Reducing Irrigation and Increasing Plant Density Enhance Both Light Interception and Light Use Efficiency in Cotton under Film Drip Irrigation

Abstract

High-density planting is an effective technique to optimize yields of mulched cotton. On the other hand, deficit irrigation is an emerging water-saving strategy in cotton cultivation, especially suitable for arid and water-scarce areas. However, the relationships between deficit irrigation, high-density planting, and regulation mechanisms of canopy light radiation and light use efficiency (LUE) in cotton is not yet clear. To clarify the mechanism of light interception (LI) and the LUE of cotton canopies, three irrigation treatments [315 (50% Fc), 405 (75% Fc, farmers’ irrigation practice), and 495 mm (100% Fc), where Fc was the field capacity] with three plant densities [13.5, 18.0 (farmers’ planting practice), and 22.5 plants m²] were applied. The findings of this research revealed that, under deficit irrigation, the above-ground dry matter (ADM) was reduced by 5.05% compared to the farmers’ irrigation practice. Over both years and across all plant densities, LI and LUE under deficit irrigation decreased by 8.36% and 4.79%, respectively, relative to the farmers’ irrigation practices. In contrast, LI and LUE for the highest irrigation level increased by 10.59% and 5.23%, respectively. In the case of the interaction (plant density and irrigation level), the ADM under deficit irrigation and high-density combination increased by 7.69% compared to the control (farmers’ irrigation × sowing practices interaction effects). The LI and LUE also exhibited an increase in 1.63% and 6.34%, respectively. Notably, the LI effect of the middle and upper cotton canopy under film drip irrigation reached 70%. A lower irrigation level resulted in a higher percentage of LI in the lower canopy region. The leaf area index, light interception rate, and extinction coefficient escalated with the increase in plant density. Under deficit irrigation treatment, the LI of the 0–30 cm canopy in high plant density settings increased by 8.6% compared to the control (farmers’ irrigation × sowing practices interaction effects). In conclusion, deficit irrigation and increased plant density improved the interception of LI and LUE of cotton canopy. These findings may help the farmers to optimize their agricultural management strategies in water-deficient areas.

1. Introduction

The advanced cotton production technology and modern cultivation methods have increased the planting area and yield of cotton in China, especially in its northwest inland cotton region. Currently, this region represents the largest cotton planting area in China, with Xinjiang contributing approximately 90% of the total yield [1]. This is mainly attributed to the abundant sunlight, short growth period, and pronounced diurnal temperature fluctuations in Xinjiang. Nevertheless, the region’s limited rainfall and high evaporation rates result in a reliance on meltwater from ice and snow for cotton irrigation, which are big constrains in crop growth [2]. Consequently, drip irrigation, plastic film mulching, and high-density planting are prevalent strategies for water conservation and enhanced yield in this region.

Since the late 1990s, the widespread adoption of drip irrigation technology under film has led to a rapid increase in cotton yield. With adequate water supply through drip irrigation, crops could exploit this advantage to enhance LI and LUE, ultimately promoting yield formation [3]. Excessive irrigation combined with fertigation could induce over-nutrition in cotton, leading to an increase in water loss and reduced water use efficiency [4]. In arid and water-deficient regions, deficit irrigation, as a new water-saving strategy, maintains the normal growth of crops by limiting stomatal opening and reducing transpiration [5]. Moreover, an appropriate water deficit could establish an optimal canopy structure and function, facilitate the transfer of assimilates to reproductive organs [6], and boost the cotton yield. Past study has also showed that planting density is an important measure that affects cotton production [7]. Reducing irrigation volume, along with optimized plant density, can further achieve the objectives of efficient water conservation, since reasonable density planting could also optimize the relationship between induction and population. This ensures the fair distribution and absorption of light energy within the cotton canopy [8], evenly distributes reproductive organs on each fruit branch, increases the effective number of bolls, and thereby improves cotton yield [9]. Low planting density could lead to a large amount of light escape from the canopy, limiting the absorption and utilization of light energy by leaves [10]. Very high planting density could lead to leaf overlapping, which could lead to closure of the internal canopy and lower LI in the middle and lower canopy [11]. This impedes the photosynthesis and accumulation of photosynthates in the lower and middle canopy leaves, and worsens the transportation of more photosynthetic products to reproductive organs, thus ultimately leading to a decrease in yield.

