The Effect of New Nano-Released 1,1-Dimethyl-Piperidinium Chloride (DPC) Drip Application on Cotton Agronomic Traits

The use of 1,1-dimethyl-piperidinium chloride (DPC) in Xinjiang is one of the necessary measures for regulating cotton populations and optimizing canopy structure. However, mechanical spraying involves operation and energy consumption, which can be achieved via drip application of water. In the present study, to investigate the effect of drip application, two types of hydrophobic nano-released DPCs were used to regulate plant type at different dosages. DPC dripping reduced plant height by more than 10 cm and plant width by more than 4 cm, and the effect improved with increasing concentration and time. The main effect of height control was at the 6th–10th nodes of the main stem, and the effect of width control was observed at the 1st and 2nd nodes of the fruit branch. The SPAD value was higher than that in the control group during the initial stages. An irregular downward trend was observed in the subsequent stages. The proline content was higher than that of the control and increased with higher concentrations of DPC. There was no significant difference in the soil DPC content at the different sites. The DPC content decreased by more than 30% at the seventh day after dripping, and the content was 2–4 μg·g−1 in each treatment. At 15 days after dripping, the DPC content was lower than 1 μg·g−1 in all treatments. Chemical regulation was conducive for providing the advantages of a high-density population. The number of harvested plants was more than 150,000 plants·hm−2, and the number of bolls per plant increased at least by one, compared with the control. The final yield of seed cotton increased by at least 300 kg·hm−2, with a maximum increase of 1672.01 kg·hm−2. Considering the different types of DPCs, nano-released types worked better than the ordinary type in terms of plant height, plant width, degradation in soil and boll formation. Nano-released DPC can play an even better role in chemical regulation with a reasonable drip in water.


Introduction
DPC is a plant growth regulator that appears to antagonize gibberellin. Cotton has unlimited growth habits, requiring complex pruning and plant type regulation in production. Regulating plant shape and canopy structure was an important measure for the "short-dense -early" cotton planting model, in which DPC was the most widely used regulator [1]. DPC is absorbed by the leaves and roots of the plant and transmitted to the whole plant. DPC can provide a guarantee for cotton dwarfing and dense planting by inhibiting the growth of the main stem, fruit branch internode elongation and terminal bud, and can achieve the purpose of shaping and simplifying pruning [2,3]. Since the mid-1980s, DPC was widely accepted by farmers because of its stable effect, safety, low price, and so on. At present, DPC is sprayed at least 5-6 times with a machine during the whole growth period. Mechanical operation could cause damage to the plant, lodging, indirect

Materials
This study was conducted at Wudui Farm, Daquan Township, Shawan City, Xinjiang from 2020 to 2022. The previous crop of the test plot was cotton. Experimental soil was loam with organic matter 11.80 g·kg −1 , alkali hydrolyzable nitrogen 52.91 mg·kg −1 , available phosphorus 19.4 mg·kg −1 , available potassium 227 mg·kg −1 . The pH of 0-20 cm top soil was 8.26 before cotton planting.
The cotton variety was Youmian F015-5. The two kinds of tested nano sustainedrelease DPC were hydrophobic microspheres and hydrophobic supramolecular complexes, which were treated with alcohol-soluble corn protein and supramolecular composite intercalated nanomaterials (also hydrotalcite, chemical composition general formula [M 2+ 1−x M 3+ x (OH) 2 ] x+ (A n− ) x/n ·mH 2 O), respectively.

