Low concentration of Paecilomyces variotii extracts promote the growth and disease resistance of pepper

Abstract

Paecilomyces variotii extract “ZhiNengCong” (ZNC) has been confirmed to be a high activity plant immune inducer, but its application effect and appropriate concentration in different crops and different environmental conditions need to be further verified. This study set up six treatments, including three ZNC concentration levels (0, 5, and 20 ng/mL) and two Ralstonia solanacearum (Rs) inoculation levels (with and without), to explore the effect of ZNC application on the growth and disease resistance of peppers. The results showed that ZNC application (particularly 5 ng/mL ZNC) increased the basal biomass of the pepper plant and promoted root growth. 5 ng/mL ZNC can improve photosynthetic performance (net photosynthetic rate, transpiration rate and chlorophyll content averagely increased 20.2%, 33.7% and 16.0%, respectively) and antioxidant enzyme activities (particularly, catalase activity averagely increased 38.9%) of peppers more efficient than 20 ng/mL. Moreover, 5 ng/mL ZNC significantly increased Rs resistance of pepper, while 20 ng/mL ZNC did not show this effect. In conclusion, 5 ng/mL ZNC had the most significant effects on the growth-promoting and disease resistance of peppers, indicating good application prospects in agricultural practice.

Keywords:

• pepper
• plant immune inducer
• ZNC
• Ralstonia solanacearum
• enzyme activity

1. Introduction

Traditional agricultural production, especially the production of economic crops such as vegetables, is characterized by high input (mainly pesticides and fertilizers) and high yield, and although it has high productive efficiency and makes significant contributions to promoting regional socio-economic development, it has also caused a series of ecological and environmental problems such as pesticide residues, water pollution, and soil degradation (Bhandari et al., 2019; Lei et al., 2018; Luo et al., 2019). As a new class of plant-targeted biogenic agents, plant immune inducers enhance plant disease/stress resistance and promote plant growth by regulating their immune or metabolic systems to induce the production of broad-spectrum and persistent metabolites (Lenka et al., 2015; Thakur & Sohal, 2013). Therefore, plant immune inducers can partially replace traditional fertilizers and pesticides, reduce agricultural inputs while increasing crop yield, which is expected to become one of the key catalysts for the green development of modern agriculture.

The excavation and application of plant endophytes and their metabolites is a hotspot in the research of plant immune inducers. Recently, a new plant-growth regulator named “ZhiNengCong” (ZNC) has attracted much attention. ZNC is a secondary metabolite extracted from the mycelium of the wild sea buckthorn endophyte Paecilomyces variotii (Lu et al., 2019), which has been confirmed to be a new plant immune inducer with ultra-high activity. ZNC has the characteristics of pure nature and high stability with a neutral pH, and thus does not weaken the using effect of other pesticides and fertilizers when mixed with it. Studies based on the model plant Arabidopsis thaliana had found that ZNC promoted plant growth by inducing growth hormone accumulation at root tips at low concentrations and enhanced plant disease resistance by activating SA signaling pathways at high concentrations (Lu et al., 2019). The growth promotion and disease resistance effects of ZNC on rice, potato, Chinese cabbage and other crops have also been confirmed (Feng et al., 2022; Q. B. Wang et al., 2020; Q. Wang et al., 2021). However, previous studies also found that the optimal concentration of ZNC was different under different crops and environmental conditions. For example, Feng et al. (2022) found that rice spraying 200 ng/mL ZNC had the highest resistance against stripe blight, while Q. B. Wang et al. (2021) found that leaf spraying 20 ~ 40 ng/mL ZNC can effectively alleviate the low-temperature stress of Chinese cabbage and promote its growth at low temperature. Therefore, the application effect and appropriate concentration of ZNC in different crops and different environmental conditions need to be further verified.

Pepper (Capsicum annuum L.) is an important vegetable and condiment all over the world, which industrial development is closely related to regional economic development and residents’ life. Pepper cultivation is vulnerable to soil-borne diseases such as bacterial wilt, which usually causes huge economic losses and has become a major obstacle to pepper production (Liu et al., 2011). At present, chemical pesticide control is the most effective measure for the prevention and control of pepper bacterial wilt, but unreasonable use of pesticides will cause the resistance of pepper to them and increase the environmental and ecological risks. Whether and at what concentration ZNC enhances the resistance of pepper to bacterial wilt has not been reported.

