Database Description

The 86 studies included in the meta-analysis were conducted during the previous five decades (1970 through 2020) in nine countries across four continents: Asia (China, Iran, and the Philippines), Australia (Australia), Europe (Denmark, Greece, and the United Kingdom), and North America (Canada and the United States). More than one-third of the studies (79%; n = 68) were conducted in three countries: the United States (36%; n = 31), Australia (24%; n = 21), and the Philippines (19%; n = 16). China, Canada, and Iran had six, five, and four studies, respectively, whereas each European country (Denmark, Greece, and the United Kingdom) had one study.

Across all studies, a total of 102 weed species belonging to 24 families were investigated for their response to water stress (Supplementary Table S1). Most of the studies investigated water-stress effects on broadleaf weeds (n = 62) followed by grass weeds (n = 23). Only one study by Hoveland and Buchanan (Reference Hoveland and Buchanan1973) investigated both broadleaf and grass weed species (Supplementary Table S1). Most of the broadleaf weed species belonged to Asteraceae (n = 22), followed by Fabaceae (n = 9), Convolvulaceae (n = 5), Amaranthaceae (n = 4), and Rubiaceae (n = 4). Similarly, the investigated grass weed species belonged to the family Poaceae (n = 24). Among weed species, hemp sesbania [Sesbania herbacea (Mill.) McVaugh] was the most frequently investigated species in four studies, followed by junglerice [Echinochloa colona (L.) Link] and redroot pigweed (Amaranthus retroflexus L.), both of which were investigated three times. All other weed species were investigated once, except eight weed species that were investigated twice: crowfootgrass [Dactyloctenium aegyptium (L.) Willd.], green foxtail [Setaria viridis (L.) P. Beauv.], itchgrass [Rottboellia cochinchinensis (Lour.) W.D. Clayton], ivyleaf morningglory (Ipomoea hederacea Jacq.), pitted morningglory (Ipomoea lacunosa L.), ragweed parthenium (Parthenium hysterophorus L.), slender amaranth (Amaranthus viridis L.), and tall morningglory [Ipomoea purpurea (L.) Roth].

Effect of Water Stress on Weed Germination

Averaged across pair-wise observations, water stress reduced weed seed germination/emergence by 90% (95% CI = −92% to −86%; Figure 2). This effect of water stress on weed germination/emergence is likely because more than one-third of the observations (i.e., n = 276 out of 759) were exposed to severe water stress (ψ_solution >−0.6 MPa), resulting in >97% germination inhibition. Seed imbibition is required for germination, and hydration levels vary by plant species (Hegarty Reference Hegarty1978), although understanding of these levels in weeds is limited (Pérez-Fernández et al. Reference Pérez-Fernández, Lamont, Marwick and Lamont2000). We observed 86% to 95% inhibition in the germination of Amaranthaceae, Asteraceae, Convolvulaceae, Fabaceae, Rubiaceae, and Poaceae weed families (Figure 3). Although differences were nonsignificant, Asteraceae was the least responsive family with 86% (99% CI = −93% to −72%) germination inhibition, while Amaranthaceae was the most responsive family with 95% (99% CI = −99% to –65%) germination inhibition due to water stress.

Figure 2. Overall water-stress effects on weed germination/emergence, growth characteristics, and seed production. The vertical black dashed line represents zero effect. The black dots are overall mean effect sizes, and the black lines are 95% confidence intervals (CIs). The values in parentheses are the number of observations followed by the number of studies for each pair-wise comparison. The mean effect sizes were considered significantly different when their 95% CIs did not include zero.

Figure 3. Overall water-stress effects on germination/emergence of grass and broadleaf weeds (top) and six weed families—Asteraceae, Fabaceae, Convolvulaceae, Amaranthaceae, Rubiaceae, and Poaceae (bottom). The vertical black dashed line represents zero effect. The black dots are overall mean effect sizes, and the black lines are 99% confidence intervals (CIs). The values in parentheses are the number of observations followed by the number of studies for each pair-wise comparison. The mean effect sizes were considered significantly different when their 99% CIs did not include zero.

