Biological Nitrogen Fixation in Water Yam {Dioscorea alata L.) and Lesser Yam {Dioscorea esculenta L.) Associated with Endophytic Diazotrophic Bacteria

摘要

Yams {Dioscorea spp.) are important tuber crops grown throughout all the tropical zones, with the highest fresh tuber production in Africa. In West Africa more than 60 million people depend on this crop for their food (Asiedu and Sartie, 2010). Lesseryam is one of the most cultivated yam species in Asian countries. Although cultivated as the dominant staple food crop in some countries (Papua New Guinea, for example), lesser yam is one of the least studied yam species (O'Sullivan and Ernest, 2007). Several research studies to improve yam tuber yield have been conducted since many decades. However, variable results and inconsistent outputs have been reported (Diby et al" 2009; Ettien et al" 2014), especially regarding the response to chemical fertilizer application. In a study that aimed to understand the growth of water yam (D. alata) in poor soil conditions and provide new guidelines for yam cultivation in Okinawa prefecture (Japan), Takada et al. (2017) speculated that, under non-fertile soil conditions, the accession A-19 of water yam could rely on biological nitrogen fixation (BNF) for its growth. It was estimated that the proportion of nitrogen fixed by this accession was about 38 % (Takada et al" 2018). Likely, Rezaei et al. (2017) reportedsome nitrogen-fixing bacteria (diazotrophs) associated with the accession E-2 in lesser yam. Recently, endophytic diazotrophic bacteria of several accessions of water yam and lesser yam were reported (Ouyabe et al" in press). However, the contribution ofthese bacterial pools to the growth and nitrogen uptake in these yam species has not been studied. The aim of this study was to evaluate the biological nitrogen fixation occurring in seven (7) accessions of yam associated with nitrogen-fixing bacteria.