This study aimed to investigate the effect of low concentration (5 ng/mL) and high concentration (20 ng/mL) ZNC on the growth, photosynthetic characteristics and antioxidant enzyme activities of pepper under the condition of with or without bacterial wilt inoculation. We hypothesized that: (1) high concentrations of ZNC can improve the resistance of pepper to bacterial wilt disease (Cao et al., 2021; Lu et al., 2019), and (2) low concentration of ZNC can promote the growth of peppers, which is characterized by higher biomass, stronger photosynthetic performance and higher antioxidant enzyme activities (Cui et al., 2022; Q. B. Wang et al., 2020).

2. Materials and methods

2.1. Experimental materials

In this study, 8-week-old screw pepper (Bolon RZ 37–94) seedlings were used for pot experiment and disease resistance test. ZNC solution (5.6 mg/ml), which is the ethanol crude extract of Paecilomyces variotii, was provided by Shandong Pengbo Biotechnology Co, Ltd (Tai’an City, Shandong Province, China). For detailed properties of ZNC, see Q. Wang et al. (2021). Soils used for pot experiment were collected from the 6-year sunlight greenhouse of Shandong Wantai Vegetable Co, Ltd in Feicheng County, Tai’an City, China, with a pH of 6.6, soil organic matter of 42.6 g/kg, available potassium of 237.6 mg/kg, and available phosphorus of 212.2 mg/kg. Ralstonia solanacearum (Rs) was used for the disease resistance test of screw pepper.

2.2. Disease resistance test

Screw pepper resistance to Rs under different ZNC concentrations (0, 5 ng/ml and 20 ng/ml) were identified referring to the national agricultural standard of China (NYT2060.2-2011, 2011). Briefly, 90 screw pepper seedlings were cultivated with substrate (which was composed of grass charcoal, vermiculite and vegetable field soil at a mass ratio of 2:1:1, and experienced a steam sterilization at 134 ºC for 30 min) in an artificial climate chamber (daytime temperature 30°C, nighttime temperature 22°C, soil moisture 90%, light 12 h, light intensity 20 klx). The seedlings were divided into 3 groups (irrigating 0, 5 ng/ml and 20 ng/ml ZNC, respectively) with 30 seedlings in each group (including 3 replicates, 10 seedlings for each replicate). For each seeding, 10 ml of Rs suspension (OD600 ranging from 0.8 to 1.0) was inoculated with pouring the wound root, and the disease resistance was investigated after 8 d ~10 days after inoculation.

For each identification material, the incidence of bacterial wilt was investigated, and the disease level was recorded according to the description of disease symptoms (Table S1). The disease index (DI) was calculated following: DI = $\frac{\sum \left(\mathrm{s}\times \mathrm{n}\right)}{\mathrm{S}\times \mathrm{N}}$×100, where s refers to the representative value for each disease level, n refers to the number of plants in each disease level, S is the representative value of the highest disease level, and N is the total number of plants investigated.

2.3. Pot experiment

2.3.1. Experimental design

A total of 6 treatments (each with 6 replicates) were set up for pot experiment, including 3 concentration gradients of ZNC solution (0, 5 ng/ml and 20 ng/ml) and 2 inoculation amounts of Rs suspension (0 and 10 ml). Details of the experiment design were shown in Table 1. Specifically, one screw pepper seedling was transplanted into a pot filled with 1.5 kg soil and then stabilized for 10 days. After the stabilization period, the ZNC solutions and Rs suspensions were subsequently added to the corresponding pots as described in Table 1. ZNC solution (0, 5 ng/ml or 20 ng/ml) was irrigated for three times (10 ml per time) at 0, 15 and 30 days after the stabilization period, respectively, and Rs suspension (OD600 ranging from 0.8 to 1.0) was inoculated for only once at day 0 using the wound root inoculating method. All plants were grown in a light incubator under the conditions as described in Section 2.2.