Plant functional groups respond differently to moisture availability (Emanuel et al. Reference Emanuel, D’Odorico and Epstein2007; Manzoni et al. Reference Manzoni, Vico, Katul, Fay, Polley, Palmroth and Porporato2011), as evidenced by greater negative responses of grasses than broadleaves to water stress (Emanuel et al. Reference Emanuel, D’Odorico and Epstein2007). Overall, germination of grass weeds was inhibited by 93% (99% CI = −97% to −83%) compared with 90% (99% CI = −93% to −84%) for broadleaf weeds (Figure 3). Mackie et al. (Reference Mackie, Zeiter, Bloor and Stampfli2019) also noted a greater impact of summer drought on grasses than forbs in their experiments across eight sites. Similarly, we observed a general trend of grass weeds being more negatively affected than broadleaf weeds across water-stress levels (Figure 4). However, water-stress effects between broadleaf and grass weeds were not significantly different due to their overlapping 99% CIs across ψ_solution subgroups.

Figure 4. The log response ratio for germination and seedling radicle length of broadleaf (green dots/line) and grass (red dots/line) weed species as a function of water-stress intensity. Water stress increased as solution osmotic potential (ψ_solution) decreased and vice versa. The subgroups for germination are 0 to −0.2, −0.2 to −0.4, −0.4 to −0.6, −0.6 to −0.8, −0.8 to −1.0, −1.0 to −1.4, and <−1.4 MPa, while the subgroups for radicle length are 0 to −0.2, −0.2 to −0.4, −0.4 to −0.6, −0.6 to −1.0, and <−1.0 MPa. Only ψ_solution-based studies were used in this analysis. For each subgroup, the solid dots and lines represent mean effect sizes and their corresponding 99% confidence intervals (CIs). The mean effect sizes were considered significantly different when their 99% CIs did not include zero. Similarly, the water-stress effects were significantly different for each subgroup and among weed types only when their 99% CIs did not overlap with one another. The fitted lines represent a four-parameter logistic regression model, and the coefficients of the models are presented in Table 2.

At low water stress (i.e., ψ_solution >−0.2 MPa), the mean water-stress effect on germination of broadleaf weeds was nonsignificant (mean = −18%, 99% CI = −41% to 14%), whereas grass weeds showed a negative effect (mean = −51%, 99% CI = −76% to −0.3%). At moderate water stress, the germination of broadleaves significantly decreased by 53% (99% CI = −69% to −29%) at ψ_solution between −0.2 to −0.4 MPa and by 84% (99% CI = −92% to −70%) at ψ_solution between −0.4 to −0.6 MPa. Similarly, the germination of grass weeds decreased by 73% (99% CI = −88% to −39%) at ψ_solution between −0.2 to −0.4 MPa and by 92% (99% CI = −98% to −67%) at ψ_solution between −0.4 to −0.6 MPa. At severe water stress, germination of broadleaf and grass weeds decreased by 97% (99% CI = −99% to −93%) and 98% (99% CI = −100% to −87%), respectively, at ψ_solution between −0.6 to −0.8 MPa. With further decrease in ψ_solution above −0.8 MPa, germination of both broadleaves and grasses was completely inhibited (i.e., >99% inhibition). We further modeled the effect of water stress on weed germination by fitting a four-parameter logistic model between mean effect sizes and mean ψ_solution for each subgroup (Figure 4). The fitted model coefficients are presented in Table 2. For both broadleaf and grass weed types, weed germination decreased with a decrease in ψ_solution (Figure 4). This indicates that the adverse effects of water stress on weed germination increased with increasing water stress.

