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Article

Effects of Two Sowing Methods on the Growth, Yield, and Quality of Hybrid Rice under Mechanical Transplantation

by
Pinglei Gao
1,
Jiahao Xiao
1,
Shiwen Deng
1 and
Qigen Dai
1,2,*
1
Jiangsu Key Laboratory of Crop Cultivation and Physiology, Jiangsu Key Laboratory of Crop Genetics and Physiology, Jiangsu Co-Innovation Center for Modern Production Technology of Grain Crops, Key Laboratory of Saline-Alkali Soil Reclamation and Utilization in Coastal Areas, Research Institute of Rice Industrial Engineering Technology, Yangzhou University, Yangzhou 225009, China
2
Joint International Research Laboratory of Agriculture, Agri-Product Safety of the Ministry of Education of China, Institutes of Agricultural Science and Technology Development, Yangzhou University, Yangzhou 225009, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(12), 2961; https://doi.org/10.3390/agronomy13122961
Submission received: 20 October 2023 / Revised: 21 November 2023 / Accepted: 29 November 2023 / Published: 30 November 2023
(This article belongs to the Section Plant-Crop Biology and Biochemistry)
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Abstract

:
Poor adaptability of hybrid-rice (Oryza sativa L.) mechanical transplanting is one of the main factors limiting hybrid-rice production. Mixed sowing ensured stronger seedlings and better mechanical transplanting quality than conventional sowing in mechanical transplanting hybrid rice. Field experiments were conducted to identify the effects of mixed and conventional sowing of hybrid rice on rice growth, yield, and quality under mechanical transplantation in Yangzhou City, Jiangsu Province, China in 2021 and 2022. Two hybrid-rice varieties, japonica rice Changyou 4 and indica rice Yuanliangyou, and two conventional rice varieties, japonica rice Nanjing 5055 and indica rice Yangdao 6 were included in this study. Both japonica and indica rice showed the following results. There were no significant differences in biomass and leaf-area index of rice under different sowing methods at the heading and maturity stages. The basic seedling and spike rate was 38.27% and 16.24% higher, respectively, in mixed sowing than those averages in conventional sowing. In addition, the spikelets per panicle of hybrid rice in mixed sowing was 10.88% greater than the average in conventional sowing, indicating better heterosis. Compared to conventional sowing, mixed sowing increased the average gel consistency and taste value by 15.86% and 28.21%, respectively, while chalkiness degree, amylose content, and protein content decreased by an average of 60.47%, 44.89%, and 36.63%, respectively. Our study showed that similar biomass and leaf-area index, large basal seedling and spike rate, and large spikelets per panicle of hybrid rice are the keys to ensuring high yield in mixed sowing. At the same time, mixed sowing improved the appearance, nutrition, and cooking/eating qualities of the rice.

1. Introduction

Rice is one of the most important grains in the world; with the continuous growth of the world’s population and the demand for food consumption, researchers have been committed to improving the potential yield of rice [1]. To achieve this goal, efforts should be made to cultivate rice varieties with high yield potentials [2]. The development of hybrid cultivars is a major approach for exploring the potential of rice yield since hybrid cultivars have a yield advantage of 10–20% over improved inbred cultivars [3]. In addition, improving crop management is equally important to maximize the yield potential of rice varieties [4].
China is a forerunner in the application of hybrid-rice technology [5]. The traditional rice-planting method in China is hand transplanting [6]. Because the hand transplanting steps are cumbersome, a large amount of labor input is required [7]. With the development of China’s economy, a large amount of the rural labor force has been transferred to cities, the cost of the rural labor force has increased, and agricultural production has gradually developed towards mechanization [8,9]. Mechanical transplanting can effectively replace hand transplanting and reduce labor input [10]. At present, the mechanical transplanting of rice has gradually become a key step in the mechanized production of rice in China [11].
An appropriate population structure is a prerequisite for achieving heterosis in hybrid rice [12,13,14]. The traditional mechanical transplanting density for rice is relatively high, which is not conducive to the expression of heterosis in hybrid rice [15,16]. Reducing the density of hybrid rice appropriately is beneficial for it to exert its heterosis [17,18]. The effective way to reduce the density of hybrid rice is to decrease the sowing quantity, but this may lead to an increase in the missing-hill rate and thus a reduction in grain yield [19]. In a previous study, we employed a mixed-sowing technology to reduce the sowing quantity of hybrid rice. Our preliminary study has shown that, compared to conventional sowing methods, mixed-sowing technology in rice mechanical transplanting not only reduces the sowing quantity of hybrid rice but also does not cause yield decline [20]. This may be due to the significantly higher panicle number of mixed-sowing technology, which compensates for the lack of spikelets per panicle. However, it is unclear whether other factors impact the yield of mixed-sowing technology.
Rice yield is determined by four components: panicle number, spikelets per panicle, filled-kernel percentage, and 1000-grain weight, and the growth characteristics of rice also significantly impact rice yield [21]. However, the relationship between rice growth and yield is complex [22]. For instance, increasing tillering may yield more panicles, but it could hinder rice growth and lead to a decrease in spikelets per panicle [23]. Increasing rice leaves can utilize more light energy, but it will not necessarily increase the photosynthetic rate of rice and may increase nutrient consumption [24]. In this regard, the rice population plays a critical role in coordinating the relationships among the yield components in rice.
In recent years, the increasing demand for higher-quality rice due to improved living standards has made rice quality a significant factor in the consumer market, with high-quality rice having a stronger market competitiveness [25,26]. Mixed sowing of rice has the potential to enhance the stress resistance, yield, and quality of rice [27,28,29]. However, current research mainly focuses on mixed sowing between conventional rice varieties, with little attention paid to the effects of mixed sowing between hybrid and conventional rice varieties on rice quality. Therefore, investigating the effect of hybrid-rice mixed sowing on rice quality is crucial for improving the market competitiveness of hybrid-rice mixed-sowing products.
To achieve this, we selected two subtypes of japonica and indica rice to investigate the effects of different sowing methods on rice growth, grain yield, and rice quality. This study aims to accomplish two main objectives: (1) to determine the factors responsible for the stable grain yield in mixed-sowing technology and (2) to examine the impact of mixed-sowing technology on rice quality.

