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Corn/soybean yield ratios estimate species × environment interactions.County yields from Kentucky, Iowa, and Nebraska (1972–2015) were used.Cropping systems had a greater effect on average ratios than productivity level.Year‐to‐year variation in the ratio decreased as the productivity level increased.Yield ratios increased significantly in only 37% of the counties (mostly in Iowa) Developing cropping systems to maintain productivity in future environments will require a better understanding of crop species × environment interactions. The ratios of county corn (Zea maysL.) and soybean [Glycine max(L.) Merr.] yields (corn/soybean) from three midwestern states (Kentucky [31 counties], Iowa [47 counties] and Nebraska [irrigated, 33 counties]) (1972 through 2015) were used to evaluate this interaction. Yield estimates (converted to a weight basis) were obtained from the National Agriculture Statistics Service. Average yield ratios were similar in Iowa (2.96) and Nebraska (3.00) in spite of higher yields in Nebraska. The highest average ratio (3.39) occurred in Kentucky counties where soybean double cropped after winter wheat (Triticum aestiviumL.) predominated. Yield ratios were remarkably stable (low CVs) over time in high‐yield environments, but stability decreased as county productivity (estimated by average corn yield) decreased (CVs increased from ∼8 to 23%). The mean yield ratios in Kentucky and Nebraska decreased significantly as the proportion of continuous corn in a county increased. The largest yield ratios in non‐double crop counties in Kentucky were associated with a larger proportion of continuous soybean in the rotation. Yield ratios increased significantly (p< 0.10) from 1972 to 2015 in only 37% of the counties (mostly in Iowa). The largest interactions between corn and soybean involved cropping systems and not fundamental physiological characteristics of these species. The consistency of mean yield ratios in rain fed and irrigated environments suggests that these crops may respond similarly to changes in precipitation.

The most consistent change in management practices associated with the increase in maize (Zea maysL.) yield in the United States since 1930 has been a steady increase in plant population. Populations increased from roughly 30,000 plants ha–1at the beginning of the hybrid era to >75,000 plants ha–1in recent years. The purpose of this review was to evaluate the potential role of sink size in historic maize yield–population relationships. The steady increase in yield (∼4×) during the hybrid era required an increase in source activity since there was no change in harvest index. The increase in grain yield was associated with an increase in kernels per unit area, so there was a concomitant increase in both source and sink. Data collected from the literature and evaluations of hybrids from different eras demonstrate that ear size (kernels per ear) of U.S. and Canadian hybrids did not increase during the hybrid era. Ears per plant did not increase above an average of one. Since ear size and prolificacy did not change, higher plant populations were needed to increase sink size (ears and kernels per unit area). This approach to analyzing population–yield relationships suggests that higher populations didn't necessarily contribute directly to higher productivity; rather, the increase in sink size avoided a sink limitation and allowed the higher productivity of the source to be translated into grain yield. Future yield increases will, therefore, require even higher populations which could create new management challenges.

The ability of soybean [Glycine max(L.) Merr.] to adjust its pod load to environmental conditions is an important, but not well understood, part of the yield production process. To better understand this process at the whole plant level, a single‐node model (SOYPOD) was extended to predict pod set of whole plants. Pod survival in SOYPODP was determined by comparing assimilate from photosynthesis (calculated from solar radiation) to pod requirements during initial pod growth. Measured profiles of flowers per node and pod and seed growth rates were used as input. Pods that do not receive adequate assimilate for a specified number of consecutive days do not survive. When inter‐nodal movement of assimilate was prohibited and the nodal assimilate production profile matched the input flower profile, pod set on the stem matched measured profiles of two field‐grown cultivars. In this form, SOYPODP accurately mimicked known responses of pod number to variation in assimilate supply, nodes per plant and individual seed growth rate. Competition for assimilate at a node‐controlled pod set, so increasing uniformity of pod development (more flowers or a shorter flowering period) increased pod set. Increasing nodal variation in the beginning of pod development reduced the sensitivity of the plant to short‐term fluctuations in assimilate supply. Variation in uniformity of pod development at a node was responsible for increases in pod set with no change in assimilate availability. The value of these increases is not clear, but it seems unlikely that they would affect yield.

The magnitude of yield gaps (YGs) (potential yield – farmer yield) provides some indication of the prospects for increasing crop yield. Quantile regression analysis was applied to county corn (Zea maysL.) yields (1972–2011) from Kentucky, Iowa, and Nebraska (irrigated) (total of 115 counties) to estimate the attainable potential yield (APY) (yield in the most favorable environments in the 40‐yr record) for each county. The YG for each year was the difference between the APY and the county yield. Potential productivity (40‐yr mean county yield) varied substantially within and among states (579–1001 g m−2). The mean relative yield gap (RYG) varied from 9 to 24% of APY and decreased linearly (p< 0.0001) as the mean county yield increased for each state. Large YGs were partially related to very low yields that occurred in some years and may reflect the ability of the soil to supply water to the crop; other soil or management factors may also be involved since irrigation did not reduce the RYG in Nebraska to zero. The highest mean county APY was 30% (Kentucky), 22% (Iowa) and 14% (Nebraska) higher than the lowest county APY. This second YG occurred in favorable environments. Reducing the first YG in Kentucky and Iowa will probably require irrigation. Reducing the second YG may be more difficult if it is related to intractable soil characteristics. These results suggest that soil conditions may play an important role in determining the size of YGs in Midwestern corn production systems.

The magnitude of yield gaps (YG) (potential yield–farmer yield) provides some indication of the prospects for increasing crop yield. Quantile regression analysis was applied to county soybean [Glycine max(L.) Merr.] yields (1972–2011) from Kentucky, Iowa, and Nebraska (irrigated) to estimate the attainable potential yield (APY) (yield in the most favorable environments in the 40‐yr record). The YG for each year was the difference between the APY and the county yield. There was substantial variation in the 40‐yr mean county yields (186–335 g m−2) within and among states. The mean county APY within each state increased linearly (P< 0.0001) in conjunction with mean county yields. The mean relative YGs varied from 9 to 24% of APY and were largest in the lowest yielding counties and they decreased linearly (P< 0.0001) as the mean county yield increased for each state. The large YGs were partially related to very low yields that occurred in some years during the 40‐yr period and these YGs may partially reflect the ability of the soil to supply water to the crop. The difference between the maximum APY in each state and each county APY defined a second YG. The largest values ranged from 12 to 19% of the maximum APY. Irrigation should partially eliminate the first YG, but the second may be more intractable to the degree it is related to soil characteristics. These results suggest that soil conditions may play an important role in determining the size of YGs.

Soybean [Glycine max(L.) Merr.] yield is often associated with the number of pods per unit area, which can be construed to suggest that the number of pod bearing structures (nodes) plays a role in determining yield. This hypothesis was tested in a 2‐yr field experiment at Lexington, KY (38° N latitude). Four cultivars that varied in relative maturity (0.7 to 5.3) and high and low populations (one cultivar) were used to create variation in nodes per square meter. The experiments were planted on 25 May 2010 and 31 May 2011 in 38‐cm rows (19 cm for the high population) and the target populations were approximately 43 (four cultivars), 26 (low population), and 78 (high) plants m−2. Irrigation was used to minimize water stress. There were four replications of each treatment in a randomized complete block design. A 30‐cm section of bordered row was harvested at maturity to determine flower scars, pods, and nodes. The photosynthetically active radiation interception was roughly 90% or above at R1 for all treatments and there was little difference in incident solar radiation or temperature among treatments during flowering and pod set. There was a two‐fold variation in nodes per square meter in both years. Pods per square meter increased in concert with nodes per square meter up to approximately 70% of the maximum node number; above that level there was no change in pods per square meter. At low node levels, pods per square meter seems to be limited by the number of nodes, but at higher node levels there was no relationship between nodes and pods.