Crop yield is intimately associated with dry matter accumulation, which relies on the canopy’s interception of light energy, because the canopy light interception capacity is strongly related to its structure [12]. Previous studies have demonstrated that optimal canopy structure could modulate light distribution within the canopy, thereby enhancing the photosynthetic production capacity of the crop population [13,14] and promoting the transport and accumulation of photosynthetic products. Adjusting the canopy structure to reduce the proportion of LI in the upper layer while increasing LI in the middle and lower layers can improve crop yield and LUE [15]. These studies highlight the significance of canopy structure adjustment in optimizing light energy utilization efficiency. Considering this aspect, some researchers have suggested that selecting genotypes with upright leaves in the top canopy to improve light distribution and photosynthetic efficiency within the canopy could be time-consuming from a breeding perspective [16]. Alternatively, adjusting agronomic measures, such as planting density and planting method, can also modify the canopy structure and enhance light energy utilization efficiency [17,18]. However, limited studies are available on light energy interception in cotton canopies under different irrigation levels and plant densities for mulch-covered drip irrigated cotton. Hence, it is important to find out whether increasing planting density could improve LUE without compromising yield under deficit irrigation for film-mulched drip-irrigated cotton.

Therefore, the present research was conducted with the objective of evaluating the relationship between LI and LUE under drip irrigation and plastic film-mulch, considering different irrigation levels and planting densities. Additionally, this study also aimed to determine the interception and spatial distribution of light energy within the canopy under deficit irrigation and high-density planting conditions.

2. Materials and Methods

2.1. Experimental Site

Two field experiments were conducted during the 2019 and 2020 cotton growing seasons at the Cotton Comprehensive Experimental Station of the Xinjiang Academy of Agricultural Sciences, located in Awat (N41°06′, E80°44′), an arid region in Northwest China. From 1991 to 2020, the annual average air temperature in this region was 10.4 °C, with an accumulated temperature sum averaging 3988 °C days annually for temperatures above 10 °C. The area receives an average of 2679 h of sunlight each year, with a frost-free period lasting an average of 211 days and an annual total precipitation of 46.7 mm. The yearly evaporation during this period was 2900 mm, rendering agriculture in this region entirely reliant on irrigation.

The soil of the experimental site was sandy loam, with a bulk density of 1.48 g cm⁻³ in the 0–40 cm soil layer and a field capacity of 28.9%. The soil contains 10.6 g kg⁻¹ of organic matter and 1.8 g kg⁻¹ of total nitrogen. The maximum and minimum air temperatures and precipitation during the crop growing season (April to October) for both 2019 and 2020 are illustrated in Figure 1. The maximum temperature, minimum temperature, and solar radiation were recorded by an automated weather station located adjacent to the experimental site (Watch Dog 2900ET Weather Station, Spectrum, Inc., Plano, TX, USA).

2.2. Experimental Design and Field Management

The field experiment was conducted in a randomized split-plot design having main plots with three irrigation treatments [315 (50% Fc, severe deficit irrigation), 405 (75% Fc, moderate deficit irrigation, farmers’ irrigation practice), and 495 mm (100% Fc, full irrigation), where Fc was the field capacity], and three plant density treatments [13.5, 18.0 (farmers’ planting practice as control treatment), and 22.5 plants m⁻²] were assigned in subplots with four replicates. The area of each subplot was 39 m² (6.5 m in length and 6.0 m in width). In these plots, the drip irrigation was applied with tubes positioned beneath a plastic film. Irrigation levels were monitored using a solenoid valve and flowmeter. To mitigate marginal effects of water movement between plots, a 50 cm narrow ditch was dug along plot boundaries, and a vinyl chloride polymer was applied. Plastic film mulching covered 81% of the plot area, with a 2.05 m film spanning three rows. All treatments had a row spacing of 76 cm. Drip lines were applied at each row, with drippers (the conventional dripper) spaced at 25 cm intervals and a flow rate of 2.1 L h⁻¹. For both crop seasons, irrigation was applied using varying irrigation levels, while other irrigation measures remained consistent with the local field practices of the region (irrigation started on 20 June and ended on 22 August, occurring every seven days, for a total of 10 irrigation events).

