Synergistic Effects of Crop Aboveground Growth and Root Traits Guarantee Stable Yield of Strip Relay Intercropping Maize

Abstract

In order to reveal the yield-increasing mechanism of relay intercropping (RI) maize with different varieties from the perspective of plant growth, source sink relationship, and root growth, a two-factor randomized block design trial was designed, which includes different maize varieties (Rongyu1210 (RY1210), Zhongyu 3 (ZY3)) and plant pattern (RI, Sole cropping (SC)). The leaf area index (LAI), dry matter accumulation and distribution, root dry weight (RDW), root length (RL), root surface area (RSA), root volume (RV), and maize yield were determined. LAI of RI RY1210 was significantly higher than that of the SC RY1210 at the filling stage and maturity stage. The dry matter accumulation of RI RY1210 ear was significantly higher than that of SC RY1210 and RI ZY3, and the RDW of RY1210 was significantly higher than that of ZY3. The ratio of RDW of RI RY1210 was higher than that of RI ZY3 in the 20–40 and 40–60 cm soil layers, respectively. The RDW, RL, RV, and RSA of RI RY1210 were significantly lower than that of sole RY1210 by 25.43%, 10.75%, 30.79%, and 23.73%, respectively, but higher than that of RI ZY3 by 143.98%, 278.29%, 54.40%, and 29.57%, respectively. The average yield of RI RY1210 was 8782.71 kg ha⁻¹, with no significant difference compared to SC, which was mainly attributed to a larger ear dry matter accumulation, higher LAI in later growth stages, larger RDW, and the ratio of roots in deeper soil layers. This study will be useful and helpful to farmers for how to select and plant high-yielding maize varieties in strip relay intercropping.

1. Introduction

Maize (Zea mays L.) is an essential grain crop, feed crop, and industrial feedstock in the world. China is a major producer and consumer of maize in the world [1]. Determining how to alleviate the contradiction between population growth and arable land reduction while increasing maize production is an inevitable demand for the green and sustainable development of China’s agriculture and for establishment of a set of crops with a high yield and high efficiency, sustainable development, multi-objective synergistic, and a simple and easily spreadable maize–soybean (Glycine max L.), a strip mixed intercropping system could be a key step to solve this problem. The strip mixed intercropping system is classified into strip intercropping and strip relay intercropping. Strip intercropping is the planting of two or more crops simultaneously in different strips on the same land, and strip relay intercropping is when late-season crops are planted by row at a late stage of growth of early-season crops, which could increase maize yield and added a season of other crops [2]. The maize–soybean strip mixed intercropping system, which has been widely applied in China, Pakistan, the United States, India, and Africa, was regarded as one of the effective pathways to ensure national food security [3].

Roots are important organs for water and nutrient uptake by plants, and root growth and development in soil is related to cultivation measures [4]. Previous studies have shown that changes in intraspecific–interspecific competitive relationships [5,6,7], and the intensity of aboveground–underground crop interactions can modify crop root morphology [8]. Maize with legumes or cereals intercropping will co-regulate root growth and development through interspecific reciprocity and could change root spatial distribution (RSD) and root architecture [9,10]. Root plasticity and interspecific complementarity can promote a maize–legume intercropping advantage [11,12,13]. Light conditions can enhance the response of intercropping crops to phosphorus stress by modifying root morphology and physiological characteristics [14,15]. In general, different cultivation measures would directly affect maize root growth by improving crops spatial layout. Furthermore, making full use of the spatial difference of root distribution and plasticity of root morphology is one of the important methods to achieve intercropping advantages complementarity. Many studies in the past have focused on intraspecific–interspecific relationships, intercropping maize with different legumes crops, intercropping advantages, and the effect of light radiation on the crop’s roots. Up to now, studies on the RSD and root architecture in response to different relay intercropping varieties have received less attention.

