SOYBEAN YIELD COMPONENT RESPONSES TO A LIGHT INTERCEPTION GRADIENT DURING THE REPRODUCTIVE PERIOD

The yield-enhancing effect of increased light interception (LI) in narrow row/late-planted soybean [Glycine mar (L.) Merr.) culture could be better understood if LI effects on the reproductive period [first flower (R1) through physiological maturity (R7)] were clarified. The primary objective was to determine yield and yield component responses to a LI gradient during R1 to R7. Field studies were conducted during 1989 and 1990 at a late planting with 'Centennial' determinate soybean using 50-cm row spacing. The test was conducted at Baton Rouge, LA, on a Mhoon silty clay soil (fine-silty, mixed, nonacid, thermic Typic Fluvaquent). Treatments were a control (no defoliation) and four partial defoliation treatments that consisted of keeping the area 12.5 cm to both sides of the interrow midline (i.e., 50% of the interrow area) free of leaf blade material until pod initiation (R3), pod elongation (R4), start of seed filling (R5), or mid-seed filling (R6.5). These treatments resulted in a LI gradient during R1 to R7, without affecting node production or leaf area index (LAI) and total dry matter (TDM) by R1. In response to reduced LI, pods per reproductive node and pod number (both formed during R3-R5) declined. Similar source/sink ratios [crop growth rate (CGR) at R5 divided-by pod number] at seed initiation were maintained across all treatments. Thus seed per pod and seed size (formed during R5-R7) were unaffected. In conclusion, in cases such as narrow rows where LI is increased throughout the reproductive period, the determining effect on yield occurs during R1 through R5. Results also indicated that the relationship between seed number and LI (during R1-R7) occurs through adjustments in pod number.