Effects of Different Irrigation Water Volumes with 1,1-Dimethyl-piperidinium Chloride (DPC) on Cotton Growth and Yield

Abstract

The cotton industry in Xinjiang, China, is limited by irrigation. In cotton production, 1,1-dimethyl-piperidinium chloride (DPC) is used as a growth regulator to improve plant shape, resistance, yield, and quality. However, few studies have investigated the mechanisms by which DPC affects the growth and yield of cotton when combined with different irrigation water volumes. In this study, a split-zone design was used to conduct field experiments over two years using Zhongmiansuo 92 and Zhongmiansuo 087. Three irrigation volumes (3750, 4500, and 5250 m³ hm⁻²; W1, W2, and W3, respectively) and four DPC applications (0, 120, 240, and 480 g hm⁻²) were evaluated. The SPAD (Soil and Plant Analyzer Development) values, photosynthesis, dry matter accumulation and partitioning, agronomic traits, yield, and water use efficiency of cotton leaves were assessed. W2 increased the chlorophyll content and stomatal opening of leaves, improved photosynthetic rates, promoted the accumulation of aboveground dry matter, and increased plant height, main stem node number, and fruit branch platform. The best yields were obtained using W1 and W2 with 120 g hm⁻² DPC and W3 with 240 g hm⁻² DPC. These results can be applied practically to improve cotton production in Xinjiang.

1. Introduction

Cotton (Gossypium hirsutum L.) is an important and widely cultivated economic crop. The development of the cotton industry has had a significant impact on the global cotton market, especially in China [1]. In 2022, the total cotton production in Xinjiang was 5.391 million tonnes, accounting for 90.2% of the national production [2]. Water resources are scarce in Xinjiang, and irrigation is a key factor limiting agricultural production. Under-film drip irrigation technology is widely used to reduce water usage and achieve efficient planting [3,4]. However, at present, due to the excessive pursuit of yield and improper management of cotton production, the amount of irrigation water being used is increasing, resulting in a series of problems such as the wasting of water resources and reduced water use efficiency [5,6]. Therefore, for the sustainable development of the cotton industry in Xinjiang, it is crucial to modify the irrigation strategy to optimize the interactions between irrigation and crop yield and improve water use efficiency while ensuring increased yield.

Various water-saving techniques and measures, such as alternate [7], limited [8], and deficit irrigation [4,6], have previously been explored with the goal of obtaining maximum yield or profit with minimum water use and improving water use efficiency. Reasonable field irrigation can promote cotton growth, balance vegetative and reproductive growth, and increase the yield. Excessive irrigation can lead to an unreasonable canopy structure distribution, with an increase in the number of green leaves in the upper canopy, which leads to intensified shading between leaves, reduced light utilization, and slower maturity of cotton bolls in the later stages of growth and development, and thus is not conducive to obtaining high yields [9,10]. Therefore, the use of appropriate irrigation volumes would optimize the cotton canopy structure, improve the efficiency of light energy utilization and accumulation of cotton biomass, increase yields, and improve water use efficiency.

Cotton exhibits an infinite growth habit, and 1,1-dimethyl-piperidinium chloride (DPC) is a plant growth regulator that acts as an antagonist to gibberellic acid activity in plants, including cotton [11]. It can restrict cell expansion and elongation, thus reducing internode length and the massive growth of vegetative organs, optimizing canopy structure, and regulating the reasonable distribution of vegetative and reproductive organs, thereby promoting increased yield [11,12,13,14]. The scientific and reasonable use of DPC can improve cotton plant shape, increase the efficiency of light energy utilization, and improve cotton yield and quality [15]. With the use of under-film drip irrigation, different irrigation volumes require corresponding doses of DPC to achieve maximum cotton productivity and high yields [16]. Some studies have shown that the appropriate use of DPC can effectively increase the number of cotton bolls in watery years, whereas in dry years, the use or excessive use of DPC can significantly reduce the number of cotton bolls [17].

The profitability of cotton planting is crucial because it can restrict farmers’ incomes and affect their enthusiasm for cotton planting. Proper irrigation with DPC application is necessary to obtain high cotton yields. Previous research on irrigation volume and DPC has primarily focused on cotton topping techniques, irrigation volume and density, and DPC and density [16]. In contrast, research on the balanced application of irrigation volume and DPC and its effects on cotton growth, yield, and water use efficiency has been limited. This study addresses the scarcity of water resources in Xinjiang and identifies the optimal combination of irrigation volume and DPC dosage suitable for cotton-growing areas in this region. Exploring the effect of the optimized combination of irrigation and DPC on cotton growth, yield, and water use efficiency has both theoretical and practical significance for cost-effective and efficient cotton production in Xinjiang.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted from 2022 to 2023 at the Huyanghe Experimental Base of the Cotton Research Institute, Chinese Academy of Agricultural Sciences (N 44°43′, E 84°48′). The experimental site was in the Northern Xinjiang region, which is characterized by an arid climate with a high number of sunshine hours and high evaporation rates. The soil type at the experimental site was grey desert soil [18], and the previous crop was cotton, which had been continuously planted at the site for more than 10 years. A 0–20 cm tillage soil layer contains 17.8% clay, 40.7% silt, and 41.5% sand, the organic matter content was 14.62 g kg⁻¹, total nitrogen was 0.82 g kg⁻¹, alkali-hydrolyzable nitrogen was 66.43 mg kg⁻¹ g, available phosphorus was 24.96 mg kg⁻¹, available potassium was 236.27 mg kg⁻¹, and pH was 7.85. In 2022, the total precipitation of cotton growing season is 47.7 mm, and the average daily temperature from April 15 to October 15 is 21.8 °C (Figure 1a). In 2023, the cumulative precipitation of the whole growing season is 96.3 mm, and the average daily temperature from April 15 to October 15 is 21.2 °C (Figure 1b). The temperature and precipitation data for the cotton growing season from April to October are shown in Figure 1.

2.2. Experimental Design

Zhongmiansuo 92 (loose type) and Zhongmiansuo 087 (compact type) were provided by the Cotton Research Institute of the Chinese Academy of Agricultural Sciences. The experiment employed a split-plot design, with irrigation volume as the main plot and DPC treatment as the sub-plot. The experiment was set up with three irrigation volumes (3750, 4500, and 5250 m³ hm⁻² [W1, W2, and W3, respectively]) and four DPC dosages (0, 120, 240, and 480 g hm⁻² [H0, H1, H2, and H3, respectively]), with each subplot containing two films and a row length of 6 m. Each treatment was replicated four times with 96 subplots.

The experiment was conducted in a 1-film, 3-row planting pattern with under-film drip irrigation, with a row spacing of 76 cm, plant spacing of 7.2–7.3 cm, and a theoretical density of 18 × 10⁴ plants hm⁻². The sowing dates were 22 April 2022 and 20 April 2023, with uniform drip irrigation for seedling emergence. Irrigation treatments started on June 10 (seedling stage) each year with an average interval of seven days. Irrigation frequency and volume were set according to the total irrigation amount for the entire growth period (

Table 1. Irrigation amount (m³ hm⁻²) and time (day/month) for each treatment during the cotton growing season from 2022 to 2023.