Table 1. Experiment design

Treatment	ZNC solution		Dosage of Rs suspension (ml)
	Concentration (ng/ml)	Volume (ml)
Control	0	10	0
ZNC5	5	10	0
ZNC20	20	10	0
Rs	0	10	10
ZNC5+Rs	5	10	10
ZNC20+Rs	20	10	10

2.3.2. Investigation on the growth of screw pepper

The plant height and stem-diameter of screw pepper were measured at the 50-day after the plant stabilization period. Then all pepper plants were harvested, in which 3 plants were randomly selected for the measurement of dry matter weight and the other 3 plants for root scanning and antioxidant enzymes determination. The former ones were divided into aboveground and underground parts, oven-dried at 70 ºC and weighed. For the latter ones, after gently shaking off the soils attached, the roots were rinsed with distilled water and being prepared for scanning root phenotype image using WinRHI-ZO (Pro.2013e) root analysis system (LaiEnDe, Weifang, China) to obtain root length, average diameter, total surface area, total volume, and other related parameters.

2.3.3. Measurements of photosynthetic characteristics and chlorophyll content

Photosynthetic characteristics (net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO₂ concentration) of screw pepper leaves were measured using a portable photosynthesis measurement system (LI-6400×T, LICOR, USA) at the 50-day after the plant stabilization period. Three upper unfolding leaves of each plant were selected for photosynthesis measurement and the measurement parameters were set as follows: leaf chamber temperature of 28°c, atmospheric CO₂ concentration of 400 μmol/mol, illuminance of 1000 μmol/(m²·s).

The chlorophyll a and chlorophyll b contents of screw pepper leaves at the 50-day after the plant stabilization period were determined by spectrophotometry (Li et al., 2000). Briefly, three subsamples of 0.2 g fresh pepper leaves were extracted with 10 ml extractant (acetone: ethanol: water at a volume ratio of 4.5: 4.5: 1.0) for 24 h in darkness, and prepared for the measurement of absorbances at 470 nm, 649 nm and 665 nm using a spectrophotometer (721–100, YuanXi, Shanghai, China).

2.3.4. Determination of antioxidant enzymes system

Antioxidant enzyme activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and the content of glutathione (GSH) were determined on fresh pepper leaves (at the 50-day after the plant stabilization period) was evaluated by the spectrophotometric method (McCord & Fridovitch, 1969). Take 1 g of screw pepper leaves into a glass homogenization tube, add 9 ml of homogenization medium for homogenization (SOD and POD use phosphate buffer saline as the homogenization medium, CAT and GSH use normal saline as the homogenization medium), grind sufficiently to homogenize the tissue, centrifuge at 2500–3500 rpm for 10 min, and take the supernatant for POD, SOD, CAT, GSH determination using the manufacturer’s protocol (JianCheng, NanJing, China). These experiments were conducted in independent triplicates.

2.4. Statistical analysis

Statistical analysis were performed using IBM SPSS 24.0 software (SPSS, Inc., Chicago, USA). The effects of ZNC concentration and Rs inoculation on response variables were determined using one-way or two-way analysis of variance (ANOVA) and treatment means were compared using LSD method (P = 0.05).The data in figures and tables are means ± standard deviation (SD).

3. Results

3.1. Screw pepper resistance to Rs under different ZNC concentrations

The effect of ZNC on pepper Rs resistance was strongly concentration dependent (Table 2). The peppers in the control had Medium Resistance (25 < DI ≤ 50, Table S2) to Rs, and that under 5 ng/ml ZNC, 20 ng/ml ZNC achieved Resistance (12.5 < DI ≤ 25), Susceptibility (50 < DI ≤ 75), respectively.

Table 2. Disease index (DI) of screw pepper under different ZNC concentrations

Treatment	Disease index(DI)
Control (DI water)	39.72 ± 1.27 b
ZNC5	23.89 ± 0.49 c
ZNC20	53.33 ± 1.67 a

Note: The identification materials are randomly arranged or arranged sequentially, and each identification material is repeated 3 times, and each replicate is 10 seedlings. Different letters represent significant differences among treatments (P < 0.05).

