Amaranthus palmeri, S. Watson

Korres, Nicholas E., Norsworthy, Jason K. & Mauromoustakos, Andy, 2019, Effects of Palmer Amaranth (Amoronthus polmeri) Establishment Time and Distance from the Crop Row on Biological and Phenological Characteristics of the Weed: Implications on Soybean Yield, Weed Science (Cambridge, England) 67 (1), pp. 126-135 : 129-131

Amaranthus palmeri biological characteristics

Amaranthus palmeri established at soybean emergence was taller ( P <0.001) than the crop at 7.3 and 10.7 WAE for 2014 and 2015, respectively ( Figure 2 ). The second A. palmeri cohort, which established 1 wk later than the crop (AMAPA-1 WAE), overgrew the crop at 17 WAE in 2014 ( Figure 2 ). In 2015, however, soybean was significantly taller ( P <0.001) than A. palmeri established at 2 WAE onward.

A significant interaction between weed establishment time and effect of distance from soybean crop on A. palmeri performance was observed at harvesting for both 2014 and 2015. More specifically, shorter establishment timings in combination with the farthest distance from the crop resulted in fewer effects on A. palmeri biological characteristics, including height ( Figure 3 ), biomass (i.e., aboveground dry weight; Figure 4 ), and seed production ( Figure 5 ) compared with late establishment times (i.e., 2, 4, 6, and 8 WAE) and shorter distances to the crop (i.e., 0 and 23 cm).

Amaranthus palmeri plants at 48 and 24 cm from the crop at 0, 1, 2, and 4 WAE were significantly taller ( P <0.001 for 2014 and 2015) than A. palmeri plants growing adjacent to the soybean crop ( Figure 3 ). These results were similar for both years. Similarly, the interaction of weed establishment time and distance from the crop exerted significant effects ( P <0.001 for 2014 and 2015) on A. palmeri plant biomass ( Figure 4 ) and seed production ( P <0.001 and P = 0.0021 for 2014 and 2015, respectively) ( Figure 5 ). Amaranthus palmeri biomass production per plant was greater for A. palmeri plants established with the crop or at 1 WAE at a 48-cm distance from the crop compared with the biomass produced by those established at 2 WAE or later regardless the distance from the crop ( Figure 4 ). In 2015, biomass production for A. palmeri that was established at 1 WAE was not different between 24 and 48 cm ( Figure 4 ).

The greater the biomass produced, the greater the seed production, especially for A. palmeri plants established at crop emergence and at a 48-cm distance from the crop row ( Figure 5 ). Amaranthus palmeri plants established at 1 WAE at a 48-cm distance produced 50% and 70% fewer seeds for 2014 and 2015, respectively, compared with A. palmeri plants in the soybean row at 0 WAE. These trends were observed for the A. palmeri cohorts established at 0 and 1 WAE. Amaranthus palmeri seed production at the other establishment times was significantly reduced irrespective of distance from the crop ( Figure 5 ).

Amaranthus species are among the most troublesome weeds in many crop production systems (Korres and Norsworthy 2017; Webster and Grey 2015). Effective control of these species, given their highly competitive ability and tendency to evolve resistance to various herbicides ( Korres et al. 2017b), often begins with understanding their biological and reproductive characteristics ( Korres et al. 2017b; Sellers et al. 2003). Differences in plant biomass production between early-season and late-season establishment of A. palmeri ( Figure 4 ) were due to changes in plant height ( Figures 2 and 3 ). Taller A. palmeri plants accumulated greater dry weight ( Fig. 4 ), hence higher seed production ( Figure 5 ). These parameters confirm the highly competitive ability of A. palmeri ( Trucco and Tranel 2011) . Toler et al. (1996) reported that the ability of Amaranthus plants to grow taller than soybean is one of the success factors of these weed species in competition with soybean. Studies by Nassiri Mahallati and Kropff (1997) on crop and weed competition for light indicated the important role of increased weed height. This is particularly illustrated by data at 7.3 WAE and 10.7 WAE, when A. palmeri overgrew the crop in 2014 and 2015, respectively ( Figure 2 ).

Amaranthus palmeri flowering

The effects of the distance from the crop on A. palmeri flowering time were not significant in both years, although the greater the distance of A. palmeri from the crop, the greater the number of plants that flowered (unpublished data). Flowering was significantly ( P <0.001) affected by establishment time independent of distance from the crop ( Figure 6 ). The shorter the weed establishment time, the greater the number of A. palmeri plants that flowered. More particularly, flowering for A. palmeri plants established at 8 WAE was 33% and 59% lower than for plants established at 0 WAE for 2014 and 2015, respectively ( Figure 6 ). Not surprisingly, the earliest established A. palmeri plants flowered within 7 WAE in 2014 ( Figure 6 ). Likewise, the earliest established A. palmeri reached full flowering between 9.8 and 11.1 WAE ( Figure 6 ) for 2014 and 2015, respectively, probably facilitated by taller A. palmeri plants ( Sosnoskie et al. 2012). Taller A. palmeri plants were able to avoid shading caused by crop canopy due to dense ground cover as indicated by the relatively strong relationship between ground cover and extinction coefficient around these dates, particularly at 10.6 and 8.4 WAE for 2014 and 2015, respectively ( Figures 7 and 8 ). The greater the ground cover, the higher the (absolute) value of the extinction coefficient, indicating a high light interception.

