Effect of defoliation intensity at different reproductive stages on seed and oil yields in sunflower (Helianthus annus L.)

In order to determine the effect of defoliation intensity at different reproductive stages in sunflower (Helianthus annus L.) on seed and oil yields, and determining the most sensitive reproductive stages to defoliation, a field experiment was conducted at Tarbiat Modares University Research field using a split plot arrangement in a RCBD with four replications in 2007 cropping season. Main plots consisted of defoliation at four reproductive stages including: star shape of inflorescence stage (R1), pollination stage (R5), seed setting initiation (R6), and end of grain filling (R7), while five defoliation intensities (0% as an undefoliated control, 25%, 50%, 75% and level removed 100%) were arranged in sub-plots. The results indicated that defoliation time and intensities had significant effects on all traits. The interaction of two factors significantly (P<0.01) reduced the grain yield, thousand seeds weight, unfilled seed and oil content. The most variation resulted from defoliation effect on number of filled seed per area and1000 seed weight, at R1 and R5 stages, consequently reduced grain and oil yields. On the bases of these results it could be concluded that sunflower crop that lost 100% of its leaves would be economically unprofitable and it is more logic to be replaced with a suitable crop adapted to the region.