The number of pods and seeds produced by soybean [Glycine max(L.) Merr.] is related to canopy photosynthesis during flowering. The effect of low photosynthesis during only a portion of flowering (growth stage R1 to R5), however, is not well defined. Two field experiments were conducted at Lexington, KY (38°N), with seeds sown in mid‐May in 0.76‐m rows (20 seeds m−1of row) and all plots irrigated as needed. In 2005/2006, plants (cultivars Pennyrile and Ripley) were shaded (60% in 2005 and 80% in 2006) for 4‐ to 9‐d periods just before or just after peak pod production. These treatments had almost no effect on seed number (significant reduction in only one of eight comparisons). In a second experiment (cv. Pennyrile, 2007/2008), shade cloth (60%) was placed over plants at growth stage R1.6 and removed at 7‐d intervals. The first 7 d of shade did not affect seed number, but 14 d of shade ending at roughly growth stage R3.0 reduced seed number by 16% and longer periods caused proportionally larger reductions. When shade cloth (80%) was put in place at 7‐d intervals (starting at R3.4) and left in place until maturity, seed number was reduced until the last treatment (put in place 4–7 d after the beginning of growth stage R6). Seed number was tolerant of short periods (4–9 d) of low assimilate supply during flowering, but could not recover from longer periods of shade (≥14 d), even when they occurred relatively early in the flowering period.

Planting date is a critical aspect of all soybean [Glycine max(L.) Merr.] production systems, but the response of yield to planting date fluctuates widely among environments. A combined analysis of many planting date experiments will provide a better estimate of the average response. Data from 28 nonirrigated planting date experiments that were conducted for more than 1 yr with at least three planting dates (a minimum of one in June) were combined by regions (Midwest, 9; Upper South, 10; Deep South, 9) for analysis. Experiments using the early soybean production system (ESPS) were not included. A segmented‐linear regression model with two segments was used to determine when yield began a rapid decline as planting was delayed. The rapid decline began on 30 May in the Midwest, 7 June in the Upper South, and 27 May in the Deep South. The rate of decline was larger in the Upper South (1.1) and Deep South (1.2) than it was in the Midwest (0.7% points per day). Yield trends before the rapid decline began showed no significant advantage for early plantings. Only 23% of the April or early (first week) May plantings exhibited higher yields than later plantings and the average advantage was 7%. The average response to planting date was remarkably similar across all regions and there seems to be no consistent advantage for ultra‐early planting dates with traditional cultivars, but there was a significant penalty for planting after the critical date in late May or early June.

Genetic control and environmental modulation of seed size operate through their influences on rate and duration of seed filling, and their interaction. To account for this interaction, here we advance an allometric model centered on the scaling exponent α calculated as the slope of the linear regression between duration and rate of grain filling in a log‐log scale. The scaling exponent allows for three types of responses: seed size is stable as a result of full compensation between rate and duration (α = −1), seed size is variable as a result of rate (α > −1), or duration‐dominated growth (α < −1). The concept was tested with 45 data sets from the literature involving nine crop species, and sources of variation including genotype, environment, and their interaction. Relative variation in seed size ranged from 5 to 274%, and the scaling exponent was strongly concentrated in the range from 0 (large, rate‐driven seed size range) to −1 (narrow seed size range due to mutually cancelled effects of rate and duration). The range of seed size declined when the scaling exponent declined from approximately 0 to −1. An α ≈ −1 (rate and duration effects cancel each other) is necessary and sufficient for small variation in seed size, whereas α ≈ 0 is necessary but not sufficient for large seed size variation. The magnitude of seed size variation is dependent on the variation in the rate of seed growth when α ≈ 0. This double condition for seed size variability is summarized in a multiple regression model with α and range of rate of grain filling as independent variables, which accounted for 73% of the variation in range of seed size.

Soybean [Glycine max(L.) Merrill] yields often decline as seed protein levels increase, but the processes responsible are not clearly understood. We compared dry matter and N accumulation by individual seeds of three high protein genotypes (K 1431, KS 4402sp, NE 3396) and three commercial cultivars (Pennyrile, Flyer, Hutcheson) in the field near Lexington, KY (38° 01′ N lat; 84° 35′ W long) for 3 yr to determine if increasing protein concentration decreased the seed growth rate. Plants were grown in 76.2‐cm rows using conventional production practices and overhead irrigation to minimize soil moisture deficits. The rate of dry matter accumulation by individual seeds (SGRDM) varied from 4.2 to 6.1 mg seed−1d−1and this variation was closely associated with mature seed size, but it was not affected by seed N concentration. The rate of N accumulation (SGRN, mg N seed−1d−1) was closely associated with seed N concentration at maturity across all genotypes and years (r2= 0.76). Thus, SGRNand SGRDMseemed to vary independently as the N concentration in mature seeds increased. The duration of seed filling (estimated by the effective filling period) was not related to mature seed N concentration. Since higher seed N concentration had no effect on the rate or duration of seed dry matter accumulation by individual seeds, the purported negative effect of seed protein levels on yield may be more closely associated with whole plant phenomena than those operating at the individual seed level.

The seed is the harvested organ in grain crops and, consequently, the accumulation of dry matter by the seed is an important component of the yield production process. The growth of individual seeds, usually characterised by the rate and duration of growth, is, at least partially, controlled by the seed itself. It is this control that gives the seed a significant role in the yield production process. Genetic variation in seed growth rate, which is substantial, is not related to yield, but variation related to environmental conditions during seed filling often is. Genetic and environmental variation in seed-fill duration is usually directly related to yield. Much is now known about the regulation of seed growth and this knowledge, if applied, may help answer some of the important questions still facing crop scientists as they try to increase yields in the future.

High temperature stress during seed filling in controlled environments reduces soybean [Glycine max(L.) Merrill] seed germination and vigor, but the effect of high temperature in the field has not been determined. Seeds of two soybean cultivars (Hutcheson, maturity group [MG] V, and DP4690, MG IV) were produced in the field in Kentucky, Mississippi, Arkansas, and Texas in 2000 to 2002. Air temperature during seed filling was monitored and brown (mature) pods were harvested, hand threshed, and all shriveled and abnormal seeds were removed before determining standard germination and vigor (accelerated‐aging germination). Mean maximum temperatures during seed filling (growth stage R5 to R7) ranged from 24.0 (Kentucky) to 37.6°C (Texas). When seed lots infected with Phomopsis longicolla(Hobbs) were removed from the analysis, standard germination and accelerated‐aging germination (AA) decreased as mean maximum temperature during seed filling increased, but the decrease was significant (P= 0.05) only for Hutcheson. Standard germination of Hutcheson declined linearly (r2= 0.49) from near 100% at 24°C to 85% at 36°C, while the decrease in AA was curvilinear (R2= 0.86) and germination reached 11% at 36°C. Seeds of Hutcheson were more sensitive to high temperature than seeds of DP4690 and seed vigor (AA) was much more sensitive to high‐temperature stress than was standard germination. Our findings support the results of experiments in controlled environments by demonstrating that high temperature during seed filling in the field, without seed infection with P. longicollaor physical injury, reduced soybean seed germination and vigor.

The effects of short periods of water stress during seed‐filling on leaf senescence, seed‐fill duration, and yield of soybean [Glycine max(L.) Merrill] are not well understood. Short stress periods were investigated in two greenhouse experiments with cultivar Elgin 87 grown in soil‐filled pots. All pots received adequate water until the beginning of growth stage R6 when a continuous water‐stress treatment (pots received 40% of the water needed to bring controls to pot capacity) was initiated and maintained until maturity. Water stress was relieved in other pots (watered as the control) after 5 or 13 d in Exp. 1 and 3 or 6 d in Exp. 2. Each treatment was replicated six to eight times in a completely randomized design. The carbon exchange rate was rapidly reduced by continuous water stress resulting in earlier maturity, significantly lower yield (39%), and smaller seeds (25–33%). The carbon exchange rate rapidly increased to near control levels in the early stress‐relief treatment, but it was always less than the control for the rest of seed filling. These plants matured sooner and produced significantly lower yields (10–23%) and smaller seeds (9–17%) than control plants. Late stress relief also reduced yield and seed size relative to the control. Yield and seed size of both stress relief treatments, however, were greater than the continuous stress treatment. Water stress‐induced acceleration of senescence could not be stopped by eliminating the stress after a short period. Short periods of water stress during seed filling may, therefore, have larger than expected effects on yield.

