Cotton (Gossypium hirsutum L.) is an economically important crop widely planted in many parts of the world, and China is the world’s largest cotton producer (Li et al., 2021). In 2021, the cotton planting area in China reached 3.03 million hectares (National Bureau of Statistics, 2021). Weeds are one of the most important factors restricting development of the cotton industry (Wang et al., 2015). The incidence of weed damage in cotton fields can be up to 56%, causing annual yield losses of 14−16% (Li et al., 2019). Weeds are an important source of biological stress for cotton, competing for light, nutrients, water and space, and this stress occurs throughout the growth period (Vulchi et al., 2022). Weeds compete with cotton, and they cast shade, severely inhibiting growth, resulting in short cotton plants, yellow leaves, and even buds falling off, thus reducing cotton yield (Wang et al., 2015). In order avoid damage caused by weeds, it is necessary to clarify the competitive effects weeds have on cotton plants in the field.

Weed stress factors affecting cotton growth include species, density, competition period, and spatial distribution (Wang et al., 2015). Numerous studies have reported competition between weeds and cotton, examples of which include giant ragweed (Ambrosia trifida), goosegrass (Eleusine indica), redroot pigweed (Amaranthus retroflexus), velvetleaf (Abutilon theophrasti), and black nightshade (Solanum nigrum). (Barnett and Steckel, 2013; Ma et al., 2015a; Ma et al., 2015b; Ma et al., 2016; Zhang et al., 2022a). These studies provided insight into competition between weeds and cotton flowers at the population level. Thus, if weeds emerge earlier or at the same time as cotton seedlings, and the growth rate of the aboveground canopy of weeds exceeds that of cotton, shading will occur, and cotton will be at a competitive disadvantage, which will eventually lead to a decline in cotton yield (Ma et al., 2015b; Ma et al., 2016). Conversely, if weeds emerge later, or if they are small and adopt a creeping form, they will be shaded by cotton, and the competitiveness of the weeds is relatively weak (Ma et al., 2015a). In addition, weeds that emerge simultaneously with or earlier than cotton are more competitive than those that emerge later in the cotton growth cycle, causing greater losses in cotton yield (Barnett and Steckel, 2013). By studying the competitive effects between weeds and cotton, the critical period and density of weed control were determined, as was the control threshold (Charles et al., 2019a; Charles et al., 2019b; Charles et al., 2020; Zhang et al., 2022a). However, there are few reports on how cotton responds to weed stress from the perspective of weed and cotton physiology (Zhang et al., 2022a). Therefore, analysing the stress effects exerted by weeds on cotton from the perspective of physiological metabolism could help to further clarify the competition mechanism between weeds and cotton.

Defence responses of plants are triggered by various biotic stresses (Steinbrenner et al., 2022). In these complex stress response systems, basic metabolism, specialized metabolism and protective enzymes are known to play important roles (Ling et al., 2022; Swinnen et al., 2022). For example, in the face of attack by most herbivorous insects, plant protective enzyme activities and basal metabolic capacity are enhanced, effectively improving tolerance and reducing damage by herbivorous insects (Castano-Duque et al., 2018; Gao et al., 2022). Although weeds cannot induce defence responses of plants through direct damage, they can stimulate defence systems through allelopathy (Li et al., 2022a). Allelopathy affects plant cell structure, photosynthesis, respiration, hormone content, receptor enzyme activity and gene expression by releasing chemicals (Kato-Noguchi et al., 2017; Qin et al., 2018; Siyar et al., 2019; Mardani et al., 2019). Allelopathic plants reduce interference of allelopathy by regulating related tissue structure, basal metabolism, secondary metabolism, protective enzyme activity and other defensive measures (Šoln et al., 2022). Especially in plant communities with population competition, plants reduce the adverse effects of allelopathy on competition through defensive responses (Uesugi et al., 2019; Chia and Bittencourt-Oliveira, 2021). Competition between weeds and plants is obviously related to this process. However, when studying competition between weeds and plants, the competition mechanism from the perspective of plant defence metabolism is not usually investigated, even though it could be useful.