2. Materials and Methods

2.1. Experimental Site

This study was conducted at two locations from 2021 to 2022. The previous crop in these two places was wheat (Triticum aestivum L.), which belongs to a subtropical temperate climate. The experimental field for 2021 was located on the Experimental Base of the Agriculture College of Yangzhou University (32°27′ N, 119°34′ E), Shatou township, Yangzhou city, Jiangsu Province. The soil type is classified as mucky soil, and its initial chemical properties were 2.101% organic matter, 0.145% total nitrogen, 0.0157% alkaline hydrolysis nitrogen, 0.0019% available phosphorus, and 13.865% rapidly available potassium on a dry-weight basis, with a pH of 7.79. The experimental field for 2022 was located at the Yangzhou University (32°39′ N, 119°42′ E), Yangzhou city, Jiangsu Province. The soil type is also classified as mucky soil, and its initial chemical properties were 2.405% organic matter, 0.132% total nitrogen, 0.0104% alkaline hydrolysis nitrogen, 0.0032% available phosphorus, and 8.15% rapidly available potassium on a dry-weight basis, with a pH of 7.69.

2.2. Experimental Design and Management

In this study, the hybrid-rice variety Changyou 4 and the conventional rice variety Nanjing 5055 were selected for the japonica rice treatment, and the hybrid-rice variety Yuanliangyou and the conventional rice variety Yangdao 6 were selected for the indica rice treatment. These rice cultivars were chosen because they are the main rice varieties in Jiangsu Province, China, and two varieties (hybrid rice and conventional rice) within the same subspecies have a similar whole growth period and plant height (Table 1).
The seedling cultivation was carried out on 18 May 2021 and 20 May 2022. In the mixed-sowing treatments, the total seeding rate was 120 g per tray (36 kg per ha), including 30 g per tray of hybrid rice and 90 g per tray of conventional rice (Table 2). The sowing rate of hybrid rice was consistent with the current hand-transplanting sowing rate in hybrid-rice production, while that of conventional rice was consistent with the current conventional rice machine-transplanting sowing rate. Consistent with the current sowing rate of mechanical transplanting in hybrid-rice production, the sowing rates of Changyou 4 and Yuanliangyou in conventional sowing treatments were both 75 g/tray (22.5 kg/ha).
The seedling cultivation was carried out by using a carpet seedling method with a 30 cm × 60 cm tray containing a commercial seedling-cultivation substrate. Each treatment was repeated with 10 trays in a randomized complete block design. An Iseki PZ60 speed transplanter ((PG63DVRF + Long mat (PG6, 63)) SET, Iseki Co., Ltd., Matsuyama, Ehime prefecture, Iyo, Japan) was selected for rice-seedling transplanting on 13 June 2021 and 2022. Each plot in the field experiment measured 28.8 m2 (3.6 m × 8 m), with three replications. All rice seedlings were transplanted by a hill spacing of 16 cm × 30 cm, with three seedlings per hill. The amounts of fertilizer in each experimental plot were 270 kg N·ha−1, 180 kg P2O5·ha−1, and 135 kg K2O·ha−1. One day before transplantation, the total of P2O5 and K2O and 81 kg N·ha−1 were applied as basal fertilizers. One week after transplanting, and at the panicle initiation and the penultimate leaf-appearance stage, 81 kg N·ha−1, 54 kg N·ha−1, and 54 kg N·ha−1 were applied, respectively. The pest-control measures and irrigation methods adopted in the field experiment followed the conventional practice in local rice production.

2.3. Data Collection

Three days after transplanting, three quadrats of 1 m2 were randomly selected in each plot to investigate the basic seedlings. Twenty hills were marked in each plot to investigate tillering at 7-day intervals between 7 days to 70 days and 130 days after transplantation. To determine the shoot biomass leaf-area index (LAI), plants from three quadrats of 1 m2 in each plot were randomly collected at the jointing, heading, and maturity stages. The leaf-area index (LAI) in each treatment was measured by using a Leaf Area Meter (LI-3100C, Lincoln, NE, USA). After oven-drying at 75 °C for 80 h, the shoot biomass of each treatment was recorded. At the maturity stage, the panicle number and grain yield were determined by five 1 m2 quadrats positioned in accordance with the five-point sampling method. Thirty hills of panicles in each plot were taken to determine the spikelets per panicle. Panicles were manually threshed, and filled and unfilled spikelets were separated and recorded by immersing them in clean water. The grain yield was calculated based on the standard moisture content of 14% for japonica rice and 12.5% for indica rice.
The spike rate, harvest index, and biomass accumulation were calculated as follows [30,31]:
  • Spike rate = Panicle number/peak seedling tillers
  • Harvest index = Grain yield/Biomass at maturity
  • Biomass accumulation (Jointing–Heading) = Biomass at heading − Biomass at jointing
  • Biomass accumulation (Heading–Maturity) = Biomass at maturity − Biomass at heading
Air-dried grains with a moisture content of 14% for japonica rice and a moisture content of 12.5% for indica rice were stored at 4 °C for 90 days. Rice-quality characteristics were assessed by the processing, appearance, nutrition, and cooking/eating qualities according to the national standard GB/T 17891-2017 [32]. A huller (SY88-TH, Shuanglong, Korea) was used to hull grain samples (150 g) twice to obtain brown rice. The milled rice was obtained by using a rice polisher (LTJM-2099, Zhejiang Bethlehem Apparatus Co., Ltd., Taizhou, Zhejiang, China) to polish the brown rice for 25 s. The brown-rice, milled-rice, and head-rice rates were assessed by their weight as a percentage of total grain weight. A grain-appearance analyzer ScanMaker i800plus (Microtek, Shanghai, China) was used to measure the grain length, width, length–width, chalkiness rate, and chalkiness degree. A Cyclotec 1093 sample mill (Foss Tecator) was used to grind grain samples into flour, which was sieved through a 100-mesh count screen to analyze gel consistency and amylose and protein content, according to Hu’s methods [33]. The taste value of the rice grains was determined using the Cooked Rice Taste Analyzer STA1A (Satake Co., Ltd., Hiroshima, Japan) according to Zhang’s methods [34].

