Effect of timing and intensity of defoliation on yield and quality of sugar beet

The effect of defoliation on root yield and quality of suagrbeet (Beta vulgaris L.) was examined in a field study in Research Station of Faculty of Agriculture, Ferdowsi University of Mashhd during 2005 growing season. The experiment was conducted as a split plot on the basis of randomized complete block design with three replications.Time of Defoliation (May 16th, May 31th, June 18th, July 9th, July 31th, and August 20th) were allocated to main plots and defoliation severity (0, 30, 60, and 100%) were assigned to subplots. Results showed that root yield, shoot and root dry weight and sugar yield were influenced by defoliation time. However, defoliation time had no significant effects on sugar white content and white sugar yield. Effects of defoliation on plant growth was more pronounced in mid season when about 1500 degree days was accumulated, compared to early or late season. Defoliation intensity resulted in a significant decrease in root yield, shoot fresh weigth, root and shoot dry weight, sugar and white sugar content, and white sugar yield compared with undefoliated control, but molass sugar was increased in all defoliation intensities. sugar and white suagr percent were significantly affected by defoliation time x intensity interaction. Complete defoliation (100%) on July 30th and Auguat 19th led to 18 and 21% reduction in sugar and 29 and 28% in white suagar, respectively. However, 100% defoliation in July 8th resulted in 6 and 8% increase in sugar and white sugar, respectively