Cotton cultivar (Xinluzhong 88 cauterized with a growth period of around 137 days, vigorous growth throughout the entire season, good uniformity, plants with a cylindrical shape, larger leaves, deep leaf color and lobes, moderately sized fruit branches, early maturing, concentrated boll opening, excellent fiber quality, and ease of picking) was sown on 19 April 2019 and 15 April 2020, with harvests taking place on 20 September 2019 and 24 September 2020. At sowing time, 450 kg ha⁻¹ of diammonium phosphate (P₂O₅, 53.8%; N, 21.2%), 225 kg ha⁻¹ of potassium sulfate (K₂O, 51%) and 150 kg ha⁻¹ of urea (N, 46.4%) were applied. An annual total of 600 kg ha⁻¹ urea was supplied via drip irrigation for all treatments. All the other cotton practices adhered to local cotton production standards.

2.3. Measurements

2.3.1. Above-Ground Dry Matter

Above-ground dry matter (AMD) samples were collected from all plots at 50, 80, 115, and 140 days after sowing (DAS) during both cropping seasons. Plant tissue samples were divided into roots and above-ground components at the cotyledonary node. Following the determination of fresh weight, roots and aboveground tissues were initially dried in an oven for 30 min at 105 °C to inactivate enzymes. Subsequently, samples were dried at 80 °C until a constant weight was achieved for dry weight determination.

2.3.2. Leaf Area Index

Leaf area index (LAI) was measured using a CI-110 plant canopy digital image analyzer (Li-Cor Inc., Lincoln, NE, USA). LAI measurements were conducted at sites with uniform growth at 70, 85, 100, 110, 120, 130, and 140 DAS. To minimize systematic error from the instrument, each measurement was replicated four times. The Bate growth function was used to simulate the growth process of leaf area index [19,20]:

$$\begin{gathered} W=A_m(1+\frac{t_e-t}{t_e-t_m}){(\frac{t}{t_e})}^{\frac{t_e}{t_e-t_m}}\hspace{1em}\hspace{1em}\hspace{1em}(t_m<t_e) \\ {C}_m=A_m{(\frac{{2t}_e-t_m}{t_e(t_e-t_m)})}^{\frac{t_e}{t_e-t_m}} \end{gathered}$$

where W is the LAI of cotton, A_m is the maximum value of the LAI, and t is the days after sowing (d). t_e is the maximum LAI of cotton (d), and t_m is the time for cotton to reach the maximum LAI rate (d). C_m is the maximum growth rate of cotton leaf area.

2.3.3. Photosynthetic Effective Radiation, Light Interception and Light Use Efficiency

Photosynthetically active radiation (PAR; μmol m⁻² s⁻¹) was measured on a sunny day from 13:00 to 15:00 using the Sunscan (Delta-T Devices, Cambridge, UK) equipment. During the measurement, the PAR value at about 0.3 m above the canopy was measured with the sensor facing upwards. The PAR transmitted inside the canopy was measured by sensors every 10 cm inside the canopy. The fractional interception of photosynthetically active radiation (FIPAR) was calculated using the method of Yao et al. [21]. The calculation method is as follows:

$$\mathrm{F}\mathrm{I}\mathrm{P}\mathrm{A}\mathrm{R}=\frac{{PAR}_{iA}-{PAR}_{iB}-{PAR}_{iC}}{{PAR}_{iA}}$$

where PAR_iA is PAR at 0.3 m above the canopy; PAR_iB is PAR at the soil surface within the canopy; and PAR_iC is PAR reflected by the canopy.

