The objectives of this study were (a): to define the effects of light, dark, and temperature on nodule activity (acetylene reduction), and (b) to establish the contributions of reserve carbohydrate and recent photosynthate to the support of nodule function. An in situ assay of nodule activity was developed for use with intact, hydroponically grown soybeans (Glyeine max IL.l Merr. cv. Calland).Nodule activity of 35-day-old plants grown in controLbd environment chambers decreased during a 10-hour dark period at 18 C, compared with actinty during the preceding and subsequent 14-hour light periods at 27 C. In contrast, plants that were maintained at a constant 27 C did not vary in nodule activity during diurnally varying dark and light exposure. Nodules of plants exposed to diurnal 18 and 27 C in 24-hour continuous dark were less active at 18 C than at 27 C. At constant 27 C, nodule activity was sustained throughout the 24-hour dark period. Thus, nodule activity was independent of short term dark periods but dependent on temperature; nodule activity was decreased at the lower temperature.Temperature also affected the nodule activity of plants maintained in the light. Exposure of shoots and roots of intact plants to the lower temperature (5 hours at 18 C) during the light period resulted in a marked decrease in nodule activity, compared with that of plants maintained at 27 C. Exposure of only the shoot portion to 18 C (roots were maintained at 27 C) resulted in a similar decrease in nodule activity.Nodules of plants exposed to 10 days of diurnally variable dark, light, and temperature had high activity in the light at 27 C and low activity in the dark at 18 C. Nodule activity of plants at a constant 27 C was not affected by diurnafly variable dark and light exposure throughout the 10-day period, altbough activity generally increased with time due to increased nodule mass. At a constant 27 C, nodules of intact plants in continuous dark sustained activity through 72 hours before declining to zero by 7 days. At diurnally varying 18 and 27 C in continuous dark, peak diurnal nodule activity was sustained through 5 days, and then declined to zero by 8 days. Mederski and Streeter (16) have also observed diurnal variation in nodule activity (C2H2 reduction); activity in the dark was at or above 50o of the maximum rates measured during illumination.Other diurnal studies have also demonstrated the maintenance of a basal level of nodule activity during dark exposure (1,4,9,12,18). These results suggest that diurnal fluctuation in nodule activity (C2H2 reduction) may reflect a change in the composite contribution of recently produced and reserve photosynthate to the support of nodule activity. The significant input of reserve photosynthate in support of nodule activity has been further demonstrated by numerous studies in which the C2H2 reduction activity of detached nodules and of excised nodulated roots has been measured (1, 2, 7, 10, 15, 18).The objectives of the present study were to define the effects of diurnal vari...

Increasing specific leaf weight (SLW) may improve leaf apparent photosynthesis (AP) in soybean [Glycine max (L.) Merr.] but screening for SLW and AP is laborious. The Objectives of this study were (i) to determine the time course of SLW and chlorophyll concentration in experimental lines selected for differences in SLW and (ii) to evaluate the potential use of the Minolta 502 SPAD meter as a rapid estimator of SLW, AP and chlorophyll concentration in leaves of soybean. In 1991 and 1992, sixteen experimental lines representing extremes in SLW were grown at Urbana, IL, and West Lafayette, IN, with three replications at each location. SPAD values, SLW and AP were measured at the R2 (full flower), R4 (full pod) and R5 (beginning seed) growth stages. In 1992 SLW, SPAD values and chlorophyll concentration were measured weekly. Seasonal patterns of SPAD values, SLW, and chlorophyll concentration were very similar through R5. After R5, SLW continued to increase but SPAD values and chlorophyll concentration declined. SPAD values and SLW were highly correlated at the R2, R4 and R5 stages at both locations and in both years. Environmental conditions during this research were not suitable for maximum AP expression, which is likely why AP and SPAD values were correlated only at the R4 growth stage at Urbana in 1992. SPAD measurements were consistent across diverse environments and effectively separated the high SLW lines from the low SLW lines. Measuring with the Minolta 502 SPAD meter is rapid, simple and non-destructive and could be an alternative method for direct selection for SLW.