Methods
According to the schematic diagram of experimental workflow of this study, a random block experiment was designed to study the effect of drip application in 2020 ( Figure 1, Table 1). Each treatment was repeated three times with an area of 40 m 2 for each plot. Two types of DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes were labeled as C1, C2, respectively. Three concentrations (L, M, H) were set with conventional DPC and blank water as the references (S1 and CK), respectively. The dosages of low (L), medium (M) and high (H) concentrations were 30 g·hm −2 , 60 g·hm −2 and 90 g·hm −2 , respectively, according to the former study. All DPCs were dripped with water on June 16, and chemical regulation was not be carried out at the later stage. Sowing adopted machine picking mode, with 6 rows of film, row spacing of 66 + 10 cm and plant spacing of 9-10 cm. Other cultivation measures such as intervillage and topping refer to the requirements of general high-yield fields. For the first dripping, the concentrations of low (L) and high (H) concentration were 30 and 60 g·hm −2 , respectively. For the second dripping, the concentrations of low (L) and high (H) concentration were 60 and 120 g·hm −2 , respectively. Based on the results of the previous two years, the absorption path after drip application was further studied with design of a random block in 2022. Two types of (C1, C2) slow-release DPC were dripped on June 6 set with the concentration of 90 g·hm −2 , and commercially available conventional DPC was set as control (S3).    In 2021, a random block design was also adopted to optimize the concentration and times of DPC. Two kinds of slow-release DPC (C1, C2) were set with two concentrations (L and H), with commercially available conventional DPC and blank water as the references (S2 and CK), respectively. DPC was dripped twice on June 4 and 28, respectively. For the first dripping, the concentrations of low (L) and high (H) concentration were 30 and 60 g·hm −2 , respectively. For the second dripping, the concentrations of low (L) and high (H) concentration were 60 and 120 g·hm −2 , respectively. Based on the results of the previous two years, the absorption path after drip application was further studied with design of a random block in 2022. Two types of (C1, C2) slow-release DPC were dripped on June 6 set with the concentration of 90 g·hm −2 , and commercially available conventional DPC was set as control (S3). Six plants were selected to investigate the agronomic traits for each plot. Plant height, stem diameter, number of true leaves, number of fruit branches, height of initial node and length of internode of main stem were investigated 5 times every 6-9 days after the application of the DPC. All the agronomic traits were measured again before harvest.

Plant Width
Plant width was measured for the first time at the first dripping of DPC, and then 5 times every 5-8 days after dripping.

Chlorophyll and Proline Content
The chlorophyll content (SPAD value) of the fourth leaf was investigated every 5-10 days after the application of DPC. On the 7th day after the dripping of DPC, the content of proline of the fourth leaf was also measured using the method of acid pyrotrione color [16]. The absorbance was determined with colorimetry at a wavelength of 520 nm.

Content of DPC in Soil and Plant
On the 3rd, 7th and 15th days after dripping of DPC, the content of residual DPC and water in soil samples was determined, and the DPC content of plants was also measured at the same time. Soil samples were taken longitudinally in three gradients from the surface layer (0-10 cm), the middle layer (10-20 cm) and the bottom layer (20-30 cm), and transversely at two sites (5 cm and 15 cm).

Yield and Components
The number of plants and bolls were investigated within the range of 6.67 m 2 for each plot, and the plant density and boll setting number per plant were calculated at the boll-opening stage. Ten plants with the same growth trend were selected to sample 15 bolls from the upper, middle and lower bolls, respectively. The boll weight and lint percentage were measured to estimate the yield of each plot.

Data Analysis
All the data were organized with Microsoft Office 2016 software. SPSS 19.0 software was taken for two factor analysis of variance, and a new multiple range test was used for multiple comparisons of the mean values between different treatments.

Regulation of Plant Height by Different Types of DPC
The main role of DPC is to control plant height. All treatments were significantly different from the control, except for S1L treatment at low concentrations in 2020 ( Figure 2A). The differences in plant height were greater than 4 cm, with a maximum of 16.1 cm. Plant height decreased with increasing concentrations; however, the difference was not significant. Among the different treatments, C1 showed the best effect. The plant height of medium and high concentrations of C1M and C1H was 72.7 cm and 71.1 cm, respectively, which was significantly different from that of S1, with a difference of more than 6.8 cm. In 2021, the difference in plant height between each treatment and the control was also significant, with a difference of more than 10 cm, and the effects of the two drip treatments were better. Plant height under S2H treatment was 57 cm, which was the lowest of all treatments. There was no difference between the same type of DPC and concentration, and the difference in plant height was less than 10 cm and 5.5 cm, respectively; however, the difference between the low concentration (C2L and S2L) and high concentration (C1H) treatments was significant, with a maximum difference of 12.2 cm ( Figure 2B). In 2022, there were no significant differences between the treatments with different types of DPC. The difference in plant height between the different treatments was only 3.1 cm. The C1 treatment with a Agronomy 2023, 13, 1543 5 of 14 single application of DPC had the best effect, with a plant height of 69.9 cm ( Figure 2C). All treatments with drip DPC application effectively controlled plant height ( Figure 3). all treatments. There was no difference between the same type of DPC and concentration, and the difference in plant height was less than 10 cm and 5.5 cm, respectively; however, the difference between the low concentration (C2L and S2L) and high concentration (C1H) treatments was significant, with a maximum difference of 12.2 cm ( Figure 2B). In 2022, there were no significant differences between the treatments with different types of DPC. The difference in plant height between the different treatments was only 3.1 cm. The C1 treatment with a single application of DPC had the best effect, with a plant height of 69.9 cm ( Figure 2C). All treatments with drip DPC application effectively controlled plant height ( Figure 3). respectively. C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1, S2 and S3 mean commercially available conventional DPC in 2020, 2021 and 2022, respectively. CK means the blank water. L, M and H mean dosages of low, medium and high concentrations, respectively. Different small letters in a column mean significant at 5% levels.