Much of the variation in yield of soybean [Glycine max(L.) Merr.] results from variation in seeds per unit area, but the physiological regulation of seed number is not well understood. The relationship between the timing of flower development and pod and seed number was evaluated in three greenhouse experiments with a girdled‐node system. Plants (‘Elgin 87’) were grown in 3.0‐L pots filled with a soil‐vermiculite mixture (2:1 v/v) and the main stem below Node 7 (one‐node treatment) or below Node 5 (three‐node treatment) was girdled when the first flower opened at Node 7. The leaves at Nodes 5 and 6 on the three‐node treatment were removed and the main stem above Node 7 was removed on all plants. All nodes in the three‐node treatment flowered at the same time and the total flower production was 68% greater than on the one‐node treatment. The three‐node treatment produced more pods (36–94%) and more seeds (36–78%) than the one‐node treatment. Girdling‐induced reductions in light saturated C exchange rate (CER) during flowering and pod set were consistently greater on the one‐node than on the three‐node treatment. However, the three‐node treatment produced more pods per unit assimilate, suggesting that the increased CER did not account for the increase in pods and seeds. The number of pods and seeds at phloem‐isolated nodes responded to synchronous flowering, suggesting that the timing of flower development may play a role in determining reproductive success in soybean.

Emergence of soybean [Glycine max(L.) Merr.] seedlings in the field is frequently less than predicted by standard germination, but the causes of this emergence failure are not well understood. This study explored the influence of soil pathogens and seed vigor on soybean seedling preemergent growth and emergence. Seed from six seedlots, representing a range in seed vigor, were planted as pregerminated and as dry seed into sterile and pathogen‐infested soil maintained at a constant soil water potential (−0.005 MPa). Final emergence (FE) and emergence rates from two planting depths (25 and 60 mm) were recorded under ambient greenhouse conditions. Once the FE stage had been reached, nonemerged seedlings were exhumed, and classified as stunted, abnormal, or dead. The FE of the high‐ and medium‐vigor seedlots was always higher than the low‐vigor seedlots, and the advantage was greatest under stressful conditions (deep planting, nonsterile soil). The FE was always lower in nonsterile than sterile soil. Seedlings emerged more slowly from deep plantings and when pathogens were present, and FE decreased as emergence was delayed in nonsterile soil. This relationship became more pronounced with low vigor seed. Planting pregerminated seeds always resulted in higher FE than dry seeds, but it did not eliminate emergence failure. Abnormal seedlings accounted for most of the emergence failure in sterile soil, but the lack of germination or of growth immediately after germination was also important in nonsterile soil. Lack of germination alone therefore does not account for lack of emergence; postgerminative, preemergent growth may be more important.

Seed number is the primary yield component in soybean [Glycine max(L.) Merrill], but the mechanisms responsible for the regulation of this component are not fully understood. Three soybean cultivars (Elgin 87, Emerald, and Essex) with genetic differences in seed growth rate and seed size were grown in the field in 1995 and 1996 to determine the relationship between photosynthesis, sink characteristics, and seed number at individual nodes. The sixth node from the bottom of the main stem was isolated by heat girdling the stem below the node to disrupt phloem continuity and by removing the part of the plant above the girdled node when flowers first opened at this node. Photosynthesis was varied by defoliating (removing approximately 0, 33, 66, 83, or 100% of the leaf area) the leaf at the girdled node. Carbon dioxide exchange rate (CER) was measured at approximately weekly intervals for up to 40 d after girdling. Girdling temporarily reduced CER in four of six comparisons and defoliation tended to increase CER. Defoliation produced large differences in nodal carbon input (NCI) per node and seed number increased rapidly as average NCI per node increased from 0 to approximately 0.10 μmol CO2node−1s−2However, there was no further increase in seed number as average NCI continued to increase to 0.5 μmol CO2node−1s−2, suggesting that isolated nodes respond differently than soybean communities. Maximum seed number per node was inversely related to cultivar differences in individual seed growth rate and seed size. The girdled node technique should prove useful to investigate seed number‐photosynthesis relationships in soybean.

An understanding of the physiological changes that have occurred in cultivar development in the past would be helpful in the continued production of high yielding cultivars of the various crop species. Field experiments using conventional cultural practices with supplemental irrigation were conducted in 1976 to determine the physiological basis for yield differences between old, low yielding soybean [Glycine max(L.) Merr.] cultivars (Lincoln' and ‘Dorman’) and new high yielding cultivars (‘Williams’ and ‘Essex’). Mechanical support was used to prevent lodging for comparison to control plots with no mechanical support. There was no lodging in Lincoln and Williams (Maturity Group III); whereas, in maturity group V, Dorman lodged significantly more than Essex in the control plots. However, there was no relationship between lodging and yield. Williams produced 34% more yield than Lincoln, primarily because of larger seed. Essex produced 88% more yield than Dorman, primarily because of more seed per unit area. Yield differences were not correlated with total shoot weights or CO2uptake rates of a single leaf during vegetative or early reproductive growth. Rates of acetylene reduction were similar for Williams and Lincoln, but Essex had a higher rate than Dorman. The yield advantage of Williams resulted partially from a longer filling period (12%), which was associated with higher CO2uptake and acetylene reduction rates late in the filling period. Essex and Dorman produced similar total shoot weights late in the filling period, so the yield advantage for Essex resulted from partitioning more photosynthate to the seed. The data suggest that yield improvement in the future may be possible by lengthening the filling period. It may not be possible to increase seed yields by increasing the partitioning of photosynthate to the seed if the partitioning in current cultivars is approaching the maximum.

A better understanding of the effects of varying the N supply to the soybean (Glycine max(L.) Merrill) be obtained by studying, after treatment, the resultant changes in N and carbohydrate levels in the various plant parts. Greenhouse and field experiments were conducted to determine the effect of varying the N supply to the plant during flowering and pod‐filling on the dry weight, total N, NO3‐, and the total available carbohydrates. The greenhouse experiment was conducted in a gravel culture hydroponics system with 5 meq NO3‐/liter m the nutrient solution. Some plants were shifted to 12.5 and 18.0 meq NO3‐/liter at initial bloom and maintained at these levels until maturity, while others were exposed to the high NO3‐levels only until the end of bloom. Treatments in the field consisted of 168 kg N/ha as NH4NO3applied at initial bloom, at the end of bloom, or at both times (total of 336 kg N/ha). Increasing the N supply to the plant during the period from initial bloom increased the dry weight of the vegetative plant parts at the end of bloom and at mid‐podfill. The effect was much greater in the greenhouse than in the field. Increasing the N supply to the plant also increased the concentration of N and NO3‐in the vegetative tissue at the end of bloom; differences in N and NO3‐were also apparent at midpodfill although the magnitude was not so great. The concentration of total available carbohydrates in the vegetative tissue decreased when the N supply to the plant increased. Treatment effects were similar in the greenhouse (non‐nodulated) and in the field (nodulated) experiment. A previous paper reported that yield was increased by increasing the N supply to the plant during flowering and pod set, primarily as a result of an increase in seed number. The increase in yield was associated with increased tissue levels of N and NO3‐and decreased levels of total available carbohydrates. The data demonstrate the importance of high tissue levels of N during the period of flowering and podset.

The growth of an individual soybean [Glycine max(L.) Merr.] seed is an important part of the yield production process. An understanding of the effect of environmental factors on the growth of individual seeds is needed to enhance our understanding of the effect of the environment on yield. Soybeans were grown in the field on a Maury silt loam soil (fine, mixed, mesic Typic Paleudalfs) for 3 years (1979 to 1981) and subjected to varying levels of moisture stress to investigate the effect of moisture stress on seed growth. Irrigated controls were compared with moisture stress treatments from planting to beginning seed fill (growth stage RS), during seed filling (growth stage R5 to R7) or from planting to maturity (severe stress). Fruits were tagged at beginning seed fill and sampled at weekly intervals to estimate the seed growth rate and the effective filling period (EFP). Although the stress treatments reduced yield and vegetative growth, the rate of seed growth (mg seed−1day−1) was not affected. The seed‐filling period (estimated as days from growth stage R5 to R7 or EFP) was shortened by the severe stress treatment in 1979 and 1981 (statistically significant only in 1979), and there was a trend for the period of R5 to R7 to be shortened by late stress in 1980 and 1981. The data suggest that the rate of individual seed growth is less sensitive to moisture stress than other plant processes. The seed filling period was more sensitive to moisture stress than seed growth rate and the effect of moisture stress on the duration of seed fill may be one way that stress reduces soybean yield.

