Light Interception Efficiency and Light Quality Affect Yield Compensation of Soybean at Low Plant Populations

Greater understanding of how soybean Glycine max (L.) Merr. yield compensation occurs across plant populations would aid research aimed at reducing optimal plant population. Objectives were to determine how net assimilation rate (NAR) and leaf area index (LAI) contribute to crop growth rate (CGR) equilibration across low, medium, and high plant populations during the vegetative (emergence–R1) and early reproductive periods (R1–R5). Determinate cultivar Delta Pine 3606 (Maturity Group VI) was planted at an optimal planting date during 1995 and 1996 at low (80000 plants ha −1 ), medium (145000 plants ha −1 ), and high (390000 plants ha −1 ) plant populations on a Commerce silt loam near Baton Rouge, LA (30°N). Yield was unaffected by plant population. Equilibration of CGR for low vs. higher plant populations near R1 was achieved through greater NAR for the low plant population during the vegetative period, created by greater light interception efficiency (LIE, light interception per unit leaf area). Although NAR equilibrated to minimal levels across plant populations near R1, low population maintained CGR parity with higher populations until R5 through greater relative leaf area expansion rate (RLAER) during the late vegetative and early reproductive periods. Higher relative leaf area expansion rate for low vs. higher plant populations resulted from increased partitioning of dry matter into branches, probably induced by greater red/far red light ratios within the canopy. In conclusion, a possible genetic characteristic conducive to low optimal plant population is greater partitioning of dry matter into branches.