The experiment was conducted in Shaya County, Aksu Region, Xinjiang, Northwest China (latitude 41.22°N, longitude 82.78°E) ( Figure 1 ). The study area exhibits a typical warm-temperate desert fringe climate, characterized by an average annual precipitation of merely 47.3 mm and a maximum annual evaporation of approximately 2000.7 mm, resulting in an evaporation-to-precipitation ratio of 42.3. Additionally, extreme weather phenomena such as wind, sandstorms, and hail are frequently influenced by neighboring desert sands and the Taklamakan Desert. Cotton stands as the primary cash crop within this locale. Soil physicochemical parameters in the study area are detailed in Table 1 . The water table was approximately 3.8 m below the surface, and the soil pH was around 7.8.

From 2021 to 2022, three different seedling emergence water volumes were used, namely 6 mm (W1), 10.5 mm (W2), and 15 mm (W3), and two drip frequencies were designed, namely low frequency (2 times, D1) and high frequency (4 times, D2). Among them, the emergence water (W1, W2, W3) was completed at the emergence stage, and the remaining irrigation water was drip irrigation at the strong seedling stage. The water irrigation (emergence water) plus water irrigation at the strong seedling stage was the water irrigation quota at the seedling stage. The low frequency treatment (D1) was drip irrigation once at the emergence stage, and the strong seedling stage and high frequency treatment (D2) was drip irrigation twice at the emergence stage and the strong seedling stage ( Table 2 ). At the same time, a local winter irrigation control treatment (CK) was designed, with an irrigation water volume of 225 mm. For CK winter irrigation treatment, the irrigation time was in November of the previous year, and due to the lower temperature than that in winter, frozen soil was formed after water irrigation to the soil, and it was thawed after the temperature rose in the following spring to ensure water consumption at the seedling stage. The irrigation effect of the soil surface was roughly the same as that of the treatment with a large amount of emerging water (W3), and the irrigation effect was better than that of the treatment with a low amount of emerging water (W1).

The experiment conducted during 2021-2022 utilized the locally common cotton variety “Yuan Cotton No. 11” as the test crop, using a consistent planting configuration of 1 film, three tubes, and six rows. The experiment was conducted in a randomized split-plot design with three replicates in each plot, and the plot length was 10 m and width was 6 m. Fertiliser application and agronomic measures for cotton in the experimental site were based on local experience. The planting area was divided into wide rows, narrow rows, and bare ground between the film. Wide rows, narrow rows, and film were spaced at 66 cm, 10 cm, and 46 cm, respectively, with a plant spacing of 10 cm ( Figure 2 ). Irrigation schedules and weather changes from 2021-2022 are shown in Figure 3 , with water irrigated approximately every ten days from the present bud stage to the bell stage, with eight irrigations in 2021 and nine irrigations in 2022.

The experiment was conducted in a completely randomized block design, with two drip frequencies (D1 and D2) and three seedling emergence volumes (W1, W2, and W3) in six treatments and three replications for each treatment. Two-factor analysis of variance (ANOVA) and multiple comparisons (p< 0.05) were performed using SPSS 25.0 to determine the differences in cotton physiological indices, yield, and quality between the treatments of emergence water volume and drip frequency. Principal Component Analysis (PCA) and correlation analysis of cotton yield and quality data using Origin2021 and plotting of correlation matrix. Using GeneCloud tools (https://www.genescloud.cn, March 30, 2024 access) on cotton physiological index interact ring heat map drawing; CAD2016 and ArcGIS 10.8 software were used to map the cotton planting pattern and the general situation of the experimental area.

From 2021 to 2022, the LAI of each treatment tended to increase and decrease with the advancing growth period, reaching a maximum at the bell stage (9th August) ( Figures 4A, B ). LAI has a consistent upward trajectory across treatments from the bud stage to the boll stage (21st May to 9th August). Notably, growth is slower during the transition from the bud stage to blooming (21st May to 5th July). Subsequently, during the boll stage (5th July to 9th August), LAI significantly increased, reaching its peak for the reproductive phase. However, leaf shedding occurs from the boll to the spathe stage (9th August to 20th September) due to insufficient water irrigation, initiating cotton spitting, and flocculation of its leaf area index.

The effects of drip frequency and emergence water on LAI were highly significant (p<0.01), and the interaction of drip frequency and emergence water volume (D×W) on LAI was substantial (p<0.05) between the two years ( Figures 4C, D ). When the frequency of dripping was the same, LAI showed a gradual increase with the increase of emergency water, and compared with the W1 treatment, LAI increased by 11.52% and 24.06% in W2 and W3 treatments, respectively. When the amount of emergence water was the same, the LAI of high frequency treatment (D2) was significantly more extensive than that of low frequency treatment (D1), and the LAI of D2 treatment increased by 13.84% compared with that of D1. In 2021-2022, the leaf area index of the CK treatment was not significantly different from that of the W3D2 treatment and was considerably more extensive than that of the remaining “DSWE” treatments.