Treatment	Date											Total
	10/6	17/6	24/6	1/7	8/7	15/7	22/7	29/7	5/8	12/8	19/8
W1	300	300	300	375	375	375	375	375	375	300	300	3750
W2	375	375	375	450	450	450	450	450	450	375	300	4500
W3	450	450	450	525	525	525	525	525	525	450	300	5250

). The irrigation system was designed with meters installed on the pipelines, and the valves were strictly closed according to the set readings. DPC was applied according to the growth stage (

Table 2. DPC dosage (g hm⁻²) and time (day/month) for each treatment during the cotton growing season from 2022 to 2023.

Treatment	Date				Total
	5/6	20/6	5/7	15/7
H0	0.00	0.00	0.00	0.00	0
H1	5.25	9.75	45.00	60.00	120
H2	9.75	20.25	90.00	120.00	240
H3	20.25	39.75	180.00	240.00	480

), with all DPC active ingredients having a content of >98% (produced by Zhongmian Xiaokang Biotechnology Co., Ltd., Zhengzhou, China). Manual top-dressing was performed on July 10, and other field management was the same as that used in high-yielding fields. Harvest dates were 5 October 2022 and 10 October 2023.

2.3. Measurement Items

2.3.1. Agronomic Traits

During the boll-opening stage, five representative cotton plants were selected consecutively from each subplot to investigate agronomic trait indicators such as plant height, stem diameter, main stem node number, and fruit branch count.

2.3.2. Dry Matter Accumulation and Distribution

During the cotton bud, flowering, and boll-opening stages, three representative cotton plant samples were selected from each experimental subplot. These were divided into four parts according to the roots (below the cotyledon node), stems, leaves, and bud bolls. The samples were placed in an oven at 105 °C for 30 min to wilt and dried at 80 °C to a constant weight. The dry matter mass of each component was weighed using an electronic balance, and the organ distribution rate was calculated.

2.3.3. Leaf SPAD Value

Using a SPAD-502 (KONICA MINOLTA, Osaka, Japan) portable chlorophyll content measuring instrument, the relative chlorophyll content (SPAD value) for third inverted main stem functional cotton leaves (inverted one leaf after topping) was measured. Five leaves per subplot were sampled, and three spots on each leaf (avoiding veins) were measured to obtain the average values.

2.3.4. Single Leaf Photosynthetic Physiology

Photosynthesis measurements were obtained using a LI-6400XT (LI-cor, Lincoln, NE, USA) photosynthesis analyzer on clear, cloudless, and windless days between 11:00 and 14:00. The net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), and transpiration rate (Tr) of the functional leaves were measured during the bud stage (15 June), flowering stage (15 July), and peak boll stage (15 August). Four leaves per subplot were measured to obtain the average values.

2.3.5. Yield and Irrigation Water Use Efficiency

During the boll-opening stage, one film was harvested to measure the seed cotton yield of each subplot. The number of plants in the producing film and the number of bolls for 10 consecutive plants in each subplot were investigated. Finally, the number of bolls per plant was determined. According to the top (inverted 1–3 fruiting branches), middle (4–7 fruiting branches), and bottom (1–3 fruiting branches) plant sections, 30 boll-opening bells were separately collected. The boll-opening bells were then weighed and ginned, and the lint cotton was weighed to obtain the weight of a single boll and the ginning yield. Irrigation water use efficiency (IWUE) was determined using the amount of irrigation during the reproductive period, and the calculation was performed using the formula: IWUE = Y/I, where Y is the seed cotton yield in kg hm⁻², and I is the total irrigation amount during the growth period in m³ hm⁻².

2.3.6. Fibre Quality

Lint cotton samples were sent to the Cotton Quality Supervision and Testing Centre of the Ministry of Agriculture for analysis (Anyang Henan, China). A fiber detector (HVI-1000) was used to measure the cotton fiber quality and the cotton fiber calibrator HVICC was used for calibration (U.S. Department of Agriculture). The measurement indexes included the average length of the upper half of the cotton fiber, neatness index, breaking specific strength, elongation, and Micronaire value.

2.4. Data Processing

Experimental data were organized using Microsoft Excel 2021and analyzed using SPSS Statistics 19 software. Analysis of variance was performed using the least significant difference method (p < 0.05 and p < 0.01), and Duncan’s multiple range test was used for inter-treatment comparisons. Graphs were generated using the Origin2022 software.

3. Results

3.1. Agronomic Traits

The results from the experiments in 2022 and 2023 showed that irrigation volume significantly affected cotton plant height, stem thickness, and fruit branch count (p < 0.01) and that DPC dosage significantly affected cotton plant height and fruit branch count (p < 0.05). There were significant differences in plant height, stem thickness, main stem node number, and fruit branch count between the two varieties assessed (p < 0.01;

Table 3. Effects of DPC dosage on the agronomic traits of cotton with different irrigation amounts (2022–2023).

Variety	Treatment		Plant Height (cm)	Stem Diameter (mm)	Node Number of Main Stem	Number Fruit Branch
2022
ZMS92	W1	H0	87.55 ab	7.99 d	13.30 ab	9.10 ab
		H1	86.55 ab	8.89 cd	13.05 ab	8.55 ab
		H2	78.65 bc	8.65 cd	12.60 b	7.95 b
		H3	70.35 c	8.07 cd	12.55 b	7.95 b
	W2	H0	89.65 ab	8.68 bcd	13.40 ab	8.95 ab
		H1	88.15 ab	9.86 bcd	13.20 ab	8.85 ab
		H2	81.10 bc	8.79 bcd	13.05 ab	8.65 ab
		H3	73.35 c	9.00 abc	12.70 ab	8.30 ab
	W3	H0	98.70 a	10.76 abc	13.95 a	9.45 a
		H1	93.60 a	10.34 ab	13.40 ab	9.30 ab
		H2	88.55 ab	9.68 a	13.15 ab	9.15 ab
		H3	86.95 ab	9.69 a	13.05 ab	8.80 ab
ZMS087	W1	H0	109.60 abc	9.08 b	13.45 abcd	8.85 abc
		H1	100.10 bcd	9.39 b	12.95 cd	8.40 abcd
		H2	95.75 de	9.05 b	12.85 de	8.10 cd
		H3	85.25 e	9.56 b	12.05 e	7.35 d
	W2	H0	110.90 abc	9.08 b	13.50 abcd	9.05 abc
		H1	101.15 bcd	9.48 b	13.35 bcd	8.50 abcd
		H2	99.15 cd	9.43 b	13.25 bcd	8.50 abcd
		H3	94.40 de	9.24 b	13.05 cd	8.15 bcd
	W3	H0	117.60 a	10.84 a	14.40 a	9.50 a
		H1	112.90 ab	9.55 b	14.10 ab	9.50 a
		H2	109.45 abc	9.37 b	14.05 ab	9.35 ab
		H3	101.9 bcd	9.31 b	13.90 abc	8.75 abc
Variance analyses
Variety (V)			**	**	*	**
Irrigation amount (I)			**	NS	NS	**
Dose of DPC (D)			*	NS	NS	*
V × I			**	NS	NS	**
V × D			NS	NS	NS	NS
I × D			NS	NS	NS	NS
V × I × D			NS	NS	NS	NS
2023
ZMS92	W1	H0	96.67 bc	9.27 d	15.00 a	10.11 ab
		H1	86.44 cd	9.43 cd	14.78 a	10.11 ab
		H2	79.33 de	9.84 bcd	14.67 ab	9.67 bc
		H3	63.89 f	8.77 d	13.67 b	8.67 c
	W2	H0	105.78 b	10.23 abcd	15.33 a	10.00 ab
		H1	93.33 c	9.58 cd	15.11 a	9.89 ab
		H2	80.78 de	9.53 cd	15.11 a	9.56 bc
		H3	72.67 ef	9.24 d	14.44 ab	9.67 bc
	W3	H0	118.78 a	11.07 ab	15.44 a	10.89 a
		H1	104.11 b	11.36 a	15.11 a	10.56 ab
		H2	88.00 cd	10.81 abc	14.89 a	10.00 ab
		H3	79.22 de	10.18 abc	14.45 ab	9.78 b
ZMS087	W1	H0	75.33 b	8.79 bcd	14.67 abc	9.56 a
		H1	72.89 b	9.05 abcd	14.22 abcd	9.55 a
		H2	59.22 cd	8.72 cd	13.44 cd	7.78 cd
		H3	51.22 d	8.04 d	13.00 d	7.11 d
	W2	H0	87.33 a	9.08 abcd	15.00 ab	10.00 a
		H1	73.33 b	9.54 abc	14.11 bcd	9.33 ab
		H2	67.45 bc	9.11 abcd	14.00 bcd	9.33 ab
		H3	58.44 cd	9.52 abc	13.45 cd	8.00 bcd
	W3	H0	93.55 a	10.19 a	15.67 a	9.89 a
		H1	87.67 a	10.22 a	14.78 abc	9.67 a
		H2	70.56 b	10.06 ab	14.44 abcd	9.56 abc
		H3	69.22 b	9.81 abc	14.33 abcd	9.00 a
Variance analyses
Variety (V)			**	**	**	**
Irrigation amount (I)			**	**	NS	**
Dose of DPC (D)			**	NS	**	**
V × I			NS	NS	NS	NS
V × D			NS	NS	NS	NS
I × D			NS	NS	NS	NS
V × I × D			NS	NS	NS	NS

