Polygonum cuspidatum, a perennial shrub-like herbaceous medicinal plant, is widely distributed across East Asia, covering China, Japan, and the Korean Peninsula (Ye et al., 2021; Sun et al., 2022). In China, it is predominantly found in the eastern, central, southern, and southwestern regions (Chen et al., 2020). It exhibits strong environmental adaptability, capable of growing in various soil types. The plants contain several pharmacologically active chemical compounds, including: (i) anthraquinone derivatives such as emodin, chrysophanol, and physcion, which exhibit antibacterial, anti-inflammatory, and antioxidant properties; (ii) stilbenes, like resveratrol, known for their antioxidant, anticancer, and anti-inflammatory effects; (iii) flavonoids, including quercetin and kaempferol, which possess antioxidant, anti-inflammatory, and immunomodulatory activities; and (iv) polysaccharides, demonstrating immunomodulatory and antitumor properties (Zhou et al., 2003; Shan et al., 2008; Zhang et al., 2013; Lee et al., 2015; Ke et al., 2023; Lai et al., 2024). P. cuspidatum extracts, particularly for resveratrol, are widely used in dietary supplements, but they lack formal GRAS (Generally recognized as safe) status from the US FDA (https://www.fda.gov/food/gras-notice-inventory). Regulatory agencies such as EFSA (https://www.efsa.europa.eu/) and WHO (https://www.fao.org/food/food-safety-quality/scientific-advice/jecfa/en/) have not established a specific acceptable daily intake for P. cuspidatum; however, resveratrol doses up to 1500 mg/day have been used in clinical studies without significant adverse effects based on EFSA. Residual components in P. cuspidatum possess certain pharmacological and physiological properties (Tian et al., 2012).

The escalating demand for the medicinal components of P. cuspidatum has led to a substantial increase in its cultivated area and production. After the extraction of active chemical compounds, the resulting P. cuspidatum residues (PRs) are typically treated as waste. Nevertheless, PRs still contain nutritional components (e.g., fiber, residual sugars, and minerals), chemical compounds (e.g., various secondary metabolites and other bioactive substances that were not fully extracted), and cellular materials (plant cell walls and other structural components) (Zhang and Liu, 2015; Zhou et al., 2018). As a result, recognizing PRs as a potential organic resource rather than just waste is crucial for the sustainable and economically viable processing of P. cuspidatum. PRs hold considerable potential for further utilization (Zhou et al., 2018). For instance, employing P. cuspidatum residues as a substitute for traditional substrates in the cultivation of Pleurotus eryngii not only mitigates the environmental and air pollution caused by the indiscriminate disposal and incineration of the residues but also reduces the production costs of the mushrooms, thereby enhancing overall economic efficiency (Chang et al., 2019; Sun et al., 2021). However, this effect was dependent on the proportion of P. cuspidatum residues used, with the most prominent effect achieved by a treatment of 20% P. cuspidatum residues in the cultivation substrate (Chang et al., 2019).

Organic amendments, such as compost, manure, biochar, and plant residues, are integral to sustainable agriculture (Matisic et al., 2024; Xu et al., 2024). They contribute to enhancing soil health, improving crop productivity, and reducing environmental impacts. Organic amendments serve as a food source for soil microorganisms, promoting microbial diversity and activity (Shu et al., 2022). This, in turn, enhances nutrient cycling, organic matter decomposition, and the production of plant growth-promoting substances (Chaudhari et al., 2021; Liang et al., 2025). In fact, P. cuspidatum residues are rich in cellulose and various mineral elements such as nitrogen, phosphorus, and potassium (Zhang and Liu, 2015), suggesting their potential as a soil amendment and organic fertilizer. Incorporating P. cuspidatum residues into the soil is reasonable to expect improvements in soil structure, increased soil fertility, and promotion of plant growth.