2.4. Data Analysis

A two-way analysis of variance (ANOVA) was conducted to assess the effects of the year and sowing method, as well as their interactions on the grain yield and its components, the seedling quality, harvest index, shoot biomass and processing, appearance, and nutritive and cooking/eating qualities. The LSD and t tests were used to assess the significance of means at a 5% level of probability. As initial pH, organic matter, total nitrogen, alkaline hydrolysis nitrogen, available phosphorus, and rapidly available potassium in the soil did not differ significantly between the two study sites, and they were close to each other, no comparison of regional factors has been made. All the above analyses were performed using SPSS v.20 (SPSS, Chicago, IL, USA) and figures were generated using Origin v. 2021 (Analytical software Northampton, Hampton, MA, USA).

3. Results

3.1. Grain Yield and Yield Components

In the treatment of japonica rice, there was no significant difference in grain yield between the mixed-sowing treatment and conventional sowing treatment in terms of year, sowing method, and their interaction (p = 0.953, p = 0.885, and p = 0.921, respectively). However, the analysis of yield components showed that the two-year average values of panicle number and 1000-grain weight of the mixed-sowing treatment were 347.06 and 26.36 g, respectively, which increased by 6.92% and 9.20% compared to the conventional sowing treatment (p < 0.01 and p < 0.01, respectively). In contrast, the two-year average values of spikelets per panicle and filled-kernel percentage of the mixed-sowing treatment were 130.97 and 82.71%, respectively, which decreased by 18.85% and 3.07% compared to the conventional sowing treatment (p < 0.01 and p < 0.01, respectively). The analysis of yield components showed that the two-year average values of panicle number of the mixed-sowing treatment were 273.81, which increased by 24.18% compared to the conventional sowing treatment (p < 0.01). In contrast, the two-year average values of spikelets per panicle, filled-kernel percentage, and filled-kernel percentage of the mixed-sowing treatment were 161.63, 85.78%, and 25.80 g, respectively, which decreased by 17.28%, 2.92%, and 3.80% compared to the conventional sowing treatment (p < 0.01, p < 0.01, and p < 0.01, respectively) (refer to Table 3 for detailed data).
Figure 1 shows that the spikelets per panicle of hybrid rice was higher with the mixed-sowing treatment compared to the conventional sowing treatment in both japonica and indica rice. For instance, in the 2021 japonica rice treatment, the number of grains per panicle of hybrid rice ranged from 160 to 180 grains under mixed-sowing treatment, which was higher than the 140 to 160 grains under conventional sowing treatment. Similarly, in the same treatment, the number of grains per panicle of hybrid rice under mixed-sowing treatment was also higher than that under conventional sowing treatment.

3.2. Basic Seedlings, Tillering Dynamics, Biomass of Shoot and LAI

There was no significant difference in basic seedlings between different sowing methods within the same subspecies over the two years (p > 0.05) (Figure 2). However, the basic seedlings of the mixed-sowing treatment increased significantly compared to the conventional sowing method within the same subspecies (p < 0.01). The average basic seedlings in the mixed-sowing treatment over the two years were 81.78 plants cm−2 and 73.35 plants cm−2 in japonica and indica rice, respectively, which were significantly higher by 42.95% and 33.58% compared to the conventional sowing treatment (p < 0.01 and p < 0.01, respectively).
Figure 3 depicts the tillering dynamics of rice in different seeding methods. In the japonica rice treatment, the tiller number of the mixed-sowing treatment was higher than that of the conventional sowing treatment on the 7th day after transplanting, but there was no significant difference in the tiller number between the two methods at the peak tiller stage (the 35th day after transplanting) (p > 0.05). However, at maturity (the 130th day after transplanting), the tiller number of the mixed-sowing treatment was higher than that of the conventional sowing treatment (p < 0.01). Similarly, the tillering dynamics in indica rice treatments were similar, but their spike rate differed. In japonica rice treatment, the spike rate of the mixed-sowing treatment was 60.51% and 60.07%, respectively, in 2021 and 2022, which were 8.79% and 7.98% higher than that of the conventional sowing treatment. In indica rice treatment, the spike rate of the mixed-sowing treatment was 49.64% and 49.33%, respectively, in 2021 and 2022, which were 24.35% and 23.82% higher than that of the conventional sowing treatment (Figure 3).
The average biomass weight at jointing for the mixed-sowing treatment was 6.60 t/ha and 7.56 t/ha in the two years, respectively. This was 6.65% and 7.69% lower than that of the conventional sowing method (p < 0.01 and p < 0.01, respectively). No differences were detected in biomass weight at heading and maturity among the years and sowing methods, and their interaction (p > 0.05). There were also no significant differences in harvest index in the same subspecies among the years and sowing methods, and their interaction (p > 0.05). However, compared with the conventional sowing method, the biomass accumulations of the japonica and indica rice treatments in the mixed-sowing treatment increased significantly from jointing to heading (p < 0.01 and p < 0.01, respectively) (Table 4).
There was a significant difference in the LAI of different seeding methods at the jointing stage (p < 0.05). In the japonica rice treatment, the average LAI of the mixed-sowing treatment in the two years was 3.31, which was 7.11% lower than that of the conventional sowing method (p < 0.05). In the indica rice treatment, the average LAI of the mixed-sowing treatment in the two years was 3.78, which was 7.81% lower than that of the conventional sowing method (p < 0.05). However, there was no significant difference in LAI at heading and maturity among the different sowing methods (p > 0.05) (Figure 4).