Note: W1, W2, and W3 indicate the irrigation treatments with 3750, 4500, and 5250 m³ hm⁻², respectively. H0, H1, H2, and H2 indicate 0, 120, 240, and 480 g hm⁻² DPC, respectively. Different lowercase letters in the same column indicate significant differences (p < 0.05). NS, *, and ** indicate p > 0.05, p < 0.05, and p < 0.01, respectively.

). In 2022, the interaction between variety and irrigation volume significantly affected plant height and the number of fruit branches (p < 0.01). Under different irrigation volumes, both varieties showed increasing trends for their agronomic trait indicators, with W3 > W2 > W1 for both years. In 2022, with the W3 treatment, ZMS92 and ZMS087 showed increases in plant height, stem thickness, main stem node number, and fruit branch count of 13.83%, 20.45%, 10.48%, 6.11% and 13.09%, 5.37%, 8.08%, 11.37%, respectively, compared with the W1 treatment. In 2023, with the W3 treatment, ZMS92 and ZMS087 showed increases in plant height, stem thickness, main stem node number, and fruit branch count of 19.54%, 16.38%, 8.26%, 4.59% and 24.10%, 16.42%, 10.64%, 8.97%, respectively, compared with the W1 treatment.

The indicators of all cotton agronomic traits under the different DPC dosages showed decreasing trends in both years, as follows: H0 > H1 > H2 > H3. In 2022, under the W1, W2, and W3 irrigation volumes, ZMS92 and ZMS087 showed decreases in plant height with the H3 treatment of 19.65%, 18.18%, 11.90% and 22.22%, 14.88%, 13.35%, respectively, when compared with the H0 treatment; main stem node number decreased by 5.64%, 5.22%, 6.45% and 10.41%, 3.33%, 3.47%, respectively; and fruit branch count decreased by 12.64%, 7.26%, 6.88% and 16.96%, 9.94%, 7.89%, respectively. In 2023, with the H3 treatment, ZMS92 and ZMS087 showed decreases in plant height of 33.91%, 31.30%, 33.31% and 32.01%, 33.08%, 26.01%, respectively, when compared with the H0 treatment; main stem node number decreased by 8.87%, 5.81%, 6.41% and 11.38%, 10.33%, 8.55%, respectively; and fruit branch count decreased by 14.24%, 3.30%, 10.19% and 25.63%, 20.00%, 9.00%, respectively. Thus, DPC had a gradual decreasing effect on cotton agronomic traits with increasing irrigation volume.

3.2. Dry Matter Accumulation and Distribution

The aboveground dry weight and dry matter distribution levels directly affect cotton yield. In this study, Dry matter accumulation fluctuates from the flowering stage, but the difference is not significant, and zms92 varieties are more sensitive to DPC response (Figure 2). With the same DPC treatments, both ZMS92 and ZMS087 exhibited an increasing trend in the aboveground dry matter weight as water increased. The aboveground dry matter weights of ZMS92 and ZMS087 with the same irrigation treatment at different fertility periods showed a trend of increasing and then decreasing with increased DPC, with the largest differences occurring at the boll-opening stage. This indicates that the interaction between irrigation volume and DPC treatment becomes increasingly stronger during the reproductive process.

In 2022 and 2023, the data showed that at the boll-opening stage, the aboveground dry matter weight of ZMS92 under different irrigation volumes followed a trend of initially increasing and then decreasing with the increase in DPC dosage, with significant differences between each DPC treatment. For the W1 and W2 treatments, the highest aboveground dry matter weight was observed with the H1 dosage, and this resulted in a significant increase of 44.53%, 37.22% and 9%, 3.94%, respectively, compared with H0. The W3 treatment peaked with the H2 dosage, with significant increases of 24.23% and 16.37%, respectively, compared with H0. Across the two-year treatment period, the highest aboveground dry matter weight was obtained using the W2H1 treatment. With the same DPC dosage, the above-ground dry matter weight for ZMS92 increased with an increase in irrigation amount. With the same water treatment, the aboveground dry matter weight of ZMS087 at the boll-opening stage followed a trend of initially increasing and then decreasing with the increase in DPC dosage, peaking with the W2H1 treatment at 113.81 g and 132.11 g, respectively. The highest dry matter weight with the W3 treatment was observed with the H1 dosage.

The data from 2022 and 2023 (Figure 3) also showed that during the bud stage, the dry matter distribution in different cotton organs was as follows: leaves > stems > bud bolls. At the flowering stage, the proportion of leaves decreased significantly, whereas the proportion of bud bolls increased notably, with no significant change in the stem proportion. By the boll-opening stage, the proportion of bud bolls further increased, whereas the proportions of stems and leaves significantly decreased. During the boll-opening stage, the proportion of buds and bolls occupied more than 60% and 50% of the upper dry matter weight, respectively, in 2022 and 2023. In both years, the two varieties showed that under the same irrigation volume, the proportion of bud bolls in the aboveground dry matter first increased and then decreased with increasing DPC dosage.