The SPAD of each treatment tended to increase and decrease with the advancing growing period, reaching a maximum at the bell stage ( Figures 5A, B ). Both emergence water and drip frequency and their interactions(D×W) significantly affected SPAD in both years(p<0.05)( Figures 5C, D ). When drip frequency was held constant, the performance of emergence water treatments in 2021-2022 followed the pattern W1< W2< W3, respectively. Similarly, when emergence water was consistent, SPAD values were significantly higher in high-frequency treatments compared to low-frequency treatments, with the sizes showing D1< D2 in 2021-2022 and no significant differences in SPAD values of W3D2 treatments compared with CK treatments. In comparison to the CK treatment, reductions in SPAD values were observed for the W1D1, W2D1, W3D1, W1D2, W2D2, and W3D2 treatments in 2021 by 14.78%, 11.26%, 5.20%, 4.43%, 2.79%, and 1.28%, respectively. Similarly, in 2022, the W1D1, W2D1, W3D1, W1D2, W2D2, and W3D2 treatments showed decreases of 15.09%, 10.32%, 4.09%, 2.51%, 1.38%, and 1.89%, respectively.

During the two years, the population physiological indexes of each treatment tended to increase and decrease as the cotton growth period progressed, and the population physiological indexes peaked at 108 days after sowing (FFS) ( Figure 8 ). When the drip frequency is the same, the differences between the different emergence water treatments are minor in the seedling and bud stages. Then the differences are gradually significant in the boll stage (108d after sowing), and then the differences decrease in the flocculating stage (170d after sowing); when the emergence water is the same, the high frequency treatment (D2) maintains a more significant level throughout the growth cycle.

Seedling emergence water, drip frequency, and the interaction between the two (D×W) had highly significant (p<0.01) effects on LAD and NAR ( Figure 9 ). At the same drip frequency, NAR and LAD roughly showed a gradual increase with increasing emergence water, and compared with W1, LAD increased by 26.01% and 93.94% in W2 and W3 treatments, respectively, and NAR increased by 27.51% and 68.58% in W2 and W3 treatments, respectively. At the same amount of emergency water, LAD and NAR were significantly more significant in the high-frequency therapies than in the low frequency treatments. They increased by 38.68% and 34.52% in the D2 treatment LAD and NAR, respectively, compared with D1. Compared with CK treatment, W3D2 treatment LAD and NAR were significantly less different; LAD and NAR increased by 3.49% and 19.06%, respectively.

Photosynthesis is the fundamental driving force for plant growth and constitutes the basis for biomass accumulation and yield formation (Evans, 2013). A higher canopy photosynthetic rate provides a solid material foundation for plant growth and development (Terashima and Hikosaka, 1995). This study found that within the appropriate range of “DSWE” water control, treatments with higher post-emergence watering levels promote cotton photosynthetic indices. With the same developmental stage factor, cotton leaf Pn and Tr values generally showed an increasing trend with increasing emergence water. Compared to W1, W2 and W3 leaf Pn increased by 17.87 and 25.90%, respectively, and W2 and W3 leaf Tr increased by 9.39 and 13.02%, respectively. Zou et al. (2022) found that insufficient irrigation water might be the main reason for the significant decrease in net photosynthetic rate, stomatal conductance, and transpiration rate of cotton leaves. Chastain et al. (2014) study confirmed this, showing that compared with the high-water treatment, the low-water treatment significantly inhibited photosynthesis, with markedly lower net photosynthetic rate, transpiration rate, and stomatal conductance indices. However, Wang et al. (2019b) argued that the primary reason might be the synergistic effect of stomatal and non-stomatal variables leading to a decline in cotton leaf photosynthetic rate, with mild water stress having a minor impact on photosynthesis. It has also been shown that stomata close during mild to moderate water stress, leading to a decrease in intercellular CO₂ concentration, resulting in a reduced photosynthetic rate. Prolonged water stress can damage photosynthetic organs, thereby reducing the net photosynthetic rate of plants (Zahid et al., 2021).

This study found that the photosynthetic characteristics of cotton under “DSWE” high-frequency treatments were significantly higher than those under low-frequency treatments. Ci and Gs were substantially larger in the high-frequency therapy with the same emergence water, increasing by 10.03% and 6.67%, respectively, compared with the low-frequency treatment. This is consistent with the findings of Liang et al. (2021), which related to photosynthetic characteristics research. Low-frequency treatments decreased net photosynthetic rate due to non-stomatal limiting factors, resulting in significantly lower maximum photochemical efficiency, photochemical quenching coefficient, and photochemical quantum yield compared to high-frequency treatments. The variation in soil water content during the growth period may contribute to the size of photosynthetic indices (Loka et al., 2011). Water significantly affects crop growth and development at each stage. Throughout the cotton growth process, especially during the blooming and boll formation stages, cotton plants exhibit vigorous growth and reproductive development, resulting in a significant increase in water demand (Sahito et al., 2015). All water is applied at once with low frequency treatments, resulting in water stress as the growth period progresses. Water stress leads to a decrease in stomatal conductance, increased respiration, obstruction of photosynthetic product transport, and reduced photosynthetic function period, thereby impairing the plant’s ability to maintain efficient photosynthetic production capacity throughout the entire growth period (Chen et al., 2022; Wu et al., 2023). Insufficient water may affect the cotton net photosynthetic rate, stomatal conductance, and other photosynthetic indices. Only appropriate “DSWE” post-emergence watering amounts and drip frequency (W3D2) can facilitate cotton leaf photosynthesis, providing a solid foundation for cotton growth and development.

