Understanding how plants alter their growth in response to interplant competition is an overlooked but complex problem. Previous studies have characterized the effect of light and water stress on soybean or common ragweed growth in monoculture, but no study has characterized soybean and common ragweed growth in mixture. A field study was conducted in 2015 and 2016 at the University of Nebraska-Lincoln to characterize the growth response of soybean and common ragweed with different irrigation levels and intraspecific and interspecific interference. The experiment was arranged in a split-plot design with irrigation level (0, 50%, 100% replacement of simulated evapotranspiration) as the main plot and common ragweed density (0, 2, 6, 12 plants m−1 row) as the subplot. Crop- and weed-free controls and three mixture treatments were included as subplots. Periodic destructive samples of leaf area and biomass of different organ groups were collected, and leaf area index (LAI), aboveground biomass partitioning, specific leaf area (SLA), and leaf area ratio (LAR) were calculated. Additionally, soybean and common ragweed yield were harvested, and 100-seed weight and seed production were determined. Soybean did not alter biomass partitioning, SLA, or LAR in mixture with common ragweed. Soybean LAI, biomass, and seed size were affected by increasing common ragweed density. Conversely, common ragweed partitioned less new biomass to leaves and increased SLA in response to increased interference. Common ragweed LAI, biomass, and seed number were reduced by the presence of soybean and increasing common ragweed density; however, seed weight was not affected. Results show that adjustment in biomass partitioning, SLA, and LAR is not the method that soybean uses to remain plastic under competition for light. Common ragweed demonstrated plasticity in both biomass partitioning and SLA, indicating an ability to maintain productivity under intra- and inter-specific competition for light or soil resources.

Vegetative growth response of round-leaved mallow to various day/night temperature regimes was studied under controlled-environment conditions to predict its potential geographic distribution and to develop an effective control program. Round-leaved mallow dry matter production was greatest with day temperatures of 18 to 26 C. Dry matter accumulation was reduced by a night temperature of 6 C but was minimally affected by night temperatures ranging from 12 to 24 C. Regression analysis predicted minimal vegetative growth at mean daily temperatures below 8 C and above 30 C, with optimum growth at 20 C. Partitioning of round-leaved mallow biomass in leaves, stems, and roots was affected by temperature. Maximum leaf weight ratio occurred at low temperatures, 10 C day and 6 C night. Stem weight ratio was greatest at a day temperature of 26 C, with night temperature having little effect. Maximum root biomass occurred with a day temperature of 18 C. Results are discussed in terms of environmental conditions that allow round-leaved mallow to be an effective competitor with crops and potential approaches for its control.

Mathematical growth analysis techniques were used to evaluate the effects of CO2 concentrations of 350, 600, and 1000 ppm (v/v) on growth and biomass partitioning in corn (Zea mays L. ‘Dekalb XL 395’), itchgrass (Rottboellia exaltata L. f.), soybean [Glycine max (L.) Merr. ‘Tracy’], and velvetleaf (Abutilon theophrasti Medic). Controlled environment chambers with day/night temperatures of 28/22 C and photosynthetic photon flux densities (PPFD) of 650 μE (microeinsteins) m-2 s-1 were used. Dry matter production in the two C3 species (soybean and velvetleaf) was increased significantly by raising the CO2 concentration above 350 ppm. In corn (a C4 species), dry matter production was least at 1000 ppm CO2 and did not differ between the 350 and 600 ppm treatments. In itchgrass (also C4), dry matter production was greatest at 600 ppm CO2 and did not differ between the 350 and 1000 ppm treatments. Increasing the CO2 concentration increased the rate of dry matter production per unit leaf area (net assimilation rate or NAR) in soybean and velvetleaf but either decreased or did not alter NAR in corn and itchgrass. At 45 days after planting, the weed/crop ratios for total dry matter production for velvetleaf/corn and itchgrass/corn were significantly greater at both 600 and 1000 ppm than at 350 ppm CO2. The weed/crop ratio for itchgrass/soybean was less at 1000 ppm than at 350 or 600 ppm CO2. Compared to the value at 350 ppm, the weed/crop ratio for velvetleaf/soybean was greater at 600 ppm and less at 1000 ppm CO2. We conclude that atmospheric CO2 enrichment probably will make weeds with the C3 photosynthetic pathway more competitive with crops having the C4 pathway. Weeds with the C4 pathway may become less competitive with crops having the C3 pathway.