3.3. Processing, Appearance, and Nutritive and Cooking/Eating Qualities

In the japonica rice treatment, the brown-rice rate, milled-rice rate, and head milled-rice rate of rice grains in the mixed-sowing treatment were significantly higher than those in the conventional sowing treatment (p < 0.01). For instance, the average brown-rice rate of rice grains in the mixed-sowing treatment was 88.51% in two years, which significantly increased by 3.45% compared to the conventional sowing method. There were no significant differences in the brown-rice rate, milled-rice rate, and head milled-rice rate of indica rice treatments among years and sowing methods, and their interaction (p > 0.05) (Table 5).
There were no significant differences in grain length, grain width, and length–width ratio of rice grains in the same subspecies among year, sowing method, and their interaction (p > 0.05). The average chalkiness rate of rice grains in the mixed-sowing treatment in the two years was 22.79% and 6.40% in japonica rice and indica rice, respectively, which significantly increased by 51.60% and 54.48% compared with the conventional sowing treatment (p < 0.01, p < 0.01, respectively). Additionally, the average chalkiness degree of rice grains in the mixed-sowing treatment in the two years was 7.97% and 1.13% in japonica rice and indica rice, respectively, which significantly increased by 37.49% and 66.27% compared with the conventional sowing treatment (p < 0.01, p < 0.01, respectively) (Table 6).
There were significant differences in the nutritive quality and cooking/eating quality of the rice among the different sowing methods. In the japonica rice treatments, the two-year average values of amylose content and protein content of the mixed-sowing treatment were 7.10% and 5.78%, respectively, which decreased by 59.35% and 39.19% compared with the conventional sowing treatment (p < 0.01 and p < 0.01, respectively). The two-year average values of gel consistency and taste value of the mixed-sowing treatment were 72.90% and 68.85%, respectively, which increased by 16.57% and 27.15% compared with the conventional sowing treatment (p < 0.01 and p < 0.01, respectively). Similarly, in the indica rice treatments, the two-year average values of amylose content and protein content of the mixed-sowing treatment were 16.64% and 6.90%, respectively, which decreased by 43.75% and 34.07% compared with the conventional sowing treatment (p < 0.01 and p < 0.01, respectively). The two-year average values of gel consistency and taste value of the mixed-sowing treatment were 55.36% and 53.65%, respectively, which increased by 15.14% and 22.65% compared with the conventional sowing treatment (p < 0.01 and p < 0.01, respectively) (Table 7).