Irrigation volume significantly affected dry matter distribution in bud bolls during the flowering and boll-opening stages. Under different irrigation volumes, the percentage of bud bolls was consistently W1 > W2 > W3, indicating that moderate drought could effectively increase the percentages of the reproductive organs. Application of DPC during the flowering and boll-opening stages effectively increased the proportion of bud bolls, but this increase diminished with the increased DPC dosages. Results from 2022 showed that during the boll-opening stage, the percentage of bud bolls was highest with the H1 dosage. In 2023, with the W1 treatment, the highest percentage of buds for ZMS92 was 60.73%, and this occurred with the H1 dosage. With the conventional W2 and excessive W3 treatments, the highest percentage of buds was 60.08% and 58.15%, respectively, and these occurred with the H2 dosage. In 2023, with the W1 and W2 treatments, the highest percentage of buds for ZMS087 was 60.81% and 58.00%, respectively, and these occurred with the H2 dosage. Whereas with the W3 treatment, the highest percentage of buds was 56.09%, and this occurred with the H1 dosage.

Among the combination treatments, ZMS92 reached maximum bud boll proportions during the boll-opening stage with the W1H1 treatment in both years, at 66.86% and 60.73%, respectively. In 2022, ZMS087 reached its highest bud boll percentage during the boll-opening stage with the W1H1 treatment, at 77.79%, and in 2023, the highest level was with the W1H2 treatment, at 60.81%. This suggests that appropriate irrigation volumes and DPC dosages can increase the percentage of bud bolls, and the compact cotton variety ZMS087 is more efficient than the loose variety ZMS92 in trans-locating dry matter to reproductive organs for cotton yield formation.

3.3. SPAD Value

The results show that from the bud stage to the peak boll stage, the chlorophyll content in the cotton leaves progressively increased (Figure 4). There were significant differences in SPAD values due to irrigation volume, DPC dosage, and variety (p < 0.01). In 2023, under different irrigation volumes, the SPAD values of ZMS92 and ZMS087 showed a decreasing trend, characterized as W1 > W2 > W3. Differences in SPAD values under different DPC dosages also reached significant levels, with both ZMS92 and ZMS087 showing a pattern of H0 < H1 < H2 < H3 across all three growth stages. During the bud stage, the SPAD values for both varieties with the H3 dosage were 20.22% and 22.01% higher than those with H0, respectively. During the flowering stage, they were 24.26% and 29.59% higher, respectively, and during the peak boll stage, they were 15.36% and 14.11% higher, respectively. The SPAD values of ZMS92 were approximately 4.66%, 3.50%, and 2.82% higher than those of ZMS087 during the bud, flowering, and peak boll stages, respectively. In 2023, significant differences were observed in the interaction effects between the main factors during the flowering stage.

The results for 2022 and 2023 are the same. At each growth stage, under different irrigation volumes, the SPAD values of the cotton leaves increased with the DPC dosage. This indicates that the use of DPC under various water treatments can promote an increase in the chlorophyll content of cotton leaves. Mild drought stress contributed to the increase in chlorophyll content in cotton leaves, with ZMS92 having higher leaf chlorophyll content than ZMS087.

3.4. Photosynthetic Characteristics

The leaf photosynthetic rate (Pn) initially increased and then decreased as the growth stages progressed and peaked during the flowering stage (Figure 5). The impact of different irrigation volumes on the Pn of cotton leaves was significant during the bud and flowering stages, showing a clear increasing trend with increased irrigation, whereas there was no significant difference during the peak boll stage. During the flowering and peak boll stages, the interactions between varieties, irrigation volume, and DPC dosage also reached significant levels. During the flowering stage, Pn reached a maximum of 24.04 µmol CO₂ m⁻² s⁻¹ for ZMS92 with the W3H1 treatment and 23.10 µmol CO₂ m⁻² s⁻¹ for ZMS087 with the W3H3 treatment. During the peak boll stage, it reached a maximum of 19.58 µmol CO₂ m⁻² s⁻¹ for ZMS92 with the W1H2 treatment and 20.10 µmol CO₂ m⁻² s⁻¹ for ZMS087 with the W3H2 treatment. The response of cotton leaf Pn to irrigation volume and DPC dosage varied across growth stages, and its photosynthetic performance was closely related to variety. The compact cotton variety maintained a high photosynthetic capacity during the later stages of growth and development. Compared with the other two growth stages, irrigation volume had the greatest impact on the photosynthetic capacity of cotton during the bud stage, and sufficient watering before this stage helped to maintain high photosynthetic capability. The application of DPC effectively enhanced the photosynthetic capacity of cotton during both the bud and peak boll stages, and the increase in the photosynthetic rate (Pn) was more pronounced with higher DPC concentrations (H2, H3).

Further analysis of the stomatal conductance (Gs), intercellular CO₂ concentration (Ci), and transpiration rate (Tr) revealed that both Gs and Tr initially increased and then decreased as the growth stages progressed, peaking during the flowering stage, whereas Ci showed a gradual decreasing trend. Across the different growth stages, Gs and Tr for ZMS92 leaves were higher than those for ZMS087 leaves. The impact of irrigation volume on Gs, Ci, and Tr was significant during the bud and flowering stages. In the bud stage, Gs gradually increased with increased DPC dosage, showing a clear upward trend, whereas Ci and Tr showed no significant changes. There was a clear interaction between irrigation volume and DPC dosage during all growth stages. Under different irrigation volumes, Gs and Tr for leaves of both varieties increased after DPC treatment at the bud and peak boll stages, whereas the trend for Ci was the opposite. The results indicate that the photosynthetic performance of cotton during different growth stages was closely related to stomatal openness, which was modulated by irrigation volume and DPC. There were obvious differences in the responses of cotton leaf stomata to irrigation amounts and DPC dosages at different growth stages.

3.5. Yield and Irrigation Water Use Efficiency

With different irrigation volumes and DPC dosages, there were significant differences in boll number per plant and seed cotton yield among cotton varieties (p < 0.01;

Table 4. Effects of DPC dosage on cotton yield and formation factors with different irrigation amounts (2022–2023).

