As population density (POP) increases in a soybean [Glycine max (L.) Merr.] crop, maximum light interception (LI) occurs earlier in the season. Earlier canopy closure would be expected to increase the cumulative radiation intercepted. We hypothesized that if radiation use efficiency (RUE) was constant across a range of population densities in a nonstressful environment, then increasing POP would increase biomass at the end of the season. To test this hypothesis, we evaluated the response of total biomass produced during the season to cumulative intercepted photosynthetically active radiation (PAR) in field experiments at Fayetteville, AR, with soybean cultivars selected from Maturity Groups (MGs) 00 to IV. Additionally, from field experiments at Keiser, AR, with MG IV soybean cultivars, we assessed the response of RUE to POP. At both locations with MG IV cultivars, a late sowing date shortened the life cycle of the crop by 13 to 25 d compared with an early sowing date, resulting in less PAR accumulated. Similarly, early maturing cultivars had less time for PAR and biomass accumulation relative to later maturing cultivars. At Keiser, in three of the four environments, RUE decreased linearly by 26 to 30% as the POP increased from 7 to 135 plants m(-2). Final biomass at the end of the season, as a function of PAR accumulated from emergence to the full-seed-size stage of development, responded linearly to intercepted PAR up to approximately 400 MJ m(-2). Above 400 MJ m(-2), the response was curvilinear with little increases in biomass>700 MJ m(-2). Our data clearly indicate that RUE decreased as POP increased and that maximum biomass production in these environments was not limited by intercepted PAR.

and water and nutrient status of the crop (Sinclair and Muchow, 1999). As population density (POP) increases in a soybean [Glycine maxAssuming that RUE is constant and that the length (L.) Merr.] crop, maximum light interception (LI) occurs earlier in the season. Earlier canopy closure would be expected to increase the of the crop cycle is not affected by POP, increasing cumulative radiation intercepted. We hypothesized that if radiation POP would expectantly shorten the time required for use efficiency (RUE) was constant across a range of population densimaximum LI, increase the total accumulation of PAR ties in a nonstressful environment, then increasing POP would infor a crop during the course of a season, and result in crease biomass at the end of the season. To test this hypothesis, we greater biomass at crop maturity. Shibles and Weber evaluated the response of total biomass produced during the season (1965) found for a MG II cultivar in Iowa that RUE to cumulative intercepted photosynthetically active radiation (PAR) was approximately constant in a year with adequate in field experiments at Fayetteville, AR, with soybean cultivars serainfall across a POP range of 6 to 52 plants m Ϫ2 . In a lected from Maturity Groups (MGs) 00 to IV. Additionally, from year with suboptimal rainfall, RUE decreased as POP field experiments at Keiser, AR, with MG IV soybean cultivars, we increased. assessed the response of RUE to POP. At both locations with MG IV cultivars, a late sowing date shortened the life cycle of the crop There have been no reports of the effects of POP on by 13 to 25 d compared with an early sowing date, resulting in less RUE in soybean at lower latitudes or across a wider PAR accumulated. Similarly, early maturing cultivars had less time range of populations than those used by Shibles and for PAR and biomass accumulation relative to later maturing cultivars. Weber (1965). We found that late-sown soybean re-At Keiser, in three of the four environments, RUE decreased linearly quired population densities considerably greater than by 26 to 30% as the POP increased from 7 to 135 plants m Ϫ2 . Final those recommended for full-season production to maxbiomass at the end of the season, as a function of PAR accumulated imize yield (Ball et al., 2000a). As POP increased, the from emergence to the full-seed-size stage of development, responded time required for the crop to intercept light completely linearly to intercepted PAR up to ≈400 MJ m Ϫ2 . Above 400 MJ m Ϫ2 ,was decreased, which shortened the time required for the response was curvilinear with little increases in biomass Ͼ700 MJ linear biomass accumulation to begin and resulted in m Ϫ2 . Our data clearly indicate that RUE decreased as POP increased and that maximum biomass production in these environments was

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