Table 2. Parameter estimates and SEs from a four-parameter logistic model fit to effect sizes for germination and seedling radicle length of broadleaf and grass weed species under water-stress gradients. ^a

^a The model is fit to solution osmotic potential–based (ψ_solution) studies only. $c$ is the lower asymptote, $d$ is the higher asymptote, $b$ is the slope at the inflection point, ψ_solution is the solution osmotic potential, and ψ_solution.50 is the solution osmotic potential at the inflection point (i.e., the ψ_solution that produces a response midway between $c$ and $d$ ).

Effect of Water Stress on Weed Growth Characteristics

Averaged across pair-wise comparisons, water stress negatively affected belowground weed growth characteristics (Figure 2). Water stress, on average, decreased seedling radicle/root length by 77% (95% CI = −89% to −54%) and root biomass by 44% (95% CI = −63% to −13%). A more intense effect of water stress on seedling radicle/root length was likely due to the use of PEG or d-mannitol solutions, with 35% of the observations (n = 19 out of 54) being exposed to severe water-stress (i.e., ψ_solution >−0.6 MPa) conditions exhibiting > 97% inhibition (Figure 4). As ψ_solution decreases, water stress increases, causing seedling radicle length to decrease progressively. At low to moderate water stress of ψ_solution between 0 to −0.4 MPa, the mean decrease in seedling radicle length was not significantly different from zero. However, further decrease in ψ_solution below −0.4 MPa resulted in a reduction in seedling radicle length compared with a no water stress condition. For instance, seedling radicle length decreased by 65% (99% CI = −82% to −31%) at ψ_solution between −0.4 to −0.6 MPa, by 97% (99% CI = −99% to −84%) at ψ_solution between −0.6 to −1.0 MPa, and by 99% (99% CI = −100% to −86%) at ψ_solution below −1.0 MPa. Although the mean water-stress effects for each subgroup were not significantly different from zero, root biomass decreased from a mean positive effect of 2% (99% CI = −28% to 44%) at low (i.e., soil moisture >60% field capacity) to a mean negative effect of 39% (99% CI = −77% to 60%) and 69% (99% CI = −92% to 15%) at moderate (i.e., soil moisture at 30% to 60% field capacity) and severe (i.e., soil moisture <30% field capacity) water stress, respectively (Figure 5). The results suggest that belowground weed growth characteristics were negatively impacted by water stress, and the magnitude of the effect intensified with increasing water stress.

Figure 5. The log response ratio for weed growth characteristics (plant height, leaf area, branches/tillers per plant, leaves per plant, root biomass, shoot biomass, and root:shoot ratio) and seed production (inflorescences per plant and seeds per plant) as a function of water-stress intensity. Water stress increased as soil moisture (% field capacity) decreased and vice versa. The green and red dots represent broadleaf and grass weed species, respectively. The solid black points and the lines represent mean effect sizes and their 99% confidence intervals (CIs) for low (>60%), moderate (30%–60%), and severe (<30% field capacity) water-stress subgroups. The mean effect sizes were considered significantly different when their 99% CIs did not include zero. Similarly, the water-stress effects were significantly different for each subgroup and among weed types only when their 99% CIs did not overlap with one another.

Water stress reduced most aboveground weed growth characteristics (Figure 2). Averaged across pair-wise comparisons, water stress decreased plant height by 36% (95% CI = −54% to −11%), leaf area by 45% (95% CI = −66% to −10%), leaves per plant by 28% (95% CI = −39% to −16%), and shoot biomass by 45% (95% CI = −58% to −29%). Although not statistically different, water stress decreased branches/tillers per plant by 27% (95% CI = −49% to 4%). Results were consistent with the findings from a recent meta-analysis by Sun et al. (Reference Sun, Wang, Chen and Ruan2020), who reported a decrease in plant growth characteristics under water stress: for instance, they observed an overall decrease in plant dry biomass by 29% due to water stress.

