Introduction
Mungbean [Vigna radiata (L.) R. Wilczek] is the major summer pulse crop grown in the states of Queensland and New South Wales of Australia. In the Australian agricultural industry, mungbean is known as an export crop, with 95% of the product being exported. Mungbean production is increasing owing to its short duration (90 to 110 d), and 1.50 × 108 kg of this crop were harvested in 2016 from Australian farms (Anonymous 2017; Rachaputi et al. Reference Rachaputi, Chauhan, Douglas, Martin, Krosch, Agius and King2015). Weeds pose a serious threat to the productivity of mungbean, and broadleaf weed control options are very limited in this crop. Weeds, such as annual sowthistle (Sonchus oleraceus L.), are one of the emerging threats to quality and yield of this valuable crop. Besides posing a strong interference, this weed has been reported to harbor insect pests such as Helicoverpa armigera Hübner and act as a host of Macrophomina phaseolina (Tassi) Goid., the causal agent of charcoal rot (Fuhlbohm et al. Reference Fuhlbohm, Ryley and Aitken2012; Gu and Walter Reference Gu and Walter1999; Widderick et al. Reference Widderick, Walker, Sindel and Bell2010).
Germination ecology studies have shown high adaptability of S. oleraceus to many cropping regions, as it can germinate at a broad range of temperatures and exhibits a high tolerance to osmotic and saline conditions (Chauhan et al. Reference Chauhan, Gill and Preston2006; Manalil et al. Reference Manalil, Ali and Chauhan2018; Widderick et al. Reference Widderick, Walker, Sindel and Bell2010). Sonchus oleraceus has no innate dormancy, produces seeds profusely (>25,000 seeds plant−1), has a pappus for efficient wind dispersal, emerges throughout the year, and has allelopathic effects and a high degree of competitiveness (Chauhan et al. Reference Chauhan, Gill and Preston2006; Gomaa et al, Reference Gomaa, Hassan, Fahmy, González, Hammouda and Atteya2014; Song et al. Reference Song, Kim, Im, Lee, Lee and Kim2017; Widderick et al. Reference Widderick, Walker, Sindel and Bell2010). These traits have made this weed the 8th most significant weed in terms of infested area (595,705 ha), 10th in terms of yield reduction (1.81 × 107 kg), and 11th in terms of economic losses worth AU$4.9 million in the Australian grain cropping regions (Balick et al. Reference Balick, Nee and Atha2000; Germishuizen and Meyer Reference Germishuizen and Meyer2003; Llewellyn et al. Reference Llewellyn, Ronning, Clarke, Mayfield, Walker and Ouzman2016; Manalil et al. Reference Manalil, Werth, Jackson, Chauhan and Preston2017; Weber Reference Weber2017). In addition, biotypes of this weed resistant to herbicides such as acetolactate synthase inhibitors, triazines, and glyphosate have been reported (Adkins et al. Reference Adkins, Wills, Boersma, Walker, Robinson, McLeod and Einam1997; Boutsalis and Powles Reference Boutsalis and Powles1995; Heap Reference Heap2019; John-Sweeting et al. Reference John-Sweeting, Preston, Baker, Walker, Widderick, van Klinken, Osten, Panetta and Scanlan2008). In Australia, the first case of glyphosate-resistant S. oleraceus was reported in 2014 (Heap Reference Heap2019), but since then several populations have evolved resistance to glyphosate, making it difficult to manage in the fallow phase and glyphosate-tolerant cotton (Gossypium hirsutum L.).
Sonchus oleraceus is a highly competitive and difficult to control weed, especially in conservation cropping systems. Competition studies have shown that the presence of 18 to 20 S. oleraceus plants m−2 in soybean [Glycine max (L.) Merr.] resulted in significant yield losses (Song et al. Reference Song, Kim, Im, Lee, Lee and Kim2017), and a similar result was also observed in wheat (Triticum aestivum L.) (GRDC 2017). The adoption of conservation agriculture systems in many cropping regions of Australia (Chauhan et al. Reference Chauhan, Gill and Preston2006) and evolution of herbicide-resistant biotypes of this weed (John-Sweeting et al. Reference John-Sweeting, Preston, Baker, Walker, Widderick, van Klinken, Osten, Panetta and Scanlan2008; Werth et al. Reference Werth, Thornby and Walker2012) have resulted in fewer weed management options available in mungbean.