Variety	Treatment		Boll Numbers Per (No.)	Boll Weight (g)	Lint Percentage (%)	Seed Cotton Yield (kg·hm−2)
2022
ZMS92	W1	H0	9.7 b	5.81 b	47.29 a	6569.85 b
		H1	10.55 ab	6.22 ab	47.08 a	6770.88 ab
		H2	10.3 ab	6.10 ab	46.76 a	6716.05 ab
		H3	10.25 ab	5.89 ab	46.19 a	6661.2 ab
	W2	H0	10.15 ab	6.05 ab	47.04 a	6633.8 ab
		H1	11.85 a	6.30 a	47.40 a	7127.23 a
		H2	11.4 ab	6.18 ab	46.41 a	7008.43 ab
		H3	11.25 ab	6.10 ab	46.27 a	6951.2 ab
	W3	H0	9.9 b	6.00 ab	46.81 a	6524.13 b
		H1	11.45 ab	6.18 ab	46.30 a	7017.6 ab
		H2	11.55 ab	6.31 a	46.30 a	7035.83 ab
		H3	10.75 ab	6.14 ab	46.06 a	6807.43 ab
ZMS087	W1	H0	11.7 cd	5.35 a	45.58 a	7063.25 de
		H1	11.95 cd	5.36 a	46.06 a	7127.23 cde
		H2	11.75 cd	5.54 a	45.52 a	7035.85 de
		H3	11.05 d	5.37 a	45.50 a	6831.18 e
	W2	H0	12.4 cd	5.42 a	45.98 a	7291.7 bcde
		H1	13.2 abcd	5.58 a	45.88 a	7492.7 abcd
		H2	13 abcd	5.54 a	45.55 a	7410.5 bcde
		H3	12.85 bcd	5.53 a	45.52 a	7392.2 bcde
	W3	H0	13.65 abc	5.50 a	45.89 a	7547.58 abcd
		H1	15.1 a	5.64 a	45.65 a	8004.43 a
		H2	14.75 ab	5.53 a	45.68 a	7839.95 ab
		H3	13.95 abc	5.51 a	45.24 a	7693.75 abc
Variance analyses
Variety (V)			**	**	*	**
Irrigation amount (I)			**	NS	NS	**
Dose of DPC (D)			*	NS	NS	*
V × I			**	NS	NS	**
V × D			NS	NS	NS	NS
I × D			NS	NS	NS	NS
V × I × D			NS	NS	NS	NS
2023
ZMS92	W1	H0	8.67 bc	6.12 abcd	45.48 abc	5802.28 bc
		H1	9.00 bc	5.96 cd	45.50 abc	5966.75 abc
		H2	7.11 c	6.17 abcd	44.76 abc	5829.70 bc
		H3	7.00 c	6.39 ab	44.28 c	5573.85 c
	W2	H0	8.67 bc	6.49 a	44.31 c	5975.93 abc
		H1	9.11 bc	6.06 bcd	45.32 abc	6147.68 abc
		H2	12.11 a	6.29 abc	44.96 abc	6597.25 ab
		H3	10.67 ab	5.89 d	45.94 a	6149.50 abc
	W3	H0	9.78 ab	6.09 abcd	45.83 abc	6423.65 ab
		H1	8.00 bc	6.29 abc	45.71 abc	6131.23 abc
		H2	10.44 ab	6.18 abcd	44.85 abc	6770.85 a
		H3	9.89 ab	6.22 abcd	44.41 bc	6743.45 a
ZMS087	W1	H0	10.00 ab	5.42 c	45.18 a	5418.53 cd
		H1	7.44 bc	5.47 bc	44.66 abc	5455.05 cd
		H2	9.67 ab	5.81 ab	44.24 bcd	5939.35 bcd
		H3	6.89 c	5.54 abc	43.63 d	5098.70 d
	W2	H0	9.45 ab	5.40 c	45.48 a	6195.18 abc
		H1	10.66 a	5.55 abc	44.88 ab	6213.48 abc
		H2	8.78 ab	5.67 abc	44.08 bcd	6268.30 abc
		H3	9.22 ab	5.67 abc	43.54 d	5966.78 bcd
	W3	H0	9.45 ab	5.72 abc	43.93 cd	6752.59 ab
		H1	11.22 a	5.81 ab	44.84 ab	7098.00 a
		H2	10.56 a	5.58 abc	44.81 ab	6926.23 ab
		H3	9.34 ab	5.83 a	43.85 cd	6981.03 ab
Variance analyses
Variety (V)			NS	**	**	NS
Irrigation amount (I)			**	NS	NS	**
Dose of DPC (D)			*	**	**	*
V × I			NS	NS	NS	NS
V × D			NS	NS	NS	NS
I × D			NS	NS	NS	NS
V × I × D			*	NS	NS	NS

Note: “W” indicates three irrigation treatments (W1: 3750, W2: 4500, W3: 5250 m³ hm⁻²); “H” indicates four types of DPC dosage (H0:0, H1:120, H2: 240, H3: 480 g hm⁻²). Different lowercase letters in the same column indicate significant differences (p < 0.05); NS, * and ** indicate no significant difference (p > 0.05), significant difference (p < 0.05), and extremely significant difference (p < 0.01), respectively.

). DPC dosage significantly affected boll number per plant and seed cotton yield (p < 0.05). Data from 2022 showed highly significant differences between varieties in the number of bolls per plant and weight per boll (p < 0.01) and significant differences in ginning yield (p < 0.05). Data from 2023 indicated significant differences between varieties in weight per boll and ginning yield (p < 0.01).

Increasing irrigation volume leads to an increasing trend in seed cotton yield for ZMS087. In both 2022 and 2023, the seed cotton yield was highest for the W3 treatment, with yield increases of 6.86%, 12.31%, 11.43%, 12.63% and 24.62%, 30.12%, 16.62%, 36.92% with the H0, H1, H2, and H3 dosages, respectively, compared with that of W1. In 2022, ZMS92 reached its highest yield of 6633 kg·hm⁻² with the W2 treatment and in 2023, the highest yield was 6424 kg·hm⁻² with the W3 treatment. Under the same irrigation water treatment, both the number of bolls per plant and the seed cotton yield of ZMS92 and ZMS087 showed a tendency to increase and then decrease with increases in the DPC dosage. In 2022, both ZMS92 and ZMS087 achieved their highest yields with the H1 dosage. In 2023, for the ZMS92 under the W1 treatment, the highest yield was with the H1 dosage, while for the W2 and W3 treatments, the highest yields were obtained with the H2 dosage. For ZMS087, the highest yield for all irrigation volumes was with the H2 dosage.

Irrigation water use efficiency (IWUE) tended to decrease with increased irrigation, and the differences were significant between the different irrigation amounts (p < 0.01) (Figure 6), with the highest IWUE occurring with the W1 treatment. Spraying DPC increased the IWUE, but the difference was not significant. These results indicate that mild drought can improve IWUE but also reduce yield. Overall, irrigation volume was a significant factor affecting yield and IWUE.

3.6. Fibre Quality

Data from 2022 and 2023 (

Table 5. Variance analysis showing the effects of irrigation amount and DPC dosage on fiber quality.

