Occurrence of Diazotrophic Endophytes in Different Portions of Sweetpotato Stems

Stem samples of three sweetpotato cultivars were taken from an experimental field two and four months after planting (MAP). The occurrence of diazotrophic endophytes in different stem portions of sweetpotato was examined by stem-pieces-incubation (SPI) and the acetylene reduction activity (ARA) method using a semi-solid modified Rennie medium. Positive responses of the stem-pieces to ARA were detected, confirming the occurrence of diazotropohic endophytes in the stems of Japanese sweetpotato cultivars. The frequency of ARA-positive portions was higher at 4 MAP than at 2 MAP. However, the site of ARA-positive portions varied between replicated stems in each cultivar. Both positive and negative responses to ARA were detected in tubes inoculated with five neighboring thinly sliced pieces from the same internode portion. These results suggest that the diazotrophic endophytes that are culturable on semi-solid modified Rennie medium were discretely localized in sweetpotato stems.