Increasing crop density and changing canopy architecture to enhance crop competitiveness are some of the methods that should be included in integrated weed management strategies in mungbean. A previous study reported that increasing mungbean density from 33 to 40 plants m−2 resulted in higher yield production in weed-free conditions (Singh et al. Reference Singh, Sekhon, Singh, Brar, Bains and Shanmugasundaram2011). In Australia, growers plant mungbean at approximately 40 plants m−2. Chauhan et al. (Reference Chauhan, Florentine, Ferguson and Chechetto2017) reported that using narrow row spacing (25 cm) in mungbean could reduce weed biomass by more than 70% compared with wider row spacing (75 cm). The development of cultural weed control strategies rather than sole reliance on chemical control strategies could be beneficial as a sustainable and environmentally friendly approach.
Previous studies showed that some glyphosate-resistant weeds such as goosegrass [Eleusine indica (L.) Gaertn.] and junglerice [Echinochloa colona L. (Link)] responded differently to environmental factors and management strategies (Ismail et al. Reference Ismail, Chuah, Salmijah, Teng and Schumacher2002; Shrestha et al. Reference Shrestha, Yang, Sosnoskie and Hanson2018). However, there is a lack of precise information about the response of glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus to the increased density of mungbean. The current study was designed to better understand the possibility of manipulating density of mungbean as a strategy for S. oleraceus management. In this study, change in growth and seed production of glyphosate-resistant and glyphosate-susceptible biotypes of this weed were quantified in competition with different mungbean densities.
Materials and Methods
Seed Collection of Glyphosate-Resistant and Glyphosate-Susceptible Biotypes
For the first study, S. oleraceus seeds were collected from Gatton, QLD, Australia (27.44°S, 152.23°E) in 2016. For the second study, two biotypes (glyphosate-resistant and glyphosate-susceptible) of S. oleraceus were used. The susceptible biotype seeds were collected from Gatton, and the resistant biotype seeds were collected from Gunnedah, NSW. Glyphosate at 720 g ae ha−1 was applied to confirm the resistance status of both biotypes. Plants of both biotypes were grown at Gatton (same conditions), and seeds collected from these biotypes were used in the second study.
Study 1
This target-neighborhood pot study was carried out to understand the effect of increasing mungbean density on S. oleraceus growth and seed production (Chauhan and Opeña Reference Chauhan and Opeña2012; Mutti et al. Reference Mutti, Mahajan, Jha and Chauhan2019). The study was conducted twice at the Gatton Research Farm of the University of Queensland, Australia, in 2016 and 2017. Plastic pots (25-cm diameter and 30-cm height) were filled with potting mix and placed in a shade house. Four to five seeds of S. oleraceus were sown in the center of each pot, and a healthy seedling was kept after thinning within 10 d of sowing. Mungbean densities of 0, 4, 8, 12, and 16 plants pot−1 were maintained by sowing mungbean seeds (‘Jade-AU’) equidistant from each other as per treatment, and at a distance of 10 cm from S. oleraceus seeds. The mungbean densities corresponded to 0, 82, 164, 246, and 328 plants m−2. Pots were subirrigated. The effect of crop competition on S. oleraceus was assessed by measuring height and number of leaves of S. oleraceus plants at a 14-d interval after sowing and continued until 56 d after sowing (DAS). A wooden scale was used to measure height from the surface to the tip of the uppermost leaf. The study was terminated at 56 DAS, when lower leaves of S. oleraceus started to turn brown. At harvest, the numbers of buds and seeds per plant for S. oleraceus were counted. The aboveground biomass of S. oleraceus was measured after drying the samples in an oven at 70 C for 72 h and separating them into leaves and stems. Height and aboveground biomass per mungbean plant were measured at harvest.
