Drastic reduction of light in cloudy days in the northern part of Iran and the presence of pod eater pests are two main reasons for the recognition of source and sink relation. The effect of source and sink on vegetative and storage organs of plant is a path for recognition of limiting factors in producing soybean plant. This experiment was conducted in order to study effect of source to sink on yield, yield components, harvest index and some agronomical characteristics of soybean. The experiment was laid out factorial in randomized complete blocks design (RCBD) with four replications. Factor A included changing of source in five levels: a1- defoliated of 25%, a2- defoliate of 50% , a3- defoliate of 75%, a4- open canopy, and a5- control (without defoliation); and factor B included changing of sink in three levels: b1- cut 25% of pods, b2- cut 50% pods, b3- cut 75% pods. Analysis of variance showed that the source and sink ratio had a significant difference on grain yield (p<0/01). The open canopy (a4) and 75% defoliate (a3) had maximum (790.4 gm-1) and minimum (458.7 gm-1) of grain yield, respectively. The grain yield of a1, a2 and a3 treatments decreased 15.1, 25, and 35% compared to the control (a5), respectively. The treatment of open canopy (a4) increased 12% yields in comparison with the control. Factor A was significant for some traits such as the number of pods, number of grain, 100 grains weight, number of lateral branches and plant height. Harvest index influenced the changes in source and sink ratio and showed significant diversity (p<0/01). The maximum and minimum of harvest index were obtained in defoliate of 75% and open canopy, respectively. Results showed that factor B had significant effect on the number of pod, 100 grain weight, seed weight in lateral branch, pods weight and the number of pod. Investigations also showed that the open canopy increased the light penetration in canopy, photosynthesis and yield. Factor B decreased the yield in order to increase in the cut of pods.