Vigorous roots could promote crop uptake of soil nutrients and water and delay leaves senescence [16], thus improving biomass and grain yield. Previous studies have shown that maize root distribution increased in longitudinal and improved nutrient uptake, which was beneficial to increase maize yield, which was owing to asymmetric intercrop species reciprocity. The compatibility of RSD of intercropping species contributed to a yield increase in faba bean maize intercropping systems due to interspecific symmetry reciprocity [5]. Under different bandwidth cultivation patterns, superior maize root architecture and spatial distribution can be achieved by different maize–soybean row ratio configurations. However, over the appropriate bandwidth, plant distance between crops becomes narrower and intraspecific competition increases. Although it is beneficial for light energy interception and the mechanical harvesting of intercropped soybeans, increased intraspecific competition is not conducive to improve a crops group yield [17,18]. Therefore, the MSSRI system under 2 m bandwidth in southwest China has a better crop group yield, and it has been practically proven that narrow and wide row spacing of 40 cm and 160 cm, respectively, has more harmonious interspecific relationships and more obvious intercropping advantages, which are increasingly acceptable by farmers [6]. Nevertheless, the question should be further clarified regarding how maize yield responds to root morphology and aboveground growth under different relay intercropping maize varieties.

The field experiment was conducted in 2019–2020. Based on years of production practice, in order to ensure that the interspecific effects are as consistent as possible, we fixed the distance between the narrow and wide rows of all treatments to 40 cm and 160 cm, respectively. The main objectives of this study were (i) to explain the differences in yield effects of different maize varieties under different farming systems from the viewpoint of maize root architecture and root spatial distribution, (ii) to elucidate how to regulate aboveground growth of maize to achieve high crop yields under different farming systems and different maize varieties, and (iii) how to mediate cooperative growth between below- and aboveground to achieve high crop yields under different farming systems and different maize varieties. Based on the maize–soybean strip relay intercropping (MSSRI) system, this study provides a new insight into the maize root morphology structure and crop yield responses to different cultivation regulation measures, which will help us to further improve the root growth characteristics of maize and provide new ideas for intensive high-yield and high-efficiency planting.

2. Materials and Methods

2.1. Site and Experimental Design

The field experiment was conducted in Sichuan Modern Grain Production Demonstration Base in Zhujia Township (29°59′ N, 104°08′ E), Renshou County, Sichuan Province, China (Figure 1). The soil was of a pH of 6.35, an organic matter of 8.64 g kg⁻¹, total nitrogen (N) of 0.73 g kg⁻¹, and available N of 69.85 mg kg⁻¹, available phosphorus of 7.35 mg kg⁻¹, and available potassium of 99 mg kg⁻¹ in the top 20 cm of the soil. The basic parameters are shown in Figure 2. The experiment was conducted with a bandwidth of 2 m, maize–soybean row ratio of 2:2, maize–soybean row distancing of 40 cm, and sole maize row distancing of 70 cm (Figure 3). The experiment was conducted in a two-factor randomized complete block: factor A: different maize varieties, Rongyu1210 (A1, compact variety) and Zhongyu 3 (A2, semi-compact variety), factor B: relay intercropping (B1) and sole cropping (B2). Thus, A1B1, relay intercropping Rongyu1210, A1B2, sole cropping Rongyu1210, A2B1, relay intercropping Zhongyu 3, A2B2, sole cropping Zhongyu 3, with three replications and a total of 12 plots, with an area of 42 m² (bandwidth 6 m × band length 7 m). The density of the maize was 60,000 plants ha⁻¹. The maize basal fertilizer was made of 600 kg ha⁻¹ calcium superphosphate (containing 12% P₂O₅) and 150 kg ha⁻¹ potassium chloride (containing 60% K₂O). A total of 270 kg ha⁻¹ pure N (carbamide 46%) was applied during the whole growth period of maize. The planting density of soybean was 12,000 plants ha⁻¹, and no fertilizer was applied during the whole growth period. Other agronomic measures were used according to crop requirements and famer′s practices. The dates of maize and soybean sowing and harvesting are shown in

Table 1. The date of sown and harvested maize and soybean.

Year	Sowing and Harvest Period (Day/Month)
	Date of Maize Sowing	Date of Soybean Sowing	Date of Maize Harvest	Date of Soybean Harvest
2019	30/3	16/6	31/7	4/11
2020	1/4	16/6	1/8	2/11

.