Study 2
This pot study was conducted in 2018 at the same location and under similar conditions as the first study. In this study, the effect of mungbean density on growth and seed production of glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus was evaluated. Mungbean densities were 0, 4, and 8 plants pot−1, which corresponded to 0, 82, and 164 plants m−2, respectively. The study included five termination times (20, 30, 40, 50, and 60 DAS). At each termination time, plant height and number of leaves per plant were measured. Plants were cut at the soil surface and dried in an oven at 70 C for 72 h before their biomass was measured. At the last termination time (60 DAS), the numbers of buds and seeds per plant of S. oleraceus, mungbean height, and aboveground and root biomass were measured.
Statistical Analyses
The first study was conducted in a completely randomized design with 10 replications. The second study was also carried out using the same experimental design but had three replications for each termination time. Before statistical analyses, the homogeneity and normality of data were checked, and ANOVA was performed using the SAS software v. 9 (SAS Institute, Cary, NC, USA). Both studies were conducted twice, and the data were pooled across the runs for each study, as no significant differences were observed between the runs. Nonlinear regression was performed using SigmaPlot software v. 14 (Systat Software, San Jose, CA, USA), and predicted values were compared using the standard errors of the means.
The effect of increasing density of mungbean on the reduction in S. oleraceus biomass and the production of buds and seeds per plant was modeled using a two-parameter exponential decay model:
$$Y\, = \,ae^{ - bx}$$
where Y is predicted biomass, buds, or seed production; a is a constant parameter; and b (slope) is the rate of reduction in biomass, buds, or seed production.
The fitness of the fitted model was ascertained in terms of R2 values. When nonlinear analysis was not possible, differences between means of the treatments were evaluated by Fisher’s Protected LSD test (P ≤ 0.05).
Results and Discussion
Study 1
Sonchus oleraceus Height and Leaf Number. Height and number of leaves per plant of S. oleraceus were significantly affected by the increase in mungbean density (Table 1). In the first 28 DAS, plant height was not affected by mungbean density. At 42 DAS, significant differences were observed between treatments. At 56 DAS, S. oleraceus plants were taller in the no-interference condition. The increase in mungbean density to 82 plants m−2 had no effect on S. oleraceus height. Mungbean density of 164 plants m−2 reduced S. oleraceus height by 21%; however, increasing the density beyond 164 plants m−2 did not further reduce height.
Table 1. Competitive effect of different densities of mungbean on Sonchus oleraceus height (cm) and leaf number per plant at different times after sowing (Study 1).
Although no significant difference was observed in the number of leaves until 28 DAS, a sharp decrease (>70%) in leaf number per plant of S. oleraceus was observed with the increased density of mungbean at 56 DAS compared with the no-competition treatment (Table 1). At 42 and 56 DAS, the lowest S. oleraceus leaf production was observed in pots sown with high densities (246 and 328 plants m−2) of mungbean.
Sonchus oleraceus Biomass. Increasing mungbean density significantly decreased S. oleraceus biomass (leaf, stem, and total shoot) (Figure 1; Table 2). The highest biomass was produced in the no-interference treatment. Increasing mungbean density from 0 plants m−2 to 82, 164, and 246 plants m−2 decreased total shoot biomass by 54%, 69%, and 86%, respectively, and no significant difference was observed between 246 and 328 mungbean plants m−2.
Figure 1. Competitive effect of different densities of mungbean on Sonchus oleraceus biomass (Study 1). Vertical bars represent standard errors (±SE) of means. Estimated parameters are presented in Table 2.
Table 2. Estimated parameters (±SEs) of a two-parameter exponential decay model, Y = ae−bx, fit to leaf, stem, and total shoot biomass per plant (g plant −1) of Sonchus oleraceus in competition with different densities (0, 82, 164, 246, and 328 plants m−2) of mungbean (Study 1).