3.2. Effects of ZNC concentration on the growth of screw pepper

Rs inoculation had an inhibitory effect on the growth of screw pepper, showing decreases of 22.4%, 18.7%, 18.8%, 10.9% and 62.8% in shoot dry weight, root length, root surface area, average root diameter and root volume, respectively (Figure 1, Tables 3 and 4). ZNC application generally promoted root growth regardless of Rs inoculation (Figure 1, Table 4). Compared to non-ZNC treatments (control or Rs treatments), ZNC5 treatments (ZNC5 or ZNC5+Rs treatments) averagely had a 15.9% higher root length and 22.1% higher root surface area, while the corresponding values for ZNC20 treatments were 15.5% and 10.3%, respectively.

Figure 1. Scanned image of pepper root system under different treatments. Control (a), ZNC5 (b), ZNC20 (c), Rs (d), ZNC5+Rs (e), ZNC20+Rs (f).

Table 3. Plant height, stem diameter, total biomass, dry matter yield and root/shoot ratio of pepper under different treatments

Treatment	Plant height (mm)	Stem diameter (mm)	Total biomass (fresh weight, g/plant)	Dry matter yield of shoot (g/plant)	Dry matter yield of root (g/plant)	Root/shoot
Control	125.36 ± 3.35 ab	3.91 ± 0.14 a	18.88 ± 1.44 ab	1.40 ± 0.09 ab	0.30 ± 0.04 a	0.17 ± 0.01 a
ZNC5	127.89 ± 7.17 b	4.13 ± 0.26 a	21.77 ± 6.99 a	1.67 ± 0.39 a	0.33 ± 0.07 a	0.20 ± 0.01 a
ZNC20	125.89 ± 14.93 ab	4.02 ± 0.51 a	23.02 ± 3.42 a	1.70 ± 0.06 a	0.31 ± 0.09 a	0.24 ± 0.01 a
Rs	117.11 ± 2.42 b	3.83 ± 0.26 a	14.04 ± 0.65 b	1.18 ± 0.20 b	0.22 ± 0.03 a	0.21 ± 0.02 a
ZNC5+Rs	120.43 ± 6.71 ab	3.96 ± 0.37 a	14.92 ± 1.16 b	1.29 ± 0.05 b	0.27 ± 0.08 a	0.19 ± 0.03 a
ZNC20+Rs	118.37 ± 10.11 ab	3.87 ± 0.18 a	14.38 ± 1.53 b	1.24 ± 0.06 b	0.24 ± 0.03 a	0.18 ± 0.01 a
Significant level (P)
Rs	NS	NS	NS	**	NS	NS
ZNC	NS	NS	NS	NS	NS	NS
Rs*ZNC	NS	NS	**	NS	NS	NS

Note: Different letters in the same column mean significant differences (P < 0.05) among treatments.

Table 4. Root morphology of pepper under different treatments

Treatment	Root length (cm)	Root surface area (cm^2)	Average root diameter (mm)	Root volume (cm^3)
Control	259.0 ± 16.2b	70.8 ± 4.1b	0.7 ± 0.0b	1.4 ± 0.1b
ZNC5	304.3 ± 11.4a	85.3 ± 3.9a	0.9 ± 0.1a	1.6 ± 0.1ab
ZNC20	301.6 ± 18.8a	81.4 ± 7.0a	0.7 ± 0.1b	1.6 ± 0.1a
Rs	213.8 ± 4.9c	58.6 ± 3.7c	0.7 ± 0.1b	0.5 ± 0.1c
ZNC5+Rs	244.3 ± 9.9b	72.5 ± 4.5b	0.7 ± 0.1b	0.6 ± 0.1c
ZNC20+Rs	244.7 ± 1.3b	61.8 ± 3.0c	0.7 ± 0.1b	0.6 ± 0.1c
Significant level (P)
Rs	**	**	**	**
ZNC	**	**	**	NS
Rs*ZNC	NS	NS	NS	NS

Note: Different letters in the same column represent significant differences (P < 0.05) among treatments.