2.2. Leaf Area Index (LAI)

LAI was calculated using the ratio of leaf area to the maize and soybean planting areas. Total LAI at the jointing stage (V6), tasseling stage (V14), filling stage (R2), and maturation stages (R6) was measured from three randomly selected plants of intercropped maize. LAI was calculated as:

$$\mathrm{L}\mathrm{A}\mathrm{I}=\frac{\rho \cdot \sum _{i=1}^n\left(L_i\right)}{S}$$

where $\rho$ is the sowing density, L is the leaf area per plant, i is the number of plants, and S is the land sampling area.

2.3. Dry Matter Accumulation

During the maize maturity stage (R6), three maize plants were chosen in each plot and each plant was divided into stem (including bracts and male panicles), leaves, and ear. All samples were packaged in paper bags and heat treated at 105 °C for 60 min, dried to a constant weight at 80 °C, and weighed to calculate the dry matter accumulation.

2.4. Root Architecture and Distribution

The maize root systems were collected at the filling stage (R2, Figure 4) and a pair of consistent plants were selected from maize strips in each plot where root distribution and root morphological were measured at different soil layers [6]. The vertical direction of the soil was divided into six layers from the surface to the subsurface, one layer for every 10 cm. The total horizontal length reaches 100 cm with the center of the maize row as the midpoint (Figure 3). All visible roots in each soil patch were carefully and meticulously manually removed and washed so as to ensure root sample integrity as much as possible, and root length (RL), root surface area (RSA), and root volume (RV) were measured with a root Perfection 2400 Photo (LA2400, Epson, Suwa, Japan). After that, the samples were heat treated at 105 °C for 60 min, dried at 80 °C to a constant weight, and weighed to obtain the root dry weight (RDW).

2.5. Yield and Yield Components

The effective panicles number (EPN) of each plot were examined at maturity. In total, twenty ears were selected from each plot according to the average weight method to investigate the number of rows and the number of seeds per row. When the sample seeds were sun dried to a constant weight, a 1000-kernel weight (TKW) was obtained using a 1/100 scale. Yield (kg ha⁻¹) = EPN × grain number per ear × TKW.

2.6. Statistical Analysis

Data were analyzed using Excel 2019 and SPSS Statistics 20.0 software (IBM Corp. in Armonk, NY, USA). All figures were plotted using Origin 2023 (Origin Lab, Northampton, MA, USA). The least significant difference (LSD) and Duncan method were used for post hoc multiple comparisons, difference significance test, and interaction analysis.

3. Results

3.1. Different Cultivation Measures to Regulate Maize Yield and Yield Components

There were significant differences in KEP, TKW, EPN, and yield between farming systems, but there were no significant differences in EPN between 2019 and 2020 (

Table 2. Different plant pattern to regulate maize yield and yield components.

Year	Treatment (T)	KPE	TKW	EPN	Maize Yield
			(g)	(ha−1)	(kg·ha−1)
2019	A1B1	504.74 ± 10.51 c	350.75 ± 3.73 a	55,535.00 ± 1261.83 b	9827.58 ± 250.40 ab
	A1B2	518.33 ± 2.38 bc	353.18 ± 2.66 a	57,171.43 ± 825.20 ab	10,467.46 ± 214.85 a
	A2B1	543.86 ± 11.38 ab	289.46 ± 5.74 b	56,666.67 ± 476.19 ab	8914.62 ± 133.28 c
	A2B2	555.73 ± 6.97 a	297.07 ± 10.03 b	58,571.43 ± 357.14 a	9662.90 ± 241.63 b
2020	A1B1	405.04 ± 4.01 d	338.59 ± 5.82 a	56,391.71 ± 277.79 ab	7737.83 ± 238.84 ab
	A1B2	415.62 ± 3.92 c	343.86 ± 4.92 a	57,965.48 ± 397.02 a	8282.46 ± 96.16 ab
	A2B1	455.27 ± 1.20 b	263.89 ± 5.25 b	56,017.86 ± 914.56 b	6735.51 ± 262.13 b
	A2B2	501.95 ± 0.93 a	277.69 ± 3.15 b	55,044.39 ± 479.67 b	7514.00 ± 80.57 c
Average	A1B1	454.89 ± 5.30 c	344.67 ± 2.67 a	55,963.35 ± 769.13 a	8782.71 ± 191.59 ab
	A1B2	466.98 ± 3.06 c	348.52 ± 3.74 a	57,568.45 ± 592.88 a	9374.96 ± 148.81 ab
	A2B1	499.56 ± 6.19 b	276.67 ± 0.98 c	56,342.26 ± 555.26 a	7825.06 ± 142.87 c
	A2B2	528.84 ± 3.83 a	287.38 ± 3.69 b	56,807.91 ± 396.42 a	8588.45 ± 82.09 b
F value	T	33.12 **	107.83 **	3.05 *	24.10 **
	Year (Y)	77.26 **	5.8 *	NS	68.27 **
	Y × T	NS	NS	NS	NS

