Soybean Leaf Morphology and Defoliation Tolerance

Abstract Plant tolerance to pest injury is an ideal component of integrated pest management (IPM) programs, because it places no selection pressure on pest populations, but tolerance is little understood and its use in pest management is limited. In field experiments in 1994, 1995, and 1996, we characterized tolerance to defoliation in ‘Clark’ soybean [ Glycine max (L.) Merr.] isolines differing in leaf morphology: 3‐ and 5‐leaflet, narrow and wide leaflet. Sequential defoliation was imposed manually at the R2 stage (full flowering), based on an insect consumption model. In 1994 and 1996, with ample rainfall, all isolines expressed yield compensation or overcompensation for defoliation as high as 75%, due to compensatory regrowth and delayed leaf senescence; in 1995, however, when rainfall was marginal, defoliation caused significant yield reduction. Equal amounts of leaf tissue removed resulted in different leaf area indices (LAIs). Wide‐leaflet isolines had higher LAI than narrow‐leaflet isolines, but the narrow‐leaflet isolines, particularly Clark‐3N, had greater light interception capacity for a given LAI level and had a higher light extinction coefficient in 1995. This shows that light was available to leaves in the lower canopy in the narrow‐leaflet isoline. Both low and high defoliation treatments caused significant yield reductions in all isolines, except that low defoliation did not significantly affect the yield of Clark‐3N in 1995. The narrow‐leaflet isolines apparently tolerated defoliation better than the wide‐leaflet isolines because the narrow‐leaflet isolines had greater light interception and maintained equal or better yields, despite similar defoliation. In addition to LAI, both canopy light interception and light extinction coefficient are important criteria for selecting cultivars tolerant to defoliation.