Scirpus planiculmis Fr. Schmidt is a perennial herb of the family Cyperaceae. In China, Korea, Japan, and other countries and regions, S. planiculmis is a common malignant weed in rice fields (Lee et al., 1991; Choi et al., 2000a). In Europe, it is considered to be a potential plant for widespread expansion (Hroudová et al., 2014). Previous studies have determined the economic threshold of S. planiculmis in rice fields (Kwon et al., 2008; Kwon et al., 2011) and screened chemical control agents (Choi et al., 2000b; Hwang et al., 2006; Park et al., 2011). However, with changes in cropping systems and cultivation patterns, S. planiculmis has become a major weed in cotton fields in Xinjiang and the Yellow River basin in China (Fang et al., 2016; Huang et al., 2020), leading to cotton yield reductions of 1.86−61.50% (Cai et al., 2020a). Due to the diversification of the breeding methods for S. planiculmis, plants can be sexually propagated through seeds and asexually propagated through bulbs (Yang et al., 2021; Zhang et al., 2022b). Strong saline-alkaline tolerance (Hroudová et al., 2014; An et al., 2021) and plastic film mulching in cotton fields provides a suitable growth environment (Hroudová et al., 2007; Liu et al., 2016), resulting in a density of up to 1.5×10⁶ S. planiculmis plants per hm² in cotton fields in Xinjiang, which severely inhibits nutrient uptake and growth of cotton (Cai et al., 2020a; Cai et al., 2021).

Compared with C₃ plants (cotton), C₄ plants (S. planiculmis) are better at absorbing CO₂, water, nutrients, and light (Li and Yang, 2020). Therefore, during competition between the two, cotton is weaker than S. planiculmis. Our previous results showed that plant height, stem diameter, number of nodes on the main stem, boll number, and fiber quality of cotton were reduced with continuous extension of damage time and increase in the density of S. planiculmis, which eventually led to a decrease in cotton yield (Cai et al., 2020b; Cai et al., 2021). During the cotton flowering and boll-forming stage, S. planiculmis reduced potassium and soluble sugar content in cotton leaves under different population densities, and inhibited the accumulation of nutrients in cotton (Cai et al., 2020a). Further studies showed that the rhizosphere exudates of S. planiculmis reduced the content of cytokinin and auxin in cotton leaves, and significantly inhibited the growth of underground parts of cotton, including main root length, lateral root length and fresh weight (Cai, 2020c). However, how cotton responds to S. planiculmis stress through physiological metabolism has not been reported.

In this study, we conducted a 2-year experiment in 2019−2020, and these were field experiment. (1) Different densities of S. planiculmis were established, and the effects on the growth and physiological metabolism of cotton were measured at different stages (seedling, budding, flowering and boll); (2) the competition period of S. planiculmis and cotton was controlled to determine the effects of S. planiculmis on the growth and physiological metabolism of cotton; (3) by changing the distribution distance between S. planiculmis and cotton, effects on the growth and physiological metabolism of S. planiculmis on cotton were determined; (4) the physiological metabolic responses of cotton to S. planiculmis were further explored under herbicide mediation. The results illuminate the physiological response mechanism of cotton to S. planiculmis from the perspective of plant defences.

The results of our study showed that with different stress patterns (density, competition period, and spatial distribution) of S. planiculmis, protective enzymes were involved in the response of cotton to S. planiculmis ( Figures 2 , 6 and 9 ). Compared with SOD and POD, cotton increase CAT activity under different stress patterns of S. planiculmis ( Figure 2 , 6 and 9 ). The protective enzyme activities of cotton and S. planiculmis were increased under low dose of herbicides ( Figure 14 , 15 ). Due to the lethal effect of herbicides on S. planiculmis, this will help cotton to further improve the protective enzyme activity to reduce the stress of S. planiculmis.