Under the condition of without Rs inoculation, the total biomass of pepper with different ZNC concentrations showed no significant difference (Table 3); compared to control without ZNC application, ZNC5 showed a significant increase of 14.9%, 17.0% and 22.2% in root length, root surface area and average root diameter, respectively (Table 4). Under the condition of with Rs inoculation, the root morphology of the ZNC5 was also better than the other treatments, and it significantly increased root length, root surface area and average root diameter by 17.5%, 20.5%and 28.6% respectively (Table 4). ZNC5 was found to be more effective than ZNC20 in promoting pepper growth and was particularly effective in promoting root growth when inoculated with Rs.

3.3. Effects of ZNC concentration on photosynthetic characteristics of screw pepper

Rs inoculation reduced the net photosynthetic rate, transpiration rate and chlorophyll content of screw pepper by an average of 51.6%, 57.1% and 13.1% respectively (Table 5). ZNC application generally improved the photosynthetic performance of the plant regardless of Rs inoculation (Figure 2, Table 5). Compared to non-ZNC treatments, ZNC5 increased net photosynthetic rate, transpiration rate and chlorophyll content by an average of 20.2%, 33.7% and 16.0%, respectively, while the corresponding increase for ZNC20 was 15.5%, 22.2% and 7.6% respectively (Table 5).

Figure 2. Chlorophyll content under different treatments. Data are shown as the mean  ± SD (n = 6). The results of two-way ANOVA was shown in the upper left corner of the figure: “Rs” represents the effect of Rs inoculation; “ZNC” represents the effect of ZNC concentration; “Rs*znc” represents the interaction effect of Rs inoculation and ZNC concentration. Different letters above the bars indicate significant differences among treatments (P < 0.05).

Table 5. Photosynthetic parameters of pepper under different treatments

Treatment	Ci (μmol mol^−1)	Gs (mol m^−2 s^−1)	A (μmol m^−2 s^−1)	E (mmol m^−2 s^−1)
Control	195.17 ± 19.85 a	30.00 ± 4.52 a	9.38 ± 2.29 b	2.68 ± 1.29 bc
ZNC5	157.83 ± 11.23 a	34.33 ± 9.40 a	12.57 ± 0.58 a	4.57 ± 0.48 a
ZNC20	162.33 ± 77.70 a	31.50 ± 8.64 a	11.42 ± 0.86 a	3.79 ± 0.70 ab
Rs	202.00 ± 34.65 a	25.33 ± 15.98 a	5.02 ± 0.57 c	1.48 ± 1.61 d
ZNC5+Rs	176.67 ± 32.00 a	27.17 ± 4.71 a	5.48 ± 0.93 c	1.70 ± 0.60 cd
ZNC20+Rs	184.50 ± 59.06 a	28.17 ± 8.28 a	5.63 ± 1.16 c	1.56 ± 0.38 cd
Significant level (P)
Rs	NS	NS	**	**
ZNC	NS	NS	**	*
Rs*ZNC	NS	NS	*	NS

Note: Ci, intercellular CO₂ concentration; Gs, stomatal conductance; A, net photosynthetic rate; E, transpiration rate. Different letters represent significant differences among treatments (P < 0.05).

The overall photosynthetic performance of screw pepper under the condition of without Rs inoculation was ZNC5≥ZNC20>ZNC0 (Table 5). ZNC5 increased the net photosynthetic rate, transpiration rate and chlorophyll content by 34.0%, 70.5% and 26.2% respectively (P < 0.05). However, under the condition of Rs inoculation, this enhancement effect of ZNC5 did not reach a significant level (P > 0.05). Generally, ZNC5 was more effective than ZNC20 in improving photosynthetic performance, particularly under the condition of without Rs inoculation.

3.4. Effects of ZNC concentrations on antioxidant enzyme activities of screw pepper

ZNC application generally improved the activity of antioxidant enzymes of the plant regardless of Rs inoculation (Figure 3). Compared to non-ZNC treatments, ZNC5 treatments increased the enzyme activities of CAT and POD by an average of 38.9% and 6.9%, respectively, and ZNC20 treatments increased these indicators by an average of 22.5% and 5.5%, respectively.

