ASSIMILATORY CAPACITY EFFECTS ON SOYBEAN YIELD COMPONENTS AND POD NUMBER

Soybean [Glycine max (L.) Merr.] yield is more restricted by assimilatory capacity (source strength) during the reproductive (R1 to R7) compared with vegetative period (emergence to R1). Although pod number (per unit ground area) is recognized as an important factor affecting yield, the period in which this yield component is source restricted has not been clearly identified. Therefore, the objective of this study was to determine this period by identifying yield components through which source strength affects pod number and finding when these yield components are determined. Field studies were conducted during 1991 and 1992 with 'Centennial' soybean at Baton Rouge, LA, on a Mhoon silty clay soil (fine-silty, mixed, nonacid, thermic Typic Fluvaquent). Treatments were partial defoliations designed to create light interception differences during R1 to R7. Yield was reduced 23% by defoliation. Harvest index, which was 0.59 for the control, ranged from 0.56 to 0.60 for the defoliation treatments. Source strength influenced pod number (per unit ground area) through branch dry matter, branch number, branch node number, and pods per reproductive node on the whole plant. Pods per reproductive node were regulated by pod initiation (pods at least 0.5 cm long) and/or abortion of initiated pods. Determination of final pod number occurred at 10 to 12 d after R5. In conclusion, pod number was source restricted from R1 to 10 to 12 d after R5. Stresses that restrict source strength during this period should be avoided to optimize pod number and yield.