SOURCE STRENGTH INFLUENCE ON SOYBEAN YIELD FORMATION DURING EARLY AND LATE REPRODUCTIVE DEVELOPMENT

Soybean [Glycine max (L.) Merr.] yield is affected more by assimilatory capacity (source strength) during reproductive (R1-R7) than vegetative (emergence to R1) growth periods, but relative sensitivities of yield components to source strength have not been clearly demonstrated. This study was conducted to verify that source strength (R1-R7) affects yield primarily through regulation of pod number. Source strength effects (R1-R6) on pod number and effects of altered source-sink strength during R5 to R7 on seed per pod and seed size were determined. Soybean cultivar Centennial was planted at an optimal date in 1991 and 1992 near St. Gabriel, LA. Source strength during R1 to R6 and source-sink strength during R5 to R7 were altered with three treatments: control, 30% shade (R1-R7), and partial pod removal 10 d after R5. Shade vs. control reduced source strength and yield 30% through a 20% reduction in pod number. Number of branch nodes and main stem pods per reproductive node, yield components shown previously to respond to source strength at late plantings, responded similarly to altered source strength at optimal planting dates. In contrast, a 28% decrease in sonrce-sink strength during R5 to R7 reduced seed per pod and seed size by only 7.7 and 4.5%, respectively. The results substantiate that source strength during R1 to R7 influences soybean yield primarily by affecting pod number and that R1 to R6 is the most responsive part of the reproductive period to altered source strength.