Refer to the calculation methods of Hamzei et al. [22] and Yan et al. [23] to calculate the light interception and light utilization efficiency of the canopy. The specific calculation formula is:

$$\begin{gathered} \mathrm{LI} =\mathit{0.5}{R}_a(1-\frac{{\mathit{PAR}}_{\mathit{iB}}}{{\mathit{PAR}}_{\mathit{iA}}}) \\ \mathrm{L}\mathrm{U}\mathrm{E}=\frac{ADM}{\mathrm{L}\mathrm{I}} \end{gathered}$$

where Ra is the total radiation interception accumulation between sampling dates (MJ m⁻²); ADM is above-ground dry matter (g m⁻²); LI is canopy light interception (MJ m⁻²); and LUE is the light utilization efficiency (g MJ⁻¹).

2.3.4. Light Extinction Coefficient

In order to analyze irrigation quantity and planting density on the influence of cotton canopy extinction coefficient, the Beer’s law is used to calculate canopy extinction coefficient (K) [24].

$$\mathrm{K}=\frac{-ln({PAR}_{iB}/{PAR}_{iA})}{LAI}$$

2.4. Statistical Analysis

Analysis of variance was carried out using a generalized linear model in SPSS 21.0 (SPSS Inc., Chicago, IL, USA). The fixed factors considered were irrigation level, plant density, and year, while replication served as a random factor. Mean values were compared using the least significant difference test at the 5% (α = 0.05) level of significance.

3. Results

3.1. Above-Ground Dry Matter, Light Interception and Light Use Efficiency

The effects of irrigation level and plant density on the ADM were significant (p < 0.05). Increasing plant density and irrigation levels led to an increase in ADM (

Table 1. Effects of irrigation level and planting density on cotton of above-ground dry matter, light interception and light use efficiency.

Year	Irrigation	Density	Above-Ground Dry Matter	Light Interception	Light Use Efficiency
	(mm)	(Plants m−2)	(g m−2)	(MJ m−2)	(g MJ−1)
2019	315	13.5	971.49 c	371.51 b	2.46 b
		18	1170.7 b	423.21 ab	2.81 ab
		22.5	1549.4 a	524.61 a	3.23 a
		SE	73.72	20.73	0.11005
	405	13.5	1115.2 c	398.91 b	2.77 b
		18	1450.5 b	505.11 a	3.12 ab
		22.5	1632.8 a	538.01 a	3.46 a
		SE	66.12	19.14	0.10771
	495	13.5	1351.6 c	463.51 b	2.89 b
		18	1605.5 b	539.41 ab	3.28 a
		22.5	1762.5 a	585.11 a	3.49 a
		SE	55.02	17.13	0.08537
2020	315	13.5	925.16 c	385.36 b	2.37 c
		18	1171.97 b	410.65 b	2.93 b
		22.5	1617.08 a	520.44 a	3.50 a
		SE	87.39	19.57	0.16
	405	13.5	1121.97 c	432.10 b	2.54 b
		18	1488.34 b	507.13 a	3.26 a
		22.5	1751.68 a	544.63 a	3.68 a
		SE	80.46	17.68	0.17
	495	13.5	1321.97 c	474.03 b	2.90 b
		18	1593.47 b	530.44 a	3.37 a
		22.5	1848.96 a	576.00 a	3.71 a
		SE	67.88	15.23	0.11
p-value
Year			0.118	0.500	0.116
Irrigation			0.000	0.000	0.000
Density			0.000	0.000	0.000
Irrigation × Density			0.000	0.000	0.485
Year × Irrigation × Density			0.998	0.995	0.890

Same small letters indicate no significant difference within same year and irrigation treatment at p = 0.05 level. The SE indicates a marginal standard error for all treatments with replicates within the same year.

). Across the three plant densities and over two years, the ADM for the low irrigation level (315 mm) was 1354.69 g m⁻², which was 5.05% lower than the control (farmer practice), while the highest irrigation level (495 mm) resulted in an ADM of 1580.69 g m⁻², 9.73% higher than the control. The ADM for the combination of the lowest irrigation level (315 mm) and the highest plant density (22.5 plants m⁻²) was 1583.68 g m⁻², which was 7.69% lower than the control.