			p Value
Year	Varieties		Fiber Length (mm)			Fiber Strength (cN/tex)			Micronaire Value			Elongation (%)
			Upper	Middle	Lower	Upper	Middle	Lower	Upper	Middle	Lower	Upper	Middle	Lower
2022	ZMS087	I	0.006 **	0.007 **	0.001 **	0.038 *	0.023 *	0.041 *	<0.001 **	<0.001 **	<0.001 **	0.188	0.014 *	0.183
		D	0.835	0.781	0.888	0.731	0.442	0.433	0.227	0.737	0.396	0.578	0.478	0.604
		I × D	0.304	0.697	0.209	0.263	0.471	0.031 *	0.892	0.460	0.833	0.863	0.543	0.781
2023		I	<0.001 **	<0.001 **	<0.001 **	0.003**	0.001 **	0.002 **	<0.001 **	<0.001 **	<0.001 **	0.78	0.012*	0.078
		D	0.653	0.284	0.582	0.407	0.101	0.888	0.222	0.462	0.480	0.578	0.004 **	0.333
		I × D	0.136	0.949	0.969	0.661	0.949	0.804	0.996	0.989	0.904	0.494	0.379	0.992
2022	ZMS92	I	0.022 *	0.047 *	0.004 **	0.125	0.09	0.316	0.036 *	0.002 **	0.002 **	0.611	0.395	0.156
		D	0.025 *	0.868	0.448	0.014 *	0.95	0.826	0.004 **	0.713	0.855	0.131	1	0.582
		I × D	0.083	0.919	0.592	0.217	0.617	0.537	0.387	0.966	0.693	0.804	0.469	0.228
2023		I	<0.001 **	<0.001 **	<0.001 **	0.042	0.343	0.196	<0.001 **	<0.001 **	<0.001 **	0.593	<0.001 **	0.022 *
		D	0.003 **	0.542	0.245	0.111	0.583	0.507	0.039 *	0.778	0.451	0.566	0.093	0.602
		I × D	0.625	0.992	0.407	0.993	0.975	0.141	0.928	0.833	0.73	0.839	0.841	0.501

* and ** indicate significant main effects or interactions of p < 0.05 and p < 0.01, respectively. I, irrigation amount; D, DPC dose; I × D, irrigation amount × DPC dose.

) show that irrigation volume significantly affected fiber length, fiber strength, and Micronaire values with significant differences in ZMS087 (p < 0.01), as well as the mid-fibre elongation rate (p < 0.05). DPC had no significant effects on the fiber quality of ZMS087. Over the two years, irrigation volume affected the fiber length and Micronaire values of ZMS92 (p < 0.01) but did not significantly affect fiber strength. DPC significantly affected the fiber length and Micronaire values of the upper bolls of ZMS92.

The data from 2022 and 2023 (

Table 6. Effects of different treatments on cotton fiber quality (2022–2023). Different lowercase letters in the same column indicate significant differences of P < 0.05.

