SOYBEAN GROWTH-RESPONSE TO PLANT-DENSITY - RELATIONSHIPS AMONG CANOPY PHOTOSYNTHESIS, LEAF-AREA, AND LIGHT INTERCEPTION

Plant spacing greatly affects leaf area, light interception, and canopy apparent photosynthesis (CAP) in soybean [GLycine max (L.) Merr.]. This study examined relationships among these variables in four population-density and row-width treatment combinations for two seasons. Wide-row (0.96 m) treatments were 3, 11, and 18 plants m-2 in 1988 and 3, 15, and 24 in 1989; narrow-row (0.43 m) treatments were 12 plants m-2 in 1988 and 13 in 1989. Narrow-row treatments tended to have greater CAP rates during early growth in 1988, but not 1989, when rates lagged behind the wide-row, high population. The reduction in CAP by the narrow-row treatment in 1989 corresponded to its reduced light interception as measured parallel to the row base. Both total radiation (300-2500 nm wavelengths) interception measured in 1988, and average photosynthetically active radiation (400-700 nm) interception measured in 1989, were linearly related to CAP prior to, but not after, canopy closure. Similarly, leaf area indices (LAI) were curvilinearly related to percent light interception until canopy closure. After canopy closure, light interception did not decline at a rate commensurate with the loss of leaf area, indicating abscission of leaves not involved in light interception. Canopy photosynthesis was not different for any treatment after Reproductive Stage R5 in either year; however, significant differences were evident between R1 and R5 in 1989. These differences corresponded to the significantly lower yield of the low population in that year. Yield differences between the medium populations of the wide and narrow rows were not explained by CAP during reproductive growth. The data indicate that when LAI is above critical levels in post-anthesis soybean canopies, factors other than photosynthesis may be involved in the response of seed yield to variation in plant density.