Cotton is an economically important crop that is very sensitive to weed competition (Wang et al., 2015). In the field, cotton growth and development are easily disrupted by weeds, resulting in dwarf plants, chlorosis, and even a decrease in the number of flowers and bolls, ultimately resulting in reduced cotton yield (Ma et al., 2016). Different weed species exert different interference effects on cotton. Herein, based on the literature, we classified weeds that compete with cotton into three categories according to plant type: low creeping weeds, comprising short plants have a poor shading effect on cotton, including goosegrass (Ma et al., 2015a); weeds with stout stems and lush branches and leaves that have a great influence on the photosynthetic utilization and morphological characterization of cotton, including Solanum nigrum (Zhang et al., 2022a), giant ragweed (Barnett and Steckel, 2013), redroot pigweed (Ma et al., 2015b) and velvetleaf (Ma et al., 2016); and intermediate weeds with a form between stout and tall plants and low creeping plants. Although members of this third class are taller, their stems are slender, and their shading effect on cotton is stronger than that of low creeping and weaker than that of stout and tall classes. S. planiculmis belongs to this third class (Cai et al., 2020a). Studies have shown that goosegrass in cotton fields does not significantly reduce the plant height of cotton, and at the maximum goosegrass density, the stem diameter and number of sympodial branches of cotton were only reduced by 6% compared with controls (Ma et al., 2015a), indicating that goosegrass had minimal effect on the morphological characteristics of cotton. However, in cotton fields containing S. nigrum (Zhang et al., 2022a), giant ragweed (Barnett and Steckel, 2013), redroot pigweed (Ma et al., 2015b) and velvetleaf (Ma et al., 2016), plant height and stem diameter of cotton were significantly affected, and plant height and stem diameter decreased regularly with increasing weed density. For example, for rows containing an extra velvetleaf plant, cotton plant height and stem diameter decreased by 5 cm and 2 mm, respectively (Ma et al., 2016). Similarly, plant height and stem diameter decreased by 2−13 cm and 2 mm, respectively, for each additional redroot pigweed per row (Ma et al., 2015b). This is due to the strong light interception effect of tall and stout weeds on cotton (Scott et al., 2000), which affects morphological characterization. Our results showed that S. planiculmis significantly reduced plant height and stem diameter of cotton only under certain density conditions (≥20 plants m^-2) at budding, and flowering and boll stages ( Figure 2 ). This indicates that under low density conditions (<20 plants m^-2), S. planiculmis does not affect the upper morphological characteristics of cotton. In addition, we found that the chlorophyll content of cotton decreased with increasing density of S. planiculmis at the flowering and boll stage of cotton, but the chlorophyll content of cotton increased under high density of S. planiculmis (80−100 plants m^-2) at the seedling stage and bud stage of cotton ( Figure 2 ). This indicates that the effect of light interception of S. planiculmis in the cotton flowering and boll stage was stronger than that in the seedling stage and budding stage, while in the seedling stage and bud stage, a high density of S. planiculmis stimulated cotton photosynthesis and increased the chlorophyll content of the crop.

In general, weed damage can affect the entire growth and development cycle of cotton, and long-term persistent stress has an adverse effect on cotton morphogenesis (Charles and Taylor, 2021). Long-term competition by weeds for light and nutrient resources has reduces the ability of cotton to absorb and utilise nutrients, manifesting as slender, short plants (Wang et al., 2015). For example, following continuous damage to cotton by S. nigrum, levels of phosphorus, potassium, soluble protein and soluble sugar were reduced, and plant height and stem diameter of cotton were decreased with increasing stress duration (Han, 2021). Similarly, during competition between S. planiculmis and cotton, the content of potassium and soluble sugar in cotton decreased with prolongation of the competition period, which was not conducive to the absorption and accumulation of nutrients by cotton, resulting in decreased plant height and stem diameter of cotton (Cai, 2020c). In addition, after continuous competition of sunflower, Japanese millet and mungbean as mimic weeds with cotton, plant height and biomass of cotton were reduced to varying degrees, which had a negative effect on the vegetative growth of cotton (Charles et al., 2019a; Charles et al., 2019b; Charles et al., 2020). In the present study, we found that when S. planiculmis was planted in April, there was no significant change in plant height, stem diameter, fresh weight, root length or chlorophyll content of cotton with prolongation of the competition period between S. planiculmis and cotton ( Figure 6 ). However, when S. planiculmis was planted in May and June, plant height, stem diameter, fresh weight, root length and chlorophyll content of cotton decreased with continuation of the competition period ( Figure 6 ). This shows that S. planiculmis had little effect on the growth and photosynthesis of cotton in the early stages, but it had a great effect on the morphogenesis and photosynthesis of cotton in the middle and later stages. Additionally, we also found that when S. planiculmis and cotton competition period for 30 days, the soluble protein content in cotton was lowest, and after 60 days the soluble protein content in cotton gradually returned to normal levels ( Figure 6 ). This may be due to long-term competition between cotton and S. planiculmis. Cotton enhances the absorption and accumulation of nitrate nitrogen, and converts nitrate nitrogen into soluble protein after assimilation in the body (Marschner, 2012; Cai, 2020c).

Most previous studies ignored the mediating effect of allelopathy on competition between cotton and weeds, but allelopathy of weeds is an objective inducing factor that affects the growth and development of cotton (Wang et al., 2022). For example, extracts of Medicago sativa, Flaveria bidentis, and Eclipta prostrata can inhibit the germination rate and radicle growth of cotton (Yuan et al., 2008; Peng et al., 2011; Yang et al., 2015). Our previous studies have shown that root exudates of S. planiculmis have no significant effect on the growth of above-ground parts of cotton, but can significantly inhibit the growth of underground parts, and high concentrations can reduce the content of cytokinin and auxin in cotton (Cai, 2020c). This indicates that allelopathy of S. planiculmis affected the growth and development of cotton by inhibiting the synthesis of cytokinin and auxin. It is worth noting that interference of root exudates of S. planiculmis on cotton is correlated with concentration, and therefore the strength of allelopathy (Li et al., 2020).

