharma et Shastry, O. granulata Nees et Arn. ex Watt and O. rhizomatis Vaughan. Among them, O. rhizomatis is an endemic species. The populations of these species are steadily decreasing due to natural and anthropogenic activities, primarily habitat loss posing the most significant threat. This PhD project aims to address 1) distribution of Sri Lankan wild rice (Oryza) species and identify the high-priority species and sites for ex situ and in situ conservation; 2) conservation status of O. rhizomatis; 3) dormancy class, level and dormancy breaking treatments of Sri Lankan wild rice (Oryza) species; and 4) the effect of dry storage and flooding in dormancy breaking of O. nivara and O. rhizomatis. I addressed these questions by: 1) mapping the distribution of Sri Lankan wild rice (Oryza) species and perform a gap analysis to determine the high-priority areas and species for ex situ and in situ conservation; 2) red list category based on the IUCN criteria for O. rhizomatis; 3) performing natural and artificial dormancy breaking treatments and categorized the type and level of dormancy; and 4) evaluating the effect of dry storage and flooding condition on seed dormancy of O. nivara and O. rhizomatis. Only 29% of O. rhizomatis populations were found within protected areas. A limited expansion of the existing network of protected areas by 1 km will permit the incorporation of around 22% of the wild rice (Oryza) populations into the protected area. Our analysis also revealed that 62% of Sri Lankan wild rice (Oryza) populations were not represented at the International Rice Research Institute (IRRI). Continuing decline of O. rhizomatis was observed in Extent of Occurrence (EOO), Area of Occupancy (AOO), quality of habitat, and number of mature individuals. The germination percentage increased with increasing dry storage time for all species. Heat treatment 50 °C for 14 days was effective to break dormancy in O. rhizomatis. All the species respond differently to dormancy breaking treatments. Scarification near the embryo significantly boosted the germination percentage to 92% for O. rhizomatis. Moreover, chemical treatments were not effective in breaking the dormancy of O. rhizomatis and O. nivara seeds. Dormancy in O. rhizomatis was released when seeds were exposed to flooding after dry storage, with shorter flooding treatment leading to higher germination. However, prolong flooding treatment (4 week) induced secondary dormancy. This behaviour is absent in O. nivara, but extended dry storage induces secondary dormancy in this species. The distribution map and gap analysis revealed that O. rhizomatis, O. eichengeri, and O. rufipogon are the high-priority species for in situ conservation while O. granulata and O. rhizomatis are high priority species for ex situ conservation. Enlargement of existing protected areas or the institution of new small-sized protected areas will be an effective method in in situ conservation of high-priority species. Based on the IUCN criterion A2c, O. rhizomatis was categorized as Vulnerable (VU). The species exhibited varying responses to dormancy-breaking treatments. The germination percentage increased with longer dry storage times. Scarification near the embryo significantly enhanced the germination percentage, reaching up to 92% for O. rhizomatis. Additionally, heat treatment at 50°C for 14 days proved effective, particularly for hulled seeds and specifically for O. rhizomatis. Among the Sri Lankan wild rice (Oryza) species O. rhizomatis, O. nivara and O. rufipogon seeds have intermediate physiological dormancy while O. eichengeri has non deep physiological dormancy. In O. nivara only the storage time significantly affected the final germination but, in O. rhizomatis, storage time, flooding time and their interaction significantly affected the final germination.