Note: Different lowercase letters represent significant differences between different treatments in the same year. Different letters represent significant differences between different planting patterns between different years. A1B1: Relay intercropping Rongyu1210; A1B2: Sole cropping Rongyu1210; A2B1: Relay intercropping Zhongyu3; A2B2: Sole cropping Zhongyu3. TKW, 1000-kernel weight; EPN, effective panicles number; KEP, Kernels per ear. * means p < 0.05, ** means p < 0.01, NS means no significant difference.

). In addition, there were no significant differences in KEP, TKW, EPN, and yield from the interaction effect of years and treatments. There were no significant differences between A1B1 and A1B2 in terms of KEP, TKW, and yield, and A2B1 was significantly lower than A2B2 in terms of KEP, TKW, and yield. There were no significant differences among the treatments in terms of EPN. The maize average yield of A1B1 was 8782.71 kg·ha⁻¹, which was not significantly different from that of the sole cropping and was increased by 12.24% compared to A2B1, where the maize yield of A2B1 was 7872.07 kg·ha⁻¹, which was significantly reduced by 8.89% compared to sole cropping.

3.2. Effect of Different Cropping Patterns on LAI of RI Maize

The different varieties of maize LAI showed significant differences between 2019 and 2020 (Figure 5). On average, maize LAI showed a tendency of increase and then decrease and reached the maximum at the tasseling stage. The average maximum maize LAI in the A1B1 treatment was 4.31, which was significantly lower than that of sole cropping by 7.31%, and maize LAI in the A2B1 treatment was 3.79, which was significantly lower than that of sole cropping by 9.55%. The maize LAI in the A1B1 treatment at the filling stage and maturity stage was significantly higher than that of sole cropping, indicating that relay intercropping RY1210 is favorable to delay leaf senescence.

3.3. Effect of Different Cropping Patterns on Dry Matter Accumulation and Distribution of RI Maize

The A1B1 and A2B1 average ear dry matter was reduced by 9.36% and 9.7%, respectively, compared to A1B2 and A2B2. A1B1 significantly increased the total dry matter accumulation per plant and the dry matter partition in ear part compared to A2B1 (Figure 6a). In particular, the A1B1 dry matter partition to the leaf was significantly different compared to A1B2, and the dry matter partitioning to the stem was not significantly different between A2B1 and A2B2 (Figure 6b). The results revealed that shoot biomass and the allocation proportion were all affected by the different maize varieties. No significant difference was found between A2B1 and A2B2 in terms of the proportion of dry matter in different parts, However, the spike dry matter accumulation of A1B1 was significantly higher than that of A1B2 and A2B1, indicating that RY1210 was more conducive to grain filling and assimilation product accumulation than ZY3.

3.4. Root Spatial Distribution and Root Architecture

3.4.1. Different Plant Pattern Regulates the Root Spatial Distribution (RSD) of Maize

The maize RSD can be regulated according to different cultivation patterns, indicating that the maize root has plasticity. Horizontally, the maize roots were roughly symmetrically distributed in the left and right soil centered on the base of maize stalk. The maize RDW was mainly distributed within a horizontal direction of 20–30 cm centered at the base of the stalk, and in the vertical direction 0–20 cm of soil. In addition, the relay intercropping pattern favored the vertical growth of maize roots compared to sole maize and had an increased tendency of deep soil root distribution (Figure 7).

