Effects of Deficit-Regulated Irrigation on Root-Growth Dynamics and Water-Use Efficiency of Winter Wheat in a Semi-Arid Area

Abstract

Water management is critical for wheat production under extreme drought conditions, and the mechanisms by which root dynamics and soil water utilization affect wheat yield are uncertain. This study was conducted in 2023–2024 under a mesophilic semi-arid climate with a two-factor partitioned experimental design, aiming to assess the response of different irrigation amounts in winter wheat crops on root growth and development, soil water utilization, and yields in different soil horizons. The results showed that variety and irrigation volume had significant effects on the spatial and temporal distribution of root and yield components, with irrigation volume having the greatest effect on yield. Compared with CK, deficit-regulated irrigation significantly promoted root penetration to deeper layers and delayed root senescence. DRWD, RLD, RSA, and RV decreased gradually with increasing soil depth, and the peaks of RLD, RSA, and RV appeared at the tassel to flowering stage, respectively; and under deficit-regulated irrigation, the contribution of the A2W4 treatment to stable yield was greater. Therefore, A2W4 is an effective water-saving irrigation method to improve grain yield and water-use efficiency under deficit-regulated irrigation.

1. Introduction

Wheat (Triticum aestivum L.) is one of the important food crops in our country, and the wheat grown in semi-arid regions of the country accounts for 60% of wheat production [1]. Drought is an important factor affecting wheat yield. With the increase in extreme drought climates globally, the wheat yield decreased up to 30% [2]. It is estimated that by around 2030, China’s population will reach up to 1.6 billion and the food requirement will reach more than 640 million tonnes, and the corresponding irrigated area will need to grow more than 9 billion ha in order to meet people’s daily food consumption needs [3].

Xinjiang is located in the interior of northwestern China and has typical semi-arid climatic conditions in the mid-temperate zone. Due to its rich light and heat resources, Xinjiang has become one of the main production areas for wheat cultivation in China. Out of which, irrigated wheat in Xinjiang accounts for about 1.1 × 10⁶ ha of planting area [4]. In recent years, agricultural production in Xinjiang has been constrained by a combination of water scarcity, serious water wastage, and fragile ecological environment [5,6,7], resulting in a serious threat to wheat production. Moisture is a necessary factor for wheat production. The average annual rainfall in Xinjiang is only about 150 mm; however, the evaporation is more than 2000 mm, and the average agricultural water consumption over the years reaches 5.33 × 10¹⁰ m³, accounting for more than 94% of the total water consumption in the region [8,9,10]. In addition, how to promote the use of high-efficiency, water-saving irrigation technology and the screening of drought-tolerant and high-yielding varieties in semi-arid areas in the context of the third green revolution in agriculture is of great importance for the sustainable and healthy development of oasis agriculture in Xinjiang.

Previous research has shown that micro-sprinkler and drip irrigation improved wheat root distribution, delayed root senescence, promoted water uptake in deeper soils, and ultimately increased wheat yields compared to conventional irrigation [11,12]. Deficit-regulated irrigation is a water-saving irrigation strategy considered to be potentially valuable in arid regions, which can improve water productivity per unit of water by regulating the amount and duration of irrigation. Deficit-regulated irrigation can achieve improvement of crop water-use efficiency and can increase and stabilize yields [13]. In addition, one or two appropriate irrigation strategies help to increase yields and water-use efficiency [14]. Highest irrigation efficiency was achieved if 750 m³ ha⁻¹ were irrigated at each of the jointing and flowering (T3) stages [15]. Irrigation at critical wheat growth periods such as the rising, jointing, heading, and flowering stages can increase the number of spikes and fruiting florets, and increase the weight of individual grains, resulting in higher seed yield [16]. The stem elongation stage is a critical stage in the water requirements of wheat, and drought occurring during jointing has severe effects on plant growth and photosynthesis [17], while a water deficit before elusion can adversely affect wheat yield [18]. In arid regions, it has been shown that a rainwater harvesting system for ridges in combination with 75 mm irrigation increased soil moisture throughout the rooting zone, while irrigation at the jointing stage and jointing and flowering stage increased yields by an average of 12.79% and 18.65% compared to no irrigation [19]. Previous studies have mainly investigated the effects of single factors such as cropping practices [20], planting density [21], irrigation volume, and irrigation frequency [22] on wheat root growth and development. Research on water-saving irrigation mainly focuses on soil properties, water-use efficiency, and yield. Liu et al. showed that crop root growth is closely related to the level of water irrigation; compared with the appropriate amount of irrigation, water deficit can promote wheat root distribution in deeper soil [23], and can increase the length of the root system of wheat at the spiking stage by 15.5%. Gajri et al. showed that wheat irrigation at the early reproductive stage can shorten the root growth and development time, thus increasing water-use efficiency and thus achieving high yields [24]. For deficit-regulated irrigation, excessive irrigation is detrimental to wheat crops’ water efficiency, and irrigated 3600 m³ ha⁻¹ from the nodulation to the filling stage is a suitable cultivation pattern for the simultaneous improvement of efficiency of yield and water use of winter wheat in this area [25]. Nevertheless, the roles of variety and irrigation in the regulation of winter wheat roots’ morphological parameters in the 0–80 cm soil layer under deficit irrigation treatments is unknown.

