Effects of Different Types of Pot-Mat Trays on the Growth of Densely Sown Seedlings and Root Morphology of Machine-Transplanted Rice

Abstract

Weak seedlings and poor growth uniformity affect the mechanical transplanting of densely sown rice seedlings. To address these issues, seedlings of the conventional japonica rice “Zhehexiang 2” were grown in a traditional flat tray (control), pot-mat tray (26 × 52 bowls; BT(26)), and pot-mat tray (30 × 58 bowls; BT(30)) to compare the effects of different specifications of pot-mat trays (BTs) on the growth and quality of mechanical transplanting of densely sown rice seedlings with 250 g/tray. The BT-raised seedlings showed improved seedling quality, with increases in the shoot and root dry weights by 7.44% and 20.11%, respectively, compared to the flat tray. Under the dense sowing rate, the plant height uniformity of the BT(26) and BT(30) treatments was significantly increased by 6.95% and 3.43%, and the root entwining force of the seedlings was 14.28% and 10.21% higher, respectively, compared with those of the control. The missing hill rate for BT-raised seedlings after mechanical transplanting was significantly reduced by 53.15%. The loss of roots during mechanical transplanting was reduced. Compared with the control, the root length, root surface area, and root number were increased, and a greater number of large roots were retained, which promoted the early development of seedlings after mechanical transplanting. The proportion of holes with two to five seedlings was higher after mechanical transplanting. The pot-mat tray divided the root growth area of seedlings, promoted the growth of the seedlings, and reduced the root loss and missing hill rate under the high sowing rate. Thus, the quality of mechanical transplanting of densely sown seedlings was improved.

1. Introduction

In China, rice is the primary staple food for more than 60% of the population. Consequently, rice production is of paramount importance for national food security [1]. With the acceleration of industrialization and urbanization in China, the mechanization of rice production has become crucial in promoting the transformation and upgrading of the rice industry. Some light and simplified technologies, such as mechanized rice transplanting and mechanized direct seeding, are now developing rapidly [2,3,4]. Seedling raising was the most important step in the rice machine-transplanting process. The integration of agricultural machinery and agronomical practices in rice production has established a comprehensive mechanized operation mode based on the mechanization of rice production, including rice seedling, soil preparation, transplanting, field management, harvesting, and so on [5,6]. Mechanized transplanting effectively reduced the input of the labor force, greatly reduced the labor intensity, and simplified the operation process [2]. However, there were still some problems in the current rice seedling raising and transplanting mode, such as high seedling raising costs and low operation efficiency [7].

Dense-seeding seedling transplanting technology was a new and efficient method developed in recent years, which greatly reduced the number of seedling trays and greatly improved the efficiency of mechanical transplanting [8,9]. In some countries with large rice planting areas, such as Japan and South Korea, a high seeding rate has been widely used in seedling raising. The seeding rate can even reach 250–300 g/tray [9,10]. Traditional machine transplanting of rice generally recommends using 120–150 g of seeds per tray and seedlings aged 20–25 days, requiring about 420 trays per hectare [11]. This conventional method demands more substrates and other materials while requiring larger greenhouse areas. In contrast, dense-seeding seedling transplanting technology elevates the per-tray seeding rate, and the number of trays per hectare decreases from 375–450 to 120–180 for transplanting; thus, the seedling substrate, greenhouse area, and other materials are reduced [9,12]. This represents one of the general methods of dense planting machine transplanting in actual agricultural production. Based on this method, a seeding rate of 250 g/tray and a seedling age of 15 d were adopted for transplantation in this study. Additionally, the seedlings were planted with a high seeding rate at the two-leaf stage, with no significant difference in growth. The seedlings were semi-autotrophic at the two-leaf stage. After transplanting, the seedlings were able to grow rapidly, and tiller formation was low, which was conducive to the development of large panicles [13,14,15]. However, increasing seeding rates may impact seedling quality. The seedlings were relatively small and weak, and the flexibility of seedling age was poor.

