Miu Tsuchiya

In rice, symbiotic N₂ fixation via nodule bacteroids does not take place naturally. Although N₂ fixation by endophytic and associative diazotrophs has been reported in rice, the main organs and seasonal regulation for the N₂ fixation have not been elucidated. In this study, seasonal changes in nitrogenase (acetylene reduction) activity and carbohydrate contents in elongated culm (EC), unelongated stem (US), and crown root (CR) were investigated in manure compost (MC)- and chemical fertilizer (CF)-applied rice. Nitrogenase activity increased after rooting (June) and reached the highest activity in US of MC-applied rice at panicle initiation (August). The sucrose content in EC continued to increase after rooting regardless of the applied materials, whereas the glucose content in US increased after rooting only in CF-applied rice, suggesting higher consumption of glucose in US of MC-applied rice. There were significant differences among bacterial microbiota in EC, US, and CR at panicle initiation. In addition, Clostridia class anaerobes were more abundant in US of MC-applied rice than in EC and CR of MC-applied rice. Such difference was not observed in US of CF-applied rice. These results suggest the suitability of US of MC-applied rice at panicle initiation as a site of N₂ fixation under anaerobic conditions.

In rice, symbiotic N fixation via nodule bacteroids does not take place naturally. Although N fixation by endophytic and associative diazotrophs has been reported in rice, main organs and seasonal regulation for the N fixation have not been elucidated. In this study, therefore, seasonal changes of nitrogenase (acetylene reduction) activity and carbohydrate contents in culm internode (IN), basal nodes (BN), and crown root (CR) were investigated in manure compost (MC)- and chemical fertilizer-applied rice. Nitrogenase activity increased after rooting, and reached the highest activity in BN of MC-applied rice at panicle initiation. In BN of MC-applied rice, the sucrose content was low whereas the higher glucose content was detected at panicle initiation. Although bacterial and diazotrophic bacterial populations were most abundant in CR, the nitrogenase activity seemed to be restricted by its trace sucrose and glucose contents. In addition, alpha-Proteobacteria and Clostridia phyla were abundant in BN-specific bacterial microbiota at panicle initiation. These results suggest that the increased N fixation in BN at panicle initiation might be sustained both by the non-structural carbohydrate supply from photosynthetic apparatus and BN-specific bacterial microbiota including diazotrophs.