Such “shade-avoidance” response, which among other parameters involves stem elongation ( Morgan and Smith 1976), may improve plant fitness by increasing capture of the most limiting resource, in this case light, under stressful conditions (Bradsh a w 1965; Sultan 1987, 2000). Under a crop canopy, both red:far-red ( R: FR) ratio and irradiance level along with the impact of these parameters on flowering timing is difficult to predict. In most cases, competition results in either no change or a delay in the onset of reproduction ( Weiner 1988). Smith and Whitelam (1997) suggested that reductions in flowering in response to reduced R: FR ratios may be adaptive, because the probability of seed production increases due to crop competition. Alternatively, changes in flowering time may be the indirect consequence of physiological trade-offs and may be unstable if adjustments in flowering initiation result in premature partitioning away from lightcapturing tissue ( Cohen 1976).

Weed fecundity and biomass are highly dependent upon time of emergence in the crop and proximity of the weed to the crop ( Clay et al. 2005; Knezevic and Horak 1998). Nevertheless, A. palmeri continued flowering until later in the season ( Figure 6 ) and produced seeds even when it emerged 6 and 8 wk after soybean emergence ( Figure 5 ), despite the increased light interception by crop canopy ( Figures 7 and 8 ).

Brown and Blaser (1968) emphasized that plant stands with low k values indicate inefficient light interception by leaves. Nevertheless, Wolf et al. (1972) stated that the extinction coefficient varies when leaf area can no longer be represented by LAI, although other non-leaf structures (e.g., stems) intercept light. In our experiments, soybean was entering into the maturing stage ( R 5 to R 7) at 12.3 to 13 WAE, with consequent initiation of leaf senescence ( Setiyono et al. 2008, 2010).

In addition, it would have been expected that A. palmeri , a species with a C 4 photosynthetic pathway ( Wang et al. 1992), would be sensitive to shade, particularly for plants emerging late in the growing season ( Buehring et al. 2002), because this species exhibits prolific growth at high light intensities ( Keeley et al. 1987; Massinga et al. 2003). Despite that, it appears that A. palmeri can tolerate shade and grow under crop canopies ( Ward et al. 2013) and evolve traits that increase its potential to grow and reproduce in various agroecosystems and environmental conditions ( Bravo et al. 2017; Korres et al. 2017a, b). Patterson (1985) reported that A. palmeri under reduced light intensity exhibited a noticeable plasticity in acclimation that enabled the plant to survive and produce viable seeds ( Jha et al. 2010). Avoiding the development of more aggressive A. palmeri biotypes and considering the consequences of evolutionary change is important in designing cropping systems and weed management strategies ( Bravo et al. 2017). The reduced A. palmeri plant biomass at the later soybean stages reflects the greater competitiveness of the crop compared with the earlier A. palmeri establishment dates, when soybean was smaller, and supports the differences in soybean yield loss among these treatments as is discussed in the following section.

Figures

Figure 2. Soybean and Amoronthus polmeri (AMAPA) height (averaged across distance from the crop) at 0, 1, 2, 4, 6, and 8 wk after soybean emergence (WAE) (i.e., AMAPA-0, AMAPA-1, AMAPA-2, AMAPA-4, AMAPA-6, and AMAPA-8, respectively). Vertical bars represent ± standard error of the mean from the analysis for comparisons within each sampling date (i.e., n =12 for 0 WAE, 24 for 1 WAE, etc.).

Figure 3. Effects of the interaction of weed establishment time and distance from the crop on Amoronthus polmeri plant height at harvest. Vertical bars represent ± standard error of the mean (SE2014 = 4.68; SE2015 =3.14) from the analysis for comparisons between weed establishment times with sample size n= 72. WAE, weeks after soybean emergence.

Figure 4. Effects of the interaction of weed establishment time and distance from the crop row on Amoronthus polmeri dry weight before soybean harvest. Vertical bars represent ± standard error of the mean (SE2014 = 1.27; SE2015 = 0.74) from the analysis for comparisons between weed establishment times with sample size n =72. WAE, weeks after soybean emergence.

Figure 5. Effects of the interaction of weed establishment time and distance from the crop row on Amoronthus polmeri seed production before soybean harvest. Vertical bars represent ± standard error of the mean (SE2014 = 2,530.27; SE2015 =1,008.30) from the analysis for comparisons between weed establishment times with sample size n= 72.WAE, weeks after soybean emergence.

Figure 6. Effects of weed establishment time on Amoronthus polmeri (AMAPA) flowering (averaged across distance from the crop) at various sampling occasions for 0, 1, 2, 4, 6, and 8 wk after soybean emergence (WAE) (i.e., AMAPA-0,AMAPA-1,AMAPA-2, AMAPA-4, AMAPA-6, and AMAPA-8 respectively) in 2014 and 2015. Vertical bars represent ± standard error of the mean (i.e., flowering of the entire A. polmeri population was evaluated at each sampling occasion) from the analysis for comparisons within each sampling date (i.e., n = 12 plots for 0 WAE, 24 plots for 1 WAE, 36 plots for 2 WAE, etc.).

Figure 7. Relationship between ground cover and extinction coefficient for each sampling date (n =12 plots) throughout the 2014 growing season. WAE, weeks after soybean emergence.

Figure 8. Relationship between ground cover and extinction coefficient for each sampling date (n = 12 plots) throughout the 2015 growing season. WAE, weeks after soybean emergence