Reduced N-15-nitrogen transport through arbuscular mycorrhizal hyphae to Triticum aestivum L. supplied with ammonium vs. nitrate nutrition

This study compared the influence of NH4+ or NO3- nutrition on the contribu tion of extraradical hyphae of the arbuscular mycorrhizal fungus Glomus mos seae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) to NH4+ or NO3- uptake b y Triticum aestivum L. 'Hano' (summer wheat) with sufficient or insufficien t N supply in semi-hydroponic culture. Roots and root-distant hyphae were s patially separated in compartmentalized pots. Although NH4+-fed plants supp lied with sufficient N had higher N concentrations than their NO(3)(-)(-)fe d counterparts, this did not favourably affect colonization rates, hyphal l ength densities or N-15 amounts transported via hyphae to the plants. Ammon ium supply did not result in higher P or reduced carbohydrate concentration s in the plants, so these factors could not explain the reduced hyphal leng ths. It was concluded that the effect of NH4+ supply on hyphal length may b e related to the reduced root growth and/or a direct effect of NH4+ on hyph al growth. Plant N deficiency reduced the percentage root length colonized, hyphal length, total N-15 uptake by hyphae and dry weight of both NO3- - a nd NH4+- fed mycorrhizal plants. This was more obvious for NO3--fed plants because plant biomass and hyphal lengths of NH4+-fed plants were relatively low in mycorrhizal plants irrespective of the IV concentration supplied. ( C) 2001 Annals of Botany Company.