4. Discussion

Hybrid rice has greatly contributed to world food production, but there are still issues with its mechanized production [35,36,37]. A potential solution to these problems is to reduce the amount of hybrid-rice seeds sowed [38,39]. To achieve this, we explored the use of mixed-sowing technology to enhance the adaptability of hybrid-rice mechanized production. Our preliminary experiments have shown that mixed-sowing technology can decrease the sowing quantity of hybrid rice while maintaining stable yields. This may be attributed to the fact that the panicle number of mixed-sowing methods is significantly higher than that of conventional sowing methods, compensating for the lower spikelets per panicle rate. However, it is not clear whether there are other factors affecting the grain yield of hybrid rice.
The impact of mixed sowing on rice yield was influenced by which varieties were mixed and their mixed-sowing ratio. The study by Teng et al. demonstrated that the yield of mixed sowing of two hybrid-rice varieties, Shenliangyou 5814 and Zhongzheyou 8, was only higher than that of single sowing of Shenliangyou 5814 under equal proportion mixing of these two varieties [40]. Our previous study indicated that, among the three japonica hybrid-rice varieties (Changyoujing 6, Jiayouzhongke 1, and Changyou 4), only when Changyou 4 was mixed with conventional japonica rice Nanjing 5055, and the sowing amount of Nanjing 5055 was not less than 90 g per tray, could the grain yield maintain an equivalent to that of hybrid-rice sowing alone [20]. Similarly, among the three indica hybrid-rice varieties (Dexiang 4013, Yuanliangyou, and Yixiangyou 2115), only when Yuanliangyou was mixed with conventional indica rice Yangdao 6, and the sowing amount of Yangdao 6 was not less than 75 g per tray, might a grain yield equivalent to that of hybrid-rice sowing alone be achieved [20]. Consistent with our previous research findings, in the present study, mixed sowing ensured a rice yield comparable to single sowing of hybrid rice. Performing the heterosis of seedlings is a necessary condition for ensuring a high yield of hybrid rice and the heterosis of hybrid rice is usually reflected in the number of spikelets per panicle, which requires a suitable growth environment [41]. Our study showed that, in the japonica rice treatment, the panicle number and 1000-grain weight of the mixed-sowing treatment were significantly higher than those of the conventional sowing treatment, covering the shortage of filled-kernel percentage and spikelets per panicle. Similarly, in the indica rice treatment, the effective panicle number of the mixed-sowing treatment was significantly higher than that of the conventional sowing method, which covered the shortage of filled-kernel percentage, spikelets per panicle, and 1000-grain weight. This might be attributed to (1) a higher sowing quantity in the mixed-sowing treatment during seedling raising, which increases the basic seedlings, and (2) a significantly higher spike rate in the mixed-sowing treatment than in the conventional sowing method. In addition, by analyzing the distribution of spikelets per panicle, we found that the spikelets per panicle of hybrid rice in the mixed-sowing treatment were higher than those in the conventional sowing method (Figure 1). This indicated that mixed sowing promoted the heterosis of hybrid rice. Wang and his partner [42] found that mixed planting of rape and leguminous green manure can improve the light-use efficiency of the crop community and promote crop growth. Agnieszka et al. [43] reported that mixed cropping of wheat and triticale can make use of the stronger competitiveness of triticale in the canopy at the reproductive growth stage to obtain a higher number of spikelets per panicle. Yuan [5] believed that increasing the number of spikelets per panicle of hybrid rice was an important way to promote the development of heterosis. Hybrid rice has a higher competitive ability than conventional rice, and the growth environment of hybrid rice in the mixed-sowing treatment is better than that in the conventional sowing method, which may be the main reason for the better expression of heterosis in hybrid rice in the mixed-planting treatment.
The tiller, spike rate, biomass, and leaf-area index of rice are closely linked to rice-grain yield [22,44]. Previous studies have shown that hybrid rice requires a reasonable population structure to fully exploit its heterosis [45]. The appropriate population structure is beneficial for rice growth [46,47]. If the hybrid-rice density is too high and the growth space is too crowded, the rice will automatically adjust and actively reduce its tillers, leading to a reduction in the spike rate [48,49]. Wu and his partner [50] found that only in the appropriate population structure can hybrid rice achieve a maximum leaf-area index and biomass. In this study, the leaf-area index and biomass of the mixed-sowing treatment at jointing were lower than those of the conventional sowing treatment, but there was no significant difference in leaf-area index and biomass between mixed sowing and conventional sowing at heading and maturity (Figure 4 and Table 4). Moreover, the biomass accumulation of the mixed-sowing treatment from jointing to heading was significantly higher than that of the conventional sowing treatment (Table 4). At the early stage of rice growth (before jointing), space will not limit the growth of hybrid rice. While conventional rice exists in the mixed-sowing treatment, the growth ability of the rice population in the mixed-sowing treatment is weaker than that in the conventional sowing method. After jointing, the space resources gradually become tense, and the hybrid rice in the mixed-sowing treatment is evenly distributed, which is more conducive to the distribution of light and space resources in the field. This deduction, to some extent, can be supported by a study on the different mechanical transplanting methods of hybrid rice by Zhang and his partner [51], who reported that the suitable population structure of hybrid rice has a high canopy-light energy utilization rate in the middle and late growth stages, which promotes the development of heterosis and is conducive to the formation of large panicles. These findings suggest the need for a greater understanding of individual plant and population growth in hybrid-rice mixed-sowing technology.
Mixed-sowing of rice can improve the field’s growth environment and enhance the utilization rate of light energy, both of which promote better rice quality [52,53]. Peng and his partner [54] discovered that mixed sowing of rice significantly increases the milled-rice rate and head milled-rice rate while reducing the chalkiness degree. Similarly, Gao [55] observed that mixed planting can lower the chalkiness rate, amylose content, and protein content of the rice grain. Kong [56] also confirmed that mixed sowing can reduce the chalkiness rate of the rice grain. Our study found that the head milled-rice rate of the mixed-sowing treatment was significantly higher, and the chalkiness rate and degree were significantly lower than those of conventional sowing (Table 5 and Table 6). Chalkiness occurs due to insufficient grain filling, resulting in brittle and easily breakable seeds [57]. Mixed sowing improves field-growth conditions and light-energy utilization, making the seeds plumper [58]. Additionally, the genetic characteristics of rice may contribute to this finding, as conventional rice typically has superior processing and appearance quality than hybrid rice, and the overall processing and appearance quality of rice improves after mixing. Numerous studies show that the amylose content in rice has a negative correlation with gel consistency and taste value [59,60,61]. In our study, the gel consistency and taste value of the mixed-sowing treatment significantly increased compared to conventional sowing, due to the lower amylose and protein contents of the mixed-sowing treatment (Table 7). Our study also found that the gel consistency and taste value of the mixed-sowing treatment increased significantly compared to conventional sowing, possibly due to the lower amylose and protein content of the mixed-sowing treatment (Table 7). This could be attributed to the fact that the content of amylose and protein in the grains of conventional rice is lower than that of hybrid rice, leading to a decrease in overall amylose and protein content after mixing. Mixed planting between crops has also been shown to reduce the amylose content of the grains [62,63]. Therefore, the interaction effect of the mixed-sowing treatment could be a reason for lowering the amylose content in rice grains.

5. Conclusions

Compared to the current conventional sowing method in the mechanical transplantation of hybrid rice, mixed-sowing technology reduced the amount of hybrid-rice seeds needed by 60% while maintaining the same grain yield. Key factors for achieving high yields in mixed sowing included a similar biomass and leaf-area index to those in conventional sowing, larger basal seedlings and spike rates, and larger numbers of spikelets per panicle of hybrid rice than those in conventional sowing. Additionally, mixed sowing decreased the chalkiness degree and amylose and protein content of the grain, while increasing the gel consistency and taste value of the grain and, therefore, enhanced the quality of the rice in terms of appearance, nutrition, and taste.