Results from the moderator analysis indicated that the negative effects of water stress on aboveground weed growth characteristics intensified with increasing water stress (Figure 5). As soil moisture, expressed as percent field capacity, became more deficit, we observed a progressive reduction in the mean effect sizes for indices related to aboveground weed growth characteristics. For example, the mean effect on weed shoot biomass decreased from a nonsignificant effect of −15% (99% CI = −40% to 21%) at low (i.e., soil moisture > 60% field capacity) to a significant effect of −39% (99% CI = −56% to −14%) at moderate (i.e., soil moisture at 30% to 60% field capacity) and −61% (99% CI =-73% to −43%) at severe (i.e., soil moisture < 30% field capacity) water stress. Similarly, we found a nonsignificant effect of low water stress on other aboveground weed growth characteristics, including plant height, leaf area, leaves per plant, and branches/tillers per plant. Growth indices were reduced at moderate and severe water stress: plant height by −24% (99% CI = −34% to −12%) and −37% (99% CI = −46% to −26%), leaf area by −43% (99% CI = −60% to −18%) and −44% (99% CI = −57% to −26%), leaves per plant by −30% (99% CI = −48% to −5%) and −47% (99% CI = −60% to −29%), and branches/tillers per plant by −23% (99% CI = −38% to −4%) and −52% (99% CI = −85% to 52%), respectively. Taken altogether, the adverse effects of soil moisture limitations on weed growth intensified with increasing water stress, as reported in a recent meta-analysis (Sun et al. Reference Sun, Wang, Chen and Ruan2020). Growth reduction is not only a direct effect of water stress but also an important adaptive mechanism (Skirycz and Inzé Reference Skirycz and Inzé2010). Plants rapidly inhibit their growth at the onset of water stress before gradually recovering and adapting to stressed conditions (Skirycz and Inzé Reference Skirycz and Inzé2010). Additionally, plants have similar multiple adaptive responses, such as generating antioxidants (Nayyar and Gupta Reference Nayyar and Gupta2006), regulating hormones (Peleg and Blumwald Reference Peleg and Blumwald2011), inducing stress proteins (Poolman et al. Reference Poolman, Blount, Folgering, Friesen, Moe and Heide2002), and improving water-use efficiency by increasing root ducts (Lee et al. Reference Lee, Jung, Jang, Jeong, Kim, Ha, Do Choi and Kim2016). Hence, although water stress will reduce weed growth, timely established weeds that can utilize soil moisture from the early onset of precipitation or soil water storage can be more competitive owing to lower resource competition (Hanson Reference Hanson2015).

Averaged across pair-wise comparisons, water stress reduced total weed biomass by 61% (95% CI = −77% to −35%; Figure 2). Although not significantly different from zero, root:shoot ratio was the only index that increased under water stress (mean = 19%; 95% CI = −13% to 63%; Figure 2). A moderator analysis further indicated that the positive effect on root:shoot ratio was mostly observed when soil moisture was maintained above 60% field capacity, that is, at low water stress (mean = 27%, 99% CI = −0.2% to 61%; Figure 5). Even under moderate and severe water stress, that is, when soil moisture was below 60% field capacity, the root:shoot ratio of weeds remained unaffected. These results indicate root growth is generally less sensitive to water stress relative to shoot growth (Sharp and Davies Reference Sharp, Davies, Hamlyn, Hamlyn, Flowers and Jones1989). Plants allocate a greater portion of assimilated dry matter to roots under water stress (Delfin et al. Reference Delfin, Drobnitch and Comas2021; Xu et al. Reference Xu, Cui, Xu, Nie, Huang and Peng2015), and the resultant increase in rooting depth allows for water extraction from deeper layers, maintaining a higher root water influx for longer durations (Chaves et al. Reference Chaves, Pereira, Maroco, Rodrigues, Ricardo, Osório, Carvalho, Faria and Pinheiro2002). Osmotic adjustment (Saab et al. Reference Saab, Sharp and Pritchard1992), higher soluble sugars and dry matter in roots (Xu et al. Reference Xu, Cui, Xu, Nie, Huang and Peng2015), increased cell wall loosening ability (Hsiao and Xu Reference Hsiao and Xu2000), and water stress–induced abscisic acid and ethylene (Sharp and LeNoble Reference Sharp and LeNoble2002; Spollen et al. Reference Spollen, LeNoble, Samuels, Bernstein and Sharp2000) are the primary mechanisms assuring greater root resilience under water stress relative to shoots. Considering plant adaptive mechanisms and the abilities of weeds to extract more water and tolerate water stress, weeds are thus expected to further intensify water-stressed conditions for crops (Griffin et al. Reference Griffin, Shilling, Bennett and Currey1989; Patterson and Flint Reference Patterson and Flint1982; Stuart et al. Reference Stuart, Harrison, Abernathy, Krieg and Wendt1984).