Based on previous studies, the aims of this study were as follows (1) to elevate the sensitivity of wheat plants to water stress during nutritive and reproductive growth; (2) to determine whether irrigation regimes can interact synergistically with root-growth rates to improve wheat yield and WUE, and (3) to determine combinations of deficit-modulated irrigation and varieties to optimize wheat yield and resource-use efficiency.

2. Materials and Methods

2.1. Experiment Site

The experiment was conducted in 2023–2024 at the Qitai Wheat Experiment Station of Xinjiang Academy of Agricultural Sciences (43°59′53″ N, 89°44′23″ E) on sandy loam soil, with the basic physicochemical properties of the 0–20 cm soil layer being 47.57 g kg⁻¹ of organic matter (OM), 67 mg kg⁻¹ of alkaline-dissolved nitrogen (AN), 14.1 mg kg⁻¹ of quick-acting phosphorus (OP), and 141 mg kg⁻¹ of quick-acting potassium (AK). The precipitation and temperature of wheat growing in different months in this experiment (provided by the local meteorological station) are shown in Figure 1. The soil capacity and field water-holding capacity of the 0 to 80 cm soil layer in the experimental plot are shown in

Table 1. Bulk density of the soil and capacity in the 0–80 cm soil layers of the experiment field.

Soil Layer (cm)	2023–2024
	Capacity (g cm−3)	Field Water-Holding Capacity (%)
0–20	1.54	25.45
20–40	157	24.30
40–60	1.58	25.22
60–80	1.56	24.34

.

2.2. Experiment Design

In the 2023 to 2024 wheat-growing season, a two-factor split-zone experimental design was used, with the main zone varieties (A) set as New Winter 22 (A1) and New Winter 18 (A2), and the subzones irrigation (W) set as 2280 m³ ha⁻¹ (W1), 2820 m³ ha⁻¹ (W2), 3360 m³ ha⁻¹ (W3), 3900 m³ ha⁻¹ (W4), 4440 m³ ha⁻¹ (W5), and 4980 m³ ha⁻¹ (CK). The plots were 12 m² (2 m × 6 m) in size and the treatments were randomly grouped in three replications each. Water infiltration was prevented by using 2 m wide protection rows between two adjacent irrigated plots, and irrigation water was measured by flow meter. Groundwater was buried at a depth of about 25 m. Water at the test site was rationed at 4500 m³ ha⁻¹ and watered eight times during the reproductive period, and the irrigation volumes for the two growing seasons are shown in Figure 2.

This test variety is the most widely planted local variety in Xinjiang, and all fields, 105 kg ha⁻¹ of pure N (urea), 140 kg ha⁻¹ of P₂O₅ (calcium superphosphate), 140 kg ha⁻¹ of K₂SO₄, and 130 kg ha⁻¹ of N were applied as a basal dressing at the pulling stage before sowing, and the winter wheat was harvested on 4 October 2023 and 1 July 2024, with a planting density of 3,000,000 ha⁻¹, and no diseases or pests occurred during the trial period. Other management measures are consistent with local field management measures.

