Effects of Changes in Source–Sink Relation in Different Planting Dates on Yield and Yield Components of Soybean Cultivars

Split-split plots experimental design based on randomized complete block design was conducted at the Baiekola Agricultural Research Center in 2010 to study the effects of changes in source–sink on the yield and yield components of soybean cultivars planted at different dates. Factors studied in this experiment included planting date, as the main plot, at two levels (June 6, June 27), cultivars (Line 032, 033, Sari or JK, and Telar or BP), as the sub plot, and five levels (including the removal of the top, the middle, and the bottom one thirds of the leaves, removal of one third of the flowers, and the control treatment as the sub-sub plot. Results of analysis of variation showed that the various cultivars were significantly different in all the studied traits at (P<0.01) probability level. Seed yield at the first planting date (184.03 g. m-2) was 11.18% lower than that at the second planting date (163.45 g.m-2). The highest seed yield was obtained in Line 033 with 219.96 g.m-2, which was statistically different from the those of all the other cultivars except that of Line 032, which was 186.19 g.m-2. The higher seed yields in Line 033, as compared with other treatments, were accompanied by the highest 1000 seed weight (242.93 g), the highest number of pods per plant on the main stem (43.31) and on the auxiliary stems (33.37), and the relatively high number of seeds per pod (2.37). Results of applying limitations of the sink–source treatments on seed yield showed that the highest seed yield was obtained in the control treatment (212.17 g.m-2). With the removal of the top one third of the leaves, the yield decreased severely so that the least seed yield in this treatment (138.08 g.m-2) was 35% less than that of the control. In the treatments of removing the middle one third of the leaves and one third of the flowers, the seed yield was reduced by 28.5 and 21.8%, respectively, as compared with the control. The least effect on seed yield was observed in the treatment of removing the bottom one third of the leaves, in which the yield was 201 g.m-2, or only 5.25% less than that of the control. The high yields in the control treatment of applying sink–source limitations were accompanied by high 1000 seed weight (220.217 g), the maximum number of seeds per pod (2.53), the maximum number of pods on the main stem (46.40), and, finally, the maximum number of pods per stem (74). The least seed yield in applying the treatment of the removal of the top one third of the leaves was also accompanied by the least 1000 seed weight (199.59 g), the fewest number of seeds per pod (2.27), the fewest number of pods per main stem (30.62), and the fewest number of pods per plant (53).