3.4.2. Different Plant Pattern to Regulate the Maize RDW and Ratio

The RDW of different maize varieties showed significant differences, with the root dry weight of RY1210 significantly higher than that of ZY3 (Figure 8). The RDW of sole cropping was significantly higher than that of relay intercropping in 0–20 soil layer, and there was no significant difference between the RDW of sole cropping and relay intercropping in 20–40 and 40–60 cm soil layers, respectively. In addition, the RDW of maize gradually decreased with an increasing soil depth. The RDW of RY1210 accounted for 58.6%, 31%, and 10.4% of the total dry weight in the 0–20, 20–40, and 40–60 cm soil layers, respectively (Figure 8d). The RDW of ZY3 accounted for 88.4%, 9.8%, and 1.7% of the total dry weight in the 0–20, 20–40, and 40–60 cm soil layers, respectively (Figure 8f). The RDW of RY1210 was significantly higher than that of ZY3 among different soil layers, especially the ratio of RDW of RY1210 was higher than that of ZY3 in 20–40 and 40–60 cm soil layers, respectively, indicating that planting suitable maize varieties will regulate the ratio of maize roots among the different soil layers and growth in deep soil layers.

3.4.3. Different Maize Varieties to Regulate the Root Morphological Structure of Maize

The RDW, RL, RV, and RSA of relay intercropping maize were significantly lower than that of sole cropping (Figure 9). Compared to sole cropping, the RDW, RL, RV, and RSA of relay intercropping RY1210 were significantly decreased by 25.43%, 10.75%, 30.79%, and 23.73%, respectively, and the RDW, RL, RV, and RSA of relay intercropping ZY3 were significantly decreased by 26.84%, 28.57%, 23.53%, and 20.46%, respectively. Compared to the relay intercropping ZY3, the RDW, RL, RV, and RSA of relay intercropping RY1210 were significantly increased by 143.98%, 278.29%, 54.40%, and 29.57%, respectively.

3.5. Crop Growth Characteristics, Maize Root Architecture and Crop Productivity Correlation Analysis

LAI, EDM, TDM, RL, RV, RSA, and RDW were significantly and positively correlated with maize yield (p < 0.001), respectively, and SDM was significantly and positively correlated with maize yield (p < 0.01, Figure 10). This indicates that maize is better in promoting the growth of RL, RV, RSA, and RDW to absorb more soil nutrients to promote LAI and total dry matter accumulation (stem, leaf, and ear) to increase the maize yield.

4. Discussion

4.1. Different Maize Varieties on the Yield of Relay Intercropping Maize

Under the MSSRI system, maize plays a major role as the dominant species in the system yield. Therefore, the key to achieving a high yield of maize–soybean strip intercropping is that the yield of intercropping maize is equivalent to that of monocropping maize, and the yield of intercropping soybean is increased by one season [3]. Previous studies have shown that the relay intercropping Chuandan 418 grain yield was significantly reduced by 11% compared to sole cropping [2], therefore, selecting the appropriate maize variety is key to achieving high group yields. Our research showed that the relay intercropping RY1210 yield showed better productivity and that there was no significant difference with sole cropping. The reasons for promoting the yield increase in relay intercropping RY1210 can be summarized as follows: (1) Plant stalks are thicker under appropriate density and appropriate N application measures, and thicker stalks have developed vascular bundles, thus increasing N translocate from leaves to ears [19], (2) Narrow-wide row maize planting has higher net photosynthesis rates because both rows of maize can intercept lateral and vertical light [20], and have increased light energy utilization by 19% compared to sole cropping [21], (3) Intercropping legumes can reduce N fertilizer application due to nodule N fixation [22], and the belowground root system can promote rhizosphere N cycling to increase the abundance of dominant bacteria and improve N utilization under the MSSRI system. In addition, maize soybean intercropping can also reduce the soil pH, increase acid phosphatase activity, and improve the efficiency of phosphorus utilization of intercropping maize [23], and (4) Maize roots have a stronger morphological plasticity compared to legumes crops, and could mediate maize root growth towards deeper soil layers (20–40 cm and 40–60 cm) to obtain more nutrients (Figure 7 and Figure 8), which gives maize a competitive advantage in the intercropping system [3], and thus compensates for group yield loss in the crops. Therefore, in MSSRI systems, plants usually regulate plant morphological structure [6], reduce growth rate [24], plant physiological metabolism [13], and reshape root architecture in response to yield changes.