Figure 3. SOD activity (a), GSH content (b), CAT activity (c) and POD activity (d) under different treatments. Data are shown as the mean ± SD (n = 6). The results of two-way ANOVA was shown in the upper left corner of the figure: “Rs” represents the effect of Rs inoculation; “ZNC” represents the effect of ZNC concentration; “Rs*znc” represents the interaction effect of Rs inoculation and ZNC concentration. Different letters above the bars indicate significant differences among treatments at P < 0.05.

ZNC5, which increased the enzyme activities of POD and CAT by an average of 6.3% and 44.8% respectively, was more effective than ZNC20 in increasing the antioxidant enzyme activities under the condition of without Rs inoculation (Figure 3). Under the condition of Rs inoculation, the enzymatic activities of SOD, POD and CAT in pepper plants were reduced by an average of 18.5%, 5.1% and 68.5%; however, the effects of different ZNC concentrations (5 ng/ml and 20 ng/ml) on the antioxidant enzyme activities of pepper were not significant (P > 0.05; Figure 3).

4. Discussion

4.1. Growth promoting effect of different concentrations of ZNC on pepper

Our study showed that ZNC application increased the biomass of pepper seedlings and promoted root growth, but at a root application concentration of 20 ng/mL ZNC, the pepper plants showed less growth promotion than those at 5 ng/mL ZNC. The promoting effect of ZNC on crop growth is in line with previous findings that the plant immune inducers Lentinan (LNT) and S-Abscisic Acid (S-ABA) have significant effects on plant growth promotion, yield increase and disease resistance (Sun et al. 2022; Iriti & Vitalini, 2021; X. Q. Wang et al., 2020; Yao et al., 2019). ZNC application can promote pepper growth probably due to the increased release of growth hormone such as indoleacetic acid (IAA), which plays an important role in plant growth regulation (Chandler et al., 2016). Cao et al. (2021) found that ZNC upregulated the expressions of three auxin-induced proteins and four small auxin genes, and increased IAA content in potatoes at concentrations of 1–10 ng/ml. Results from root scanning showed that the root system of pepper plants treated with 5 ng/ml ZNC was more well-developed, with a 17.5% increase in root length and 14.3% increase in root diameter, as well as a slight increase in total root volume and mean root diameter relative to the control treatment, which is consistent with Q. Wang et al. (2021) who found that ZNC at lower concentrations (10 ng/mL) had a significant promotion effect on both primary root length and lateral root number in eggplant.

ZNC application affects antioxidant enzyme activity in plants, and it has been shown that ZNC can up-regulate antioxidant-related genes and regulate related enzyme activities (Cui et al., 2022). Our study found that the antioxidant enzyme activities under ZNC treatment (particularly ZNC5 treatment) were generally higher than those under the control treatment, with significant increases in CAT and POD activities. The photosynthetic characteristics of the plants in the ZNC treatment were also better than the control treatment, where the application of 5 ng/ml ZNC increased chlorophyll content, net photosynthetic rate and transpiration rate by 26.2%, 34.0% and 70.5% respectively. Previous studies have shown that ZNC can enhance the photosynthetic capacity of plants by promoting the uptake of nitrogen and phosphorus via the root system, thereby enhancing the activities of the photosynthesis-related enzymes such as PEPC and ATP synthase (Ma et al., 2022; Saia et al., 2019).

4.2. Disease resistance effect of different concentrations of ZNC on pepper

It has been shown that plant-growth regulator induce resistant active substances in plants infested with pathogens, such as some metabolites and proteins, and that these active molecules are picked up by plant epidermal cells and produce some resistance to the disease (Abeysekara et al., 2016; Qiu et al., 2017). Some other studies found that plant-growth regulator agents could activate relevant genes in the plant, induce a series of metabolic regulatory responses in the plant, increase enzymes or phenolic disease-resistant substances in the plant, and improve host plant tolerance to biotic and abiotic stresses by increasing antioxidant activity (Hamilton et al., 2012; Keen et al., 1972). This was also supported by the finding in this study that ZNC increased the activity of antioxidant enzymes such as POD in pepper seedlings inoculated with Ralstonia solanacearum.