The irrigation level and plant density exerted significant effects on the LI and LUE of the cotton (p < 0.05). Over the two-year experiment, both LI and LUE in the cotton field increased with rising irrigation levels and planting densities (Table 1). At the same planting density, LI and LUE under low irrigation decreased by 8.36% and 4.79%, respectively, compared to the control, while LI and LUE under high irrigation increased by 10.59% and 5.23%, respectively (Figure 2 and Figure 3). In terms of the interaction between the two factors, the LI and LUE of the low irrigation and high-density combination over two years were 1.63% and 6.34% higher than those of the control, respectively.

3.2. Leaf Area Index

Both irrigation level and planting density employed a significant effect on the LAI (p < 0.05). The two-year experiment demonstrated that the LAI of low irrigation (315 mm) decreased by 8.3% and 19.0%, respectively, compared to the control (405 mm) and high irrigation (495 mm) (Figure 4). Increasing planting density significantly enhanced the leaf area index. Under the combined effect of the two factors, the LAI of low irrigation and high-density treatments increased by 4.7% compared to the control combination (405 mm and 18.0 plants m⁻² in farmers’ practice).

The analysis of growth characteristics related to the leaf area index (LAI) (

Table 2. Effects of irrigation level and planting density on fitting parameters of leaf area index.

Year	Irrigation	Density	Am	Cm	te	tm
	(mm)	(plants m−2)		m2 m−2 d−1	d	d
2019	315	13.5	2.42 b	0.037 a	120.36 a	81.22 a
		18	2.55 ab	0.034 a	122.98 a	74.09 a
		22.5	2.86 a	0.040 a	120.18 a	75.26 a
		SE	0.079	0.001	0.947	1.63
	405	13.5	2.58 b	0.037 a	122.24 a	79.94 a
		18	2.71 ab	0.038 a	118.85 a	74.67 a
		22.5	2.97 a	0.042 a	121.95 a	79.23 a
		SE	0.072	0.001	0.789	1.13
	495	13.5	2.79 b	0.041 a	121.81 a	81.45 a
		18	3.24 a	0.045 a	125.39 a	79.43 a
		22.5	3.17 a	0.043 a	122.54 a	74.62 a
		SE	0.079	0.001	1.16	1.63
2020	315	13.5	2.30 c	0.034 c	119.29 a	76.54 a
		18	2.81 b	0.040 b	117.86 a	72.94 a
		22.5	3.24 a	0.046 a	118.21 a	74.21 a
		SE	0.127	0.002	0.729	1.639
	405	13.5	2.71 b	0.040 b	119.99 a	78.51 a
		18	3.01 b	0.043 b	117.52 b	71.72 a
		22.5	3.51 a	0.053 a	114.09 c	71.47 a
		SE	0.136	0.002	0.805	1.599
	495	13.5	3.10 b	0.045 b	120.59 a	78.68 a
		18	3.43 b	0.050 ab	115.39 b	72.58 a
		22.5	4.09 a	0.061 a	115.69 b	73.42 a
		SE	0.155	0.003	0.903	1.844
p-value
Year			0.000	0.000	0.000	0.007
Irrigation			0.000	0.000	0.374	0.772
Density			0.000	0.000	0.058	0.001
Irrigation × Density			0.540	0.706	0.472	0.822
Year × Irrigation × Density			0.015	0.126	0.037	0.452

Days after sowing is indicated as ‘d’. Same small letters indicate no significant difference within same year and irrigation treatment at p = 0.05 level. The SE indicates a marginal standard error for all treatments with replicates within the same year.

