his study was aimed to investigate dual effects of irrigation regimes and N fertilizer rates on some agronomic traits (with emphasis on yield qualitative and quantitative characteristics) and finding optimized irrigation level and N application rate for two canola (Brassic napus L.) cultivars. For this purpose, two variety of canola (Zarfam and Modena), four irrigation regimes including 30%, 45%, 60% and 75% (I1–I4) of maximum allowable depletion (MAD) of available soil water (ASW) and four nitrogen rates (viz. 0, 90, 180 and 270 kg N ha−1 (N1–N4) were involved in Karaj, Iran for two successive years (2007–2008). Our results revealed special fertilizer threshold for each irrigation regime in respect to seed yield. Response rate to fertilizers was ceased in lower fertilizer rates by prolonging irrigation. The response rate showed a decrease of 15.4%, 17.2% and 30.7% in I2, I3 and I4 in comparison with I1, but I2 response to fertilizer ceased in higher N rate as Ncritical (189.8 kg N ha−1). This implies that I2 improved response of canola cultivars to N fertilizer, which was accompanied by its higher WUE. Also, all estimated Ncriticals for all irrigation levels were higher than the current recommendation of 130 kg N ha−1. This show the capability of increasing canola cultivars yield in study region by reasonable increasing of fertilizer rate (decreasing gap between recommended N rate and estimated values) in advisable irrigation regime (I2). Cultivars tended to respond similarly to irrigation and nitrogen for seed yield in both years, but Zarfam was more efficient than Modena in respect to response to diverse treatments.

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