2.3. Measurements and Data Analysis

2.3.1. Soil Moisture Content

Soil samples were collected by soil auger, 3 days after irrigation at pre-sowing, rising, jointing, botting, heading, flowering, grain-filling, and maturity stages; samples were taken from each 20 cm soil layer to a depth of 80 cm. Soil moisture was determined by the desiccation method.

2.3.2. Determination of Root System Characteristics

The field trial was set up as an in situ soil column test. A replicated field trial area was selected and mapped prior to soil preparation. The soil at 80 cm depth was ploughed to the surface in four layers: 0–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm. The 0 to 80 soil layers were blended and the bulk density of each layer determined. Soil columns were constructed according to the method of Wu et al. [26]. Root samples were collected from the 0–80 cm soil layers in each plot at the jointing, botting, heading, flowering, and grain-filling stage, with 1 soil layer for every 20 cm depth [27]. Root parameters (root length, root surface area, and root volume) were obtained from the root analysis software (WinRHIZO2008) for the different soil layers after treatment.

Root dry weights were obtained by blotting with absorbent paper and drying in an oven at a constant temperature of 80 °C. Dry-weight density of roots (DRWD, g m⁻³), length density of roots (RLD, cm cm⁻³), surface area of roots (RSA, m² ha⁻¹), and volume of roots (RV, m³ ha⁻¹) in different soil layers were determined by Equations (1)–(4) [28,29,30,31], respectively.

$$DRWD={\displaystyle \frac{M}{V}}\times {10}^6$$

(1)

$$RLD={\displaystyle \frac{L}{V}}$$

(2)

$$RSA={\displaystyle \frac{A}{S}}\times {10}^8$$

(3)

$$RV={\displaystyle \frac{V^{\prime }}{S}}\times {10}^8$$

(4)

Total root dry weight (TDRW, g m⁻²), total root length (TRL, cm cm⁻²), total root surface area (TRSA, m² ha⁻¹), and total root volume (TRV, m³ ha⁻¹) were defined as the unit of soil area in different layers of soil. The sum of root weight, root length, RSA, and RV for different soil areas [32], respectively (5)–(8) were determined by the following:

$$TDRW={\displaystyle \sum _{i=1}^n{M}_i}/S\times {10}^4$$

(5)

$$TRL={\displaystyle \sum _{i=1}^n{L}_i}/S$$

(6)

$$TRSA={\displaystyle \sum _{i=1}^n{A}_i}/S\times {10}^8$$

(7)

$$TRV={\displaystyle \frac{{\displaystyle \sum _{i=1}^n{V}_i^{\prime }}}{S}}\times {10}^8$$

(8)

2.3.3. Grain Yield, ET, and WP

Wheat grain yield was measured from a 6 m² area per plot at maturity. Groundwater recharge and runoff were negligible at this experimental site. Crop water consumption (ET) (9) and water-use efficiency (WUE) (10) were calculated using the following soil–water balance equation [32,33].

ET = irrigation + precipitation + soil water consumption

(9)

WUE was defined as follows:

WUE (kg ha⁻¹) = wheat grain yield/water consumption during the reproductive period

(10)

2.3.4. Statistical Analysis

The data were arranged using Microsoft Excel 2016 software, analyzed by ANOVA, and plotted using R.4.2.1 R language software, and multiple comparisons were performed using the Least Significant Difference (LSD) method for significant (p < 0.05) differences [34].

3. Results

3.1. Effects of Deficit-Regulated Irrigation on Soil Water Consumption Characteristics and Its Water-Use Efficiency

The total water consumption of wheat exhibited a notable decline with a reduction in irrigation volume under a range of deficit-regulated irrigation treatments. Conversely, the water-use efficiency demonstrated an ‘N’ pattern (

Table 2. Effect of deficit-regulated irrigation on total water consumption at different fertility stages of wheat.