) revealed that both A_m and C_m were significantly influenced by planting density and irrigation level (p < 0.05). At low irrigation (315 mm), A_m and C_m were decreased by 8.0% and 9.2%, respectively, compared to the control (405 mm). Increasing planting density significantly enhanced A_m and C_m. Under the combined effects of the two factors, compared to control combination (405 mm and 18.0 plants m⁻² in farmers’ practice), the A_m and C_m of the low irrigation and high-density treatments increased by 5.5% and 5.8%, respectively.

3.3. The Fractional of Light Interception

The fraction of intercepted photosynthetically active radiation (FIPAR) of the canopy was significantly affected by both irrigation level and plant density (p < 0.05), exhibiting a similar pattern across the two years (Figure 5). Across all plant densities and years, the FIPAR under low irrigation was 3.0% and 5.5% lower than that under the control and high irrigation, respectively. Increasing planting density significantly enhanced FIPAR. Concerning the interaction of the two factors, the FIPAR of the low irrigation and high plant density treatment was essentially consistent with the control (farmer irrigation and plant density), reaching 0.9.

3.4. Light Extinction Coefficient

The light extinction coefficient was significantly influenced by both irrigation level and plant density (p < 0.05). Across all plant densities and years, the light extinction coefficient in low irrigation (315 mm) was 2.1% and 3.7% lower than that of the control (405 mm) and high irrigation (495 mm), respectively (Figure 6). Increasing planting density significantly enhanced the light extinction coefficient, while the light extinction coefficient in low irrigation and high plant density treatment was same as that of the control (farmer irrigation and plant density), reaching 0.92.

3.5. Spatial Distribution of Light

Under drip irrigation and film mulching conditions, 70% of the light interception (LI) primarily indented within the 30–90 cm range of the canopy (Figure 7). During the two cropping seasons, higher irrigation levels correlated with increased intensive areas of canopy light energy interception. With low irrigation, the light energy interception rate within the 30–90 cm canopy region reached 0.6, which was 12% lower than the control. However, a 14.9% increase was observed in the 0–30 cm canopy region. Under the combined influence of both factors, the light energy interception within the 0–30 cm range for the low irrigation and high-density treatment reached 19.2%, showing an increase of 8.6% compared with the control treatment.

4. Discussion

In drip-irrigated cotton fields under plastic film-mulching, a 20% decrease in irrigation level and a 20% increase in planting density compared to current farmers’ practices led to a 7.69% and a 1.63% increase in above-ground dry matter accumulation and light energy interception, respectively. This resulted in a 6.7% increase in LUE. This optimization strategy allows farmers to cultivate more cotton in arid and water-scarce areas. Under drip irrigation, LI was primarily concentrated within the 30–90 cm canopy range. Lower irrigation levels resulted in a higher LI content in the lower canopy regions. Leaf area index, light interception rate, and extinction coefficient increased with plant density increases. Under low irrigation conditions, the LI of the 0–30 cm canopy with high density increased by 8.6% compared with control. Therefore, the loss of light energy caused by reducing the irrigation level can be compensated through increasing the planting density, thereby improving LUE in cotton crop.

4.1. Effects of Irrigation Quota and Planting Density on Above-Ground Dry Matter

The ADM forms the material basis for cotton yield. Maintaining ADM within a suitable range was conducive to synchronizing the inconsistency between vegetative growth and reproductive growth, which might help to establish a reasonable population basis for high cotton yield [25]. Some past studies have shown that population ADM was related to planting density [26]. As planting density increases, population ADM initially increases and then decreases [27], but the distribution proportion of photosynthetic product accumulation in the reproductive organs of a single plant decreases during the later growth stages, which might not help in yield formation. However, deficit irrigation could effectively control plant vegetative growth and achieve ideal plant type and root-shoot ratio [28]. In this experiment, under the condition of 76 cm equal row-spacing planting, reducing the irrigation level significantly decreased the ADM, but increasing the planting density compensated for this loss. These results are in line with the findings of Zhang et al. [29]. A moderate water deficit could adjust the transfer of crop population growth center and the growth and development of certain tissue and organs, enabling the entire plant growth and development to cope with water scarcity in a more economical and rational manners. This, in turn, promotes the yield formation in densely planted cotton.