	Treatment	Fiber Length (%)			Uniformity Index (%)			Fiber Strength (cN/tex)			Micronaire Value			Elongation (%)
Varieties
		Upper	Middle	Lower	Upper	Middle	Lower	Upper	Middle	Lower	Upper	Middle	Lower	Upper	Middle	Lower
2022
ZMS087	W1H0	27.8 bc	31.13 abc	32.73 abc	83.95 a	85.78 a	86.03 a	27.30 ab	33.10 ab	34.73 abc	4.48 ab	4.08 abc	3.95 ab	6.60 a	6.70 b	6.73 a
	W1H1	28.08 bc	30.73 c	32.05 abc	85.00 a	85.08 a	85.55 a	28.13 ab	33.00 ab	33.08 c	4.48 ab	4.18 ab	4.15 a	6.60 a	6.70 ab	6.73 a
	W1H2	28.18 bc	31.38 abc	31.53 c	85.58 a	85.88 a	85.65 a	27.75 ab	32.60 ab	32.83 c	4.48 ab	4.18 ab	4.05 a	6.60 a	6.70 ab	6.70 a
	W1H3	27.93 bc	30.90 abc	31.6 c	84.18 a	85.95 a	86.08 a	26.80 ab	31.60 b	32.75 c	4.68 a	4.15 ab	4.15 a	6.60 a	6.70 ab	6.70 a
	W2H0	28.08 bc	30.88 bc	31.88 bc	85.33 a	85.63 a	85.83 a	27.08 ab	32.23 ab	34.15 abc	4.28 bcd	4.23 a	3.88 ab	6.60 a	6.68 ab	6.73 a
	W2H1	27.25 c	31.23 abc	32.03 abc	84.18 a	85.63 a	85.98 a	26.53 b	34.33 a	33.13 c	4.43 abc	4.15 ab	3.93 ab	6.60 a	6.70 ab	6.75 a
	W2H2	28.15 bc	31.88 abc	32.30 abc	83.83 a	85.83 a	86.25 a	28.28 ab	33.10 ab	33.55 bc	4.25 bcd	4.18 ab	4.03 a	6.63 a	6.70 ab	6.73 a
	W2H3	28.18 bc	31.50 abc	32.73 abc	85.15 a	85.45 a	85.28 a	27.83 ab	33.10 ab	33.78 abc	4.35 bcd	4.10 abc	3.88 ab	6.60 a	6.70 ab	6.73 a
	W3H0	28.45 abc	31.93 abc	32.9 ab	84.08 a	85.58 a	86.20 a	28.23 ab	32.80 ab	33.33 bc	4.10 d	3.73 d	3.60 b	6.60 a	6.73 ab	6.73 a
	W3H1	29.58 a	32.50 ab	33.20 a	85.03 a	85.83 a	85.15 a	28.70 a	33.88 ab	35.38 ab	4.13 cd	3.88 bcd	3.63 b	6.63 a	6.75 ab	6.75 a
	W3H2	28.90 ab	31.80 abc	32.98 ab	84.83 a	85.10 a	85.28 a	28.13 ab	33.20 ab	33.58 bc	4.10 d	3.80 cd	3.80 ab	6.63 a	6.70 ab	6.73 a
	W3H3	28.45 abc	32.58 a	33.03 ab	84.88 a	84.90 a	85.68 a	28.50 a	34.48 a	35.68 a	4.25 bcd	4.05 abc	3.78 ab	6.63 a	6.73 ab	6.78 a
ZMS92	W1H0	27.33 bc	29.45 a	29.33 b	84.63 a	86.8 a	86.75 ab	26.98 b	29.85 a	30.20 a	5.28 a	5.15 a	4.95 abc	6.70 a	6.75 a	6.80 a
	W1H1	27.63 bc	29.78 a	29.78 ab	85.90 a	87.1 a	86.93 ab	27.78 ab	30.18 a	30.00 a	5.28 ab	5.23 a	5.20 abc	6.70 a	6.80 a	6.80 a
	W1H2	27.20 bc	29.58 a	30.15 ab	85.03 a	86.7 a	86.33 ab	27.95 ab	30.78 a	30.78 a	4.95 bcd	5.13 a	5.28 a	6.70 a	6.78 a	6.80 a
	W1H3	27.23 bc	29.75 a	29.83 ab	85.63 a	86.4 a	87.00 ab	26.55 ab	30.43 a	30.25 a	5.08 abcd	5.23 a	5.23 ab	6.70 a	6.78 a	6.80 a
	W2H0	27.25 bc	29.33 a	30.65 a	85.53 a	86.5 a	86.05 b	27.35 a	30.15 a	31.05 a	5.08 abcd	5.00 a	4.90 aac	6.70 a	6.80 a	6.80 a
	W2H1	26.73 c	29.48 a	30.00 ab	84.98 a	86.5 a	86.68 ab	26.68 ab	29.18 a	30.70 a	5.23 abc	5.25 a	5.05 abc	6.70 a	6.78 a	6.80 a
	W2H2	28.08 ab	29.68 a	30.23 ab	86.48 a	87.1 a	87.68 a	28.75 ab	29.60 a	30.83 a	4.88 d	5.18 a	4.95 abc	6.73 a	6.78 a	6.80 a
	W2H3	28.15 ab	29.78 a	30.18 ab	85.13 a	86.9 a	87.03 ab	27.85 ab	29.08 a	29.35 a	5.13 abcd	5.18 a	5.13 abc	6.70 a	6.78 a	6.80 a
	W3H0	27.73 bc	29.83 a	30.75 a	85.95 a	87.0 a	86.63 ab	27.20 a	29.93 a	30.90 a	5.20 abc	4.78 a	4.88 abc	6.70 a	6.80 a	6.80 a
	W3H1	27.33 bc	29.93 a	30.18 ab	86.20 a	86.1 a	86.53 ab	27.50 ab	30.48 a	30.83 a	4.98 abcd	4.75 a	4.68 bc	6.70 a	6.78 a	6.80 a
	W3H2	29.18 a	30.35 a	31.08 a	86.18 a	85.8 a	86.53 ab	28.95 a	30.15 a	30.73 a	4.93 cd	4.73 a	4.63 c	6.73 a	6.80 a	6.80 a
	W3H3	28.25 ab	29.38 a	31.05 a	85.55 a	87.2 a	87.03 ab	28.75 a	30.95 a	31.43 a	5.08 abcd	4.93 a	4.73 abc	6.70 a	6.80 a	6.80 a
2023
ZMS087	W1H0	30.65 ab	30.15 cde	29.90 ed	84.63 a	83.95 b	84.78 a	31.30 bc	30.88 cd	31.65 ab	4.68 abc	4.70 abc	4.53 abc	6.73 a	6.75 ab	6.75 a
	W1H1	29.30 c	29.63 e	29.30 d	84.10 ab	83.30 b	84.65 ab	30.55 bc	30.73 d	30.85 b	4.88 ab	4.78 ab	4.55 ab	6.70 a	6.70 b	6.75 a
	W1H2	29.78 bc	30.03 de	29.78 ed	84.10 ab	84.98 ab	85.20 ab	31.05 bc	31.03 cd	31.38 ab	4.88 ab	4.90 a	4.90 ab	6.73 a	6.78 a	6.78 a
	W1H3	29.23 c	29.65 e	30.10 cde	82.63 b	83.68 ab	84.28 b	30.43 c	31.73 abcd	31.48 ab	4.95 a	4.90 a	4.60 a	6.70 b	6.70 b	6.78 b
	W2H0	30.18 abc	31.13 abcd	30.50 bcde	83.35 ab	84.15 ab	84.83 ab	31.75 abc	31.78 abcd	32.33 ab	4.20 cd	4.53 abc	4.30 cd	6.73 a	6.80 a	6.78 a
	W2H1	30.20 abc	30.38 cde	30.60 abcde	83.68 ab	83.38 ab	84.98 ab	31.63 abc	31.25 bcd	32.33 ab	4.50 abcd	4.53 abc	4.43 abcd	6.70 a	6.70 b	6.78 a
	W2H2	30.75 ab	31.10 abcd	30.93 abcd	83.65 ab	84.18 ab	84.15 ab	32.28 abc	32.03 abcd	32.33 ab	4.53 abcd	4.68 abc	4.40 abcd	6.70 a	6.78 a	6.80 a
	W2H3	30.38 abc	30.78 bcde	30.68 abcde	84.30 ab	84.28 ab	84.38 ab	32.38 abc	32.20 abc	32.48 ab	4.43 bcd	4.70 abc	4.38 bcd	6.70 a	6.75 ab	6.80 a
	W3H0	30.58 ab	31.90 ab	31.78 ab	84.40 a	83.93 ab	84.83 a	31.58 abc	32.65 ab	32.05 ab	4.05 d	4.25 c	3.98 d	6.70 a	6.80 a	6.80 a
	W3H1	31.43 a	31.40 abc	31.40 abc	83.68 ab	83.53 ab	84.48 ab	32.23 abc	32.00 abcd	32.73 a	4.25 cd	4.28 c	3.93 cd	6.73 a	6.75 ab	6.78 a
	W3H2	31.08 ab	31.90 ab	31.63 ab	83.35 ab	84.18 ab	84.85 ab	33.53 a	32.10 abcd	33.03 a	4.20 cd	4.38 bc	4.05 cd	6.70 a	6.78 a	6.80 a
	W3H3	30.88 ab	32.20 a	32.05 a	83.33 ab	84.23 a	84.70 ab	32.60 ab	32.80 a	32.55 a	4.23 cd	4.25 c	3.98 cd	6.70 a	6.80 a	6.80 a
ZMS92	W1H0	28.13 d	28.15 c	27.43 d	84.15 a	83.83 a	83.68 c	28.95 a	28.83 a	28.80 ab	5.65 a	5.75 a	5.60 ab	6.78 a	6.78 d	6.83 ab
	W1H1	28.43 cd	28.43 bc	27.7 d	84.60 a	84.55 a	83.95 bc	29.25 a	29.15 a	29.13 ab	5.63 a	5.65 ab	5.73 a	6.80 a	6.80 cd	6.80 b
	W1H2	28.43 cd	28.00 c	28.0 cd	83.98 a	85.30 a	85.48 a	29.33 a	28.58 a	28.23 ab	5.50 a	5.75 a	5.73 a	6.80 a	6.78 d	6.80 b
	W1H3	29.65 abc	28.80 abc	28.6 abcd	83.58 a	84.48 a	84.20 abc	29.95 a	29.70 a	30.08 ab	5.50 a	5.48 abc	5.38 abcd	6.80 a	6.78 d	6.78 b
	W2H0	28.88 cd	29.33 abc	28.7 abcd	84.80 a	84.55 a	83.85 c	29.10 a	29.35 a	28.98 ab	5.45 a	5.53 abc	5.40 abc	6.80 a	6.83 bcd	6.83 ab
	W2H1	29.15 cd	29.30 abc	27.7 d	84.75 a	85.13 a	84.65 abc	29.15 a	29.35 a	27.93 b	5.30 a	5.48 abc	5.33 abcd	6.80 a	6.88 ab	6.83 ab
	W2H2	29.40 bcd	28.73 abc	28.3 bcd	84.73 a	84.40 a	84.70 abc	29.58 a	29.43 a	29.08 ab	5.23 a	5.63 ab	5.53 abc	6.80 a	6.83 bcd	6.80 b
	W2H3	29.70 abc	29.40 abc	29.7 abc	84.88 a	84.90 a	85.35 ab	30.33 a	29.58 a	30.25 a	5.18 a	5.38 abc	5.18 abcde	6.85 a	6.83 bcd	6.85 ab
	W3H0	29.10 cd	30.28 a	29.7 abc	85.05 a	84.95 a	84.63 abc	29.83 a	29.63 a	29.85 ab	4.88 a	5.15 bc	4.75 de	6.83 a	6.90 a	6.85 ab
	W3H1	29.50 bcd	29.98 ab	30.0 ab	84.03 a	84.80 a	85.15 abc	30.35 a	29.73 a	29.98 ab	4.73 a	5.13 bc	4.58 e	6.78 a	6.90 a	6.90 a
	W3H2	30.73 ab	29.95 ab	30.3 a	84.63 a	84.73 a	85.00 abc	30.20 a	29.30 a	30.33 a	5.00 a	5.05 c	4.98 bcde	6.83 a	6.85 abc	6.85 ab
	W3H3	30.93 a	30.33 a	29.6 abc	84.93 a	84.20 a	85.05 abc	30.68 a	29.78 a	29.03 ab	4.78 a	5.23 abc	4.93 cde	6.83 a	6.85 abc	6.83 ab

) show that with increasing irrigation volume, the fiber length and fiber strength of ZMS92 and ZMS087 increased as follows: W3 > W2 > W1. The fiber length and strength of ZMS92 and ZMS087 with the W3 treatment were increased by 4.21%, 4.58% and 2.13%, 3.88%, respectively, compared with the W1 treatment. The Micronaire value showed a decreasing trend, and ZMS92 and ZMS087 were 7.78% and 10.80% lower, respectively, with the W3 treatment than the W1 treatment. The Micronaire value of ZMS087 reached 4.03–4.51 with the different water treatments, and the difference between the two varieties was significant. The fiber elongation rate was not sensitive to DPC response, but ZMS92 was greater than that of ZMS087.