Varieties	Treatment	Total Water Consumption (g m−2)
		Rising	Jointing	Botting	Heading	Flowering	Filling
A1	W1	56.49 a	66.62 a	57.06 a	43.77 a	30.99 a	66.91 a
	W2	64.50 b	69.63 b	62.30 b	71.04 b	34.74 b	70.65 b
	W3	67.64 c	75.91 c	69.35 c	81.55 c	41.94 c	147.28 c
	W4	70.05 d	82.78 d	74.42 d	94.54 d	61.57 d	92.00 d
	W5	69.42 e	89.52 e	83.43 e	110.42 e	92.11 e	96.88 e
	CK	66.42 f	85.85 f	99.48 f	137.05 f	124.75 f	104.86 f
A2	W1	60.30 a	52.00 a	45.77 a	69.94 a	21.72 a	60.64 a
	W2	71.76 b	64.16 b	49.12 b	76.50 b	30.73 b	72.79 b
	W3	76.00 c	73.64 c	59.24 c	92.14 c	53.42 c	86.91 c
	W4	77.85 d	78.82 d	65.05 d	102.09 d	65.01 d	99.20 d
	W5	81.52 e	86.20 e	76.00 e	105.31 e	82.04 e	103.15 e
	CK	85.62 f	75.66 f	94.98 f	92.59 f	105.21 f	70.24 f
Significance level
A		***	***	***	***	***	***
W		***	***	***	***	***	***
A × W		***	***	***	***	***	***

Note: Different lower case letters in the same column indicate significant (p < 0.05) differences between treatments with different tillage practices or different irrigation rates for the same tillage practices. Same as below. *** Extremely significant difference at p < 0.001. The same below.

and

Table 3. Effect of deficit-regulated irrigation on water-use efficiency at different fertility stages of wheat.

Varieties	Treatment	WUE (kg ha−1 mm−2)
		Rising	Jointing	Botting	Heading	Flowering	Filling
A1	W1	73.46 a	62.29 a	72.73 a	94.81 a	133.89 a	62.02 a
	W2	69.77 b	64.63 b	72.23 b	63.34 b	129.54 b	63.70 b
	W3	76.88 c	68.51 c	74.98 c	63.76 c	123.99 c	35.31 c
	W4	81.37 d	68.86 d	76.59 d	60.29 d	92.58 d	61.96 d
	W5	102.28 e	79.31 e	85.11 e	64.30 e	77.09 e	73.29 e
	CK	93.34 f	72.22 f	62.32 f	45.24 f	49.70 f	59.13 f
A2	W1	83.76 a	97.12 a	110.33 a	72.20 a	232.56 a	83.27 a
	W2	72.47 b	81.05 b	105.86 b	67.98 b	169.19 b	71.44 b
	W3	71.05 c	73.33 c	91.15 c	58.61 c	101.09 c	62.13 c
	W4	84.14 d	83.10 d	100.70 d	64.16 d	100.76 d	66.03 d
	W5	64.41 e	60.91 e	69.076 e	49.85 e	63.99 e	50.90 e
	CK	56.64 f	64.10 f	51.06 f	52.38 f	46.10 f	69.05 f
Significance level
A		***	***	***	***	***	***
W		***	***	***	***	***	***
A × W		***	***	***	***	***	***

Note: *** Extremely significant difference at p < 0.001. The same below.

). In comparison to the CK, the total water consumption and water-use efficiency of wheat exhibited an ‘M’ pattern throughout the reproductive process. The highest total water consumption was observed at the filling stage, with an average of 89.64, while the highest water-use efficiency was recorded at flowering, with a value of 122.47. The treatment resulted in a maximum value of 98.20 for the water-use efficiency (WUE). The water-use efficiencies of A2 varieties exhibited an average increase of 34.29%, 20.99%, 11.74%, and 12.90%, respectively, compared to A1 varieties under the W1, W2, W3, and W4 treatments. This indicated that A2 varieties were able to utilize water more efficiently and improve water-use efficiencies under deficit-regulated irrigation conditions.