Interference of allelopathy on plants is related to the distance between plants (Li et al., 2022b). In this study, we analysed this effect by changing the distance between S. planiculmis and cotton. We found that plant height and stem diameter of cotton did not change significantly under different distance conditions, indicating that allelopathy of S. planiculmis did not affect the growth of aboveground parts of cotton, consistent with our indoor research results (Cai, 2020c). However, the difference is that fresh weight and root length of cotton did not change significantly with a difference in distribution distance ( Figure 9 ), which may be due to the low density of flat stalks (eight plants) in this study. The allelopathic intensity of S. planiculmis was weak at this density, which could not significantly inhibit fresh weight and root length of cotton, while S. planiculmis at higher density (≥20 plants/m²) caused significant inhibition. This also indicates that allelochemicals of S. planiculmis can inhibit growth of the underground parts of cotton at high concentrations (Cai, 2020c).

Under allelochemical stress, receptor plants can alleviate damage caused by reactive oxygen species (ROS) by altering enzyme activity via a self-protective mechanism (Li et al., 2022a). Recent studies have shown that the allelopathic effect of Artemisia argyi water extract (AAWE) can lead to membrane system rupture and ROS release in rice, and under low AAWE concentration, an increase in SOD activity can compensate for a decrease in CAT and POD activities, which ensures that ROS levels remain relatively stable. However, with increasing AAWE concentration, POD and CAT activities continued to decrease, and MDA content increased, eventually leading to ROS imbalance (Li et al., 2022a). Our study showed that under different densities of S. planiculmis, SOD activity of cotton was decreased at each period, while CAT activity was increased, and POD activity was increased in the budding stage and the flowering and boll stage, whereas MDA content was decreased significantly in the flowering and boll stage ( Figure 2 ). This shows that POD and CAT are protective enzymes that play a major role in cotton in response to the density effect of S. planiculmis. Only CAT activity showed an increasing trend with prolongation of competition period ( Figure 6 ). This indicates that CAT in cotton plays a continuous role in resisting S. planiculmis under long-term competition. Regarding the distribution of S. planiculmis, only CAT activity was increased at different distance ranges ( Figure 9 ). Therefore, we believe that an increase in CAT activity plays a key role in the responses of cotton to S. planiculmis stress.

We further explored the effects of herbicide application on physiological metabolism of S. planiculmis and cotton, and found that protective enzyme activities of both species were improved after herbicide application ( Figure 14 and 15 ). The herbicide used in this study was selective and had a good control effect on S. planiculmis (Peng et al., 2012; Tian and Yu, 2020). Although the protective enzyme activity of S. planiculmis increased after application of low doses of herbicide, this is a normal response of weeds to herbicides (Peng et al., 2013a; Peng et al., 2013b). However, herbicides do not kill cotton; rather, they only stimulate protective enzyme activities (Peng et al., 2013a; Peng et al., 2013b). This indicates that during interaction between cotton and S. planiculmis, herbicides may gradually weaken the stress advantage of S. planiculmis, enhance the self-protection ability of cotton, and improve the resistance of cotton to S. planiculmis. Therefore, we believe that when controlling S. planiculmis, application of low-dose selective herbicides may help cotton to resist stress caused by weeds. Our previous studies showed that application of low-dose insecticides, fungicides and plant growth regulators can increase protective enzyme activities of cotton, and help resist pest-associated stress (Zhang et al., 2022c; Zhang et al., 2022d; Zhang et al., 2022e). Clearly, a low-dose pesticide can stimulate the defence response of cotton. On the one hand, pesticides can directly act on pests, and on the other hand, they can enhance the defence ability of cotton against pests, and resistance to pests is promoted through this dual role. This knowledge helps to explain pest resistance, and supports the sustainable development of agricultural production management.

