Physiological performance of iron-coated primed rice seeds under submerged conditions and the stimulation of coleoptile elongation in primed rice seeds under anoxia

Although seed priming is well known to be an effective method for enhancing seed vigor and seed performance, the efficacy of priming (soaking in water and re-drying) on the physiological performance of iron-coated rice (Oryza sativa L.) seeds under submerged conditions, and the physiological mechanisms of coleoptile elongation of the primed rice seeds under anoxia, have not been well elucidated. In the present study, primed and unprimed rice seeds with or without iron-coating were produced. We examined the physiological performance of these rice seeds and the physiological efficacy of priming on coleoptile elongation under anoxia. For the primed rice seeds, seed germination, coleoptile elongation, seedling emergence and the establishment of iron-coated rice seeds all improved considerably, which resulted in an increase of plant height and dry weight for the iron-coated primed rice seeds. Increases in alpha-amylase activity and glucose concentration over time were significantly higher in the primed rice seeds than in the unprimed rice seeds. Under anoxia, the alpha-amylase activity and soluble sugar concentration were significantly higher in the primed rice seeds than in the unprimed rice seeds. The glucose and fructose concentrations in the coleoptiles were significantly higher in the primed rice seeds than in the unprimed rice seeds, suggesting that the stimulated coleoptile elongation is partly related to the increased availability of soluble sugar from the seed to the coleoptiles. The increased availability of soluble sugar from the rice seeds to the coleoptiles can be assumed to help maintain glycolytic flux and alcoholic fermentation under the submerged conditions. These results clearly indicate the efficacy of priming on iron-coated rice seeds, and that the coleoptile elongation of the primed rice seeds can be partly ascribed to the sugar availability from the rice seeds to the coleoptiles. Therefore, the use of primed rice seeds for iron coating is highly effective to induce the emergence and establishment of stable seedlings in direct rice sowing systems.