It has been shown that 1–10 ng/mL ZNC has a positive effect on both disease resistance and growth promotion in plants, with 10 ng/ml ZNC performing best in terms of oomycete resistance in potato (Cao et al., 2021; Lu et al., 2019). The results of this study for resistance to Ralstonia solanacearum in pepper seedlings showed that different concentrations of ZNC had different effects on disease resistance in pepper, specifically, 5 ng/ml ZNC significantly increased Rs resistance in pepper, while 20 ng/ml ZNC did not show this effect and even increased the risk of disease (Table 2). The application of 20 ng/ml ZNC increased the risk of pepper bacterial wilt, which is inconsistent with the results of studies in potato crops (Cao et al., 2021), suggesting that the effect of ZNC in enhancing crop disease resistance varies according to crop type. It has been suggested that high concentrations of plant-growth regulator increase the risk of disease possibly because the alkylated derivatives produced at high concentrations reduce their resistance-inducing properties and can even be toxic to plants (Smiglak et al., 2016). The latest research showed that ZNC contain a super-high activity substance, 2’-deoxyguanosine (Lu et al., 2023), to promote disease resistance even at concentrations as low as 1–10 ng/ml, which could be attributed to the activation of bursts of reactive oxygen species, callose deposition and mitogen-activated protein kinase phosphorylation (L. Wang et al., 2022; Lu et al., 2019).

5. Conclusion

This study showed that root application of 5 ng/mL ZNC had a significant disease resistance- and growth-promoting effect on pepper. It increased total root length, net photosynthetic rate, and chlorophyll content by 15.9%, 20.2%, and 7.6%, increased antioxidant enzymes CAT and POD by 38.9% and 6.9%, respectively, and increased resistance to Ralstonia solanacearum by one level, indicating good application prospects. However, it was also found that high concentrations of ZNC may have an inhibitory effect on crop growth, suggesting that the concentrations used in field production should be strictly controlled. Future research needs to further explore the optimal application concentration of ZNC on different crops on the one hand, and to combine it with the production of agricultural inputs such as fertilizers to find lighter and simpler application methods on the other hand.

Correction

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Acknowledgments

The authors would like to thank Professor Ding Xinhua for providing ZNC solution.

Disclosure statement

The authors have no relevant financial or non-financial interests to disclose.

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/23311932.2023.2195975

Additional information

Funding

This work was financially supported by the Major Science and Technology Innovation Projects in Shandong Province [Grant numbers: 2020CXGC010803 and 2021CXGC010801].

Notes on contributors

Wenjing Yin

Wenjing Yin is a postgraduate student from the Shandong Agricultural University (SDAU). She carried out this research during pursuing a master's degree in agriculture.

Yujing Wei

Yujing Wei is a postgraduate student of agricultural sciences at SDAU.

Guihua Li

Guihua Li is an economist of Tai'an Urban Management Comprehensive Service Center.

Yunlong Wang

Yunlong Wang is a postgraduate student from the China Agricultural University.

Yanhong Lou

Yanhong Lou, Hong Pan, Quangang Yang, Guoqing Hu, and Hui Wang are all associate professors at the SDAU.

Hong Pan

Yanhong Lou, Hong Pan, Quangang Yang, Guoqing Hu, and Hui Wang are all associate professors at the SDAU.

Quangang Yang

Yanhong Lou, Hong Pan, Quangang Yang, Guoqing Hu, and Hui Wang are all associate professors at the SDAU.

Guoqing Hu

Yanhong Lou, Hong Pan, Quangang Yang, Guoqing Hu, and Hui Wang are all associate professors at the SDAU.

Hui Wang

Yanhong Lou, Hong Pan, Quangang Yang, Guoqing Hu, and Hui Wang are all associate professors at the SDAU.

Yuping Zhuge

Yuping Zhuge is a professor at the SDAU.