Author Contributions

Data curation, J.X. and S.D.; formal analysis, J.X. and S.D.; funding acquisition, P.G. and Q.D.; methodology, Q.D.; writing—original draft, P.G.; writing—review and editing, P.G. and Q.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Key Research and Development Project(2021YFD1700803), Key Research and Development Program of Jiangsu Province (D21YFD17008), Jiangsu Natural Science Foundation (BK20220565), the Key Research and Development Program of Jiangsu Province (BE2019343), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Distribution of spikelets per panicle among different subspecies with different sowing methods in 2021 and 2022. (a) Distribution of spikelets per panicle of japonica rice treatments in 2021; (b) Distribution of spikelets per panicle of japonica rice treatments in 2022; (c) Distribution of spikelets per panicle of indica rice treatments in 2021; (d) Distribution of spikelets per panicle of indica rice treatments in 2022. Q1 indicates the first quartile, which is equal to the 25% spikelets per panicle arranged from small to large in this treatment. Q3 indicates the third quartile, which is equal to the 75% spikelets per panicle arranged from small to large in this treatment. The data (n = 30) is the average value of thirty independent experiments.
Figure 1. Distribution of spikelets per panicle among different subspecies with different sowing methods in 2021 and 2022. (a) Distribution of spikelets per panicle of japonica rice treatments in 2021; (b) Distribution of spikelets per panicle of japonica rice treatments in 2022; (c) Distribution of spikelets per panicle of indica rice treatments in 2021; (d) Distribution of spikelets per panicle of indica rice treatments in 2022. Q1 indicates the first quartile, which is equal to the 25% spikelets per panicle arranged from small to large in this treatment. Q3 indicates the third quartile, which is equal to the 75% spikelets per panicle arranged from small to large in this treatment. The data (n = 30) is the average value of thirty independent experiments.
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Figure 2. Basic seedlings of japonica treatments (a) and indica rice treatments (b) with different sowing methods in 2021 and 2022. Data (n = 3) are presented as mean values ± SDs. Bars with the same lowercase letters indicate no significance according to the t test at a 5% level of probability.
Figure 2. Basic seedlings of japonica treatments (a) and indica rice treatments (b) with different sowing methods in 2021 and 2022. Data (n = 3) are presented as mean values ± SDs. Bars with the same lowercase letters indicate no significance according to the t test at a 5% level of probability.
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Agronomy 13 02961 g003aAgronomy 13 02961 g003b
Figure 3. Tillering dynamics of different sowing methods among different subspecies in 2021 and 2022. (a) Tillering dynamics of japonica rice treatments in 2021; (b) tillering dynamics of japonica rice treatments in 2022; (c) tillering dynamics of indica rice treatments in 2021; and (d) tillering dynamics of indica rice treatments in 2022. Data (n = 3) are presented as mean values ± SDs.
Figure 3. Tillering dynamics of different sowing methods among different subspecies in 2021 and 2022. (a) Tillering dynamics of japonica rice treatments in 2021; (b) tillering dynamics of japonica rice treatments in 2022; (c) tillering dynamics of indica rice treatments in 2021; and (d) tillering dynamics of indica rice treatments in 2022. Data (n = 3) are presented as mean values ± SDs.
Agronomy 13 02961 g003aAgronomy 13 02961 g003b
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Figure 4. Leaf-area index (LAI) of different sowing methods among different subspecies in 2021 and 2022. (a) LAI of japonica rice treatments in 2021; (b) LAI of japonica rice treatments in 2022; (c) LAI of indica rice treatments in 2021; and (d) LAI of indica rice treatments in 2022. Data (n = 3) are presented as mean values ± SDs. Bars with the same lowercase letters indicate no significance according to the t test at a 5% level of probability.
Figure 4. Leaf-area index (LAI) of different sowing methods among different subspecies in 2021 and 2022. (a) LAI of japonica rice treatments in 2021; (b) LAI of japonica rice treatments in 2022; (c) LAI of indica rice treatments in 2021; and (d) LAI of indica rice treatments in 2022. Data (n = 3) are presented as mean values ± SDs. Bars with the same lowercase letters indicate no significance according to the t test at a 5% level of probability.