Regulation of Internode Length of the Main Stem by Different Types of DPC
The effect of DPC on plant height was mainly reflected in the control internode length of the main stem. In 2020, compared with the control, the length of the 6th to 10th internodes was significantly shorter in all treatments ( Figure 4). The largest difference was 1.9 cm in the eighth internode in S1L treatment, followed by 1.6 cm in the seventh internode in C2H treatment. The total length between the 6th and 10th nodes from C1L to S1H treatment was 4.0 cm, 3.7 cm, 2.9 cm, 3.1 cm, 4.9 cm, 3.8 cm, 3.6 cm, 4.4 cm and 3.9 cm shorter than that of the control, respectively. all treatments. There was no difference between the same type of DPC and concentration, and the difference in plant height was less than 10 cm and 5.5 cm, respectively; however, the difference between the low concentration (C2L and S2L) and high concentration (C1H) treatments was significant, with a maximum difference of 12.2 cm ( Figure 2B). In 2022, there were no significant differences between the treatments with different types of DPC. The difference in plant height between the different treatments was only 3.1 cm. The C1 treatment with a single application of DPC had the best effect, with a plant height of 69.9 cm ( Figure 2C). All treatments with drip DPC application effectively controlled plant height ( Figure 3).

Regulation of Internode Length of the Main Stem by Different Types of DPC
The effect of DPC on plant height was mainly reflected in the control internode length of the main stem. In 2020, compared with the control, the length of the 6th to 10th internodes was significantly shorter in all treatments ( Figure 4). The largest difference was 1.9 cm in the eighth internode in S1L treatment, followed by 1.6 cm in the seventh internode in C2H treatment. The total length between the 6th and 10th nodes from C1L to S1H treatment was 4.0 cm, 3.7 cm, 2.9 cm, 3.1 cm, 4.9 cm, 3.8 cm, 3.6 cm, 4.4 cm and 3.9 cm shorter than that of the control, respectively.

Regulation of Internode Length of the Main Stem by Different Types of DPC
The effect of DPC on plant height was mainly reflected in the control internode length of the main stem. In 2020, compared with the control, the length of the 6th to 10th internodes was significantly shorter in all treatments ( Figure 4). The largest difference was 1.9 cm in the eighth internode in S1L treatment, followed by 1.6 cm in the seventh internode in C2H treatment. The total length between the 6th and 10th nodes from C1L to S1H treatment was 4.0 cm, 3.7 cm, 2.9 cm, 3.1 cm, 4.9 cm, 3.8 cm, 3.6 cm, 4.4 cm and 3.9 cm shorter than that of the control, respectively.
In 2022, all DPC treatments were dripped in the period of internode elongation at the sixth to seventh nodes, so the internode length of the first to fifth nodes under each treatment changed to <2 cm, the length tended to be fixed and the impact was limited ( Table 2). These results are consistent with those for 2020. However, the length of the internode gradually decreased by 3 cm from the sixth node. The internode length of nodes 6-9 was shortest under C1 treatment and longest under S3 treatment. The length difference between the eighth node of the C1 and S3 treatments was the largest, up to 2.7 cm (Table 2).
In 2022, all DPC treatments were dripped in the period of internode elongation at the sixth to seventh nodes, so the internode length of the first to fifth nodes under each treatment changed to <2 cm, the length tended to be fixed and the impact was limited ( Table  2). These results are consistent with those for 2020. However, the length of the internode gradually decreased by 3 cm from the sixth node. The internode length of nodes 6-9 was shortest under C1 treatment and longest under S3 treatment. The length difference between the eighth node of the C1 and S3 treatments was the largest, up to 2.7 cm (Table 2). . Difference between the internode length of each treatment and the corresponding internode length of the control in 2020. Note: C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1 means commercially available conventional DPC in 2020. L, M and H mean dosages of low, medium and high concentrations, respectively.