Increasing leaf apparent photosynthesis (AP) may improve seed yield in soybean [Glycine max (L.) Merr.]. Selection for high specific leaf weight (SLW) may increase AP. The objectives of this study were to (i) incorporate extremes in SLW from exotic germplasm into adapted soybean lines with high agronomic potential, (ii) compare the time course of SLW in lines known to vary in SLW, and (iii) evaluate the relationships among SLW, AP, and seed yield. In 1991 and 1992, 16 genotypes representing extremes in SLW were grown at Urbana, IL, and West Lafayette, IN. The SLW and AP were measured at the R2, R4, and R5 growth stages. At the R5 growth stage in 1992, AP was also measured at approximately twice ambient CO2 concentrations. A subset of lines was sampled weekly for SLW from R1 until R7. High SLW lines displayed a greater rate of increase in SLW than low SLW lines beginning after R2 and continuing until R6. Significant class mean differences (P < 0.01) in SLW were observed on all sample dates across locations. Class mean differences in AP at ambient and twice ambient CO2 concentrations were observed at Urbana, although ranking of SLW classes was inconsistent across years. Leaf rugosity was negatively correlated with AP and seed yield. A high SLW line with low rugosity, LG89‐1917, expressed high AP and produced yields very similar to the adapted cultivars Chapman and Elgin 87. Lines with high SLW and low leaf rugosity may hold the greatest potential for high AP and yield improvement.

A yield advantage of some cultivar mixtures has been observed in soybean (Glycine max (L.) Merr.] in that their yield has exceeded the weighted mean of the component cultivars grown in pure stands. However, no previous work has evaluated the performance of mixtures of soybean cultivars differing in both height and relative maturity and grown in a noncultivatable, narrow‐row production system. The objective of this 2‐yr field study was to evaluate the performance of mixtures of short‐determinate (early ‘Gnome’ or late ‘Elf’) and tall‐indeterminate (early ‘Century’ or late ‘Williams’) cultivars grown in 0.13‐m row widths. Ratios of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100 were used for the components in each two‐cultivar mixture. A significant (a = 0.05) quadratic component was observed in regression analysis of yield of Elf‐Century mixtures, indicating overcompensation. The predicted mean yield for the highest yielding ratio, 54 Elf/46 Century, was 11% greater than the pure stand yield observed for its higher yielding component, Elf, and 12% greater than the weighted mean yield of 54 Elf/46 Century based on purestand performance. The quadratic component was significant at a = 0.10 for Gnome‐Century mixtures, indicating undercompensation as compared with the pure‐stand performance of Gnome and Century. Elf‐Williams and Gnome‐Williams mixtures showed no significant regression components. Diversity in relative maturity, as well as height, may have contributed to overcompensation by Elf and Century grown in mixtures in this narrow‐row environment. This combination of the short‐late Elf with the tall‐early Century was the most diverse of the cultivar pairs evaluated in mixture.

Optimization of relative vegetative and reproductive dry matter accumulation may provide a means of enhancing soybean [Glycine max (L.) Merr.] productivity. This 2‐year field study measured the effect of artificially hastened flowering on the accumulation of dry matter and nitrogen in the seed and stover of eight soybean cultivars. Exposure to short‐ photoperiod treatment (10‐h photoperiods from 18 June through 8 July at Urbana, IL) hastened flowering and physiological maturity by an average of 5 and 17 days, respectively. Despite this decrease in season length, short‐photoperiod treatment resulted in lower seed yield only in the relatively short‐season cultivars Wells and Corsoy in each of the 2 years, and in the early isoline of Clark (L62‐1932) in 1 year. In five later‐maturing cultivars, seed yield potential was maintained despite the abbreviation of season length in response to treatment. Dry matter harvest index was increased by treatment in six of the eight cultivars, and contributed to yield retention in all but the earlfiest cultivars. Total N accumulation was decreased by treatment in the early‐maturing cultivatars Wells, Corsoy, and L62‐1932 and unaffected in the latermaturing cultivars. Photoperiod treatment had no effect on nitrogen harvest index in any of the cultivars evaluated. Efforts to optimize the partitioning of dry matter in soybean through genetic or chemical means may be most successful if directed towards reproductive timing in late‐maturing genotypes where vegetative duration is excessive.