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Table 1. Whole growth period, plant height, and seed-germination percentage of the experimental rice varieties.
Table 1. Whole growth period, plant height, and seed-germination percentage of the experimental rice varieties.
Subspecies VarietiesWhole Growth Period (d)Plant Height (cm)Germination
Percentage (%)
Japonica riceChangyou 4163109.591.62
Nanjing 5055160106.590.05
Indica riceYuanliangyou138111.691.24
Yangdao 6145115.092.83
Table 2. The sowing rate of each variety in each treatment during seedling cultivation.
Table 2. The sowing rate of each variety in each treatment during seedling cultivation.
SubspeciesVarietiesSowing MethodHybrid Rice (g tray−1)Conventional Rice (g tray−1)Total Sowing Amount of Rice
(g tray−1)		Hybrid Rice (kg ha−1)Conventional Rice (kg ha−1)
Japonica riceChangyou 4 mixed with Nanjing 5055MS3090120927
CS7507522.50
Indica riceYuanliangyou mixed with Yangdao 6MS3090120927
CS7507522.50
MS and CS represent mixed sowing and conventional sowing, respectively.
Table 3. Grain yield and yield components of different sowing methods.
Table 3. Grain yield and yield components of different sowing methods.
SubspeciesYearSowing MethodGrain Yield
(t ha−1)		Panicle
Number m−2		Spikelet
per Panicle		Filled-Kernel
Percentage (%)		1000-Grain
Weight (g)
Japonica rice2021MS9.98 ± 0.32 a346.80 ± 6.79 a130.00 ± 3.61 b82.70 ± 0.21 b26.47 ± 0.34 a
CS10.10 ± 0.62 a323.17 ± 5.48 b161.67 ± 4.06 a85.37 ± 0.35 a24.17 ± 0.09 b
2022MS10.06 ± 0.37 a347.31 ± 3.11 a131.93 ± 4.15 b82.71 ± 0.21 b26.24 ± 0.10 a
CS10.08 ± 0.58 a326.02 ± 5.52 b161.11 ± 8.99 a85.28 ± 0.34 a24.11 ± 0.08 b
Two-way ANOVA
Yearnsnsnsnsns
Sowing methodns********
Year × Sowing methodnsnsnsnsns
Indica rice2021MS9.90 ± 0.17 a274.00 ± 6.99 a161.00 ± 5.20 b85.95 ± 0.51 b25.82 ± 0.08 b
CS9.72 ± 0.09 a221.07 ± 4.79 b195.00 ± 8.74 a88.48 ± 0.16 a26.82 ± 0.13 a
2022MS9.88 ± 0.15 a273.61 ± 5.04 a162.26 ± 6.85 b85.61 ± 0.37 b25.78 ± 0.08 b
CS9.79 ± 0.07 a219.91 ± 4.63 b195.79 ± 12.08 a88.23 ± 0.74 a26.81 ± 0.08 a
Two-way ANOVA
Yearnsnsnsnsns
Sowing methodns********
Year × Sowing methodnsnsnsnsns
MS and CS represent mixed sowing and conventional sowing, respectively. Data (n = 3) are presented as mean values ± SDs. The values with the same lowercase letters within the same subspecies and column indicate no significant differences at a 5% level of probability. ns and ** indicate no significance and significance, respectively, according to the LSD test at a 5% level of probability.
Table 4. Shoot biomass, harvest index, and biomass accumulation of the sowing methods.
Table 4. Shoot biomass, harvest index, and biomass accumulation of the sowing methods.
SubspeciesYearSowing
Method		Biomass (t ha−1)Harvest
Index		Biomass Accumulation (t ha−1)
JointingHeadingMaturityJointing–
Heading		Heading–
Maturity
Japonica rice2021MS6.59 ± 0.21 b12.26 ± 0.27 a20.92 ± 0.15 a0.455 ± 0.003 a5.67 ± 0.09 a8.87 ± 0.49 a
CS7.05 ± 0.09 a12.14 ± 0.14 a21.05 ± 0.44 a0.441 ± 0.009 a5.08 ± 0.09 b8.72 ± 0.26 a
2022MS6.61 ± 0.11 b12.14 ± 0.23 a21.12 ± 0.14 a0.456 ± 0.003 a5.53 ± 0.15 a8.99 ± 0.22 a
CS7.09 ± 0.10 a11.94 ± 0.17 a20.77 ± 0.24 a0.445 ± 0.005 a4.84 ± 0.09 b8.64 ± 0.34 a
Two-way ANOVA
Yearnsnsnsnsnsns
Sowing method**nsnsns**ns
Year × Sowing methodnsnsnsnsnsns
Indica rice2021MS7.58 ± 0.11 b13.25 ± 0.30 a22.35 ± 0.50 a0.447 ± 0.010 a5.67 ± 0.22 a9.10 ± 0.40 a
CS8.23 ± 0.10 a13.25 ± 0.08 a22.25 ± 0.36 a0.455 ± 0.007 a5.02 ± 0.06 b8.90 ± 0.51 a
2022MS7.54 ± 0.14 b13.45 ± 0.08 a22.35 ± 0.27 a0.448 ± 0.005 a5.91 ± 0.09 a8.90 ± 0.40 a
CS8.15 ± 0.07 a13.33 ± 0.08 a22.45 ± 0.31 a0.457 ± 0.006 a5.18 ± 0.08 b9.00 ± 0.45 a
Two-way ANOVA
Yearnsnsnsnsnsns
Sowing method**nsnsns**ns
Year × Sowing methodnsnsnsnsnsns
MS and CS represent mixed sowing and conventional sowing, respectively. Data (n = 3) are presented as mean values ± SDs. The values with the same lowercase letters within the same subspecies and column indicate no significant differences at a 5% level of probability. ns and ** indicate no significance and significance, respectively, according to the LSD test at a 5% level of probability.
Table 5. Processing quality of rice grains with different sowing methods.
Table 5. Processing quality of rice grains with different sowing methods.
YearSubspeciesSowing
Method		Brown-Rice Rate (%)Milled-Rice Rate (%)Head Milled-Rice Rate (%)
Japonica rice2021MS88.55 ± 0.13 a73.36 ± 0.46 a64.61 ± 1.86 a