Plant width Regulation by Different Types of DPC
Chemical regulation can be used to shape compact plants. In 2020 and 2021, the plant width decreased with increasing DPC concentrations in all treatments ( Figure 5). The effect of C1H treatment with high concentration was the best, and the plant width was 38.3 cm and 33.3 cm in 2020 and 2021, respectively ( Figure 5A,B). There was no significant difference between the treatments at the same concentration. Apart from C2 in 2020, the plant widths of the other treatments with low and high concentrations showed significant differences, with an average difference of more than 4.0 cm. Compared with the control, the  Figure 5A,B). There were no significant differences between the low concentration and control treatments. Plant width decreased under medium concentration treatments, but the difference was not statistically significant. In 2022, the plant width under C1 treatment was the smallest, and there was no significant difference between the different types of DPC ( Figure 5C).

Plant width Regulation by Different Types of DPC
Chemical regulation can be used to shape compact plants. In 2020 and 2021, the plant width decreased with increasing DPC concentrations in all treatments ( Figure 5). The effect of C1H treatment with high concentration was the best, and the plant width was 38.3 cm and 33.3 cm in 2020 and 2021, respectively ( Figure 5A,B). There was no significant difference between the treatments at the same concentration. Apart from C2 in 2020, the plant widths of the other treatments with low and high concentrations showed significant differences, with an average difference of more than 4.0 cm. Compared with the control, the plant width was significantly reduced in high concentration treatments, and it decreased by 2 cm and 4.0 cm in 2020 and 2021, respectively ( Figure 5A,B). There were no significant differences between the low concentration and control treatments. Plant width decreased under medium concentration treatments, but the difference was not statistically significant. In 2022, the plant width under C1 treatment was the smallest, and there was no significant difference between the different types of DPC ( Figure 5C).

Effect of Different DPC Types on Agronomic Traits
Compared with the control, agronomic traits, such as stem diameter, height of the first node of the fruit branch, and length of the first and second nodes of the fourth fruit branch showed a downward trend under each drip application of different forms of DPC, but there were no significant differences in the three years of the experiment (Table 3). Stem diameter was between 10.00 and 11.50 mm. The height of the initial nodes of fruit branches was mostly between 24.0 and 25.0 cm. Compared with the control, the length of the first and second nodes of the fourth fruit branch could be reduced by 4.0 cm and 1.8 cm at most, respectively. Significant differences were detected in the numbers of true leaves and fruit branches of the main stem in 2020. C1L, C1H, C2L, S1H and the control showed significant differences in the number of true leaves on the main stem, with a maximum difference of 0.9 for C2L. There was a significant difference in the number of fruit branches between C1L and the control, with a maximum difference of 0.7.

Effect of Different DPC Types on Agronomic Traits
Compared with the control, agronomic traits, such as stem diameter, height of the first node of the fruit branch, and length of the first and second nodes of the fourth fruit branch showed a downward trend under each drip application of different forms of DPC, but there were no significant differences in the three years of the experiment (Table 3). Stem diameter was between 10.00 and 11.50 mm. The height of the initial nodes of fruit branches was mostly between 24.0 and 25.0 cm. Compared with the control, the length of the first and second nodes of the fourth fruit branch could be reduced by 4.0 cm and 1.8 cm at most, respectively. Significant differences were detected in the numbers of true leaves and fruit branches of the main stem in 2020. C1L, C1H, C2L, S1H and the control showed significant differences in the number of true leaves on the main stem, with a maximum difference of 0.9 for C2L. There was a significant difference in the number of fruit branches between C1L and the control, with a maximum difference of 0.7.