The growth rate and duration of an individual seed are important parameters in the yield production process of a grain crop. Experiments were conducted in the CSIRO phytotron at Canberra, Australia to investigate the effect of temperature on the rate and duration of seed growth and associated plant characteristics in soybeans [Glycine max(L.) Merr.]. Soybeans (‘Fiskeby V’) were grown at 24/19 C until beginning seed growth and shifted to 18/13, 24/19, 27/22, 30/25, and 33/28 C (day/night temperature, 8‐hour day) until maturity. Seed growth rates (SGR) were estimated by harvesting seeds at 5‐day intervals, and from the growth rate of excised cotyledons cultured in nutrient media for 96 hours. The SGR of seed developing on the plant increased from 6.1 to 7.9 mg seed−1day−1as air temperatures increased from 18/13 to 27/22 C, but there was no further change as the temperature increased to 33/28 C. Excised cotyledons showed a similar growth rate response to temperature. Exposing the plants to high temperatures (33/28 C) during the period of flowering and pod set reduced SGR (36%) regardless of the temperature during the subsequent seed growth period, suggesting that seed growth is sensitive to temperature levels at these early stages. Seed respiration on a dry weight basis (mgCO2h−1g−1) was not affected by temperature. The duration of seed growth was not affected by temperatures of 24/19 to 30/25 C but was reduced by 3 days at 33/28 C, and this was associated with accelerated leaf senescence as shown by leaf yellowing and reductions in CO2exchange rate. Final seed size was reduced from 200 to 151 mg seed−1at 18/13 and 33/28 C. The data suggest that SGR and duration are relatively insensitive to temperatures ranging from 24/19 to 30/25 when these are imposed after flowering and pod development. The reduction in the duration of seed growth at high temperatures may be one mechanism by which high temperatures reduce yield.

The quality of soybean [Glycine max(L.) Merr.] seed at harvest is dependent on the field production environment during development, maturation, and storage on the plant. These investigations were conducted to evaluate the effects of field weathering after physiological maturity (maximum dry seed weight) on seed viability and vigor. Five production environments were compared for two cultivars (‘Cutler 71’ ‐ 1973, ‘Kent’ and Cutler 71 ‐ 1974 and 1975). Seed was hand‐harvested at physiological maturity, harvest maturity (first time the seed dried to less than 14% moisture content), and at regular intervals for up to 3 months after harvest maturity. All seed were evaluated for viability (standard germination test) and vigor (accelerated‐aging germination test). Physiological maturity occurred at a seed moisture content of approximately 55% for both cultivars. The time interval from physiological maturity to harvest maturity (desiccation period) ranged from 10 to 20 days and was closely related (R2= 0.88) to the mean open pan evaporation during the period. Seed viability and vigor were highest at physiological maturity and remained high (>80% germination) until harvest maturity for all but one production environment. Viability was maintained at these high levels for 1 to 2 months following harvest maturity for four of five production environments. However, seed vigor declined rapidly, reaching levels that were significantly (α = 0.05) less than those at harvest maturity within 4 to 39 days after harvest maturity. In four of the production environments, seed vigor reached a level of less than 50% germination within 1 month of harvest maturity. The time required for seed vigor to significantly decline following harvest maturity was closely related to mean air temperature (R2= 0.90), mean minimum relative humidity (R2= 0.94), and precipitation per day (R2= 0.75). The wide range in seed vigor observed shortly after harvest maturity emphasizes the importance of timely harvest of seed fields and evaluation of seed viability and vigor soon after harvest.

Changes in the proportion of the total biomass harvested have made an important contribution to the increase in yield in several grain crops. Field experiments were conducted for several years on Typic Argiudoll and Typic Paleudalf soils to investigate the effect of cultivars, growth habit, and environmental conditions on the partitioning of assimilate between vegetative and reproductive plant parts in soybean [Glycine max(L.) Merr.] during flowering and fruit set. The experiments included the following variables; cultivars from maturity group II to V, including both indeterminate and determinate growth habits, optimum and delayed plantings, and moisture stress. Data were collected from growth stage R1 to R6 on vegetative growth characteristics (nodes on main stem, total nodes, and plant height), pod development, and weight of vegetative and reproductive plant parts. Partitioning coefficients were calculated by dividing fruit weight by the total aboveground biomass. The nodes on the main stem and on the branches reached a maximum at R5 in all cases. Determinate cultivars produced fewer nodes on the main stem between R1 and R5; however, they produced more branch nodes and the total node production was greater than the indeterminate cultivars. The number of pods containing developing seed increased until approximately R6, suggesting that the production of new vegetative sinks (node production) in soybean stopped before the total fruit load was established. The partitioning coefficients increased rapidly from < 5% at growth stage R3 to ≥ 30% at R6. The partitioning coefficient at any growth stage varied only slightly across the variables included in the experiments. This suggests that variation in fruit and seed number may be more closely related to variations in crop growth rate than to variations in the allocation of assimilate between vegetative and reproductive plant parts.

Mechanistic models of soil water uptake have differed in the form of the water uptake term of Richard's equation. One of the most widely used uptake terms is that proposed by Hillel et al., although a detailed study comparing predicted soil water contents with those observed experimentally has been lacking. The purpose of this work was to develop a model that would test the accuracy of this water uptake term on a seasonal basis at various levels of water stress. Simulated soil water contents were compared with water contents observed biweekly in a 3‐yr field experiment that included three moisture treatments: irrigated, normal rainfall, and plastic covered. When a longitudinal resistance was included, 69 to 88% of the predicted soil water contents fell within the 95% confidence limits of the observed values from the irrigated and normal rainfall treatments in 3 yr. In the plastic‐covered treatment, however, only 17 to 49% of the predicted water contents were within the confidence limits. We conclude that the uptake term is accurate under normal and well watered conditions, but not under severe water‐stress conditions. Sensitivity analysis showed that the model was sensitive to net radiation, LAI, and rainfall and insensitive to hydraulic conductivity and root density.

Seed size (mass per seed) is an important yield component in soybean [Glycine max(L.) Merr.]; however, little is known of the variation in seed size within a seed lot. Consequently, the variation in seed size within seed lots collected from greenhouse and field experiments was investigated. Treatment variables included in the experiments were planting dates, cultivars (determinate and indeterminate), and source‐sink alterations applied during seed filling. Seed size distributions were determined using round‐hole screens ranging from 3.2 to 8.7 mm in diameter at intervals of 0.8 mm. All seed lots investigated exhibited a wide range in seed size from approximately 90 to greater than 200 mg seed−1. However, 40 to 80% of the seeds stayed on a single screen. The seed size of seeds on a single screen varied significantly among cultivars, planting dates, and years. Determinate cultivars tended to have a greater proportion of the seed on a single screen, but the range in seed size was similar to that exhibited by indeterminate cultivars. Treatments that altered mean seed size usually affected the proportions of the various sized seeds in the lot, more than the range in seed size. Variations in seed size within a seed lot were not associated with differences in seed density, specific seed coat mass, or seed N concentration. The data indicate that variation in mean seed size can be caused by variations in the proportion of the various sized seed in the seed lot, with relatively small changes in the range in seed size.

Seeds from an individual soybean [Glycine max(L.) Merrill] plant exhibit a range in seed mass, the cause of which is not well understood. This research was conducted to investigate the relationship between the time of flower opening and seed mass. Five soybean cultivars (Cumberland, Harper, DeSoto, Douglas, and Essex) were grown in the field using conventional cultural practices in 1984 and 1985. Fully open flowers were marked with acrylic paint at growth stages R1 to R4.5. Fruits from marked flowers were harvested at maturity and seed dry mass determined. The range in seed mass was estimated using round‐hole screens. The rate of seed development was monitored by determining when seeds reached 3 mm in length and when the pod turned yellow. Mean mass of seeds from flowers opening at R4 or R4.5 was less than mass of seeds from earlier flowers, and this was associated with a shorter seed filling period. Seed mass was not consistently related to time of flowering for flowers opening at R1 to R3. There was a range in seed mass within a time of flower opening, and most of this variation was associated with variation in seed growth rate. Applying fruit removal treatments when pods from the marked flowers reached maximum size indicated that the maximum potential mass of seeds from late flowers was much larger than the normal mass. The data indicate that the time of flowering accounts for only a portion of the variation in seed mass of a soybean plant.

