Effect of Different Defoliation Intensities at Plant Reproduction Stages on Seed Yield and Oil Percentage of Two Sunflower Hybrid Cultivars

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.