3.2. Effects of Deficit-Regulated Irrigation on Yield and Its Components in Drip-Irrigated Winter Wheat

As shown in Figure 3, deficit-regulated irrigation had a significant effect on wheat yield and its components during the wheat reproductive period. The number of spikes per hectare, number of grains per spike, thousand grain weight, and yield of wheat showed an increasing and then decreasing trend with the decrease in irrigation, and reached maximum values of 1995.00, 28.29, 47.61, and 7100.04, respectively, under W5 treatment. The number of spikes per hectare, number of grains per spike, and thousand grain weight of wheat showed a significant trend of increase in the W5 treatment as compared to CK with an average increase of 13.33%, 3.66%, and 3.73%, respectively, and the deficit irrigation treatment showed a significant increase in its yield and its components as compared to CK, 3.66 and 3.73 per cent. The deficit-regulated irrigation treatments showed an average decrease of 13.33, 3.52, 7.49, and 0.42 percent in the number of spikes per hectare, number of grains per spike, and weight of thousand grains per hectare and yield, respectively, as compared to CK.

3.3. Effects of Deficit-Regulated Irrigation on Root Development Indices and Root Architecture

As shown in Figure 4a,b, deficit-regulated irrigation significantly affected total root dry weight (TDRW) and total root length (TRL), which showed a significant decreasing trend with decreasing irrigation water. Compared with CK, TDRW decreased by 22.43% in deficit-regulated irrigation treatments, with the largest decrease of 63.42% in W1 treatment, and TRL increased by 14.92%, with the largest increase of 43.21% in W5 treatment. Compared with the A1 variety, the TDRW and TRL of the A2 variety increased by an average of 58.55% and 47.00%, respectively, under the deficit-regulated irrigation treatment.

3.4. Effects of Regulated Deficit Irrigation on Root Growth of Winter Wheat under Drip Irrigation

As shown in

Table 4. Effects of deficit-regulated irrigation on the TRSA of winter wheat.

Varieties	Treatment	TRSA (×103 m2 ha−1)
		Rising	Jointing	Botting	Heading	Flowering	Filling
A1	W1	7.54 a	21.70 a	5.09 a	6.81 a	2.76 a	6.29 a
	W2	9.33 b	23.29 b	5.22 b	8.74 b	4.91 b	7.75 b
	W3	9.79 c	24.51 c	5.55 c	8.68 c	7.35 c	7.51 c
	W4	9.11 d	30.54 d	8.12 d	11.32 d	26.03 d	9.13 d
	W5	20.34 e	42.34 e	15.09 e	21.16 e	25.32 e	21.87 e
	CK	14.00 f	27.50 f	12.81 f	15.45 f	10.53 f	14.95 f
A2	W1	13.10 a	34.53 a	7.45 a	9.21 a	9.19 a	10.24 a
	W2	12.55 b	29.11 b	8.24 b	10.61 b	8.96 b	10.93 b
	W3	14.22 c	31.78 c	8.25 c	11.20 c	7.80 c	10.64 c
	W4	21.33 d	47.22 d	13.62 d	15.32 d	11.95 d	18.37 d
	W5	14.01 e	30.19 e	11.51 e	8.92 e	7.58 e	11.70 e
	CK	11.73 f	22.16 f	8.71 f	7.96 f	7.32 f	10.26 f
Significance level
A		***	***	***	***	***	***
W		***	***	***	***	***	***
A × W		***	***	***	***	***	***

Note: Different lower case letters in the same column indicate significant (p < 0.05) differences between treatments with different tillage practices or different irrigation rates for the same tillage practices. Same as below. *** Extremely significant difference at p < 0.001. The same below.

and

Table 5. Effects of deficit-regulated irrigation on the TRV of winter wheat.