Mechanized transplanting is mainly divided into traditional mat seedling and pot seedling planting [2]. The pot seedling method includes an independent pot for each seedling, and the root system is complete and not entangled. The mat seedlings are raised through a flat tray, and the roots are entangled with each other [16,17]. Rice seedlings need to form blanket seedlings before they can be used for mechanical transplanting [17,18]. To meet the requirements for rice mechanical transplanting, pot-mat seedling technology was developed, which combined the advantages of bowl seedlings and blanket seedlings [19]. The pot-mat seedling machine-transplanting technology was convenient for a transplanter to accurately take the seedlings from a bowl by cultivating the corrugated blanket seedlings on the blanket and under the bowl [9]. The roots of the pot-mat seedlings were independent of each other, and the roots in the bottom bowl were 18.2% higher than those of the traditional mat seedlings. The root quantity, stem width, white root number, and shoot dry weight of pot-mat seedlings were all superior to that of general mat seedlings [20]. Pot-mat seedlings could reduce the missing-transplanting seedling rate and injury root rate of seedlings and promote the early emergence of tillers [19].

At present, rice production in China is facing the problem of the high production cost of machine transplanting [7]. The technology of dense sowing seedlings could provide a basis for the upgrading of China’s rice industry. However, the machine-transplanted seedling quality was far inferior to the traditional seeding rate of seedlings under a high seeding rate [5,11]. The cultivation of high-quality rice seedlings is a key research topic in machine-transplanted rice. High-density seedling transplanting was generally suitable for cutting small seedling blocks due to the high seeding rate [9]. The transplanters need to be adjusted to the minimum amount of seedlings to take seedlings, such as selecting the horizontal seedling extraction amount (i.e., the number of times the transplanter horizontally moves laterally to pick up seedlings during machine transplanting) to 26 or 30. However, some shortcomings hinder high-density seedling mechanical transplanting, such as considerable root damage and slow regreening [19]. Therefore, in this study, two specifications of pot-mat tray (BT) were used: 26 bowls and 30 bowls, corresponding to 26 and 30 times the lateral seedling pick-up times of the dense seedling transplanter, respectively. This study evaluated the application of the technology for mechanical transplanting of densely sown blanket rice seedlings, which aims to further improve the quality of seedlings and mechanical transplanting. However, owing to the high seeding rate required, the competition between seedlings is increased, the nutrients obtained by each seedling are reduced, seedling growth is inhibited, and the quality of the seedlings is weakened [8,11]. To provide a theoretical basis and technical reference for cost reduction and efficient cultivation of rice, the effects of different sizes of BTs on the growth of densely sown rice seedlings and the root morphological characteristics of the seedlings after mechanical transplanting were studied.

2. Materials and Methods

2.1. Experimental Materials

The experiment was performed in an artificial climate chamber at China Rice Research Institute (CNRRI) in Fuyang District, Hangzhou Zhejiang Province, China, in 2024. The conventional japonica rice cultivar ‘Zhehexiang 2’ was selected as the test material. The 1000-grain weight was 25.2 g. Control (CK) uses the traditional flat tray used by the mainstream 30 cm row spacing transplanter. The basic parameters of the pot-mat trays (BTs) were as follows: BT(26) comprised 26 bowls in the horizontal direction × 52 bowls in the vertical direction, and thus the total number of bowls per tray was 1352 bowls, and BT(30) comprised 30 bowls in the horizontal direction × 58 bowls in the vertical direction, and thus the total number of bowls per tray was 1740 bowls. The length and width of the two specifications of BT were consistent with CK. The length of the inner side of the tray was 58 cm, and the width was 28 cm; thus, the total area per tray was 1624 cm² [21]. The seedling tray and the seedling root system are shown in Figure 1. A schematic diagram of a seedling tray is shown in Figure 2.

2.2. Experimental Design

The experimental seeding rate (dry grains) was 250 g/tray. Three types of seedling trays, namely CK, BT(26), and BT(30), were used for seeding. A total of three treatments and three replicates for each treatment were included.