Understanding the cause of soybean [Glycine max(L.) Merr.] seed deterioration during storage will assist in the development of accurate laboratory tests to evaluate seed vigor. This study was designed to investigate the changes that occur in mitochondria! respiration during the early stages of soybean seed deterioration. High‐quality seed lots of ‘Desoto’ and ‘Union’ soybean were stored at 25 °C and 115 g kg‐1seed moisture. Seed quality (germination and vigor) and conductivity of leachate from embryonic axes were measured at monthly intervals during storage. Mitochondrial State 3 (ST 3) respiration rates and respiratory control ratios (RCR) were measured from excised axes of samples after 24 h of imbibition. Germination remained high for seeds of both cultivars throughout storage; however, seed vigor (as measured by accelerated aging) declined significantly after 7 and 10 months for Union and Desoto, respectively. Conductivity of the leachate from excised axes for both cultivars increased early during storage prior to detectable changes in whole seed vigor. State 3 respiration rates of mitochondria from axes decreased throughout storage and RCR values declined after 2 months for Union and after 4 months for Desoto. Decreases in ST 3 respiration rates and RCR in both cultivars preceded measurable declines in whole‐seed vigor and correlated with increases in the conductivity of axes leachate.

Peroxidative changes in seed lipids are thought to be a major cause of seed deterioration during storage. This study was conducted to determine the fatty‐acid composition of soybean [Glycine max(L.) Merr.] seeds, axes, and mitochondria during the early stages of deterioration. High‐quality seed lots of ‘Union’ and ‘Desoto’ soybean were stored at 25 °C and 115 g kg‐1 seed moisture for 10 and 12 months, respectively. Seeds were sampled at frequent intervals and the fatty‐acid composition of storage and membrane lipids from seeds, axes, and mitochondria were determined. There were no detectable changes in the fatty‐acid composition of storage lipids from whole seeds or axes during the storage period for either seed lot, despite the substantial decline in seed vigor. There was a slight decrease in the percentage of unsaturated fatty acids in the membrane lipids of excised axes from Desoto seeds. The percentage of unsaturated fatty acids in the mitochondria from excised axes declined early during storage. The double‐bond index (DBI) of lipids from mitochondria of the Union axes fell from 1.76 at the beginning of storage to 1.60 after 10 months. Similar declines of 1.79 to 1.70 after 1 yr in storage were seen for the Desoto axes. Mitochondria! respiration and DBI were highly correlated for both seed lots. These data suggest that the decreases in mitochondrial respiration during storage may be associated with peroxidative changes in mitochondrial lipids and that these changes occurred prior to loss in seed vigor.

The number of seeds per unit area is an important yield component in soybean [Glycine max(L.) Merr.]; however, the mechanisms responsible for the regulation of this yield component are not well understood. Field experiments were conducted at Lexington, KY (3 yr), and at Taian, China (1 yr), to investigate the relationship between net canopy photosynthesis and seeds per unit area using genotypes with differences in individual seed growth rates (SGR). At Lexington, shades (30 and 63% reduction in insolation) were placed over plots from growth stage Rl until maturity to create differences in canopy photosynthesis. Planting dates (early and late) and row spacing (wide and narrow) were used at Taian to create differences in canopy photosynthesis. Crop growth rate (CGR) was measured between growth stage Rl and RS as an estimate of net canopy photosynthesis. Yield, seeds per m2, and SGR were also measured. Within each genotype, there was a linear relationship between CGR and seeds per m2across treatments and years. Within an experiment, seeds per m2at a constant CGR was inversely related to genotypic differences in SGR. A partitioning coefficient (γ) was estimated by dividing the total sink demand (seeds per m2✕ SGR) by CGR. There were no apparent genotypic differences in γ however, γ decreased linearly as CGR increased. The data suggest that the model proposed by Charles‐Edwards, which describes seeds per m2as a direct function of canopy photosynthesis and a partitioning coefficient and an inverse function of assimilate flux to individual seeds, accurately describes the regulation of seeds per m2in soybean.

The effect of varying source‐sink ratios on the distribution of 14C labeled assimilate in soybeans (Glycine max(L.) Merr.) was investigated in a greenhouse experiment. Varying source‐sink ratios were created by pod (approximately 50%) or leaf removal (two side leaflets from each trifolioliate) at the end of flowering. A single leaf was exposed to 14CO217 days after application of the treatments. Pod removal reduced seed yield/plant and increased seed size, whereas leaf removal reduced yield, the number of seed/plant, and seed size. Seven hours after labeling a greater percentage of 14C remained in the leaf in the pod removal treatment (72%) than in either the leaf removal (55%) or the control (59%), and specific leaf weight was higher in the pod removal treatment. The distribution of 14C 24 hours after labeling was not greatly affected by the treatments. The pod removal treatment had a greater percentage of 14C in the labeled leaf, in the stem both above and below the node of the labeled leaf, and less in the seed. Removal of the leaves did not affect the percentage of 14C in the labeled leaf after 24 hours or the distribution of the 14C in the plant, probably because the source‐sink ratio was similar to that of the control. The primary effect of altering the source‐sink ratio was on the movement of 14C labeled assimilate out of the leaf.

No‐till planting of corn (Zea maysL.) seed may result in delayed germination and slower seedling growth. The objective of this research was to evaluate the effect of seed vigor on plant growth and yield of corn planted in various surface residues without tillage. Four seed lots of B73 ✕ Mol7, which had good germination (88‐98%) but a range in seed vigor, were no‐till planted into soft red winter wheat (Triticum aestivumL.), tall fescue (Festuca arundinaceaL.) sod, and corn residue at early, midseason, and late planting dates in 1982 and 1984. Vegetative plant weight, plant height, and leaf area were measured at the four‐collar leaf stage and at 50% silking. Grain yield was measured at maturity. The weight, leaf area, and height of plants from the low‐vigor seed lots were consistently less than plants from the high‐vigor seed lots in all treatments at the four‐collar stage. Similar trends were seen at silking; however, the differences were much smaller and usually not significant. In most cases, seed vigor had no effect on grain yield. The few treatment combinations with reduced yields for the low‐vigor seed lots also had reduced seedling emergence. Thus, there was no direct effect of seed vigor on yield; however, the effect of seed vigor on seedling emergence can affect yield if less‐than‐optimum plant populations are obtained.

Delayed seed germination and slow seedling growth commonly occur following no‐till planting of corn (Zea maysL.) seed. This investigation was conducted to evaluate the effect of seed vigor on seedling emergence under a wide range of no‐tillage soil environments. Four seed lots of B73 ✕ Mol7, which had good germination (88‐98%) but a range in seed vigor, were no‐till planted into soft red winter wheat (Triticum aestivumL.), tall fescue (Festuca arundinaceaL.) sod, and corn residue at three planting dates in 1982 to 1984. Stress (low soil temperature and high soil moisture) occurred at the early planting date (mid‐April) in 2 of 3 yr, while near‐ideal soil conditions occurred at the other planting dates. Maximum soil temperatures at the early planting date following no‐tillage into wheat stubble and corn stalks were similar to conventional tillage, but much greater than no‐tillage into tall fescue sod. The low‐ and medium‐vigor seed lots had consistently lower emergence than high‐vigor seed lots in all tillage systems and planting dates. Emergence following conventional tillage was consistently greater than emergence following all no‐tillage treatments; however, this difference was significant only at the early planting date in 1982 and 1983. Under the stress conditions that occurred in the early planting date in 1982 and 1983 following no‐tillage, the high‐vigor seed lots had significantly greater emergence than low‐vigor seed lots. Thus, selecting seed lots with high germination and vigor would be beneficial for early plantings in no‐tillage systems.

