To study the impact of plant density on Chenopodium quinoa (c.v. CICA-17) achene yield and its relationship with morphology, leaf anatomy and gas exchange in the absence of water stress, field trials were conducted at 1995 m asl in Northwestern Argentina. Two plant densities were evaluated; low density (LD) 7.2 plants/m (120 240 pl/ha) and high density (HD) 27.9 plants/m (465 930 pl/ha). HD treatment caused light competition, inducing morphological and anatomical changes in Quinoa plants. Plants grown under HD conditions showed decreases in plant height and stem diameter, lower stomatal dimensions and densities, and thinner leaf blades. Compensation strategies such as increases in specific leaf area and a higher number chloroplasts per palisade cell were observed, nevertheless these changes did not fully compensate C absorption and gas exchange limitations, therefore limiting the uptake of N and P and resulting in a 53.2% lower yield of HD compared to LD. Considering the ability of quinoa plants to change its morphology and anatomy, further studies with intermediate plant densities are necessary in order to determine if it is possible to achieve higher yields and to increase the efficiency in the use of the resources.

Field studies were conducted in 2019 and 2020 to compare the effects of shade cloth light interception and Palmer amaranth (Amaranthus palmeri S. Watson) competition on ‘Covington’ sweetpotato [Ipomoea batatas (L.) Lam.]. Treatments consisted of a seven by two factorial arrangement, in which the first factor included shade cloth with an average measured light interception of 41%, 59%, 76%, and 94% and A. palmeri thinned to 0.6 or 3.1 plants m−2 or a nontreated weed-free check; and the second factor included shade cloth or A. palmeri removal timing at 6 or 10 wk after planting (WAP). Amaranthus palmeri light interception peaked around 710 to 840 growing degree days (base 10 C) (6 to 7 WAP) with a maximum light interception of 67% and 84% for the 0.6 and 3.1 plants m−2 densities, respectively. Increasing shade cloth light interception by 1% linearly increased yield loss by 1% for No. 1, jumbo, and total yield. Yield loss increased by 36%, 23%, and 35% as shade cloth removal was delayed from 6 to 10 WAP for No. 1, jumbo, and total yield, respectively. F-tests comparing reduced versus full models of yield loss provided no evidence that the presence of yield loss from A. palmeri light interception caused yield loss different than that explained by the shade cloth at similar light-interception levels. Results indicate that shade cloth structures could be used to simulate Covington sweetpotato yield loss from A. palmeri competition, and light interception could be used as a predictor for expected yield loss from A. palmeri competition.

Transpirations of cotton and velvetleaf plants competing for water and light were estimated with heat balance stem flow gauges in a controlled environment. Cotton and velvetleaf plants were grown together in pots and monitored during fluctuations in soil water, irradiance, air temperature, and humidity. Shading from other plants imposed light competition on velvetleaf or cotton. Velvetleaf used significantly more water than cotton given ample water and light. Transpiration by both species was reduced in dry soil, but velvetleaf transpiration was reduced primarily by leaf abscision, while cotton leaves did not abscise. Both cotton and velvetleaf plants transpired significantly less when shaded. Leaf water potentials indicated that shaded plants experienced reduced evaporative demand suggesting that shading reduced plant stress in dry soil. Competition for water was strongly influenced by competition for light.

Field experiments were conducted in 1990 and 1991 at Lethbridge, Alberta, to determine if narrower row spacings and increased plant densities than commonly practiced would improve safflower's competitive ability with weeds. A factorial set of treatments of safflower at two row spacings of 11 and 22 cm and six densities ranging from 10 to 192 plants m−2 grown weed free or infested with green foxtail (Setaria viridis (L.) Beauv.). Decreasing safflower row spacing from 22 to 11 cm slightly improved competition with green foxtail, but increasing safflower density had a much greater effect. Weed-free safflower biomass and seed yield plateaued at 70 and 84 plants m−2 in 1990 and 1991, respectively. However, safflower infested with 500 plants m−2 of green foxtail increased in biomass and seed production up to 100 plants m−2 in 1990 and 156 plants m−2 in 1991. At these high densities, weedy safflower yielded less than weed-free safflower but three to four times more than achieved at lower stand densities. Concurrently, high safflower density reduced green foxtail biomass up to 72% and seed yield up to 85%. Dense safflower stands developed a closed canopy early in the season and shaded green foxtail more than thin stands.

In field plots, Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea (Link) W. F. Wight] competed strongly with yellow nutsedge (Cyperus esculentus L.), reducing its dry weight and the number of plants and tubers with no loss of dry weight to the millet. Because Japanese millet did not shade the nutsedge for more than half the day, the main competition was thought to be ‘root’ competition (nutrient and water) with light competition as a minor factor. The greenhouse studies verified that roots were the primary area of competition. However, light competition between the two species was stronger in the greenhouse than in field plots. Also, when confined to limited light and soil volumes with a 1-month growing period, Japanese millet was reduced in dry weight by nutsedge competition.

Kudzu (Pueraria lobata Ohwi # PUELO) was grown from seeds in controlled-environment chambers at 350, 675, or 1000 μl·1−1 CO2. Biomass and leaf area production, morphological characteristics, and growth analysis components were determined at 14, 24, 45, and 60 days after emergence. At 60 days, plants grown at 1000 μl·1−1 CO2 had 51% more biomass, 58% longer stems, and 50% more branches than plants grown at 350 μl·1−1 CO2. Plants grown at 675 μl·1−1 CO2 were intermediate. Growth analysis components indicated that CO2 enrichment increased growth by compounding effects due to increased net assimilation rates and increased leaf area duration. Relative growth rates were not significantly affected. The large CO2-induced increase in stem height versus stem diameter is in marked contrast to previously reported responses of woody erect growth forms. Possible ecological implications for competitive abilities are discussed.

An improved understanding of the effects of nitrogen (N) on crop–weed interactions is needed for the development of integrated weed management systems where responsible use of N fertilizers is considered. Field experiments conducted in 1999 and 2000 at two locations in eastern Nebraska quantify the effects of N and increasing duration of weed interference on corn growth and development. A naturally occurring population of weeds was allowed to compete with the corn crop for increasing lengths of time and at three rates of N application (0, 60, and 120 kg N ha−1). Weed interference and withholding applied N increased the time to 50% silking by an average of 3.9 and 2.9 d, respectively. Regardless of treatments, relative growth rates of corn leaf area and biomass were maximized between the V1 and V2 growth stages of corn and increased linearly with N rate but were affected to a lesser extent by weed presence. The improvement in early season corn growth with addition of N resulted in greater leaf area, biomass, and height, which improved the competitive ability of corn against weeds. Reductions in maximum corn leaf area and height due to weed interference usually began earlier and were more extensive at reduced rates of N. Partitioning of biomass to reproductive structures increased with N during reproductive stages, likely contributing to greater harvest indices at the end of the season. Results from this study indicate that the effects of N fertilization on early-season crop growth provided a competitive advantage for corn relative to weeds, thereby increasing the length of time that weeds could compete with a crop before removal was required, but further research is needed to identify mechanisms regarding improved crop tolerance to weeds.