In addition, compared to relay intercropping ZY3, relay intercropping RY1210 has: (1) A sufficient source supply. Larger LAI promotes the photosynthesis of maize, and photosynthetic products are temporarily stored in the leaves (source), and stored carbohydrates (glucose and starch) are transferred to the ear (sink) when maize enters the reproductive stage, and the larger source supply provides a guarantee for crop yield increase [25]; (2) Energy supply. A higher accumulation of dry matter above the ground indicates that the crop’s energy supply is more adequate, while a higher energy supply and rational distribution to various organs can ensure maize growth and development [26]; (3) The percentage of dry matter accumulation in the stalks and leaves was significantly higher than that of sole cropping (Figure 6), indicating that the plants were able to photosynthesize more efficiently, that the manufactured photosynthetic products were transported to various organs by vascular bundles in stalks, that nutrients were transferred to the ears through the proximity transport pathway, and that nutrients and energy were accumulated, which contributed to the increase in grain filling and crop yields. What is more noteworthy is that RY1210 TKW increased significantly by 24.58% compared to ZY3 TKW (Table 2), which is the vital reason for the yield increase in relay intercropping RY1210. So, how do RY1210 kernel width, kernel length, kernel perimeter, and kernel surface area respond to the mechanism of yield effect? This question should be further explored.

4.2. Different Maize Varieties on LAI and Dry Matter Accumulation of Relay Intercropping Maize

Higher LAI and dry matter accumulation were the base for yield formation [27]. In this study, different maize varieties showed different LAI and the maximum LAI of relay intercropping RY1210 was 4.31 (Figure 5), which was significantly decreased by 7.31% compared to sole cropping. the maximum LAI of relay intercropping ZY3 was 3.79, which was significantly decreased by 9.55% compared to sole cropping, which was consistent with the results of the previous study [28]. As a compact maize variety, RY1210 maintains the characteristic of leaves close to the stem by reducing the angle of the leaves, thus allowing more sunlight to penetrate the soil surface and improving the light energy interception and dry matter accumulation of maize leaves in the lower canopy of the relay intercropping maize [29], so it can capture and utilize the light energy more efficiently than ZY3 (semi-compact maize variety), and it has a more advantageous advantage in the utilization of light energy [30].

Prolonging the leaf area duration could enhance the source productivity, and the enhancement of leaf green-keeping ability could improve leaf photosynthetic capacity and biomass accumulation, especially during the late grain filling stage [31], which was consistent with previous research. In this study, the dry matter accumulation of RY1210 was higher than that of ZY3. At the same time, the dry matter ratio in the stem and leaves of relay intercropping RY1210 was significantly higher than that of sole cropping. Conversely, relay intercropping ZY3 was significantly lower than that of sole cropping (Figure 5). The LAI of RY1210 was significantly higher than that of sole cropping at the filling and maturing stages, and RY1210 maintained a higher LAI at the later stages of the reproductive stage compared to ZY3 (Figure 5). Photosynthetic assimilates translocation to the ear and accelerates leaf senescence during the kernel filling stage. In addition, the mutual shading of maize leaves weakens photosynthesis under the strip relay intercropping system, especially during the later stages of maize growth [16]. Hence, shortfalls in the carbohydrates and nutrients supply might be compensated by enhanced rates of leaf senescence and remobilization of carbohydrates and nutrients to the maize ears [32]. In this study, the strip relay intercropping planting pattern significantly delayed leaf senescence during the reproductive growth stage of maize. This delayed leaf senescence may result from the superior photosynthetic characteristics under the maize soybean strip relay intercropping system because the two rows of maize could receive direct and lateral light, simultaneously [33]. Meanwhile, the relay intercropping system increased the leaf nitrogen content and maintained its leaf green color and delayed the process of leaf senescence compared to sole planting [34]. Our results are consistent with the results of previous studies.