Information on factors influencing the rate and duration of seed growth is needed to understand completely yield production. This study was undertaken to determine the relationship of seed size and the timing of pod development to the rate and duration of seed growth in soybeans (Glycine max(L.) Merrill). Field studies were conducted for 2 years (1974‐75) on a Maury silt loam soil (Typic Paleudults) utilizing five cultivars of varying seed size: ‘Kanrich’, 234 mg/seed; ‘Williams’, 171 mg/seed; ‘Cutler 71’, 165 mg/seed; ‘Custer’, 129 mg/seed; and ‘Essex’, 114 mg/seed. Each year the rate of dry matter accumulation in the seed was measured on the first and the last pods to develop on each cultivar. Kanrich had the highest seed growth rate (7.96 mg seed−1day−1); Essex the lowest rate (3.64 mg seed”1 day1), and the other cultivars had rates intermediate between Kanrich and Essex. Seed growth rates were relatively constant across the early and late pods. The duration of the effective filling period (EFP) was less for the late pods on the indeterminate cultivars (Kanrich, Williams, Cutler 71, and Custer), and this resulted in slightly smaller seed. There was no difference in the length of the EFP among the five cultivars used. Air temperatures during the filling period were lower in 1974 than in 1975, and the low temperatures were associated with low seed growth rates, a long EFP and large seed. These data suggest that the rate of seed growth is partially determined by the genetic makeup of the seed. The direct relationship between seed growth rates and seed size provides an explanation of the negative correlations frequently observed between seed size and the number of seed produced by the plant.

The redistribution of nitrogen from vegetative to reproductive plant parts of soybeans (Glycine max(L.) Merr.) may be one of the factors causing leaf senescence and, consequently, it may have a direct influence on yield. Experiments were conducted for 2 years in a greenhouse hydroponics system to investigate the influence of N removal from the nutrient solution at various intervals during pod filling on the redistribution of N from vegetative plant parts to the seed and on leaf senescence. Plants were harvested at early podfill and at maturity, and all abscised leaves and petioles were collected. Subsequently, total N determinations were made on the abscised leaves and petioles, roots, pods, stems, and seed, thereby permitting an estimation of the redistribution of N. Removal of N early in the pod‐filling period reduced yield, primarily as a result of smaller seed. It hastened maturity and increased the proportion of N in the seed that came from redistribution. Early N removal resulted in earlier leaf and petiole abscission, but none of the N removal treatments had a major effect on the concentration of N in the abscised material. The portion of the N in the seed that came from redistribution ranged from approximately 20% (controls) to approximately 60% (early N removal treatments). Leaves and pod walls were the major sources of redistributed N. Raising the N level in the nutrient solution above that of the control during the filling period had no effect (relative to the control) on any of the parameters measured. The data suggest that the redistribution of N from vegetative to reproductive plant parts influence plant senescence; however, other factors are involved.

The number of seed produced by a soybean (Glycine max(L.) Merrill) community is one of the important determinants of yield. Since soybeans usually abort a high percentage of their flowers and pods, this study was undertaken to investigate the influence of N nutrition on flower and pod abortion. Experiments in which the N supply to the soybean plant was varied during flowering and pod filling were conducted in the greenhouse and field. The greenhouse experiment was conducted in a hydroponics system with 5 meq NO3‐/liter in the nutrient solution. Some plants were shifted to 12.5 or 18.0 meq NO3‐/liter at initial bloom and maintained at these levels until maturity, while others were exposed to the high NO3‐/levels only until the end of bloom. Treatments in the field consisted of (1) 168 kg N/ha as NH4NO3applied at initial bloom, (2) 168 kg N/ha at the end of bloom, and (3) 168 kg N/ha at both growth stages (total of 336 kg N/ha). The field experiments were conducted on a maury silt loam soil (Typic Paleudalts). Increasing the N supply to the plant during the period from initial bloom to the end of bloom increased yield 33% in the greenhouse and 28 to 32% in the field. Yields were not affected by high levels of N during the filling period. In the greenhouse experiment and one field experiment all of the yield increase resulted from an increase in seed per plant, which was due to an increase in the number of nodes and a reduction in flower and pod abortion. Flower abortion ranged from approximately 55% on the controls to approximately 45% on the treatments giving the maximum yield increase. In a second field experiment, the yield increase came partially from an increase in seed size and partially from an increase in seed number. The data demonstrate the importance of an adequate supply of N to the plant during the period of flowering and pod set for maximum yield.

Rate of hypocotyl elongation of ‘Cutler’ and ‘Lee 68’ soybean (Glycine max(L.) Merr.) cultivars was measured at several temperatures and incorporated into a prediction model for initial soybean growth. The rate of hypocotyl elongation was found to be a function of temperature and the length of the hypocotyl. Hypocotyl elongation was extremely slow at 10 C, and the seed did not germinate at 40 C. The rate of hypocotyl elongation reached a maximum at approximately 30 C and increased with increasing hypocotyl length for both cultivars. Elongation rates of Lee 68 were consistently higher than those of Cutler. Prediction equations were developed from the data and used to predict the hours to 50% emergence as a function of soil temperature and depth of planting. These equations provided a reliable estimate of emergence times for each cultivar when compared with the actual data under a range of soil temperature conditions in the field and in the laboratory. Theoretical results from the equations indicated that the optimum time to plant soybeans so that emergence would occur in the least amount, of time was when the soil temperature was in the range of 25 to 35 C, provided no other factors were limiting emergence.

The ability of an individual soybean [Glycine max(L.) Merr.] seed to utilize assimilate is an important part of the yield production process. Cotyledon cell number is a major determinant of genetic differences in seed growth rate. Experiments were conducted in the field and greenhouse with ‘McCall’, ‘Williams’, and ‘York’ cultivars to determine if source‐sink alterations would affect cotyledon cell number. Source‐sink alteration treatments (66% defoliation, 80% depodding, and 63% insolation reduction) were applied when tagged fruits were 20 to 25 mm long. Seeds from the tagged fruits were harvested at maturity and cotyledon cells were counted. Defoliation or shading reduced cotyledon cell number by 21 to 55% for McCall and York in the greenhouse and Williams in the field. Depodding increased cotyledon cell number by 26 to 102%. Cotyledon growth rates in an in vitro culture system increased with increasing sucrose concentrations and reached a maximum at 100 to 200 mM. Cotyledon growth rates in vitro with saturating levels of sucrose and asparagine were closely correlated with cotyledon cell number across treatments and genotypes. The data indicate that cotyledon cell number in soybean is influenced by the physiological environment during the cell division phase of seed development and that both cotyledon cell number and assimilate supply are important in determining seed growth rate.

To facilitate the development of forages with mineral composition optimal for ruminant production, knowledge of the biochemical and physiological processes determining genotypic differences in nutrient uptake is needed. Experiments were conducted to identify the mechanism(s) that regulated the difference in leaf blade K concentration between two selections of Lolium× Festucahybriderivatives. Selection differences in rates of K uptake werestimated by measuring Rb uptake by excised roots, and the selectivity of cation uptake for K over Na was determined using intact tillers grown in modified Hoagland's solutions containing varying ratios of K:Na. Differences in Rb uptake by excised roots were of sufficient magnitude to explain differences in leaf blade K concentration. Results indicate differences in K and Na uptake between these two selections reflected an alteration in cation uptake selectivity by roots. Perturbing the plasma membrane integrity by Ca depletion treatments eliminated the difference between selections in K uptake but not in Na uptake. Differences in K uptake were associated with differences in the stimulation of the plasma membrane ATPase by K. These results were interpreted in terms of at least two transport processes contributing to the differences in leaf blade cation concentrations, one process on the plasma membrane affecting K accumulation and another affecting Na accumulation, possibly on the tonoplast membrane.

The rate of kernel dry weight accumulation and filling period duration of corn (Zea maysL.) have been suggested as significant factors that influence yield potential. To determine the importance and usefulness of these kernel characteristics, environmental and genetic variations should be investigated. The objectives of this study were to measure the effects of plant density on the kernel growth components and observe the inheritance of these characteristics in the F1hybrids. Kernel dry weight accumulation rate and effective filling period duration (EFPD) were examined in three corn inbreds and their three F1hybrids at two plant densities. Kernel dry weight accumulation rate was not affected by plant density; however, EFPD was 2.5 days less at the high plant density. The resultant, smaller kernels along with a reduced number of kernels resulted in a 20% yield reduction per plant at the higher plant density. Yield per plant and per unit area in response to changes in plant density were altered more by kernel number per plant than by kernel size. A significant increase in kernel size contributed to the greater F1yields although the major advantage of the hybrids was due to more kernels per plant. In two of the three F1hybrids, the larger kernel size was due solely to longer EFPD's since the rate of dry weight accumulation of one of the parental inbreds was the same as that of the F1's. In the other F2hybrid, both rate and EFPD were greater in the F1genotype than for either parental inbred. These data show that EFPD but not the rate of kernel growth is influenced to a limited extent by plant density and both are under genetic control. Thus genetic modification of these kernel growth components should be feasible.