CS85.62 ± 0.35 b65.30 ± 0.46 b48.60 ± 1.94 b
2022MS88.47 ± 0.16 a73.38 ± 0.25 a63.94 ± 1.67 a
CS85.49 ± 0.12 b64.97 ± 0.28 b49.27 ± 0.95 b
Two-way ANOVA
Yearnsnsns
Sowing method******
Year × Sowing methodnsnsns
Indica rice2021MS83.04 ± 0.21 a71.09 ± 0.14 a51.11 ± 0.95 a
CS83.35 ± 0.06 a71.53 ± 0.13 a52.30 ± 0.57 a
2022MS83.11 ± 0.22 a71.25 ± 0.61 a51.14 ± 0.89 a
CS83.47 ± 0.07 a71.50 ± 0.11 a52.60 ± 0.87 a
Two-way ANOVA
Yearnsnsns
Sowing methodnsnsns
Year × Sowing methodnsnsns
MS and CS represent mixed sowing and conventional sowing, respectively. Data (n = 3) are presented as mean values ± SDs. The values with the same lowercase letters within the same subspecies and column indicate no significant differences at 5% level of probability. ns and ** indicate no significance and significance, respectively, according to the LSD test at a 5% level of probability.
Table 6. Appearance quality of rice grains with different sowing methods.
Table 6. Appearance quality of rice grains with different sowing methods.
YearSubspeciesSowing
Method		Grain Length (mm)Grain Width (mm)Length–WidthChalkiness Rate
(%)		Chalkiness Degree
(%)
Japonica rice2021MS4.47 ± 0.01 a2.65 ± 0.01 a1.71 ± 0.03 a23.18 ± 0.33 b6.44 ± 0.23 b
CS4.59 ± 0.10 a2.71 ± 0.02 a1.77 ± 0.03 a46.31 ± 0.33 a14.09 ± 0.30 a
2022MS4.48 ± 0.04 a2.66 ± 0.01 a1.72 ± 0.06 a22.40 ± 0.68 b6.35 ± 0.35 b
CS4.48 ± 0.04 a2.67 ± 0.02 a1.76 ± 0.01 a47.86 ± 1.75 a14.03 ± 0.51 a
Two-way ANOVA
Yearnsnsnsnsns
Sowing methodnsnsns****
Year × Sowing methodnsnsnsnsns
Indica rice2021MS6.34 ± 0.03 a2.11 ± 0.01 a3.09 ± 0.02 a8.08 ± 0.67 b1.19 ± 0.15 b
CS6.46 ± 0.10 a2.10 ± 0.03 a3.05 ± 0.05 a12.88 ± 0.97 a3.37 ± 0.07 a
2022MS6.42 ± 0.02 a2.09 ± 0.01 a3.1 ± 0.05 a7.85 ± 1.08 b1.06 ± 0.31 b
CS6.41 ± 0.05 a2.10 ± 0.01 a3.06 ± 0.03 a12.62 ± 0.65 a3.33 ± 0.21 a
Two-way ANOVA
Yearnsnsnsnsns
Sowing methodnsnsns****
Year × Sowing methodnsnsnsnsns
MS and CS represent mixed sowing and conventional sowing, respectively. Data (n = 3) are presented as mean values ± SDs. The values with the same lowercase letters within the same subspecies and column indicate no significant differences at 5% level of probability. ns and ** indicate no significance and significance, respectively, according to the LSD test at a 5% level of probability.
Table 7. Nutritive quality and cooking/eating quality of rice grains with different sowing methods.
Table 7. Nutritive quality and cooking/eating quality of rice grains with different sowing methods.
YearSubspeciesSowing
Method		Amylose Content (%)Gel Consistency (mm)Protein Content (%)Taste Value
Japonica rice2021MS7.09 ± 0.02 b72.89 ± 0.24 a5.74 ± 0.08 b68.7 ± 1.8 a
CS17.05 ± 0.62 a62.12 ± 0.50 b9.47 ± 0.08 a53.3 ± 0.9 b
2022MS7.11 ± 0.24 b72.91 ± 0.02 a5.82 ± 0.09 b69.0 ± 1.7 a
CS17.88 ± 0.50 a62.95 ± 0.62 b9.54 ± 0.10 a55.0 ± 1.7 b
Two-way ANOVA
Yearnsnsnsns
Sowing method********
Year × Sowing methodnsnsnsns
Indica rice2021MS16.58 ± 0.50 b55.42 ± 0.50 a6.83 ± 0.11 b54.3 ± 0.7 a
CS24.17 ± 0.30 a47.83 ± 0.30 b10.39 ± 0.12 a41.7 ± 0.9 b
2022MS16.70 ± 0.61 b55.30 ± 0.61 a6.97 ± 0.15 b53.0 ± 1.0 a
CS23.67 ± 0.30 a48.33 ± 0.30 b10.54 ± 0.17 a41.3 ± 0.7 b
Two-way ANOVA
Yearnsnsnsns
Sowing method********
Year × Sowing methodnsnsnsns
MS and CS represent mixed sowing and conventional sowing, respectively. Data are mean values (n = 3) ± standard deviation. Data (n = 3) are presented as mean values ± SDs. The values with the same lowercase letters within the same subspecies and column indicate no significant differences at 5% level of probability. ns and ** indicate no significance and significance, respectively, according to the LSD test at a 5% level of probability.
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MDPI and ACS Style

Gao, P.; Xiao, J.; Deng, S.; Dai, Q. Effects of Two Sowing Methods on the Growth, Yield, and Quality of Hybrid Rice under Mechanical Transplantation. Agronomy 2023, 13, 2961. https://doi.org/10.3390/agronomy13122961

AMA Style

Gao P, Xiao J, Deng S, Dai Q. Effects of Two Sowing Methods on the Growth, Yield, and Quality of Hybrid Rice under Mechanical Transplantation. Agronomy. 2023; 13(12):2961. https://doi.org/10.3390/agronomy13122961

Chicago/Turabian Style

Gao, Pinglei, Jiahao Xiao, Shiwen Deng, and Qigen Dai. 2023. "Effects of Two Sowing Methods on the Growth, Yield, and Quality of Hybrid Rice under Mechanical Transplantation" Agronomy 13, no. 12: 2961. https://doi.org/10.3390/agronomy13122961

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