Effect of Water Stress on Weed Seed Production

Water stress decreased weed seed production or fecundity. Averaged across pair-wise comparisons, water stress decreased inflorescences per plant by 48% (95% CI = −49% to −46%) and seeds per plant by 64% (95% CI = −73% to −52%) relative to the unstressed condition (Figure 2). A moderator analysis indicates that both inflorescences and seeds per plant decreased with increasing water stress (Figure 5). The inflorescences per plant decreased by 76% (99% CI = −81% to −71%) at severe water stress of <30% field capacity to 34% (99% CI = −53% to −8%) at moderate water stress of 30% to 60% field capacity, and 20% (99% CI = −38% to 2.5%) at low water stress of >30% field capacity. Similarly, the seeds per plant decreased by 88% (99% CI = −95% to −70%) at severe water stress of <30% field capacity to 50% (99% CI = −65% to −31%) at moderate water stress of 30% to 60% field capacity, and 27% (99% CI = −43% to −7%) at low water stress of >30% field capacity. These results suggest that weeds can continue to produce flowers and seeds to some extent under water-stressed conditions. When water is limited, plants often shorten their vegetative growth and accelerate to rapid flowering and seed production to attain senescence (Bernal et al. Reference Bernal, Estiarte and Peñuelas2011; Franks et al. Reference Franks, Sim and Weis2007; Sherrard and Maherali Reference Sherrard and Maherali2006).

Publication Bias and Sensitivity Analysis

The distribution of individual effect sizes for various indices related to weed germination, growth characteristics, and seed production is shown as density plots in Figure 6. All indices had narrow distributions and slightly offset from zero, indicating a negative effect, except root:shoot ratio, which showed a slightly positive effect of water stress. Nonetheless, density plots for all indices show nearly symmetrical distribution, indicating no publication bias for any of the indices considered in the meta-analysis (Light and Pillemer Reference Light and Pillemer1984; Sterne and Harbord Reference Sterne and Harbord2004).

Figure 6. Density plots depicting the distribution of the individual effect sizes for all 12 response variables considered in this meta-analysis: (A) weed seed germination/emergence; (B) radicle/root length, plant height, and leaf area; (C) branches/tillers per plant, leaves per plant, inflorescences per plant, and seeds per plant; and (D) total biomass, root biomass, shoot biomass, and root:shoot ratio.

