Effects of different amounts of nitrogen fertilizer as Urea (0, 40, 80, 120, 160 and 200 kg pure N/ha) on the yield, quality and storability of three edible onion cultivars (Sefid Kashan, Topaz and Ghermez Azarshahr) were investigated. The experiments were conducted at experimental fields (with clay - sandy soil), and laboratories of the Department of Horticulture, Univ. of Tehran. An split plot design with four replications was used for the experiment. Effect of nitrogen fertilizer on the yield and bulb mean weight was significant at 1% level, and the highest yield and bulb mean weight was obtained at 120 kg N/ha. The increase in yield and mean bulb weight was correlated with the increase in length and weight of the aerial leaves. Further more, characters like length and width of the bulbs, as well as the thickness of the bulb neck were affected by the nitrogen treatments. Nitrogen had positive and significant effects on the total soluble solids (TSS) and dry weight (DW) of the bulbs as quality factors and the increase of the N up to 120 kg/ha resulted to increase in TSS and DW of the bulbs and aerial leaves. Nitrogen at 160 and 200 kg/ha resulted to the decj.ease in TSS and DW. Nitrogen had negative effects on the firmness of the bulbs and weight loss percentage during the storage period and using of more than 40 kg/ha of nitrogen resulted to the decrease in firmness and increase in sprouted bulbs and weight loss. There was not any significant interaction between the levels of nitrogen and different onion cv., and all the three cultivars were responsing similarly to the different levels of nitrogen. For the studied characters, “Sefid Kashan” cultivar was better than the other two and therefore cultivation of this cultivar is recommended for the places with the environmental conditions of this experiment, and with the use of 80-120 kg/ha of nitrogen fertilizer.
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