Response of onion plants to arbuscular mycorrhizae 1. Effects of inoculation method and phosphorus fertilization on biomass and bulb firmness

Onion (Allium cepa) plants were grown in pots in two types of irradiated so il, mineral and organic. Onion development was observed under two or three levels of P fertilization, and three methods of arbuscular mycorrhizal fung us inoculation with two fungus species. In mineral soil, preinoculated onio n plants had a higher biomass than non-inoculated control plants or plants inoculated with either colonized root segments or spores. Fungus species ha d no differential effect on dry biomass or final bulb diameter. Preinoculat ed onion plants reached marketable size (> 25 mm. bulb diameter) 2-3 weeks earlier than those inoculated by either of the other two methods. Non-inocu lated onion plants remained stunted. Bulbs of onions inoculated with Glomus versiforme were firmer than those inoculated with G. intraradices. Increas ing P fertilizer rates had a significant positive linear effect on the P ti ssue concentration of plants inoculated with G. intraradices or G. versifor me, but no effect on bulb firmness. The P tissue concentration of inoculate d plants was significantly higher than that of non-inoculated controls, and in inoculated plants, it differed among inoculation methods. The P tissue concentration was higher in onion plants inoculated with G. versiforme than in those inoculated with G. intraradices. In organic soil, the dry biomass of preinoculated plants was higher than that of plants inoculated by root segments. The highest root colonization levels were obtained under a low so il P level with G. intraradices, and with the root segment method of inocul ation with G. versiforme.