Varieties	Treatment	TRV (m3 ha−1)
		Rising	Jointing	Botting	Heading	Flowering	Filling
A1	W1	1.32 a	21.70 a	5.09 a	6.81 a	2.76 a	6.29 a
	W2	1.97 b	23.29 b	5.22 b	8.74 b	4.91 b	7.75 b
	W3	2.21 c	24.51 c	5.55 c	8.68 c	7.35 c	7.51 c
	W4	2.14 d	30.54 d	8.12 d	11.32 d	26.03 d	9.13 d
	W5	5.37 e	42.34 e	15.09 e	21.16 e	25.32 e	21.87 e
	CK	3.53 f	27.50 f	12.81 f	15.45 f	10.53 f	14.95 f
A2	W1	2.27 a	34.53 a	7.45 a	9.21 a	9.19 a	10.24 a
	W2	2.09 b	29.11 b	8.24 b	10.61 b	8.96 b	10.93 b
	W3	2.93 c	31.78 c	8.25 c	11.20 c	7.80 c	10.64 c
	W4	4.16 d	47.22 d	13.62 d	15.32 d	11.95 d	18.37 d
	W5	3.34 e	30.19 e	11.51 e	8.92 e	7.58 e	11.70 e
	CK	2.69 f	22.16 f	8.71 f	7.96 f	7.32 f	10.26 f
Significance level
A		***	***	***	***	***	***
W		***	***	***	***	***	***
A × W		***	***	***	***	***	***

Note: Different lower case letters in the same column indicate significant (p < 0.05) differences between treatments with different tillage practices or different irrigation rates for the same tillage practices. Same as below. *** Extremely significant difference at p < 0.001. The same below.

, deficit-regulated irrigation had a significant impact on total root surface area (TRSA) and total root volume (TRV), which showed a significant decreasing trend with decreasing irrigation volume. Compared with CK, TRSA increased by 6.86% under deficit-regulated irrigation treatments, with the highest increase of 40.79% in TRSA under W5 treatment, and TRV decreased by 10.03%, with the highest decrease of 46.04% in TRV under W5 treatment. Compared with the A1 variety, the average increase in TRSA and TRV of the A2 variety under deficit-regulated irrigation treatment was 35.13% and 50.38%, respectively. Whereas, the average increase in TRV was in the order of W1 (132.03%), W4 (92.86%), W2 (46.13%), W3 (26.46%), and W5 (−45.58%).

Throughout the growth period, the root system parameters under the different treatment combinations showed an upturned “V”-shaped trend, initially increasing and then declining. Under deficit-regulated irrigation treatments, all root system parameters peaked during the jointing stage. The A1W5 and A1W1 treatments had the highest and lowest values of the four root morphology parameters (TDRW, TRL, TRSA, TRV), respectively. The figures (Figure 5a–d) illustrate the distribution of root phenotypic parameters in soil layer 0 to 80, indicating that DRWD, RLD, RSA, and RV progressively decreased with increasing depth of soil layer at the same growth stage. Compared to the control (CK), the deficit-regulated irrigation enhanced the root system parameters by 6.45%, with the A1W5 treatment showing the maximum increase of 24.19%. DRWD, RLD, RSA, and RV in the 0–80 cm layer of the soil under the different treatment combinations showed an increasing and then a decreasing trend during the whole reproductive period of the wheat. The root system parameters varied with soil depth. RLD, RSA, RV, and DRWD peaked at the jointing and booting stages, respectively, in the 0 to 20 cm soil layer. In contrast, maximum RLD, RSA, RV, and DRWD in the 20–80 cm soil layer occurred during the heading and flowering stages, respectively. Moreover, compared to the A1 variety, the A2 variety showed an average increase of 45.64% in root system parameters under deficit irrigation treatment.

3.5. Effects of Deficit-Regulated Irrigation on Root Growth of Winter Wheat under Drip Irrigation

As shown in Figure 6, deficit-regulated irrigation had a significant effect on TDRW, TRL, TRSA, and TRV indexes in winter wheat, and different deficit irrigation mainly promoted the accumulation of dry matter in TDRW. Compared with CK, deficit irrigation had a significant effect on root morphology and structure, and deficit irrigation treatments had a significant effect on TDRW and TRSA, followed by TRL and TRV. Under deficit-regulated irrigation treatments, compared with CK, A1W5 and A2W4 treatments had significant enhancement effects on root morphology indexes, and with the advancement of reproductive process, the TRL, TRSA, and TRV reached the maximum value at the nodulation stage of the A2 variety W5 treatment with 2.28, 47.22, and 14.85, respectively, and the TDRW reached the maximum value in the W5 treatment at the irrigation stage of the A1 variety, with 615.72.