Sensitivity analysis identified a few influential studies for some of the indices investigated (Figures 7 and 8). For example, an exclusion of Zollinger and Kells (Reference Zollinger and Kells1991) from the data set increased overall effect size estimates from −36% (95% CI = −54% to −11%) to −23% (95% CI = −29% to −15%) for plant height and from −45% (95% CI = −66% to −10%) to −33% (95% CI = −46% to −17%) for leaf area. Similarly, with the exclusion of Chauhan (Reference Chauhan2013), the magnitude of overall effect size estimates increased from −27% (95% CI = −49% to 4%) to −15% (95% CI = −28% to −0.2%) for branches/tillers per plant and from −28% (95% CI = −39% to −16%) to −15% (95% CI = −28% to −0.2%) for leaves per plant. This was likely because these studies reported drastic impacts of water stress on weed growth characteristics; for example, Chauhan (Reference Chauhan2013) observed a 56% reduction in leaf area at 12.5% field capacity compared with the control (i.e., 100% field capacity). Therefore, the exclusion of these studies caused a deviation in the overall effect size estimates. In contrast, the exclusion of Chadha et al. (Reference Chadha, Florentine, Chauhan, Long and Jayasundera2019) decreased the magnitude of the overall effect size estimates for weed total biomass by 9% from −61% (95% CI = −77% to −35%) to −70% (95% CI = −72% to −68%). However, we determined that the main conclusions of this meta-analysis are robust, because (1) overall effect sizes for all other indices (germination/emergence, radicle/root length, inflorescences per plant, seeds per plant, root biomass, shoot biomass, and root:shoot ratio) were not sensitive to any given study, (2) overall effect sizes estimated using the Jackknife procedure after excluding influential studies fall within 95% CI of their original overall effect size estimates, and (3) drastic effects of severe water stress on plants are not uncommon, and these effects have caused the resultant change in magnitude of the aforementioned overall effect sizes.

Figure 7. Results from the sensitivity analysis depicting variations in the overall effect size estimates (mean ± 95% confidence intervals [CIs]) of water-stress effects on (A) weed germination/emergence, (B) seedling radicle/root length, (C) plant height, and (D) leaf area when a particular study is omitted from the analysis. The vertical black solid and dashed lines represent overall effect sizes (mean ± 95% CIs) with all studies included.

Figure 8. Results from the sensitivity analysis depicting variations in the overall effect size estimates (mean ± 95% confidence intervals [CIs]) of water-stress effects on (A) branches/tillers per plant, (B) leaves per plant, (C) inflorescences per plant, (D) seeds per plant, (E) total biomass, (F) root biomass, (G) shoot biomass, and (H) root:shoot ratio, when a particular study is omitted from the analysis. The vertical black solid and dashed lines represent overall effect sizes (mean ± 95% CIs) with all studies included.

Lessons Learned, Evidence Gaps, and Future Research Considerations

The meta-analysis based on 1,196 observations from 86 studies accomplished in this study is the first to assess the integral and quantitative response of 102 weed species to water stress. This is also the first study to evaluate the holistic effect of water stress on 12 response variables associated with germination, growth, and seed production of weeds, and concurrently differentiate germination response of grasses and broadleaf weeds. We found a generally negative response of weeds to water stress, and our findings underscore and strengthen the previously held notion that water stress inhibits plant growth and performance. The germination of grass weeds might be slightly more sensitive to water stress compared with broadleaf weeds. Moreover, weed germination is completely inhibited at ψ_solution below −0.8 MPa, and a minimum of −0.09 MPa for grass weeds and −0.32 MPa for broadleaf weeds is required to inhibit their germination by half. Similarly, a minimum of −0.50 MPa is required to reduce seedling radicle length of broadleaf weeds by half. Plant height demonstrates inhibition by about one-fourth, inflorescences per plant by one-third, and seeds per plant by one-half under moderate water stress of 30% to 60% field capacity. In general, weed fecundity was found to be suppressed to a larger degree than growth morphology under water stress. For instance, weed root biomass and shoot biomass were inhibited by 61% to 69%, whereas fecundity (inflorescences per plant and seeds per plant) was inhibited by 76% to 88% under severe water stress of <30% field capacity relative to unstressed (i.e., 100% field capacity) conditions. Our findings that weeds will germinate, survive, grow, and reproduce and will continue to be competitive and problematic in managed agronomic systems, even under intense drought or water-stressed conditions, cannot be ignored. As cropping systems continue to experience extreme weather events more than at any time in the past, future studies should investigate, evaluate, and promote the adoption of multiple diverse strategies aimed at effectively managing weeds under water-limited conditions as an integral component of integrated weed management programs. This study identifies distinctive, adaptive behavior (i.e., ability to compete for water) of weeds that can inform predictions of emergence, survival rate, and possible shifts in weed communities in water-limited regions and periods. Finally, quantitative information gathered in this meta-analysis will be helpful in modeling and/or predicting multidimensional responses of weeds to water stress.