The leaf area index is one of the most important parameters determining the accurate calculation of the rate of canopy photosynthesis and is an essential indicator of the quality of the crop population (Pauli et al., 2017; Bunce, 1989). Relevant studies have found minimal variation in LAI among different water control treatments during the seedling period. As the growth period progresses, the LAI of each treatment gradually increases, reaching its maximum value during the blooming and boll formation stages, and the differences among treatments also become increasingly significant (Li et al., 2020; Cetin et al., 2023), which is consistent with the LAI-related research in this experiment. Through variance analysis, this study found that within the appropriate range of “DSWE” water control, the LAI of cotton significantly increases with the increase in the emergence of water, and high-frequency water control can effectively enhance the LAI of cotton. LAI increased by 11.52% and 24.06% in W2 and W3 treatments, respectively. When the amount of emergence water was the same, the LAI of high frequency treatment (D2) was significantly more extensive than that of low frequency treatment (D1), and the LAI of D2 treatment increased by 13.84% compared with that of D1. Some scholars have found that increasing irrigation quotas and frequencies can promote the nutritional growth of cotton and enhance the LAI during the budding and blooming stages (Kalaydjieva et al., 2015). Therefore, besides irrigation water quantity and drip frequency, factors such as planting density and mulching methods may also need to be considered when assessing the LAI of cotton.

Zhao et al. (2016) have shown that the physiological indicators of the cotton population are significantly higher with larger irrigation quotas, which are more conducive to cotton dry matter accumulation and ultimately increasing yields. Consistent with these findings, our experiment observed that the physiological indicators of the cotton population gradually increased with the increase in post-emergence watering. Among different drip frequency treatments, those with higher frequencies exhibited more considerable physiological indicators. Compared with W1, LAD increased by 26.01% and 93.94% in W2 and W3 treatments, respectively, and NAR increased by 27.51% and 68.58% in W2 and W3 treatments, respectively. Increased by 38.68% and 34.52% in the D2 treatment LAD and NAR, respectively, compared with D1. The differences observed in physiological indicators among treatments in our experiment may be related to cotton emergence rate and leaf area index. The emergence rate was lower in treatments with higher irrigation quotas, affecting the cotton population structure. However, the cotton canopy received richer natural resources, such as better ventilation and light in these treatments, promoting early photosynthesis and dry matter production (Feng et al., 2013). In contrast, differences in emergence rates among treatments with different drip frequencies were relatively small, making the soil moisture environment in the main root zone a determining factor for cotton population physiological indicators (Ballester et al., 2021).

In this study, the yield of cotton seed cotton lint increased gradually with the increase of seedling emergence water, and the yield of high frequency treatment was significantly larger. Compared with low frequency treatment (D1), the yield of high frequency treatment (D2) lint and seed cotton increased by 15.89%, 13.64%, 23.60%, and 18.17%, respectively, in 2021 and 2022. This is consistent with the findings of Ballester et al. (2021) and Hill et al. (2024) regarding the effect of different irrigation frequencies on cotton yield. Cotton boll number and yield varied under different water stress conditions. Mild water stress during the entire growth period had a minimal impact on cotton yield, while continuous water stress significantly reduced the number of bolls per plant. Boll formation concentrated on cotton plants’ lower and middle fruiting branches, resulting in fewer bolls on the upper branches, leading to decreased cotton yield (Cheng et al., 2021; He et al., 2022). Consistent with these findings, our experiment suggests that the reduction in post-emergence watering may concentrate the main nutrient and reproductive growth of cotton plants on the lower and middle parts, where the root system and lower fruiting branches and leaves grow vigorously. Conversely, the upper fruiting branches and leaves receive less water irrigation, affecting the net photosynthetic intensity and leading to a decrease in non-structural carbohydrate content, exacerbating the shedding of cotton bolls and consequently reducing yield.

The results showed that drip frequency and emergence water had significant effects on cotton fiber length, evenness, textile parameters, and FQI but had no significant effects on fiber strength, elongation, and maturity. As post-emergence watering and drip frequency increased, the average length of the upper half of the fiber, fiber uniformity, textile parameters, and Fiber Quality Index (FQI) all increased to varying degrees. The differences in fiber strength, elongation, and maturity among the various “DSWE” water control treatments were relatively small. However, findings by Dai et al. (2022) and Gao et al. (2021) differed; they found that reduced water irrigation could increase cotton fiber uniformity and elongation, thus improving cotton quality. Studies by Thorp et al. (2020) found that the decrease in irrigation volume had an increasing impact on the average length of the upper half of cotton fibers. This was mainly manifested as the aggravation of water stress with decreasing irrigation volume, leading to a significant decrease in cotton fiber quality. Furthermore, it was shown that water stress at any stage resulted in reduced fiber uniformity in cotton. Therefore, cotton quality can be influenced by different irrigation methods, cultivation practices, growing environments, and cotton varieties.