Leaf senescence and N redistribution during seed filling may limit soybean [Glycine max(L.) Merr.] yield by restricting the seed filling period. Experiments were conducted in the field and greenhouse to evaluate the relationship between seed N demand and leaf senescence. Plants (cv. McCall) were grown under shade cloth (which reduced insolation by 63%) from R1 to R5 (beginning seed fill), then the shade was removed from some plants with the objective of increasing photosynthesis, seed dry matter, and N accumulation. Experiments included plants grown in soil with no nodules (NO−3as primary source), plants grown in soil with many nodules (N from NO−3and N2fixation), and in a hydroponics system (NO−3as primary N source) where N was removed from the media at R5. Increasing photosynthesis at R5 increased seed dry matter accumulation and the response was larger when N was available. Seed N accumulation also increased if N was available. Increasing photosynthesis had no effect on seed N accumulation when N was not available to the plant. Temporal changes in chlorophyll and N of an upper leaf were measured to characterize leaf senescence. Increasing photosynthesis did not accelerate leaf senescence in any of the treatments. If N was available, N uptake increased when photosynthesis increased. If N was not available, seed N concentration decreased. The data suggest that leaf senescence does not occur because of seed N demand, but may be regulated by processes in the leaf.

The yield production of soybean [Glycine max(L.) Merr.] is partly determined by the flower and pod abortion process. Since the causes of abortion are not well understood, field experiments were conducted over 2 yr to study the relationships of assimilate concentrations and photosynthate partitioning to flower and immature pod abortion. Soybean (cv. Kent) was grown in the field during two seasons using conventional cultural practices. Flowers that opened at R1 (early flowering) and R3 (late flowering) were marked and the abortion levels were determined. In the second year immature pods present just after early and late flowering were also marked and abortion determined. Flower and immature pad samples were taken on the day of marking for chemical analysis. Early flower abortion averaged 40%, and late flower abortion averaged 80%. Early immature pod abortion was 15Yo but late immature pod abortion was 57%. In both years the concentrations of ethanol‐soluble carbohydrates, starch, ethanol‐soluble N, ethanol‐insoluble N, NO−3, Mg2+, K+, and Ca2+in the fully open flowers or in immature pods was not related to the abortion percentages exhibited by the flowers or pods. The ethanolsoluble carbohydrate concentration ranged from 71 to 107 g kg−1in flowers and from 40 to 45 g kg−1in immature pods. Starch concentration ranged from 24 to 29 g kg−1in flowers and 18 to 30 g kg−1in immature pods. The results did not support the hypothesis that low assimilate concentrations are the major cause of abortion in soybean. During both early and late flowering, a single leaf located in the middle of the main stem that subtended fully open flowers or immature pods was labeled with 14CO, at both 0 and 3 days after flower opening. Flowers exhibiting low abortion percentages contained a greater percentage of the total 14C recovered (0.049 to 0.095%) as well as a greater specific activity of 14C (0.018 to 0.027Yo mg−1flower) than those exhibiting high abortion percentages (0.014 to 0.024% and 0.005 to 0.009% mg−1flower, respectively). Immature pods exhibiting low abortion also contained greater percentages of 14C recovered (0.15 to 0.20%) than those exhibiting higher abortion (0.012%). Although it is difficult to distinguish between a cause and a symptom of abortion, abortion seems to be at least related to the lack of photosynthate partitioning to flowers and immature pods.

Soybean [Glycine max(L.) Merr.] seed produced from late maturing cultivars or following delayed planting of early maturing cultivars is often exposed to frost prior to maturation and harvest. Since little information is available regarding the effect of freezing temperatures on seed germination, freezing experiments were conducted with intact plants, and detached pods in controlled environmental chambers. Pods at three developmental stages (green, yellow, and brown) were exposed to temperatures of −2 to −12 C for up to 32 hours. There was a curvilinear relationship (R2= 0.94) between seed moisture and the freezing temperature of seed with the freezing point decreasing from −2 C at a moisture percentage of 68% (green pods) to −20 C at 30% moisture (brown pods). Seed exposed in detached pods supercooled 2 to 5 C below the freezing point temperature at all development stages. Immature seed in green pods exposed on the plant froze at their freezing point (−2 C); however, seed in yellow or brown pods exposed on the plant supercooled 2 to 4 C below their freezing point. Seed injury did not occur when the seed remained in the supercooled state; however, ice formation resulted in seed injury and death. Immature soybean seed in green pods (65% moisture) were not injured by freezing temperatures of approximately −2 C; however, seed in yellow pods (physiological maturity, 55% moisture) showed significant reductions in seed germination and vigor following an 8‐hour exposure at −7 C. Germination of seed in brown pods (approximately 35% moisture) was reduced only by exposure to −12 C. These results indicate that the temperature required to cause freezing injury to soybean seed decreases during seed development and maturation. When soybean seed have reached physiological maturity (maximum dry weight) temperatures substantially below 0 C are required to cause injury.

The relationship between the vigor of soybean [Glycine max(L.) Merr.] planting seed and yield of the resulting crop has not been clearly determined. Studies were conducted in 1975, 1976, and 1977 to evaluate the relationship between seed vigor and yield in soybeans by experimental techniques that duplicate as closely as possible practices followed by commercial producers of soybeans. The cultivars ‘Cutler 71’ and ‘Kent’ were used in 1975 and 1976, and ‘Williams’ and Kent in 1977. Three to six lots of seed of each cultivar were selected to provide a wide range in seed vigor and maintain a high viability (standard germination). Two seeding rates were used, 16 and 33 viable seed/m (13 and 26 viable seed/m in 1977) in 3‐row plots with 76 cm between the rows (66 cm in 1975). Standard germination of the 27 lots included in the experiment ranged from 77.0 to 98.5% and averaged 90%. The 4‐day germination was closely related to the standard germination but averaged 6.3 percentage points lower. The accelerated‐aging germination averaged 69.5% and ranged from 18.0 to 97.5%. Field conditions at planting were near optimum and field emergence was high for all seed lots in all 3 years of the experiment. Thus, there was no relationship between seed vigor and emergence or final stand, regardless of cultivar or planting rate. Seed vigor had no effect on yield regardless of cultivar or planting rate. In the absence of stand differentials, there appears to be no yield advantage associated with the utilization of high vigor planting seed.

A visual indicator of physiological maturity (maximum dry seed weight) in soybean [Glycine max(L.) Merrill] would be useful to researchers and soybean producers. Experiments were conducted utilizing two cultivars in the greenhouse and six cultivars in the field to characterize the color of soybean seeds and pods at physiological maturity. In greenhouse experiments where plants were exposed to 14CO2as they neared maturity, it was found that the 14C content of the seed decreased as the yellow coloration of the seed coat increased. Practically no 14CO2was recovered from yellow seeds, regardless of the color of the pod, suggesting that the seed stops importing assimilates and is physiologically mature when it is completely yellow (free of green color). Normally, yellow seeds were found only in yellow or brown pods. The respiration rate of seed, measured as CO2evolution, declined rapidly as the seedcoat began to turn yellow and reached a low level when the seed were completely yellow at a seed moisture content of 55 to 60%. Physiological maturity of an entire plant, estimated from dry seed weight data in the field, occurred when 26% of the seed were yellow and 35% of the pods were yellow or brown. The seed moisture content ranged from 54 to 62%. The date of physiological maturity estimated in the field was slightly early in comparison to complete physiological maturity (all seed completely yellow) due to the sampling variation associated with estimating dry seed weight and the variation in pod maturity on an individual plant. The data suggest that the occurrence on the main stem of one normally colored mature pod per plant represents a useful and acceptable indicator of physiological maturity, even though all the pods or seed were not completely yellow at this stage of development.