Sonchus oleraceus Bud and Seed Production. The highest number of buds (83 buds plant−1) and seed production (9,480 seeds plant−1) of S. oleraceus were observed in the no-interference treatment (Figure 2). Competition of mungbean with this weed resulted in an exponential reduction in S. oleraceus bud number and seed production per plant. The reduction trend (b parameter) was similar for the number of buds and seed production per plant. Although increasing mungbean density to 82 and 164 plants m−2 reduced the number of seeds by 55% and 78%, respectively, a significant number of seeds were produced, even at the mungbean density of 328 plants m−2 (1,185 seeds plant−1).
Figure 2. Competitive effect of different densities of mungbean on Sonchus oleraceus number of buds per plant (A) and seed production per plant (B) at 56 d after sowing (Study 1). Vertical bars represent standard errors (±SE) of means.
Mungbean Height and Biomass. Mungbean height was not significantly influenced by the increasing densities of mungbean plants; however, biomass (g m−2) increased significantly (Table 3). Increasing the mungbean density from 82 to 164 plants m−2 increased mungbean biomass by 22%. Increasing the mungbean density beyond 164 plants m−2 had no significant effect on mungbean biomass, and no significant differences were observed between 246 and 328 plants m−2 treatments.
Table 3. Effect of Sonchus oleraceus competition with mungbean on mungbean height and aboveground biomass (Study 1).
a NS, nonsignificant.
Study 2
Height and Leaf Number of Glyphosate-Resistant and Glyphosate-Susceptible Biotypes of Sonchus oleraceus. The competitive effect of different densities of mungbean on glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus height and number of leaves per plant was significant (Table 4). In general, the susceptible biotype was taller than the resistant biotype. In both biotypes, there was no significant difference between mungbean density treatments for plant height up to 30 DAS. At 60 DAS, competition offered by the mungbean density of 82 plants m−2 resulted in a 22% reduction in the susceptible S. oleraceus height; however, no significant differences were observed between the densities of 82 and 164 plants m−2. Crop interference had no effect on glyphosate-resistant biotype height at 60 DAS.
Table 4. Competitive effect of different densities of mungbean on plant height and leaf number per plant of glyphosate-resistant and glyphosate-susceptible Sonchus oleraceus biotypes at different harvest times (Study 2).
In general, the susceptible biotype produced more leaves per plant than the glyphosate-resistant biotype (Table 4). The effect of increasing mungbean density on the number of leaves was not significant until 20 DAS in the glyphosate-resistant biotype and 30 DAS in the susceptible biotype. After these time periods, the number of leaves per plant in the glyphosate-resistant and glyphosate-susceptible biotypes was reduced by 38% and 45% at 82 mungbean plants m−2 and 46% and 58% at 164 mungbean plants m−2,respectively, in comparison with the no-interference treatment.
Biomass of Glyphosate-Resistant and Glyphosate-Susceptible Biotypes of Sonchus oleraceus. Aboveground and root biomass of glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus was reduced with the increase in mungbean density at all harvest times (Table 5). In general, the glyphosate-susceptible biotype produced more aboveground and root biomass than the glyphosate-resistant biotype. The highest aboveground and root biomass in both biotypes was observed in the no-interference condition. At the mungbean density of 82 plants m−2, 10% and 23% reductions in aboveground biomass of the glyphosate-resistant and glyphosate-susceptible biotypes, respectively, were recorded. The corresponding reductions in root biomass were 31% and 49% for the glyphosate-resistant and glyphosate-susceptible biotypes, respectively, suggesting that underground competition was more pronounced than aboveground competition.
Table 5. Competitive effect of different densities of mungbean on aboveground shoot and root biomass of glyphosate-resistant and glyphosate-susceptible Sonchus oleraceus biotypes at different harvest times (Study 2).
