Water stress may be imposed in sugar beet to increase some qualitative traits and water use efficiency and to prevent excessive top plant portion growth without significant decrease in yield. This study was conducted to determine the appropriate irrigation interval on the basis cumulative water evaporation from standard evaporation pan ( irrigation treatments on growth indices and qualitative and quantitative yields of sugar beet. The experiment was conducted in 2005 at the Lavark Agricultural Field station, Isfahan University of Technology located near Nejaf-abad using a randomized complete block design with split plot lay out with four replications. The study included four irrigation treatments (irrigation after 70, 85, 100 and 115 mm cumulative pan evaporation) as the main plot and two monogerm cultivars (zarghan and shirin) as subplots. The effects of irrigation treatments on top dry weight was non-significant in 79, 100 and 224 day after planting samplings (DAPS),but was significant in 121 (corresponding to the maximum top dry weight occurrence) up to 203 DAPS. In these sampling, top dry weight decreased by about 30.24% as irrigation was delayed from 70 to 115 mm pan evaporation. The effect of cumulative top dry weight was only significant at 5% level of probability in 121 and 203 DAPS. In these sampling, Zarghan produced 15.79% higher top dry weight than shirin. The effects of irrigation treatments on root dry weight was significant in all sampling, but the effect of cultivar was non-significant. Delay in irrigation reduced root dry weight. The effects of irrigation treatments on leaf area index (LAI) was significant at 5% level of probability in 79 DAPS and at 1% level of probability in other sampling. LAI was reduced as irrigation was delayed. The effect of cultivar in LAI was significant in 79, 100 and 121 DAPS. The LAI of Zarghan was about 0.5 units higher than that of Shirin in these sampling. Crop growth rate decreased as irrigation was delayed from 70 to 115 mm pan evaporation. However, irrigation delay more than 85 mm pan evaporation had negligible effect on net assimilation rate and relative growth rate. Irrigation treatment significantly reduced root yield. The maximum root yield (77.5 t ha-1) was obtained with 70 mm treatment and the minimum root yield (54.37 t ha-1) was obtained with 115 mm treatment. Cultivars did not differ significantly for root yield. The maximum (17.19%) and minimum (15.02%) sugar c
