No abstract

Short‐season soybean [Glycine max (L) Merr.] production systems, such as double cropping and late sowing, require high populations to optimize yield, but effects of high populations on seed number and seed mass are unknown. We evaluated plant population effects on yield compensation, stability of harvest index, assimilate partitioning for seed number, and seed‐filling characteristics for 2 yr near Keiser, AR. The study had two cultivars, two levels of irrigation, and three row spacings that each had five levels of population ranging from 6 to 134 plants m−2 Increasing population reduced yield per plant but increased yield per unit area. Harvest index was relatively constant across populations for a given year and irrigation regime, and yield was closely associated with biomass at maturity. At high populations, plants maintained individual seed mass by reducing the proportion of shell mass per pod. Final individual seed mass, seed growth rate (SGR), and the length of effective filling period did not change with increasing population for irrigated or nonirrigated treatments. Reductions in yield caused by low population density were due to low seed number. Seed number per square meter was directly proportional to the ratio of crop growth rate (CGR) to SGR. For short‐season production, high populations ensured early canopy coverage and maximized light interception, CGR, and crop biomass, resulting in increased seed number and yield potential.

Water deficit in soybean [Glycine max (L.) Merr.] results in the accumulation of the products of N2 fixation (ureides) in shoots, and this may lead to feedback inhibition of N2 fixation. Manganese is required for ureide degradation in leaves, and it was hypothesized that increased leaf Mn+2 would alleviate ureide accumulation during drought, lessen feedback inhibition, and prolong N2 fixation. In a growth chamber experiment, ureides supplied through roots decreased N2 fixation in well‐watered plants, and a soybean cultivar with demonstrated tolerance of N2 fixation to water deficit (Jackson) had a lesser concentration of shoot ureides following exogenous ureide application than a cultivar sensitive to water deficit (KS4895). In a greenhouse experiment, N2 fixation in Jackson under moderate water deficit was not different from the control in the absence or presence of soil‐applied Mn+2, whereas N2 fixation in KS4895 was 30% of the control in the absence of soil‐applied Mn+2 and 111% of the control in the presence of soil‐applied Mn+2 Increased N2 fixation in KS4895 with soil‐applied Mn+2 under water deficit was associated with decreased shoot ureide concentration. In field experiments, Jackson consistently had lesser shoot ureide concentrations than did KS4895, and enzymatic degradation of ureides was greater for Jackson on one date than for KS4895. We concluded that ureides inhibit N2 fixation, that genetic variation in the ability to degrade ureides may be important in drought tolerance, and that increased leaf Mn+2 concentration promotes ureide breakdown and prolongs N2 fixation under water deficit.

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

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.