It cannot be ignored that intraspecific competition of weeds is always accompanied by interactions between weeds and cotton (Wang et al., 2015). In previous reports, it was confirmed that during competition between cotton and giant ragweed (Barnett and Steckel, 2013), goosegrass (Ma et al., 2015a), redroot pigweed (Ma et al., 2015b) or velvetleaf (Ma et al., 2016), intraspecific competition of weeds led to a decrease in the biomass of individual plants. Specifically, the stem diameter of redroot pigweed and velvetleaf decreased with increasing density, while plant height increased (Ma et al., 2015b; Ma et al., 2016). We found that with increasing population density, plant height of S. planiculmis tended to increase, while stem diameter showed a downward trend, and fresh weight and root length first increased then decreased ( Figure 1 ). Moreover, S. planiculmis responds to intraspecific competition by regulating photosynthesis, soluble protein synthesis, and protective enzyme activities, via a self-regulatory strategy characteristic of weed populations (Li et al., 2022b).

Weeds ultimately impair cotton yield by altering cotton plant morphology and nutrient metabolism (Wang et al., 2015). In this study, with increasing S. planiculmis density, cotton boll number, single boll weight and yield were diminished ( Figure 4 ). In addition, continuous damage by S. planiculmis also led to a decrease in boll number, single boll weight and yield ( Figure 8 ). However, with increasing distance between S. planiculmis and cotton, boll number, single boll weight and yield were less affected ( Figure 11 ). Using equation fitting, we previously determined that the control threshold of S. planiculmis in cotton fields was 20 plants m^-2, and the duration of damage should be less than 30 days (Cai et al., 2020a, 2021). Therefore, we suggest that S. planiculmis should be controlled in the field to avoid significant loss of cotton yield.

Moreover, temperature and rainfall are considered to be important climatic factors affecting plant growth (Khan et al., 2022; Kharivha et al., 2022). In order to adapt to abiotic stresses such as drought, high temperature and low temperature, plants also reduce the impact of climatic conditions on themselves by regulating defense responses (Akpinar and Cansev, 2022; Vital et al., 2022). Obviously, when studying the stress of weeds on cotton in the field, extreme climate may have an impact on cotton response to weed stress. Therefore, climate factors need to be included in this consideration. We documented changes in temperature and precipitation during the cotton growing stage (seedling to boll opening) from 2019 to 2020 ( Figure S1 ). These climatic data reflect the characteristics of large temperature difference between day and night and low rainfall in the test area, which belongs to the typical temperate continental arid climate. Temperature difference led to the acceleration of plant growth and the occurrence of phenological events (Talukder et al., 2014; Lymperopoulos et al., 2018). To adapt to the environment with large temperature difference, cotton improves the adaptability to temperature difference by regulating photosynthesis, respiration and physiological metabolism (Wang et al., 2016, 2018). In order to reduce the effect of temperature difference on cotton, we chose plastic film mulching to maintain soil temperature. Especially in low temperature conditions, the film has played a role in raising soil temperature, which can reduce the temperature difference on cotton. Studies have shown that with the strengthening of drought stress, cotton plant height, stem diameter and other morphological indicators decreased, and by regulating photosynthesis and protective enzyme activity in a sustained manner to respond strongly to drought (Tian et al., 2017). In this study, we made up for the lack of rainfall and avoided the occurrence of drought stress by drip irrigation. In addition, the film played a good role in moisture conservation. Therefore, we think that the effects of climatic factors on cotton growth and physiology can be reduced by applying reasonable cultivation and management measures.

In summary, using S. planiculmis and cotton as an example, we analysed the stress effect of S. planiculmis on cotton under different densities, competition periods and distribution conditions, from the perspective of morphogenesis, physiological metabolism and yield, and revealed the defence pathway of cotton in response to S. planiculmis that involves regulation of protective enzyme activities. We concluded that CAT plays a key role in protecting enzymes involved in defence responses. On this basis, we further explored the effect of low-dose herbicide on the interaction between cotton and S. planiculmis. The results showed that low-dose herbicide could help cotton resist stress caused by S. planiculmis by elevating the activities of protective enzymes in cotton. Therefore, we believed that when attempting to control S. planiculmis, low doses of herbicide may protect cotton and control weeds. This strategy is also applicable to the control of other pests. In the future, we will further explore the response strategy of cotton to S. planiculmis from the perspective of gene regulation, and thereby obtain a deeper explanation for the stress mechanism of cotton in response to S. planiculmis.

The raw data of the presented results of this study are available on request to the corresponding author.

QZ wrote the manuscript; JW designed the study; JP conducted experiments; QZ and JP analyzed data. All authors contributed to the article and approved the submitted version.

This research was supported by the National Natural Science Foundation of China (Grant No. 31260435, 31660519), and the Xinjiang Corps Major Science and Technology Project (Grant No. 2018AA06-02).

We thank reviewers and editors for valuable comments and suggestions on the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.