Effect of Different Types of DPC on SPAD and Proline Leaf Content
In 2020 and 2022, the SPAD values of the last four leaves from the top showed a downward trend under all treatments from the first ten days of June (Figures 6 and 7). In 2020, the SPAD value of the control remained low, whereas the SPAD values under each treatment remained between 55 and 59. From 19 to 28 June, the SPAD value under each treatment tended to increase with concentration, and C2H treatment exhibited the highest value; however, the trend was not obvious in the later period ( Figure 6A). In 2022, the SPAD value of the S treatment was always low for the three types of DPC, while that of C1 remained high. The difference between the two values reached a maximum of 7.2 on 15 June, and then decreased ( Figure 6B). In 2020, the proline content under different types of DPC treatments was higher than that of the control, and proline content tended to increase with increasing concentration. C1H, C2M and C2H were significantly different from the control, with a maximum difference of 2.59 ug·g −1 ( Figure 7A). However, there were no significant differences between the different dosage forms in 2022 ( Figure 7B).

Adsorption and Transfer of Different Types of DPC in Soil and Plants
The soil DPC content was very low after three days of drip application, and the content at different sites and depths was about 3-6.00 µg·g −1 (Figure 8). There was no significant difference in the content of different types of DPC at the same site and depth seven days after drip application, and there was no significant difference in DPC content at the horizontal 5 cm and 15 cm sites. The DPC content at the 15 cm site was slightly higher, and the maximum average difference under C2 treatment was 0.87 µg·g −1 . The DPC content decreased by approximately 30% within 3-7 days of drip application. The soil content under each treatment decreased significantly, reaching <1.00 µg·g −1 after 15 days of drip application. The DPC content under S3 treatment was the lowest at both sites, and the minimum was only 0.25 µg·g −1 . Although there were differences among different depth treatments, there was no apparent trend. During the test period, plant DPC content was 150-200 µg·g −1 , about 50 times higher than that in soil (Figure 9). Figure 6. SPAD value of the last four leaves of cotton in each treatment. Note: (A,B) mean the SPAD value of different treatments in 2020 and 2022, respectively. C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1 and S3 mean commercially available conventional DPC in 2020 and 2022, respectively. CK means the blank water. L, M and H mean dosages of low, medium and high concentrations, respectively.
(A) (B) Figure 7. Proline content of cotton under various treatments. Note: (A,B) mean the proline content of different treatments in 2020 and 2022, respectively. C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1 and S3 mean commercially available conventional DPC in 2020 and 2022, respectively. CK means the blank water. L, M and H mean dosages of low, medium and high concentrations, respectively. Different small letters in a column mean significant at 5% levels.

Adsorption and Transfer of Different Types of DPC in Soil and Plants
The soil DPC content was very low after three days of drip application, and the content at different sites and depths was about 3-6.00 μg·g −1 (Figure 8). There was no significant difference in the content of different types of DPC at the same site and depth seven days after drip application, and there was no significant difference in DPC content at the horizontal 5 cm and 15 cm sites. The DPC content at the 15 cm site was slightly higher, and the maximum average difference under C2 treatment was 0.87 μg·g −1 . The DPC content decreased by approximately 30% within 3-7 days of drip application. The soil content under each treatment decreased significantly, reaching <1.00 μg·g −1 after 15 days of drip application. The DPC content under S3 treatment was the lowest at both sites, and the minimum was only 0.25 μg·g −1 . Although there were differences among different depth treatments, there was no apparent trend. During the test period, plant DPC content was 150-200 μg·g −1 , about 50 times higher than that in soil (Figure 9). respectively. C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1 and S3 mean commercially available conventional DPC in 2020 and 2022, respectively. CK means the blank water. L, M and H mean dosages of low, medium and high concentrations, respectively. Different small letters in a column mean significant at 5% levels. . Figure 9. Content of DPC in plants at different stages after dripping. Note: C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S3 means commercially available conventional DPC in 2022. Different small letters in a column mean significant at 5% levels.

Effect of Different Types of DPC on Cotton Yield
Based on the analysis of cotton yield and its constituent factors, it was found that the yield and number of bolls per plant could be increased by spraying them with DPC. Compared with the control, the number of bolls per plant increased by 0.4-1.5 in under DPC treatment. The highest number of bolls per plant under C2L was 7.3 and 6.0 in 2020 and 2021, respectively. The increase in single-boll weight was not significant. In 2020 and 2021, the highest values were 5.33 g and 5.27 g under S1H and C1H treatments, respectively. The maximum difference between treatments was 0.08 g in the same year. In 2020, the yield of the control was 5582.82 kg·hm −2 , which was 16.03% lower than that under C1H treatment, which had the highest yield, and 15.13% lower than that under C2H treatment. The yield of each treatment showed an increasing trend with increasing DPC concentration. In 2021, the control yield was 5566.28 kg·hm −2 , 19.08% lower than that under C2L treatment, which had the highest increase in yield, and 6.06% lower than that under S2H treatment, which had the lowest increase. In 2022, the number of bolls per plant, weight per boll, and yield under slow-release treatment were higher than those under normal treatment; however, the differences were not significant.