Nevertheless, there is no direct evidence indicating that returning the residue of P. cuspidatum rhizomes after active ingredient extraction to the field has significant benefits for the plant growth. If PRs can enhance the growth and bioactive compound production of P. cuspidatum plants, this approach would simultaneously reduce fertilizer usage and lower the costly waste disposal expenses and environmental risks associated with PRs. The objective of this study was to analyze the effects of P. cuspidatum residues returning to field on the growth, active compound (polydatin, resveratrol, and emodin) levels, and soil structure of P. cuspidatum plants, so as to provide technical guidance for the subsequent comprehensive utilization of P. cuspidatum residues.

In this study, the application of PR treatments had demonstrated significant enhancements in various plant growth parameters. This finding aligns with previous studies highlighting the positive impact of organic amendments on plant growth and biomass production (Agegnehu et al., 2016; Wan et al., 2024). However, the PR treatments dramatically altered plant growth performance and biomass production, depending on used doses of PRs. Among them, the PR2500 treatment exhibited the superior performance than other PR treatments and compound fertilizer treatment, which could be attributed to the optimal balance of nutrients and organic matter provided by this dose, which likely facilitated robust root development and nutrient uptake (Schulz et al., 2013; Dai et al., 2020). This also suggested that the PR2500 treatment not only enhances aboveground biomass but also significantly promotes root development, which is crucial for overall plant health and resilience. However, the PR4000 (high-dose) treatment also promoted growth, albeit to a lesser extent, indicating that higher doses of PRs may not necessarily induce to proportional increases in plant growth, possibly due to nutrient saturation, potential phytotoxicity from excessive organic matter or secondary metabolites, and allelopathic effects (Zhang et al., 2021).

The SPAD value is a reliable indicator of leaf chlorophyll content, which is directly related to photosynthetic capacity and plant health (Markwell et al., 1995). The PR1500 and PR2500 treatments significantly increased the SPAD value, compared to the CK treatment, suggesting that these treatments effectively enhanced chlorophyll content, potentially improving the plant’s photosynthetic efficiency (Gitelson et al., 2002). In contrast, the PR4000 treatment did not significantly affect the SPAD value, indicating a potential saturation or inhibitory effect at higher application rates, which warrants further investigation.

Leaf gas exchange parameters are critical for assessing plant photosynthetic performance and water-use efficiency (Deng et al., 2024). The low-dose PR (PR1500) treatment also showed significant improvements in the net photosynthetic rate, transpiration rate, and intercellular CO₂ concentration, and the moderate-dose PR (PR2500) treatment exhibited the most pronounced effects on the net photosynthetic rate, transpiration rate, intercellular CO₂ concentration, and stomatal conductance. Among all treatments, the PR2500 treatment showed the best results and was significantly superior to the compound fertilizer treatment. This indicated that PR2500 can serve as an alternative to chemical fertilizers for enhancing the growth and improving leaf gas exchange in P. cuspidatum. The change in gas exchange by PR2500 treatment aligned with the enhanced growth parameters observed in this treatment, indicating a strong correlation between physiological processes and growth outcomes (Dai et al., 2020; Cao et al., 2023). This suggested that a suitable dose of PRs can enhance photosynthetic capacity, possibly through the release of organic matter and nutrients (Chan et al., 2007). Further research is needed to clarify changes in leaf photosynthetic efficiency after PRs treatment through morphological observations (such as stomatal density) and chlorophyll fluorescence parameters. In contrast, the PR4000 treatment did not significantly alter any of the leaf gas exchange variables. This could be due to a potential inhibitory effect at higher application rates, where an excess of nutrients or organic matter may interfere with physiological processes (Zhang et al., 2021).