4. Discussion

Water is the most limiting resource for the growth and development of cotton [19]. Excessive water supply can lead to excessive accumulation of organic matter formed by cotton photosynthesis in vegetative organs, resulting in late maturation of cotton bolls, which is not conducive to the formation of yield [20]. Moderate drought reduces cotton plant height, leaf area, and total biomass by promoting the conversion of vegetative growth to reproductive growth, thus improving the harvest index and water use efficiency. Moderate drought also promotes plant root system development, enhancing water absorption and utilization efficiency [21]. There are also studies that have shown a positive correlation between seed cotton yield and irrigation volume [22,23]. The application of DPC increases yield by increasing the weight of individual bolls and accelerates cotton maturity [24]. In this study, the seed cotton yield of ZMS087 reached 8004 kg·hm⁻² in 2022 with an irrigation volume of 5250 m³ hm⁻² and chemical control of 120 g·hm⁻² DPC (W3H1), which was 12.31% higher than the mild drought treatment W1H1 and 6.05% higher than W3H0 (Table 4). In 2023, the yield with the W3H1 treatment was 7098 kg·hm⁻², with increases of 30.99% and 30.12%. The yield increase of ZMS92 was lower than that of ZMS087. The highest yields for both varieties were obtained with the H2 treatment in 2023. This indicates that irrigation and DPC provide suitable moisture levels and population structures for cotton growth. Furthermore, there were differences among the varieties. In addition, the different climatic conditions in the two study years may have led to different responses to DPC, with increased irrigation reducing IWUE and mild water deficit conditions increasing IWUE [25,26]. In this study, as IWUE increased, seed cotton yield decreased. Based on local irrigation water prices, the economic benefits of increased yield significantly outweigh the costs of irrigation.

Cotton yield is closely related to biomass accumulation and allocation. Biomass accumulation is an important indicator of crop photosynthetic capacity, which is the basis for yield formation [27,28,29]. In this study, irrigation volume significantly affected biomass accumulation and allocation at the flowering and boll-opening stages, with the proportion of buds, flowers, and bolls showing W1 > W2 > W3 under different irrigation volumes. Moderate drought effectively increases the proportions of the reproductive organs in cotton. DPC has a bidirectional regulating effect on cotton growth. If the dose is too low, it will not achieve the desired chemo-regulatory effect, whereas if the dose is too high, it will inhibit nutrient translocation [14]. It remains unclear whether DPC increases or decreases the sensitivity of cotton to drought [30]. Fernandez et al. evaluated the interactions between DPC management and irrigation treatments for crop biomass allocation [31]. In this study, spraying DPC resulted in an initial increase and then a decrease in biomass accumulation, effectively increasing the proportion of bud bolls during the flowering and boll-opening stages.

Cotton yield formation is also influenced by photosynthetic capacity, chlorophyll content, and plant hormones. Mild water deficit results in a decrease in cotton Pn, primarily due to reduced stomatal openness, which leads to an insufficient supply of CO₂, with stomatal limitation becoming the dominant factor. Furthermore, severe water deficit also damages the chloroplast structure and reduces their number, which synchronously limits cotton Pn due to both stomatal and non-stomatal factors. Consequently, this contributes to the production of photosynthesized products and their transport to the reproductive organs [32]. The canopy’s photosynthetic capacity plays a crucial role in the reproductive development of cotton bolls during the mid to late stages [33]. In this study, SPAD values decreased with increasing irrigation and increased with increasing DPC dosage during all reproductive periods. The decrease in SPAD value after increasing irrigation volume could be the result of the dilution effect.

Alongside genetic factors [34], cotton fiber quality is also influenced by environmental conditions and field management practices [35,36]. Fiber length and strength are important indicators of fiber quality. Some studies have found that fiber length increases with irrigation volume before slightly decreasing [37], whereas other studies have shown that irrigation volume has a positive effect on fiber length [38,39]. Water shortage decreased fiber strength [40,41], and mild drought shortened cotton fiber length, whereas the fiber strength ratio was not affected by irrigation [22]. This study showed that irrigation increased the overall length and strength of the cotton fibers and decreased the Macronaire value (Table 6). Fiber strength increased by 5.75% and 6.31% in 2022 and 2023, respectively, with increasing irrigation. Mert [42] and Dagdelen et al. [22] found that fiber elongation rates increase with irrigation volume. Wen et al. demonstrated that irrigation volume had no effect on fiber elongation [43], which is in agreement with the results of this study. Furthermore, the effects of DPC, which vary among varieties, had no significant effect on fiber quality in this study. Some studies have indicated a negative correlation between fiber quality and seed cotton yield [44,45]. Furthermore, some researchers suggest that when yield and quality cannot be optimized simultaneously, priority should be given to crop yield under drought stress conditions [46]; thus, fiber quality should not be improved at the expense of seed cotton yield [47]. The cotton industry is an important pillar of the global economy, with a wide range of participation and high concentration around the world. Cotton yield, quality, cost and benefit are the basis of evaluating the competitiveness of a country’s cotton industry [48].

5. Conclusions

This study shows that both irrigation volume and DPC dosage influence cotton agronomic traits, physiological characteristics, biomass accumulation and allocation, yield quality, and irrigation water use efficiency. Furthermore, mild drought was shown to improve water use efficiency but also reduce yield. Increased irrigation could effectively regulate stomatal openness, increase the photosynthetic rate of cotton leaves, and promote dry matter accumulation and cotton yield. However, it also reduced water use efficiency and reproductive organ proportions. The application of DPC can, to some extent, shape the appropriate cotton plant type, increase the translocation of dry matter to reproductive organs, improve the utilization efficiency of resources such as water and light, and thus improve cotton yield. However, the effect of irrigation volume on yield was greater than that of spraying DPC. Overall, the results show that the appropriate dose of DPC varies depending on the irrigation conditions. Specifically, with irrigation volumes of 3750 m³ hm⁻² or 4500 m³ hm⁻², the application of 120 g hm⁻² DPC results in optimal yield increases, whereas at 5250 m³ hm⁻² the application of 240 g hm⁻² DPC is optimal. The effects of chemical controls were also correlated with cotton varieties.