DPC Application Mode
DPC application is very common, and its control technology has become necessary for cotton production in Xinjiang. At present, DPC is mainly applied via foliar spraying with a mechanical device 5-6 times during the whole growth period [17,18]. The comprehensive mechanization level of cotton cultivation and harvesting is expected to reach 94% by 2022. To adapt to the mechanized light and simple mode, more than 90% of cotton fields have integrated water and fertilizer treatments. Drip application of DPC could further promote this integration and reduce the cost of cotton planting. Coating seeds with slow-release materials can regulate cotton growth and increase yield [19]. Zhang [20] found that DPC drip application significantly reduced the plant height and internode length of the main cotton stem in a concentration-dependent manner. Therefore, the drip application method is feasible. Our previous experiment also showed that seedling height

Effect of Different Types of DPC on Cotton Yield
Based on the analysis of cotton yield and its constituent factors, it was found that the yield and number of bolls per plant could be increased by spraying them with DPC. Compared with the control, the number of bolls per plant increased by 0.4-1.5 in under DPC treatment. The highest number of bolls per plant under C2L was 7.3 and 6.0 in 2020 and 2021, respectively. The increase in single-boll weight was not significant. In 2020 and 2021, the highest values were 5.33 g and 5.27 g under S1H and C1H treatments, respectively. The maximum difference between treatments was 0.08 g in the same year. In 2020, the yield of the control was 5582.82 kg·hm −2 , which was 16.03% lower than that under C1H treatment, which had the highest yield, and 15.13% lower than that under C2H treatment. The yield of each treatment showed an increasing trend with increasing DPC concentration. In 2021, the control yield was 5566.28 kg·hm −2 , 19.08% lower than that under C2L treatment, which had the highest increase in yield, and 6.06% lower than that under S2H treatment, which had the lowest increase. In 2022, the number of bolls per plant, weight per boll, and yield under slow-release treatment were higher than those under normal treatment; however, the differences were not significant.

DPC Application Mode
DPC application is very common, and its control technology has become necessary for cotton production in Xinjiang. At present, DPC is mainly applied via foliar spraying with a mechanical device 5-6 times during the whole growth period [17,18]. The comprehensive mechanization level of cotton cultivation and harvesting is expected to reach 94% by 2022. To adapt to the mechanized light and simple mode, more than 90% of cotton fields have integrated water and fertilizer treatments. Drip application of DPC could further promote this integration and reduce the cost of cotton planting. Coating seeds with slow-release materials can regulate cotton growth and increase yield [19]. Zhang [20] found that DPC drip application significantly reduced the plant height and internode length of the main cotton stem in a concentration-dependent manner. Therefore, the drip application method is feasible. Our previous experiment also showed that seedling height could be controlled by applying DPC after sowing [21]. However, researchers have also explored problems, such as the necessary large increase in DPC and the difficulty in determining the best time for drip application. However, with the rapid development of nanotechnology, the application of nanomaterials could achieve slow release and reduce DPC adsorption [22]. Simultaneously, the promotion and application of "dry seeding and wet drainage" technology could also achieve chemical control of DPC drip application in the early stages of cotton development, and reduce the number of spraying operations by 1-2 times [1]. In addition, the low price, low toxicity and minimal harm to the soil of DPC ensures the feasibility of drip application. The present results showed that drip application of DPC was easier to operate, and was effective for plant regulation.