3.6. Correlation Analysis of Root-Growth Parameters and Yield of Winter Wheat under Drip Irrigation

As shown in Figure 7, winter wheat yield and root morphology parameters were highly significant and positively correlated (p < 0.01) under deficit-regulated irrigation treatments. Yield was highly significant with TDRW, TRV, TRSA, TRL, DRWD, TDRW, RSA, RLD, and RV, where RV, TRV, and TRSA had larger correlation coefficients, all of which were 0.96. TRL and TDRW and their yields attained highly significant differences, but with smaller correlation coefficients, 0.82 and 0.85, respectively.

4. Discussion

The root system is a vital organ in plants, serving a multitude of functions. These include the uptake of water and nutrients, the storage of metabolites, the provision of anchorage, the provision of mechanical support, and the interaction with the soil environment [35]. In a study conducted by Zhang et al., it was demonstrated that the soil layer between 0 and 20 cm is the region where roots exhibit the most vigorous growth and development [36], and is also the layer with the greatest root length, root dry weight, and root length density; compared with 3750 m³ ha⁻¹ irrigation conditions, extreme drought conditions promote the increase in the deep root volume to a certain extent, which contributes to the utilization of the deep soil water by the root system of wheat under drought conditions and improves the water-use efficiency, but it results in a lower yield, whereas overflooding of 4500 m³ ha⁻¹ could lead to inhibition of root growth, prompting a decrease in root vigor and early root decline, affecting water uptake and ultimately resulting in the lowest water-use efficiency and lower yields. The present study has demonstrated that the beneficial impact of deficit-regulated irrigation on root structure is evident in the following key areas: All growth parameters in the 0–80 cm soil layers peaked at nodulation, whereas TDRW, TRL, TRSA, and TRVs peaked at tasseling to flower stage in W1. Root parameters peaked sooner in deficit-irrigated treatments, and all growth parameters in soil layers 0–80 cm peaked at nodulation, whereas TDRW, TRL, TRSA, and TRV peaked later in W1 treatment. This may reflect the improved soil water use, WUE, and irrigated water-use efficiency during the later fertile stages following deficit-irrigated treatments [37]. Tiller root conformation in cereal crops is considered to be the most complex root system in cereal crops, and its irrigation level and timing have a significant effect on its root growth. It has been shown that irrigation applied too late in the reproductive period is detrimental to root growth in the later stages of wheat fertility, whereas deficit-regulated irrigation applied early in the reproductive period is beneficial for obtaining high yielding and efficient root morphology and architecture. Research has shown that water control during wheat nodulation promotes deep root growth, photosynthate partitioning into roots, tillering, and canopy structure [38]. It has been shown that the W1 irrigation pattern can delay deep rooting senescence and enhance DRWD, RLD, RSA, and RV during nodulation, thereby enhancing soil penetration and deep-soil-moisture uptake and utilization [39]. Precipitation plays a small role in yield formation of winter wheat in Xinjiang, and under the condition of consistent soil water storage, the main influence on wheat yield formation is the amount of irrigation, while excessive irrigation can reduce wheat yield and water-use efficiency [40]. Additionally, the biological activity of the root system in deep soils is enhanced by appropriate irrigation [41].