As soybean [Glycine max(L.) Merr.] seeds develop and mature, they gradually desiccate to cause a switch in metabolic processes from a developmental to a germination phase. This investigation was conducted to determine when fresh (nondesiccated), immature soybean seeds acquire the ability to germinate and develop into normal seedlings. Soybean pods were detached from ‘DeSoto’ plants at four developmental stages ranging from full seed (first time the seed filled the locule) to yellow pod [physiological maturity (PM)] in 1981 to 1983. Freshly harvested seeds were immediately evaluated for seed moisture, dry seed weight, viability (radicle protrusion through the testa), germination (normal seedling development), and respiration. Seed and axis dry weight accumulation and moisture loss were nearly identical throughout development. Near maximum viability (93%) of freshly harvested seeds occurred when the seed first filled the locule (35% of final dry seed weight); however, maximum germination (development of normal seedlings) did not occur until maximum accumulation of dry seed weight (PM). Respiration during germination of fresh seed harvested before PM declined prior to radicle emergence without a concomitant decrease in seed moisture, followed by an increase in respiration typical of a germinating seed. Contrary to previous reports, freshly harvested immature soybean seeds did not need a desiccation period prior to expression of maximum viability and normal seedling development.

The spacial distribution of plants in a crop community is an important determinant of yield. However, the mechanisms responsible for the observed responses to changing plant density are not well understood. Field experiments utilizing a systematic design that made it possible to evaluate a wide range of plant densities in a single experiment were conducted with an indeterminate (‘Cumberland’‐ 2 yr) and a determinate (‘Pixie’‐1 yr) soybean [Glycine mar(L.) Merrill]. Population densities used in the analyses varied from 0.6 to 24 plants m−2. At low population densities, where there was no interplant competition, yield increased in direct proportion to increases in plant density. At plant densities providing interplant competition, the rate of yield increase was reduced. Yield of the indeterminate cultivar increased as plant density increased above the density required for 95% insolation interception at growth stage R5. The increase in yield was a result of increases in seed m−1in both years along with seed size in 1985. However, the determinate cultivar exhibited maximum yield at the plant density that provided 95% insolation interception at growth stage R5. The data for the indeterminate cultivar confirms a recently published theoretical analysis of soybean yield responses to plant density. The data suggest that plant densities higher than those required to maximize insolation interception by growth stage R5 may be required for maximum yields of indeterminate cultivars.

Soybean [Glycine max(L.) Merr.] yield has been positively associated with seed‐filling period, and soybean genotypes have been identified that exhibit significant differences in seed‐filling period. This experiment was conducted to obtain heritability estimates of the seed‐filling period duration based on effective filling period (EFP) and growth stage measurements, to determine the effect of stem termination type on the heritability estimates, and to measure the correlations between seed‐filling period length, and maturity and seed size. Data were collected on four populations in the F2, F2:3, and F2:4generations from 1983 through 1986 at Lexington, KY on a Maury silt loam (fine, mixed, mesic Typic Paleodalfs) soil. Parent (F2:3)‐progeny (F2:4) regression gave nonsignificant heritability timates for EFP in all four populations (−0.11–0.22), highly significant estimates for duration of the R5 to R7 period in three of the four populations (0.16–0.63), and highly significant estimates all populations for duration of the R1 to R8 period (0.37–0.82). Seed size was rarely correlated with filling‐period duration. For the four populations, maturity was positively correlated with R1 to R8 in three, with R5 to R7 in two, and with EFP in none. Stem termination class did not affect heritability estimates. In populations segregating for stem termination, however, determinate genotypes were preferentially identified as having long R5 to R7 periods. The EFP was not a useful selection criterion in these segregating populations. For the duration of the R5 to R7 period to be a useable selection criterion, selection will need to be performed in separate subpopulations of the different stem termination types with attention being paid to the maturity of the genotypes selected.

Although drought and defoliation stress have been shown to reduce soybean [Glycine max(L.). Merr.] yield, little information has been published regarding their effects on soybean seed quality. Field experiments were conducted in 1986, 1987, and 1989 to evaluate the effect of drought and defoliation (1989 only) stress during soybean seed development on seed germination and vigor. Essex (MG [maturity group] V) and Union (MG III) were grown in 1986 and 1987, and Harper (MG III) and McCall (MG 00) in 1989. Moisture treatments were either well watered or drought stressed during seed development (R5 to R7). In 1989, a total defoliation treatment was also imposed at R6 as an additional stress factor. There were significant reductions in yield and yield components following drought stress in all 3 yr and following defoliation in 1989. Leaf conductance and transpiration also decreased in the drought stress treatments. There was no effect of drought stress on seed germination or seed vigor as measured by accelerated aging germination and the cold test across the four cultivars (determinate and indeterminate) and 3 yr. In 1989 slight changes in 3‐d germination and conductivity occurred for some drought stress treatments. Most of this response, however, was related to increased occurrence of hard seed, which does not represent an indication of a change in vigor. Seed germination and vigor were significantly reduced for small, flat, shriveled, and underdeveloped seeds that only occurred following defoliation. These seeds represented a small portion of the seed lot that would normally be removed during conditioning. The data suggest that drought stress would have no effect on seed germination or vigor, unless the stress was severe enough to produce shriveled, flat, underdeveloped seeds.

There have been few comparisons of the relationship between laboratory estimates of soybean [Glycine max(L.) Merr.] seed vigor and field emergence under adverse soil conditions. These studies were conducted in 1973‐75 to compare the results of standard germination, two vigor tests (accelerated‐aging germination and 4‐day germination), and several vigor indices to field emergence in varying soil environments. Each vigor index was computed by converting the results of each laboratory test into a vigor rating ranging from 0 to 10. The three vigor indices evaluated and the tests used in each were: VR‐1 (standard germination, 4‐day germination, and accelerated‐ aging germination), VR‐2 (standard germination and accelerated‐aging germination) and VR‐3 (standard germination and 4‐day germination). The prediction accuracy (those seed lots that emerged in the same performance level in the field as predicted in the laboratory) was determined for each laboratory test and index. All laboratory tests consistently overpredicted field emergence when seed was planted in adverse soil conditions (severe crusting and cold soil temperatures). The prediction accuracy for all seed lots averaged across years with adverse field conditions was very low (8.3%) for standard germination and improved to 46.7% for accelerated‐aging germination and 56.9% for 4‐day germination. The prediction accuracy increased to 65.0% and 68.3% for the VR‐3 and VR‐1 vigor indices for the same comparisons. The VR‐1 index, using all three laboratory tests provided a better estimate of field emergence than either vigor index (VR‐2 and VR‐3) when only two tests were used. The prediction accuracy of the VR‐1 vigor index was approximately 25% higher than averaging the results of the same three tests. The vigor index concept offers the possibility of combining additional laboratory parameters that relate to seed and seedling vigor.

The redistribution of N from the vegetative plant parts and pod walls to the seed of soybeans [Glycine max(L.) Merr.] during reproductive growth can be a major source of N for the developing seed. It has been suggested that the redistribution of N may be associated with leaf senescence. Experiments were conducted to evaluate the redistribution of N and the changes in photosynthesis in two soybean cultivars that differed in the duration of seed fill. ‘Williams’ and ‘Lincoln’ were grown in the field using conventional cultural practices for 2 years and total N, protein (1 NNaOH extractable), and CO2uptake rate (measured with a 14CO2technique) were determined for two leaf positions (last fully expanded and 10th node) during reproductive growth. Total N (mgdm‐2) in the last fully expanded leaves began to decline at growth stage R6 and declined steadily until leaf abscission occurred. Total N in the leaves from the 10th node began to decline sooner but the levels in the leaves at abscission were the same as in the last fully expanded leaves. The decline in protein generally followed a pattern similar to total N. The CO2uptake rate increased to a maximum during the beginning of seed fill (growth stage R5) and started to decline early in growth stage R6. There was no difference between cultivars in the relationship between CO2uptake rate and the levels of leaf protein. The earlier senescence of the short filling period cultivar (Lincoln) was associated with more rapid decline in CO2uptake rate compared with Williams.