Effect of DPC on the Agronomic Traits
DPC was used to reshape the plant structure by regulating cotton plant height and width, thereby improving the canopy structure. Researchers have found that DPC could regulate the daily growth of the main stem in a concentration-dependent manner [23,24]. Synergistic DPC can reduce plant height, reduce the number of fruit branches, and make the leaves upright and the stem thick, making the cotton plant more compact [25][26][27]. All of the above conclusions were drawn under spraying conditions. This experiment used drip application to explore the regulatory effect on the plant height, key main stem node position affecting plant height and plant width step-by-step. Plant height and width could also be controlled via drip application of DPC. Plant height and width were reduced by 10 cm and >4 cm, respectively. This effect improved with increasing concentration and frequency ( Figure 2). Nodes 6-10 are the main stem nodes for height control. The first and second nodes of the fruit branches are the main nodes for width control (Figure 4 and Table 2). In actual production, some fruit branches do not have a second node owing to a variety of characteristics; therefore, the second node has a significant impact on plant width.

DPC Residue in Soil
DPC is a degradable and safe pesticide with little environmental impact. According to the Test Guidelines for Environmental Safety Assessment of Chemical Pesticides, the maximum allowable residue standard of DPC formulated in the United States is 1 ppm. DPC can be easily adsorbed by soil, which reduces its migration and percolation. Li found that the half-life of DPC in soil was 8.2 days, the disappearance rate was >98% after 15 days of application and the minimum detectable concentration was 0.28 ppm [28]. Soil temperature and humidity significantly affected DPC degradation. The average DPC degradation rate in the soil was 3.0-3.5%·d −1 when the temperature was 25 • C and the humidity was 60-70% of the saturated water capacity. The half-life was approximately 8 days, and the time required for 95% degradation was 30-35 days. Tian found that part of the DPC was absorbed by the seed embryo, and another part diffused into the soil and was absorbed by the seedling root system [29]. In this study, the DPC content in the soil was determined after drip application. The difference between DPC contents at different sites was small and irregular, indicating that DPC decreased by more than 30% after seven days of water diffusion (Figure 8). The DPC content was between 2 and 4 µg·g −1 . Although slow-release and increased DPC dosages were used, its soil content under each treatment was significantly reduced to <1.00 µg·g −1 after 15 days, which complied with the safety standards ( Figure 6).

Effect of DPC on Cotton Yield
A single plant and population structure under the control of DPC is more conducive to "short and dense" planting, reducing canopy depression, promoting vegetative growth transformation to reproductive growth, improving dry matter accumulation and increasing yield [30]. Peng and Xu demonstrated that an appropriate amount of DPC could maintain good ventilation and light transmittance in the middle of a cotton field, increase the number of bolls per plant and increase the weight of the bolls per plant [31,32]. Luo also studied the effects of DPC on the canopy structure and cotton yield characteristics at different densities and found that DPC could increase the leaf inclination angle, shape a more compact plant type and improve the canopy structure [33]. DPC could shorten the bud emergence time of fruit nodes by 0.4-2.9 days in different parts of the fruit [34]. The present experiment proved that the yield still increased under dripping DPC, and the key factor was to control and realize the population advantage of high density. The density was increased to more than 150,000 plants·hm −2 under DPC regulation, followed by optimization of the canopy to facilitate boll formation. The number of bolls per plant increased by more than one compared with the control, and the final yield increased by 6-19%. Among the different test dosage forms, treatment with nano-released DPC achieved a higher yield than that of conventional DPC (Table 4). Note: C1 and C2 mean DPC treated with hydrophobic microspheres and hydrophobic supramolecular complexes, respectively. S1 and S3 mean commercially available conventional DPC in 2020 and 2022, respectively. CK means the blank water. L, M and H mean dosages of low, medium and high concentrations, respectively. Different small letters in a column mean significant at 5% levels.

Conclusions
With the results of 3 years of experiment, adopting the drip application method with water could meet the requirements of chemical control; achieve a light, simple and convenient integrated application of water, fertilizer and medicine; and have no harm to the soil environment. Compared with the control, drip application of DPC could reduce the cotton plant's height by more than 10 cm, reduce the plant width by more than 4 cm, increase at least one boll per plant and improve the yield by more than 300 kg·hm −2 . DPC residue in soil was less than 1.00 µg·g −1 . The effect of nano-released DPC was greater than that of common DPC in regulating plant height, plant width, boll formation and yield increasing. However, due to limitations in the application time with water dripping, the duration of the drug effect was still not long enough, resulting in insignificant differences from ordinary DPC and unclear advantages.
With the continuous progress of planting and nano-released technology, drip application of DPC with water would be more accurate and effective with spray assistance.