To determine the effects of different intensities of leaf removal at different reproduction stages of sunflower on seed yield and oil percentage, and the most sensitive stages to defoliation, an experiment was carried out by split-spilt plot design in randomized complete block with 3 replications at research farm, College of Agric. Urmia University, in 2004. The main factor, sub factor and sub-sub factor in our experiment were: 1. two cultivars, Uroflor and Alstar, 2. defoliation at four reproduction stages, star shape of inflorescence (R₂), pollination stage (R₅), seed setting initiation (R₆) and final period of seed setting (R₇), and 3. four defoliation intensities ( 0% as an undefoliated control, 25%, 50% and 75% ). Seeding was accomplished on May 23. Average triple factor interactions verified that different defoliations at R₂ stage have significantly reduced seed yield and oil percentage of both Uroflor and Alstar cultivars. Out of this reduction, 75 percent defoliation at R₂ was the most, while at R₅ and R₆ stages 50 and 75 defoliation caused significant reduction on seed yield in comparison with control. At R₇ stage different defoliations had no significant effect on seed yield of Uroflor cultivar, while, on Alstar cultivar, 75 percent defoliation at R₇ stage caused significant difference in relation to control. Also average triple interaction among factors showed that none of defoliation percentages had significant effect on oil percentage. In view of the fact that, seed development and filling occurred after defoliation at R₂ and R₅ , the most variation resulted from defoliation of valued traits like number of filled seed per head and weight of 1000 seeds, observed at R₂ and R₅ stages and consequently reduced seed and oil yields. On the bases of this experiment it could be concluded that sunflower cultivation with losses of 50 and 75 percent will be economically unprofitable and it will be more desirable to replace it with any other suitable crop in the region.

Keywords: Sunflower, Defoliation, Seed yield, Oil percentage.

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