Effects of defoliation intensity and timing on quantitative and qualitative yields of sugar beet

Leaf area reduction affects the yield of crops by decreasing photosynthesizing area and whereby, affecting growth and development processes. Artificial defoliation is used to simulate the damages of different factors on leaves. The present study was carried out to determine the effects of different levels of defoliation at different vegetative growth stages on quantitative and qualitative yields of sugar beet and also, to determine the most sensitive growth stage to defoliation at Motahari Research Station of Sugar Beet Seed Institute (SBSI), Karaj, Iran in 2010. The study was carried out as a factorial experiment based on a Randomized Complete Block Design with four replications. Two factors were studied including growth stage at four levels (cotyledon until true two-leaf stage, about 12-leaf stage, about 37-leaf stage and about 55-leaf stage) and defoliation intensity at five levels (0, 25, 50, 75 and 100% defoliation). It was found that the highest effect of defoliation intensity on root yield was observed at 37-leaf stage, while the highest effect of defoliation on the loss of sugar percentage was observed at the end of growing season (57-leaf stage). In total, the greatest loss of white sugar yield was observed at 37 and 55-leaf stages. 100% defoliation at 37 and 55-leaf stages resulted in significant loss of white sugar yield compared to control. White sugar yield was decreased by 31% by 100% defoliation at 37-leaf stage and by 21% by 100% defoliation at 55-leaf stage. In addition, it was revealed that the increase in Na and amino-N due to the defoliation decreased coefficient of extraction and consequently, increased molasses percentage and decreased extractable sugar per-centage.