The impact of distinct deficit-regulated irrigation techniques on the root morphology of winter wheat in Xinjiang’s characteristic semi-arid climatic conditions remains unclear. It was found that the appearance of the maximum values of root parameters in each treatment showed an overall delay with the increase in irrigation volume and soil depth (Figure 8). The deep RLD, RSA, and RV exhibited delayed flowering under the W1, W2, W3, and W4 treatments, while the DRWD demonstrated delayed filling under the W5 and CK treatments. It can be hypothesized that the observed effects may be attributed to the impact of water deficit during the initiation period on soil bulk weight and soil water content in the 0–20 cm layer of the top soil. This resulted in an increase in soil porosity, field water-holding capacity, and growth of primary and secondary roots in the 0–20 cm soil layer, as evidenced in previous studies [42]. Conversely, the root-growth parameters were found to be influenced by soil bulk weight [43] and soil water content [44], as demonstrated in other research. This indicates that the implementation of deficit-regulated irrigation has the potential to stimulate root development by enhancing soil structure via the modification of irrigation water content. Prior studies have concentrated on the impact of an individual factor—either variety or watering volume—on the root development of wheat. Nevertheless, there is little literature on the effect of dual factors and their interactions on the root structure of wheat roots under deficit-regulated irrigation. Tennant et al. showed that wheat root stunting is mainly caused by depletion of the crops’ soil moisture, and that root morphology and structure are significantly affected with deeper soil depth, and that root morphology and structure of root systems of different wheat varieties are subjected to wide variations [45,46]. ShahzĀD et al. found that under arid wheat-cropping systems, deficit-regulated irrigation of 1500 m³ ha⁻¹ can significantly promote the growth of root morphology in soils within the 0–40 cm soil layer under 200 mm of precipitation, thereby increasing grain yield [47]. However, the regulatory effect of deficit-regulated irrigation on root growth was predominantly observed in the root system below 20 cm in the present study. An optimal distribution of root-growth characteristics was observed within the 0–80 cm soil layer, contingent upon the A2W4 treatment. In the deep layer, the peak appearance of RLD, RSA, and RV was delayed as a result of this treatment. Furthermore, this study demonstrated that the peak of deep root growth occurred later under the A2W4 treatment, and the rate of decline of post-peak decay was slowed down. Therefore, we concluded that a good spatio-temporal distribution of the root system is the most important morphological or physiological factor that leads to high yields.

It was concluded by Meng et al. that the application of irrigation at the overwintering and jointing stages of wheat is conducive to the accumulation and utilization of dry matter in the later stages of wheat growth. This finding is supported by evidence cited in reference [48]. Ma et al. showed that deficit-regulated irrigation at the nodulation and flowering stages was beneficial for improving wheat water-use efficiency and its yield [49]. However, multiple irrigation treatments based on two varieties were added in our study. The W4 and W5 treatments significantly improved total water consumption, water-use efficiency, yield, and its components in wheat as compared to CK. The results of the two-factor treatments showed that the A1W5 treatment had the highest yield. Liao et al. showed that strong, high, and consistent seed yield was positively correlated with high and consistent deep root growth [50]. In the present study, both root-growth parameters and seed yield showed highly significant positive correlations, which may be due to the positive correlation between root-growth parameters and water-use efficiency, which in turn is in general agreement with Zhang et al. [51]. It is hypothesized that root-growth parameters may significantly affect the absorption of water and soil nutrients in winter wheat, which in turn regulates the number of spikes and thousand grain weight, and ultimately affects seed yield. Among them, the A2W4 treatment was found to be instrumental in retarding the aging process of roots, sustaining greater root biomass and length, and enhancing the root system architecture (RSA) during the later phases of growth. These effects are crucial for achieving optimal yield outcomes.

5. Conclusions

Under the extreme climatic conditions in Xinjiang, there was a positive correlation between soil water content, root water uptake, and seed yield and root structure and distribution characteristics of winter wheat. A2W4 can be used as a water-saving and stable-yield-cultivation model for winter wheat, which has moderate root growth and dis-attribution in the middle layer and a large number of roots in the surface and bottom layers. In this growing regime, the peak of deep root growth occurred later and the decay following the peak was delayed. By leveraging this root distribution characteristic, it is possible to effectively increase the water consumption of the middle and lower soil layers, maintaining a large amount of root biomass in the late reproductive stage, realizing the water demand during the filling period, and improving the water-use efficiency; thus, dry matter production and seed yield can be promoted. In summary, the proportion of deep roots has an effect on wheat grain yield potential. Improvement of deep soil root distribution and uptake capacity during cultivation can help to achieve high yields and make the most of the water used.