Nitrogen Fixation in Basal Nodes of Rice Unelongated Culm Sustained by Carbohydrate Contents and Bacterial Microbiota

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.