Seed Dormancy Mechanisms in Wild Rice (Zizania aquatica)

Freshly harvested seeds of wild rice (Zizania aquatica L.) require 3 to 5 months of moist storage at 1 to 3 to induce germination. Dormancy lasting more than 1 year has been noted. These periods of dormancy pose problems to plant breeders desiring multiple generations per year and to growers desiring to change varieties in establish fields. The purpose of this research was to determine the role of the pericarp in seed dormancy, the existence of germination inhibitors, and the influence of gibberellin and kinetin on germination of dormant wild rice seeds. The pericarp of seeds harvested the previous day was scraped off, punctured, or cut at several locations on the seed. Germination occurred only when the treatments were made directly over or very near the embryo, indicating mechanical resistance by the pericarp. Soil collected from fields 1 and 2 years out of production was screened for seed. The pericarps of nongerminating seeds were randomly punctured, increasing germination 33 and 79% respectively. This suggests an impermeable pericarp. Freshly harvested seeds from which the pericarp was either scraped or not scraped were germinated in aqueous extracts of the pericarp, and hulls (lemma and palea) from freshly harvested seeds. Aqueous extracts of the pericarp reduced germination 77% while aqueous extracts of the hulls reduced germination 84%, compared to scraped seed germinated in water. Aqueous extracts of the hulls from seeds stored for 1 year had little influence on germination when used as germination media for freshly harvested scraped seeds. The hulls were removed from some freshly harvested seeds and not from others before storing for 1, 2, and 4 weeks in water at 1.5 C. Seedling survival after 30 days was significantly reduced when lemmas and paleas were left on the seeds 1 or 2 weeks during storage in water at 1.5 C. These experiments support the contention of growth regulators in the hulls of freshly harvested seed. All combinations of 0, 0.01, 0.1, 1, and 5μM solutions of gibberellic acid (GA₃) and kinetin were applied germinated dehulled, punctured seeds. Before seeds were dehulled and punctured, they were stored in plastic containers at 1.5 C for 90 days. Germination increased from 36 to 51% as GA₃ concentrations increased. Kinetin alone had little influence on germination except in combination with GA₃. Less etiolated seedlings were obtained when kinetin was included in GA₃ treatments. The addition of 5μM GA₃ + 1μM kinetin increased germination of freshly harvested, scraped seeds from 29 to 76%. Wild rice appears to have multiple mechanisms of seed dormancy. The seed pericarp exhibits mechanical resistance and impermeability. Water soluble germination inhibitors appear to be present in hulls and pericarp, and gibberellic acid concentrations are low in freshly harvested seed. Freshly harvested seed can be germinated by dehulling and scraping, permitting multiple generations per year in breeding programs. Persistence of dormant seeds in fields will present problems in introducing new varieties.