This current meta-analysis identified critical gaps in the existing evidence base and provides directions for reporting data standards and future research avenues:

• Numerous studies included in the meta-analysis lack information on variability within sampling populations such as SD, SE, CV, or LSD. Within-treatment uncertainty statistics are critical for the robust characterization of confidence in reported effect size estimates. Therefore, we ask researchers to report these statistics along with replication size and treatment means in each study. Such a practice will allow reasonable quantitative analysis and information integration.

• Meta-analyses require independence, and quantitative results from the same group of researchers/programs/collaborator networks are treated as a possible source of dependence (Stevens and Taylor Reference Stevens and Taylor2009). Certain authors/research groups might be more likely to find certain results due to their use/preference of specific methodological elements (protocols, populations, experimental environments, instrumentation, etc.) or bias in performing the experiment, analyzing data, or reporting results (Danchev et al. Reference Danchev, Rzhetsky and Evans2019). This can violate the assumption of independence between effect sizes, potentially distorting the results of the meta-analysis (Moulin and Amaral Reference Moulin and Amaral2020). In the present study, we found that two investigators conducted 69% (n = 11 out of 16) of studies in the Philippines, and one of those investigators was involved in 75% (n = 3 out of 4) and 81% (n = 17 out of 21) of studies in Iran and Australia, respectively. In total, one investigator authored or coauthored 41% (n = 36 out of 86) of the studies included. Therefore, there might be a potential source of systemic dependence due to the heavy contribution by a single research group in the meta-analysis. Authorship dependence has been reported to impact effect sizes (Abou-Setta et al. Reference Abou-Setta, Rabbani, Lix, Turgeon, Houston, Fergusson and Zarychanski2019; Moulin and Amaral Reference Moulin and Amaral2020; Singh et al. Reference Singh, Grann and Fazel2013). We therefore encourage the global community of weed scientists to assess water stress effects on weeds (using local water availability regimes, soil types, cropping systems, etc.). This will ultimately help to develop a more diverse, robust, reliable, and conclusive understanding of weeds’ performance, population dynamics, and potential weed shift patterns in an altered climate.

• Water-stress effects on weed seed germination and seedling radicle length were mostly studied using PEG or d-mannitol solutions of varying osmotic potential in petri dishes. However, such solutions may not realistically represent soil–water–seed interactions occurring in fields. As a result, the observed effects on weed seed germination and seedling radicle length with PEG or d-mannitol solutions may not necessarily translate in field conditions. Thus, field-sampled soils are more suitable as test media compared with potting mix or solutions, in the interest of transferability. Supporting this hypothesis, Camacho et al. (Reference Camacho, Heitman, Gannon, Amoozegar and Leion2021) observed varying responses of seed germination of multiple weed and crop species between PEG versus soil test media as well as among different soil textural groups under the same water potentials. They further indicated that total seed germination is better characterized as a function of soil hydraulic conductivity rather than soil water potential. Therefore, if the goal is to test field seed germination with regard to soil moisture availability rather than determining the relative susceptibility of multiple weed or crop species to drought stress, future research should use soils of varying textures as test media.

• A systematic search of studies for meta-analysis identified most of the evidence base toward water-stress effects on weed seed germination/emergence. In total, water-stress effects on weed germination/emergence were investigated in 84% of the studies (i.e., n = 73 out of 86) accounting for 60% of the total observations (i.e., n = 759 out of 1,196) included in this meta-analysis. We only found a few studies that investigated water-stress effects on weed growth characteristics and seed production, thereby limiting detailed quantitative synthesis. For example, data were insufficient to elucidate how water-stress intensity expressed as soil water potential (ψ_soil) will impact weed seed germination, growth morphology, and fecundity under field conditions. We only found four ${{\unicode{x03C8}}_{{\rm{soil}}}}$ -based water-stress studies that resulted in 77 observations for 12 weed indices. Out of 1,196 total observations, 62 studied root and shoot, and 77 studied inflorescence and weed seed production. Roots and shoots are important in assessing the impacts of water stress on plant functioning and seed production is important to assess the ability to reproduce; hence, we suggest conducting more ${{\unicode{x03C8}}_{{\rm{soil}}}}$ -based water-stress studies (Singh et al. Reference Singh, Kukal, Irmak and Jhala2022) for investigating weed growth and especially seed production. Such studies will essentially unveil the relative fitness and adaptability of weeds to water stress compared with crops and will predict weed seedbank size and infestation in water-limited field conditions.