Bud and Seed Production of Glyphosate-Resistant and Glyphosate-Susceptible Biotypes of Sonchus oleraceus. The number of buds and seed production per plant of both biotypes of S. oleraceus were adversely affected by the increased densities of mungbean (Figure 3). The highest number of buds and seed production per plant were observed for the susceptible biotype in the no-interference treatment. When grown in competition with 82 mungbean plants m−2, the number of buds per plant of the glyphosate-resistant and glyphosate-susceptible biotypes was reduced by 34% and 35%, respectively. Corresponding values for seed production per plant were 39% and 40% for the glyphosate-resistant and glyphosate-susceptible biotypes, respectively, under this treatment. The glyphosate-resistant biotype produced 24% fewer seeds than the susceptible biotype in the no-competition treatment. Although increasing mungbean density from 0 to 164 plants m−2 sharply reduced seed number, a large number of seeds were produced, even at the mungbean density of 164 plants m−2 (>4,000 seeds plant−1 in both biotypes).
Figure 3. Competitive effect of different densities of mungbean (MB) on number of buds per plant (A) and seed production per plant (B) of glyphosate-resistant (R) and glyphosate-susceptible (S) biotypes of Sonchus oleraceus (Study 2). Vertical bars are LSD at the 5% level of significance.
Mungbean Height and Biomass. Increasing the mungbean density from 82 to 164 plants m−2 had a significant effect on mungbean aboveground and root biomass per square meter, but no significant difference was observed for mungbean height (Table 6). Mungbean aboveground and root biomass increased with greater mungbean density. The highest aboveground and root biomass accumulation by mungbean occurred at the density of 164 plants m−2 when grown in competition with the glyphosate-resistant biotype of S. oleraceus. The glyphosate-resistant biotype posed less competition on mungbean in comparison with the susceptible biotype.
Table 6. Effect of glyphosate-resistant and glyphosate-susceptible biotypes of Sonchus oleraceus competition with 82 and 164 mungbean plants m−2 on mungbean height and aboveground and root biomass per plant (Study 2).
a NS, nonsignificant.
Increasing crop competitiveness through increased crop plant density has been suggested as an integral part of integrated weed management strategies. The number of leaves per plant of S. oleraceus was adversely affected by the increase in mungbean density. However, S. oleraceus height was not affected by the increase in mungbean density in the first study. In the second study, at the final harvest, competition offered by the mungbean density of 82 plants m−2 resulted in a 22% reduction in susceptible S. oleraceus height, but there was no significant difference between biotypes and mungbean density for plant height up to 30 DAS. Crop interference had no effect on the glyphosate-resistant biotype height at 60 DAS. Chauhan and Abugho (Reference Chauhan and Abugho2012) reported that an increase in rice density had a negative effect on longfruited primrose-willow [Ludwigia octovalvis (Jacq.) P. H. Raven], as height of this weed was significantly reduced (by 41%), but the increase in rice density had no effect on spiny amaranth (Amaranthus spinosus L.). Reductions in S. oleraceus height and number of leaves per plant could be attributed to competition for light, space, and nutrients (Davis et al. Reference Davis, Johnson and Wood1967; Khaliq et al. Reference Khaliq, Matloob and Chauhan2014). In the present study, even though S. oleraceus height and number of leaves per plant were reduced by the increased densities of mungbean, greater competition was not effective in completely suppressing S. oleraceus, as the weed grew taller and was not adequately suppressed.
Sonchus oleraceus biomass was significantly reduced with the increase in mungbean density. Increases in mungbean density from 0 to 246 plants m−2 resulted in an 86% reduction in total shoot biomass; no significant difference was observed between densities of 246 and 328 mungbean plants m−2 in the first study. Burke et al. (Reference Burke, Schroeder, Thomas and Wilcut2007) reported that Palmer amaranth (Amaranthus palmeri S. Watson) biomass was reduced as a result of competition with peanut (Arachis hypogaea L.). The results of the second study showed that the root biomass reduction was higher than the aboveground shoot biomass reduction. Zimdahl et al. (Reference Zimdahl2007) claimed that in a resource-constrained condition, plants experience intense competition. Despite the no-water limitation, as subirrigation was used in the present study, root competition was higher than aboveground competition. This might be attributed to the contrasting growth habits of the tested crop and weed species. The spreading nature of mungbean and the taller growth habit of S. oleraceus could be a reason for lower aboveground competition. Many studies have shown that crop competition for light, space, and nutrients through increased seeding density and narrow row spacing has an adverse effect on weeds (Acciaresi and Chidichimo Reference Acciaresi and Chidichimo2007; Weiner et al. Reference Weiner, Griepentrog and Kristensen2001). Our results indicate that when weeds such as S. oleraceus are able to grow taller than the main crop, reliance on increased crop density alone may not be effective, and additional weed management is warranted.