4.2. Effects of Irrigation Quota and Planting Density on Leaf Area Index and Light Interception

The canopy serves as the carrier for plant light energy interception, and LAI is considered as a crucial factor of a population’s light-energy absorption efficiency. Similarly, the LAI is also the most active, easily modifiable, and controllable factor [30]. Low LAI could lead to a significant light escape loss, while high LAI may result in reduced canopy aeriation and light transmittance [26]. Consequently, properly adjusting planting density can enhance the degree of overlap between plants and leaves in the middle and lower canopy regions, minimizing light escape loss and improving the LUE of the canopy population. However, increasing planting density without implementing other agronomic adjustments may cause excessive LAI and increased overlap of upper canopy leaves, leading to low light transmittance in the middle and lower canopy regions and significantly reducing LUE in these areas. Mao et al. [31] discovered that increasing planting density in tandem with growth regulator concentration can modify canopy structure, render the upper canopy more compact, reduce shading at the canopy top, enhance photosynthetically active radiation transmittance at the canopy top, and improve the overall LAI. The present study found that under conventional irrigation levels, reducing water consumption by 20% while simultaneously increasing planting density weakened the light energy interception effect of the canopy during the cotton growth period. However, the proportion of LI in the lower canopy region significantly increased, resulting in an overall higher LUE for the cotton field compared with other treatments. It was inferenced that reducing irrigation levels under high-density planting conditions not only increased the cotton population LAI, advanced canopy closure timing, and augmented plant canopy light energy interception [32], but also promoted more compact cotton canopy growth during the full bud stage, improving the proportion of LI in the middle and lower canopy regions [17]. This further compensated for the canopy closure deficiency in the middle and lower canopy regions caused by high-density planting. Additionally, high-density planting might counterbalance the light energy loss resulting from deficient irrigation [33]. Under the combined control of these factors, more light energy could be transmitted to the middle and lower canopy regions, providing favorable light conditions for the leaves. However, the photosynthetic physiological characteristics of the leaves in these regions remain unclear, necessitating further research to investigate the effects of planting density and irrigation level on photosynthetic characteristics and the stress resistance physiology of the plant population.

4.3. Effects of Irrigation Quota and Planting Density on Light Use Efficiency

Crop production is a population process, and coordinating the relationship between population and environment is very crucial for optimizing the yield and LUE [32]. Under these conditions, adjusting the canopy light interception (LI) is also an essential means to achieve these improvements. Xue et al. [11] found that increasing LI could significantly enhance cotton yield and LUE, particularly by raising the proportion of LI at the bottom of the canopy. However, when LI levels become too high, cotton yield may not increase significantly, and in some cases, it may even decrease. This phenomenon could be attributed to the high percentage of LI in the upper canopy, resulting in a severe shading in the middle and lower canopy regions [22]. Insufficient light in these areas may impede the normal growth of cotton bolls, leading to a decline in the boll formation rate of cotton plants [34]. Simultaneously, under medium- and low-density conditions, reducing irrigation levels can improve canopy ventilation and light transmission. However, the low-population biomass and the number of bolls per unit area can lead to decreased LUE and can even reduce the yield. Therefore, under high-density conditions, reducing deficit irrigation by 20% water consumption could not only enhance LI in the canopy, but also improve the structural distribution of the canopy. In the present study, cotton plants increased effective green leaf area during the reproductive growth stage, which lead to higher canopy photosynthetic rates to facilitate the transfer of assimilates to the reproductive organs, improving cotton yield and LUE.

5. Conclusions

In the oasis cotton region of Xinjiang, adopting an irrigation level of approximately 315 mm under a planting density of approximately 22.5 plants m⁻² for mulch-covered drip irrigation cotton can optimize the distribution of light energy within the canopy. This optimization leads to improved cotton yield and light use efficiency under the tailored irrigation and density conditions. The findings of this research can assist farmers, in this region and in other areas of cotton growing with similar climatic conditions, in conserving water and enhancing their agronomic management strategies under water scarcity conditions.