Showy crotalaria (Crotalaria spectabilis Roth) was grown in a controlled-environment greenhouse with a day/night temperature of 32/26C at full sunlight and under 40, 60, or 90% shade. Shading significantly reduced height, dry-matter accumulation, leaf production, leaf-area expansion, axillary-branch development, reproductive development, and the partitioning of plant biomass into stems. Shading increased the partitioning of plant biomass into leaves and the leaf area ratio. Constant shading or shading from 0 through 25 days after planting reduced the leaf area duration (LAD) proportionally more than the net assimilation rate (NAR). Shading from 25 to 67 days after planting reduced NAR and LAD equally. In other experiments reducing the day/night temperature below 32/26C significantly reduced plant height, leaf area, and the rate of leaf production. Dry-matter production decreased significantly at temperatures below 29/23C, because of substantial reduction in LAD. Showy crotalaria flowered and produced seed at temperatures ranging from 23/17C to 32/26C. The results indicate that the geographical distribution of showy crotalaria in the United States probably will continue to increase.

The objectives were to compare the growth of black and eastern black nightshade and develop predictive models for their growth. Black and eastern black nightshade were planted on June 3 and 23, and July 12 in 1988 and May 28, June 17, and July 10 in 1989. Exponential models were strongly correlated with the growth of both species across all planting dates. Eastern black nightshade produced more biomass than black nightshade, although weight of berries was equivalent. Leaf area per plant 60 d after planting was five times greater for eastern black nightshade than black nightshade. Eastern black nightshade responded to self-shading by increasing specific leaf area.

Texas panicum was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. After 33 days, maximum plant height, leaf area, and dry weight occurred at 34/26 C. The greatest number of tillers and leaves occurred at 34/16 C. Partitioning of plant biomass into stems increased with increasing night temperature, while partitioning into leaves tended to decrease. Partitioning of biomass into roots decreased with increasing night temperature at day temperatures of 19 or 24 C but not at 34 C. Plant dry matter production (DMP) was highly correlated with both leaf area duration (LAD) and net assimilation rate (NAR) during the first 2 weeks of growth. During the subsequent 3 weeks, DMP was highly correlated with LAD but not with NAR. The low-temperature threshold for growth of Texas panicum was approximately 11 C. Texas panicum attained 25% of its maximum growth at an average daily temperature of 21 C, 50% at 24 C, and 75% at 26 C. Texas panicum is unlikely to become a highly competitive weed outside the southern and southwestern United States.

Soybean and ivyleaf morningglory were grown alone and together to quantify water use and growth after treatment with soil-applied chlorimuron plus metribuzin, imazaquin, or no herbicide. Soybean water consumption was 22 ml pot−1 day−1 or more when grown alone or with another soybean, regardless of herbicide treatment. Ivyleaf morningglory grown alone or with another ivyleaf morningglory consumed ≥ 32 ml water pot−1 day−1 when no herbicide was applied. Treatment with chlorimuron plus metribuzin or imazaquin reduced ivyleaf morningglory water consumption to ≤ 4 ml pot−1 day−1. Water use was reduced with either herbicide when soybean and ivyleaf morningglory were grown together, indicating little or no water consumption by ivyleaf morningglory. Ivyleaf morningglory height, root length, leaf area, and dry matter were reduced by imazaquin or chlorimuron plus metribuzin. Soybean leaf area was also reduced by herbicide treatment or the presence of another plant. Soybean biomass was not reduced in any planting combination, except when two soybean were grown together and treated with imazaquin. Nontreated ivyleaf morningglory partitioned more biomass into the shoots than roots. Ivyleaf morningglory responded to herbicide treatment by partitioning more biomass into roots.

The C4 weed johnsongrass [Sorghum halepense (L.) Pers. ♯3 SORHA] and the C3 crop soybean [Glycine max (L.) Merr. ‘Ransom’] were grown separately and in inter- and intraspecific competition at 350 and 675 ppm CO2 in controlled environment chambers with 29/23 C day/night temperatures and 950 μE·m-2 · s-1 PPFD (photosynthetic photon flux density). In the absence of competition, the higher CO2 concentration stimulated dry matter accumulation, leaf area expansion, net assimilation rate, and leaf area duration of soybean more than that of johnsongrass. The plant relative yield (PRY) of soybean in competition with johnsongrass increased, and the PRY of johnsongrass in competition with soybean decreased, as the CO2 concentration was increased from 350 to 675 ppm. Thus, the competitiveness of the C3 crop with the C4 weed increased with increasing CO2 concentration. Relative yield totals were not significantly different from 1.0, indicating that the two species were competing for the same resources. With the increases in global atmospheric CO2 concentration predicted for the next 50 to 100 yr, the competitiveness of C3 crops with C4 weeds could be increased.