Height, number of leaves, and biomass of S. oleraceus were significantly reduced with crop interference in most cases, likely leading to the observed decrease in number of buds and seed production per plant. Seed production was greatly reduced, but even at the highest mungbean density tested (328 plant m−2), a substantial number of seeds (>2,000 plant−1) were produced by S. oleraceus. A reduction in the reproductive output of weeds in cereals in response to competition by increasing densities of crops has been reported in several studies (Kolb and Gallandt Reference Kolb and Gallandt2013; Mashingaidze et al. Reference Mashingaidze, Van Der Werf, Lotz, Chipomho and Kropff2009). The reduction in weed seed production due to increased crop competition could be related to the reduction in weed biomass. A sound understanding of the weed seedbank dynamic is important to devise sustainable weed management strategies. Reduction in seed output by S. oleraceus in competition with mungbean can have important implications for seedbank persistence, because this weed exhibits no dormancy, is effectively dispersed by wind, and emerges throughout the year (Chauhan et al. Reference Chauhan, Gill and Preston2006; Widderick et al. Reference Widderick, Walker, Sindel and Bell2010). Germination of this weed is highly dependent upon the presence of light (Chauhan et al. Reference Chauhan, Gill and Preston2006). Increasing crop density to prompt faster crop canopy closure would prevent light transmission to the soil surface, and hence could be useful in preventing or delaying the emergence of this weed during the growing season.
Although the glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus responded similarly to increased mungbean density, the plants of the susceptible biotype grew taller and produced more leaves, biomass, and seeds than the glyphosate-resistant biotype. The mechanisms and effects of being resistant to glyphosate vary between plant species (Glettner and Stoltenberg Reference Glettner and Stoltenberg2015). For example, Kaspary et al. (Reference Kaspary, Lamego, Cutti, Aguiar, Rigon and Basso2017) stated that the glyphosate-resistant biotype of hairy fleabane (Erigeron bonariensis L.) produced more seeds than the susceptible biotype. In contrast, Brabham et al. (Reference Brabham, Gerber and Johnson2011) reported 25% less seed production in glyphosate-resistant giant ragweed (Ambrosia trifida L.). Our results demonstrated that although the seed production of the glyphosate-resistant biotype of S. oleraceus was 24% less than that of the susceptible biotype, a large number of seeds were produced, even under crop interference (>4,000 seeds plant−1). Maternal condition and genetic diversity are major factors affecting weeds’ response to environment (Bajwa et al. Reference Bajwa, Chauhan and Adkins2018; Gioria and Pyšek, Reference Gioria and Pyšek2017). The observed differences between the glyphosate-resistant and glyphosate-susceptible biotypes could be attributed to genetic diversity between these biotypes. In this study, the effect of maternal conditions on seed production was removed by growing plants in the same environment.
The findings of this study corroborate the previous work of Chauhan et al. (Reference Chauhan, Florentine, Ferguson and Chechetto2017), who reported decreased weed biomass with an increased density of mungbean. Nevertheless, for a prolific seed producer like S. oleraceus, sole reliance on increased crop density to suppress weed biomass does not seem effective for long-term weed management. Hence, the increased crop-density component needs to be integrated with PRE herbicides. Initially, PRE herbicides would keep the crop weed-free, and then the dense crop would shade and suppress late-emerging S. oleraceus seedlings. In the field, increased crop density may restrict mechanical operations, and thus threshold density levels under field conditions need to be optimized. Mungbean grain yield was not evaluated in the current study, as the weed matured earlier than the crop. In future studies, mungbean grain production should be considered to better understand the effect of weed competition.
Acknowledgments
This work was supported by a grant from Grains Research Development Corporation (GRDC), Australia, under project UA000156. No conflicts of interest have been declared.