4.3. Different Maize Varieties on Root Morphology of Relay Intercropping Maize

The RDW of relay intercropping RY1210 was significantly higher than that of ZY3 (Figure 9). In addition, compared to the relay intercropping ZY3, the relay intercropping RY1210 could optimize the root spatial distribution and root conformation in the deep soil layer, mediate the vertical growth of the maize root system, and positively feedback the maize yield effect. The explanation may be (1) that in the MSSRI system, the crop distance is narrower than sole cropping, which results in an increased intraspecific competition among individual plants, and the relay intercropping maize would obtain water and soil nutrients from the deep soil layer for adaptive growth to ensure the plant’s normal growth and development. In addition, the relay intercropping maize root will reshape the root configuration and adaptive growth, and the distribution tendency of the root in the deep soil layer increases to improve the plant’s resistance to lodging characteristics [19]. (2) The relay intercropping RY1210 has a large LAI, which could photosynthesize through the leaves, intercepting the sunlight energy into chemical energy, and produce nutrients and energy to supply the root growth, while the aboveground-mediated carbon metabolism provides nutrients for the root growth and development [14,15]. On the other hand, the aboveground of the plant regulates the growth and development through the synthesis and secretion of phytohormones, such as auxin and gibberellin, which could promote root growth to a certain extent [35]. Appropriate maize varieties are conducive to the root system extension to the deep soil layer; therefore, aboveground–belowground coupling mutual coordination mediates the plant canopy structure, coordinates intraspecific competition and aboveground–belowground relationship [6], and can coordinate the reasonable transportation and distribution of the source-flow-sink relationships, optimize the maize root morphology, the proportion of root spatial distribution and the root dry weight, and provide adequate moisture and nutrient supply for the high maize yields, promoting root flourish and crop yield increase.

Previous studies have shown that in a MSSRI system, root plasticity can be improved by regulating interspecific distance to alter competition relationships, and that RDW, RL, RSA, and RV are lower in relay intercropping maize than sole cropping [6]. The gravity of relay intercropping maize plants also can respond to different narrow row spacing, which was used to regulate root morphological plasticity by regulating indole-3-acetic acid level and reducing N uptake capacity, resulting in lower RDW, RSA, and RV in relay intercropping maize than in sole maize [35]. Our results showed that relay intercropped maize was significantly lower in RL, RV, and RSA than sole maize, which was consistent with previous studies [6,35]. In contrast, under the maize soybean intercropping systems, the crops yield advantage obtained through greater leaf area level, RL, and root biomass density [36], and root length density, root dry weight density, and root surface area density were significantly higher in intercropping maize than sole crops [9,11]. This was because relay intercropping had a shorter co-growth period of about 50 days compared to the intercropping patterns, while the co-growth period of intercrops under the maize–soybean intercropping system was about 120 days. Therefore, the compatibility of crops roots under the intercropping system promotes maize lateral root growth and increased the root length density ratio, which established the basis for high yield in the system [5]. In our research, it is noteworthy that RY1210 was significantly higher than ZY3 in root configuration (RL, RV, RSA) and RDW, and that the higher yielding relay intercropping 1210 was more favorable for root distribution and elongation in deeper soil layers (Figure 7 and Figure 8), where superior root growth characteristics promotes aboveground absorption and utilization for soil nutrients, while the superior aboveground canopy structure also provides energy supply for the underground root growth, and the aboveground–underground coordination guarantees high crop yields (Figure 11).

5. Conclusions

The average maize yield of relay intercropping RY1210 was 8782.71 kg ha⁻¹, with no significant difference compared to sole cropping. Compared to ZY3, RY1210 could still maintain higher LAI in the later reproductive stage, along with a higher total dry matter accumulation, which increased the translocation of photosynthesis assimilation products to the ear, improved the TKW, and increased crop yield, and these superior plant characteristics significantly improved the RDW, RL, RV, RSA, and the spatial distribution of the deeper soil layers root. In the future, this study will be useful and helpful to farmers for selecting and planting high yielding maize varieties in strip relay intercropping.