Growth dynamics, water relations, and photosynthesis of soybean [Glycine max (L.) Merr. ‘Ransom’], common cocklebur (Xanthium pensylvanicum Wallr.), jimsonweed (Datura stramonium L.), prickly sida (Sida spinosa L.), sicklepod (Cassia obtusifolia L.), smooth pigweed (Amaranthus hybridus L.), spurred anoda [Anoda cristata (L.) Schlect.], and velvetleaf (Abutilon theophrasti Medic.) were compared in a controlled-environment greenhouse programmed for 32C day and 23C night temperatures. Net photosynthetic rates, net assimilation rates, and water-use efficiency on a whole-plant or single-leaf basis were greatest in the C4-plant, smooth pigweed. Total dry-matter production at 29 days after planting was greatest in common cocklebur and least in jimsonweed. Interspecific differences in dry-matter production were highly positively correlated with leaf area duration and negatively correlated with net assimilation rate. Threshold leaf water-potential levels causing stomatal closure varied among species. The stomata of jimsonweed were the most sensitive to water stress and those of prickly sida were the least sensitive.

Soybean [Glycine max (L.) Merr. ‘Tracy′] and two associated weeds, sicklepod (Cassia obtusifolia L.) and showy crotalaria (Crotalaria spectabilis Roth), were grown in controlled-environment chambers with day/night temperatures of 29/23 C, photosynthetic photon flux density (PPFD) of 600 μE·m−2·s-1, CO2 concentrations of 350 or 675 ppm, and one-eighth or one-half strength Hoagland's nutrient solution applied three times daily. Leaf areas and dry weights of plant parts were determined at 1, 3, and 5 weeks. Stomatal resistances, transpiration rates, leaf water potentials, and leaf chlorophyll contents were measured, and net assimilation rates (NAR) and leaf area durations (LAD) were calculated. In all species, growth in 675 ppm CO2 enhanced dry-matter production through increases in both NAR and LAD. The increased dry-matter production with one-half strength compared to one-eighth strength Hoagland's solution was, however, caused by increased LAD. Stomatal conductances and transpiration rates decreased in 675 ppm CO2, but were not affected by nutrient level. High CO2 concentration or low nutrient level generally decreased leaf chlorophyll content per unit area. Growth enhancement by high CO2 was greater in one-half strength than in one-eighth strength Hoagland's solution.

The effects of shade on dry-matter production, leaf area, and biomass partitioning in purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (C. esculentus L.) were determined in a controlled-environment greenhouse with a day/night temperature regime of 32/26 C. For comparison with a full-light treatment, screening provided 40, 70, and 85% shade. Shading significantly reduced height of yellow nutsedge and dry-matter production, leaf-area production, and rhizome and tuber formation of both species. Shading decreased the partitioning of plant biomass into tubers and rhizomes and increased partitioning into leaves. Yellow nutsedge had a higher net-assimilation rate than purple nutsedge at all levels of shading, but the greater leaf-area duration or total amount of leaf area present in purple nutsedge resulted in greater dry-matter production by that species. The responses of the two species showed no differences in shade tolerance.

Kyllinga species are becoming more common throughout the southeastern United States. Two species, Kyllinga brevifolia and Kyllinga squamulata, in particular are prevalent weeds in turfgrass. To better understand these weeds, growth chamber studies determined the growth of K. brevifolia, K. squamulata, and Cynodon dactylon × Cynodon transvaalensis as influenced by three temperature regimes (33/24, 25/17, 19/11 C day/night, respectively). Temperature influenced almost all aspects of Kyllinga species growth. Plant height of both Kyllinga species increased nearly twofold after 8 wk at high temperatures. Plants were mowed each week to 2.5 cm; both species produced more than twice as many clippings by 8 wk at high (33/24 C) temperatures than at low (19/11 C) temperatures. Destructive analysis at 8 wk revealed that K. brevifolia shoot and root weight increased with decreasing temperature, whereas K. squamulata shoot and root weights were not affected by temperature. Shoot weight percentage for both Kyllinga species increased from 59% in medium temperatures to 69% in high temperatures. K. brevifolia shoot weight percentage decreased to 53% in low temperatures, whereas K. squamulata shoot weight percentage increased to 72%. K. brevifolia inflorescences formed at 2, 3, and 5 wk in high, medium, and low temperatures, respectively, whereas K. squamulata flowered immediately in all temperatures. C. dactylon × C. transvaalensis and Kyllinga species growth were similar within each temperature regime throughout the 8-wk study.

Small broomrape is a parasite of several broadleaf plant species. Consequences of small broomrape infestation in host cropping systems include seed contamination, reduction in crop seed yield, and host plant death. The effect of small broomrape parasitism on the biomass partitioning of its primary host, red clover, has not been documented. Greenhouse experiments were conducted to determine the relationship between small broomrape and red clover biomass accumulation. Total biomass of parasitized red clover plants was 15 to 51% less than nonparasitized red clover plants. Small broomrape parasitism reduced the amount of dry matter allocated to red clover inflorescences by 50 to 80%. Small broomrape dry matter accumulation was strongly related to total red clover–small broomrape dry matter accumulation. Small broomrape attachment number per red clover plant was a poor indicator of relative small broomrape dry weight accumulation. The results of this study indicated that small broomrape accumulated resources from red clover at the greatest expense to the economically important reproductive tissues.