IAA, a primary auxin, promotes cell division in meristematic tissues, such as root apical meristems, and also influences the differentiation of cells into specialized tissue, such vascular tissues, which plays a role in root development (Woodward and Bartel, 2005; Liu et al., 2023, 2024). ZR, as a cytokinin, promotes cell division by stimulating the transition from the G2 phase to the M phase of the cell cycle, often in coordination with auxins (Mok and Mok, 2001). The PR1500, PR2500, and PR4000 treatments all led to significant increases in root IAA levels, compared to the CK treatment. IAA is a key auxin that promotes cell elongation and is essential for root development and architecture (Roychoudhry and Kepinski, 2022; Rong et al., 2023). The substantial increase in IAA levels, particularly under the PR2500 treatment, suggests that this treatment would boost root growth and development by stimulating auxin biosynthesis or reducing its degradation (Ljung et al., 2005). Similarly, the PR1500, PR2500, and PR4000 treatments also significantly boosted root ZR levels, respectively. ZR is a cytokinin that plays a vital role in cell division, shoot development, and delaying senescence (Sakakibara, 2006). The significant increase in ZR levels, especially under the PR2500 treatment, indicated that this treatment enhanced shoot growth and overall plant vigor by promoting cell division. The application of PR2500 resulted in the most significant increases in both IAA and ZR levels in roots, which aligned with the superior growth performance observed under this treatment. Root IAA and ZR levels were collectively significantly positively correlated with root biomass, suggesting that these hormones enhanced root growth, leading to improved nutrient and water uptake and overall plant vigor. This also suggested that PR amendments can enhance the levels of IAA and ZR in the roots, which synergistically accelerate the production of root biomass. The results underscores the potential of organic amendments like HRs to enhance plant hormonal levels, thereby promoting plant growth and development (Batista-Silva et al., 2024). Emodin, in vitro, was found to inhibit the growth of a small amount of soil bacteria and fungal species (Singh et al., 1992), while resveratrol appears to have no significant effect on soil microbial populations (Sohn et al., 2015). Therefore, PRs may indirectly enhance auxin and cytokinin levels by improving soil nutrient availability or modulating enzyme activities in hormone biosynthesis pathways, not microbial activity, which requires further experimental validation. As a result, PRs, especially PR2500, can be a viable alternative to traditional fertilizers, offering both hormonal and growth benefits. In this study, the application of PR treatments significantly affected the levels of key bioactive compounds and resveratrol-associated gene expression in roots of P. cuspidatum plants. The PR1500, PR2500, and PR4000 treatments led to significant increases in root polydatin levels, respectively, compared to the CK treatment. Polydatin, a resveratrol glucoside, is known for its antioxidant and anti-inflammatory properties (Du et al., 2013). The increase in polydatin levels in roots suggests that these treatments could elevate the production of the active component, which also potentially enhance the plant’s defense mechanisms against oxidative stress and pathogens (Li et al., 2021; Imtiyaz et al., 2024). Similarly, these treatments significantly increased root emodin levels, respectively, with PR2500 showing the most prominent effect. Emodin is a natural anthraquinone with potential therapeutic properties, including anti-inflammatory and anticancer activities (Dong et al., 2016). The substantial increase in emodin levels, particularly under the PR2500 treatment, indicates that this treatment could enhance the plant’s emodin synthesis, potentially leading to improved medicinal properties (Izhaki, 2002). Generally, as the rhizomes of P. cuspidatum increase in size, the proportion of secondary xylem and secondary phloem also increases (Liu and Hu, 2001). Resveratrol and its glycosides are primarily concentrated in the pith, secondary phloem, and periderm of rhizomes, while emodin is mainly distributed in the secondary xylem of roots (Liu et al., 2015). Since fertilization and various PR treatments significantly increased root biomass, this also implies that the levels of major active components, such as polydatin, emodin, and resveratrol, would increase accordingly.

