penter and Board, 1997a, 1997b). Crop growth rate equilibration across plant populations occurs through Greater understanding of how soybean [Glycine max (L.) Merr.] adjustments of LAI and/or NAR (Hunt, 1978). Greater yield compensation occurs across plant populations would aid research aimed at reducing optimal plant population. Objectives were to deter-NAR [and also relative growth rate (RGR)] in low mine how net assimilation rate (NAR) and leaf area index (LAI) compared with normal plant populations during the late contribute to crop growth rate (CGR) equilibration across low, mevegetative and early reproductive periods is sometimes dium, and high plant populations during the vegetative (emergence-Jim Board, Dep. of Agronomy, Louisiana Agric. Exp. Stn., LSU Abbreviations: CGR, crop growth rate [g m Ϫ2 (land area) d Ϫ1 ]; LAI, Agric. Ctr., Baton Rouge, LA 70803. Approved for publication by leaf area index; LER, leaf expansion rate [cm 2 m Ϫ2 (land area) d Ϫ1 ]; the Director of the Louisiana Agric. Exp. Stn. as manuscript no. 99-09-LI, light interception (%); LIE, light interception efficiency (%); 0559. Received 13 Oct. 1999. *Corresponding author (jboard@agctr. NAR, net assimilation rate [g m Ϫ2 (leaf area) d Ϫ1 ]; RGR, relative lsu.edu). growth rate (g g Ϫ1 d Ϫ1 ); RLAER, relative leaf area expansion rate [cm 2 m Ϫ2 (leaf area) d Ϫ1 ]; TDM, total dry matter m Ϫ2 [g m Ϫ2 (land area)].

Soybean [Glycine max (L.) Merr.] yield is more restricted by assimilatory capacity (source strength) during the reproductive (R1 to R7) compared with vegetative period (emergence to R1). Although pod number (per unit ground area) is recognized as an important factor affecting yield, the period in which this yield component is source restricted has not been clearly identified. Therefore, the objective of this study was to determine this period by identifying yield components through which source strength affects pod number and finding when these yield components are determined. Field studies were conducted during 1991 and 1992 with ‘Centennial’ soybean at Baton Rouge, LA, on a Mhoon silty clay soil (fine‐silty, mixed, nonacid, thermic Typic Fluvaquent). Treatments were partial defoliations designed to create light interception differences during R1 to R7. Yield was reduced 23% by defoliation. Harvest index, which was 0.59 for the control, ranged from 0.56 to 0.60 for the defoliation treatments. Source strength influenced pod number (per unit ground area) through branch dry matter, branch number, branch node number, and pods per reproductive node on the whole plant. Pods per reproductive node were regulated by pod initiation (pods at least 0.5 cm long) and/or abortion initiated pods. Determination of final pod number occurred at 10 to 12 d after R5. In conclusion, pod number was source restricted from R1 to 10 to 12 d after R5. Stresses that restrict source strength during this period should be avoided to optimize pod number and yield.

In many crop models, light intercepted by a canopy (IPAR) is calculated from a Beer's Law equation: IPAR = PAR × [1 − exp(−k × LAI)], where k is the extinction coefficient, PAR the photosynthetically active radiation, and LAI the leaf area index. The first objective of this study was to investigate the effect of row spacing on k for corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.], and sunflower (Helianthus annuus L.) to provide information for modeling. Data from literature and from an experiment conducted at Temple, TX, were evaluated. The second objective was to investigate effects of time of day and stage of crop development on k for different row spacings. Seeds of all four species were sown in rows 0.35, 0.66, or 1.00 m apart. Measurements of canopy light interception were taken near solar noon on two dates before anthesis. At anthesis, extinction coefficients were determined at 0845, 1015, and 1145 h (solar time). The extinction coefficient showed a linear decrease as row spacing increased. For each crop, the effect of row spacing on k was described by one linear regression for most data. Stage of crop development and stage of development × row spacing interaction did not significantly affect k during the period of measurements. The effect of time of day was significant for all four crops, and the time of day × row spacing interaction was significant for soybean and sunflower. Thus, modeling light interception for different row spacings should account for these effects.

A major agronomic problem in the southeastern USA is low yield of late-planted soybean [Glycine max (L.) Merr.]. This problem is aggravated by the adverse effect of waterlogging on crop growth. Our objectives were to identify soybean growth stages sensitive to waterlogging; identify yield components and physiological parameters explaining yield losses induced by waterlogging; and determine the extent of yield losses induced by waterlogging under natural field conditions. Greenhouse and field studies were conducted during 1993 and 1994 near Baton Rouge, LA, (30 degrees N Lat) on a Commerce silt loam. Waterlogging tolerance was assessed in cultivar Centennial (Maturity Group VI) at three vegetative and five reproductive growth stages by maintaining the water level at the soil surface in a greenhouse study. Using the same cultivar, we evaluated the effect of drainage in the field for late-planted soybean. Rain episodes determined the timing of waterlogging; redox potential and oxygen concentration of the soil were used to quantify the intensity of waterlogging stress. Results of the greenhouse study indicated that the early vegetative period (V2) and the early reproductive stages (R1, R3, and R5) were most sensitive to waterlogging. Three to 5 cm of rain per day falling on poorly drained soil was sufficient to reduce crop growth rate, resulting in a yield decline from 2453 to 1550 kg ha-1. Yield loss in both field and greenhouse studies was induced primarily by decreased pod production resulting from fewer pods per reproductive node. In conclusion, waterlogging was determined to be an important stress for late-planted soybean in high rainfall areas such as the Gulf Coast Region.

Increased light interception is considered the main factor explaining greater yield in narrow‐ compared to wide‐row spacing in soybean [Glycine max (L.) Merr.]. Controversy exists, as to when during the growth cycle the greater light interception of the narrow rows has an enhancing effect on yield. The objectives of this study were to analyze crop growth rate and yield components to determine the importance of greater light interception during vegetative (E‐R1), early reproductive (R1‐R5), and late reproductive (R5‐R7) periods to increased yield in narrow‐row culture. Field studies were conducted during 1989 and 1990 at a late (July) planting date with ‘Centennial’ soybean (determinate, Maturity Group VI) at row spacings of 100, 75, 50, and 25 cm. The test was conducted at Baton Rouge, LA on a Mhoon silty clay soil (fine‐silty, mixed, nonacid, thermic, Typic Fluvaquents). Yield increased with greater light interception as row spacing was reduced from 100 to 75 and 50 cm. Although greater light interception occurred throughout the growing season in narrow compared to wide rows, increased crop growth rate occurred only during vegetative and early reproductive periods. The main factors responsible for increased yield in narrow rows were greater fertile node production and increased pod per fertile node. These yield components are primarily determined during the vegetative and early reproductive periods. In conclusion, analyses of crop growth rate and yield components revealed that greater light interception during the vegetative and early reproductive periods was responsible for increased yield in narrow‐row culture.