Knowledge of how plants will partition their new biomass will aid in understanding competition between crops and weeds. This study determined if the amount of biomass partitioned to the root versus the shoot can be predicted from tissue carbon [C] and nitrogen [N] concentrations and the daily gain in C (GC) and N (GN) for each unit shoot and root biomass, respectively. Pots measuring 28 cm diameter and 60 cm deep were embedded in the ground, and each contained one plant of either corn or velvetleaf. Each plant received one of three nitrogen treatments: 0, 1, or 3 g of nitrogen applied as ammonium nitrate in 2001 and 0, 2, or 6 g of nitrogen in 2002. Measurements of total above- and belowground biomass and tissue [C] and [N] were made at 10 different sample dates during the growing season. Fraction of biomass partitioned to roots (Pr) was predicted from [C], [N], GC, and GN. Accurate prediction of the fraction of biomass partitioned to roots versus shoots was evaluated by comparing observed and predicted Pr across all treatments. The coordination model has potential as a reliable tool for predicting plant biomass partitioning. Normalized error values were close to zero for corn in 2001 and 2002 and for velvetleaf in 2001, indicating that biomass partitioning was correctly predicted.

Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited.

Laboratory experiments were conducted to determine the effects of optimum temperature, depth of planting, and simulated moisture stress on smellmelon germination and subsequent emergence. Smellmelon germination increased with temperatures ranging from 20 to 30 C, with the highest germination of 83% occurring at 30 C. Germination percentages decreased to 49 and 15% with temperatures of 35 and 45 C, respectively. Emergence from depths of 9 to 15 cm was less than 25% for all temperatures. Highest emergence from depths of 1 to 6 cm occurred with 30 and 35 C. Smellmelon germination was reduced from 81 to 61% when the osmotic potential was reduced to − 0.2 MPa and was further reduced to 48 and 7% with solutions of − 0.4 and − 0.6 MPa, respectively. Field studies were conducted to evaluate smellmelon biomass production in 1999 and 2000 when competing with cotton (Gossypium hirsutum L.). End of season-smellmelon leaf area values were 16,280 and 22,053 cm2 per plant in 1999 and 2000, respectively, or 67 and 117 g of leaf dry weight per plant for 1999 and 2000, respectively. In 1999, smellmelon produced 60 g of stem dry weight per plant, and it increased to 75 g in 2000. Optimum germination temperature for smellmelon coincides with those for many annual crops. These data indicate that smellmelon germination and emergence can occur under a wide variety of environmental conditions. On the basis of biomass production values, smellmelon may also be a very efficient competitor with many cultivated crops.

Junglerice is one of the world's most problematic C4 grass weeds present in dry-seeded rice in many countries. A screenhouse study was conducted to determine the effect of four rice planting densities (0, 100, 200, and 400 plants m−2) and four nitrogen (N) rates (0, 50, 100, and 150 kg ha−1) on the growth and morphological plasticity of junglerice. Junglerice plant height was reduced by 15 to 35%, tiller number by 54 to 77%, leaf number by 61 to 85%, leaf area by 69 to 90%, leaf biomass by 63 to 88%, stem biomass by 70 to 92%, and inflorescence biomass by 66 to 94% at rice planting densities ranging from 100 to 400 plants m−2 relative to the junglerice plants grown alone. However, all these growth parameters increased with increasing N rates. Junglerice biomass increased by 125 to 472%, whereas rice biomass increased by 122 to 285% with the application of 50 to 150 kg N ha−1. Additional N favored junglerice biomass production relative to rice. Rice crop interference (200 to 400 plants m−2) reduced junglerice growth and biomass and overshaded the junglerice plants when no N was applied. Increasing N application resulted in taller plants and higher biomass of junglerice, while it reduced root-shoot weight ratio. These results suggest that increasing N rate increased the competitive ability of the junglerice over rice regardless of crop planting density. Information generated in this study could be useful in devising appropriate combinations of planting density and fertilizer management strategies for cultural junglerice management, particularly in situations where junglerice species are more responsive than rice to N.

The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of nitrate (NO3−), ammonium (NH4+) or glutamine (Gln) for plant growth were estimated. Based on these estimates, the effects of these forms of nitrogen (N) on the carbon balance of plants and on shoot–root biomass allocation were evaluated. The results indicated that NO3− as an N source for plant growth is not substantially more expensive to utilize than either NH4+ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is also suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and hence on shoot–root biomass allocation.