Resveratrol (3,5,4’-trihydroxystilbene) is a naturally occurring stilbenoid that has attracted significant attention due to its wide range of health benefits and therapeutic potential (Baur and Sinclair, 2006). It is commonly found in various plant species, with P. cuspidatum being one of the richest sources of this compound (Baur and Sinclair, 2006; Deng et al., 2024). Resveratrol’s potential health benefits have led to its use in dietary supplements, functional foods, and pharmaceuticals (Ke et al., 2023). In the present study, PR1500 and PR2500 treatments significantly elevated root resveratrol levels, respectively, compared to the CK treatment. Moreover, the PR2500 treatment exhibited a significantly greater enhancement in the levels of the three active components in P. cuspidatum roots than the chemical fertilizer treatment. This strongly suggests that the PR2500 treatment can serve as a promising viable alternative to chemical fertilizers for field production of P. cuspidatum. Nevertheless, the lack of significant change in resveratrol levels after the application of PR4000 suggested that there may be a complex relationship between the PR4000 treatment and resveratrol metabolism. It could be that PR4000 treatment did not have a significant impact on resveratrol synthesis or accumulation, because excessive application of organic residues could cause microbial imbalance, nutrient immobilization, or root toxicity (Turmel et al., 2015). There was a significantly positive correlation between resveratrol levels and PcRS gene expression in roots, which provides a mechanistic insight into the observed increase in resveratrol levels. Since the RS is a key enzyme in the biosynthesis of resveratrol and its glycosylated form polydatin (Jeandet et al., 2010), the up-regulation of PcRS gene, especially under the PR2500 treatment, is likely to be directly related to the increased resveratrol and its derivative production. This up-regulation likely drives the metabolic flux towards the stilbene biosynthesis pathway, resulting in higher resveratrol accumulation. The observed increase in resveratrol levels under PR treatments highlights the potential of these strategies to optimize the production of bioactive compounds in P. cuspidatum, which could have significant implications for the commercial production in P. cuspidatum.

Nevertheless, the present study also observed that PR1500, PR2500, and PR4000 treatments significantly suppressed the expression levels of the PcPKS1 gene in roots. It is documented that RS can compete with PKS for the same substrates, p-coumaroyl-CoA and malonyl-CoA. In this process, PKS catalyzes the reaction of the butenone-intermediate molecule at the C6 and C1 positions to form naringenin chalcone, while RS catalyzes the reaction of the butenone-intermediate molecule at the C7 and C2 positions to form resveratrol (Liu, 2012). Root resveratrol levels were significantly negatively correlated with root PcPKS1 expression. The suppression of PcPKS1 under PRs treatments may indicate a shift in the plant’s metabolic pathways towards the production of resveratrol, potentially at the expense of other secondary metabolites (e.g., naringenin chalcone) (Verpoorte and Memelink, 2002), although this was not demonstrated in this study. The observed suppression of PcPKS1 and the concomitant increase in resveratrol levels under PR treatments potentially suggest that these interventions redirect metabolic flux towards stilbene biosynthesis. Environmental and biochemical factors can influence the partitioning of metabolic resources in plants (Verpoorte and Memelink, 2002). This underscored the intricate balance between competing biosynthetic pathways in P. cuspidatum.

The application of PR2500 emerged as the most effective treatment for enhancing plant growth, biomass production, photosynthetic activities, and root active component levels in P. cuspidatum. The improvement in root biomass was associated with the elevation of root IAA and ZR levels, and the increase in root resveratrol levels was associated with the up-regulation of root PcRS gene expression. An increase in the application rate of PRs from PR2500 to PR4000 led to a reduction in the physiological activity, growth, and active component levels in P. cuspidatum. It suggests that organic amendments like PRs can be a viable alternative to traditional fertilizers, offering both growth promotion and a shift in metabolic pathways towards the production of beneficial compounds like resveratrol. Therefore, the PR2500 treatment is recommended for the cultivation of P. cuspidatum. The application of PRs as fertilizer could reduce waste management costs while providing a low-cost organic amendment. In addition, PRs decrease input costs for farmers. However, processing (e.g., composting, pelletizing) and transportation costs of PRs must be factored in for large-scale use. PR’s nutrient content may vary, necessitating standardization. Slow nutrient release of PRs may not meet high-demand crop needs without supplementation. Long-term field studies are needed to assess whether PR application leads to cumulative benefits or potential negative consequences such as nutrient imbalances, soil microbial communities, and organic matter accumulation.