• Seed size and depth of occurrence in soil (burial depth) are the other important factors that influence the relative effect of water stress on weed germination/emergence (Cordeau et al. Reference Cordeau, Wayman, Reibel, Strbik, Chauvel and Guillemin2018; Tanveer et al. Reference Tanveer, Tasneem, Khaliq, Javaid and Chaudhry2013). Larger seeds can have a greater advantage over smaller seeds, as they have higher food reserves, leading to greater emergence, rooting depth, and survival under increasingly dry conditions (Harrison et al. Reference Harrison, Regnier, Schmoll and Harrison2007; Leishman and Westoby Reference Leishman and Westoby1994; Tanveer et al. Reference Tanveer, Tasneem, Khaliq, Javaid and Chaudhry2013). Likewise, weed seeds that are buried deeper can exhibit a considerably greater emergence rate than seeds closer to or on the surface during dry soil conditions (Cordeau et al. Reference Cordeau, Wayman, Reibel, Strbik, Chauvel and Guillemin2018). Although we did not quantitatively address these factors in our meta-analysis due to limited consideration given to these factors in the included studies, exploring the role of covariates such as the average seed size of weed species and their burial depth could be a promising avenue for future research that supplements existing lessons from this meta-analysis.

• Plant functional characteristics such as leaf/root/seed traits govern their differential response to water stress across plant functional groups (Lopez-Iglesias et al. Reference Lopez-Iglesias, Villar and Poorter2014). Leaf traits such as lower specific leaf area (Pérez-Ramos et al. Reference Pérez-Ramos, Volaire, Fattet, Blanchard and Roumet2013), and seed traits such as heavy and rapidly germinating seeds (Merino-Martín et al. Reference Merino-Martín, Courtauld, Commander, Turner, Lewandrowski and Stevens2017) favor drought survival. Similarly, the response of root traits to drought varies among plant functional groups, for example, grass weeds can increase their root diameter and specific root surface area and decrease root tissue density to produce thicker roots for better nutrient and water acquisition, while herbs can decrease their specific root surface area and root length to increase root carbon allocation and water uptake (Lozano et al. Reference Lozano, Aguilar-Trigueros, Flaig and Rillig2019). Essentially, droughts can modify plant communities, species distribution, diversity, and richness (Evans et al. Reference Evans, Byrne, Lauenroth and Burke2011; Garssen et al. Reference Garssen, Verhoeven and Soons2014; Olivares et al. Reference Olivares, Svenning, van Bodegom and Balslev2015), and the magnitude of response to drought is determined by the distribution and composition of plant species and functional groups (Kuiper et al. Reference Kuiper, Mooij, Bragazza and Robroek2014; Zweifel et al. Reference Zweifel, Rigling and Dobbertin2009). It is therefore important to highlight, acknowledge, and understand the role of plant functional traits and their intraspecific as well as interspecific variations in mediating drought response. The existing literature has limited data on water-stress response of broadleaf versus grass weeds to their below- and aboveground growth characteristics as well as seed production. Future studies in weed science research communities should prioritize understanding the differential response of these general weed types to water-stress gradients. This is needed for the accurate and robust characterization of the shift patterns among weed species/functional groups in water-limited environments, thereby enabling us to design effective weed management programs for sustainable farming.