2.3. Experimental Methods

Before sowing, the seeds were disinfected with 80% ethylicin (fungicide:water 1:2000, v/v) for 24 h, then washed three times with clean water, soaked in water at 20 °C for 48 h, and wrapped in a wet towel to accelerate germination. At 30 °C, the seeds germinated after 1–2 days. The test substrate was provided by the China Rice Research Institute. The basic physical and chemical properties were as follows: pH value was 5.6, bulk density was 0.53 kg/dm³, total nitrogen content was 2.01 g/kg, available phosphorus content was 321.2 mg/kg, available potassium was 307.4 mg/kg, and organic matter content was 69.02 g/kg. When sowing, 2.6 L of soil was placed in each tray, and the seeds were evenly spread on the soil surface and covered with 1 L of soil. After sowing, the tray was folded for 2 days, and the seedlings were incubated in the artificial climate chamber after emergence. The temperature regime in the climate chamber was set as follows: 07:00–10:00, 28 °C; 10:00–15:00, 32 °C; 15:00–17:00, 29 °C; and 17:00–07:00, 23 °C.

2.4. Experimental Items

2.4.1. Seedling Quality

At 5, 10, and 15 d after emergence, 20 seedlings were randomly selected from each treatment to determine the plant height and leaf age. The plant height was determined as the highest point from the root to the top of the seedling, measured with a ruler. The seedlings were divided into aboveground and belowground parts. The seedlings were killed at 105 °C for 30 min and then dried to constant weight at 80 °C [22].

2.4.2. Uniformity of Seedling Population

Before mechanical transplanting, 30 seedlings were selected from each treatment. The plant height of the seedlings was measured with a ruler, and the plant height uniformity (U_PlH) was calculated using the following formula [23]:

$$U_{PlH}=1-\left(S_{PlH}/\overline{X_{PlH}}\right)\times 100;$$

$$S_{PlH}=\sqrt{1/(n-1){\displaystyle \sum }_{i=1}^n{\left(X{i}_{PlH}-\overline{X_{PlH}}\right)}^2}$$

where S_PlH is the standard deviation of plant height, X_iPlH is the measured plant height, and $\overline{X_{PlH}}$ is the average measured plant height.

2.4.3. Root Entwining Force

At 15 d after seedling emergence, a spring scale was used to measure the root entwining force. Each tray was cut into 15 cm × 15 cm seedling blocks, fixed at each end, and the seedling block was pulled from one end with the spring scale hook. When the seedling block broke, the maximum value displayed by the spring scale was termed the root entwining force [11]. Measurements for each treatment were conducted with three replicates.

2.4.4. Transplanting Quality

At 15 d after seedling emergence, a rice 6-row Yanmar dense seedling transplanter was used to perform mechanical transplanting. The transplanting data are as follows: the spacing between plants and rows is 30 × 14 cm, the horizontal seedling extraction amount was 26 and 30, and the longitudinal picking seedling quantity was 12 mm. After transplanting, the number of seedlings per planting hole and the number of missing seedlings per hole were recorded, and the missing hill rate and the proportion of seedlings per hole were calculated. The missing hill rate refers to the absence of seedlings in a planting hole after mechanical transplanting. The missing hill rate (%) = number of missing holes/total number of holes surveyed × 100. The plant height of seedlings was measured with a ruler, and the plant height ratio was calculated. The plant height ratio refers to the proportion of the number of seedlings under a certain plant height to the total number of seedlings measured. The proportion of plant height of 0–5 cm, 5–12 cm, 12–16 cm, and 16–20 cm was calculated. The plant height ratio (%) = number of seedlings per plant height/total number of seedlings measured × 100.

2.4.5. Root Morphology

Before mechanical transplanting, 20 seedlings were randomly selected for each treatment. Immediately after mechanical transplanting, 10 consecutive seedlings were randomly selected per treatment. The roots were washed to remove soil particles, and then the individual roots of each seedling were separated. The roots were scanned using a digital scanner (EPSON V700, manufactured by Seiko Epson Corporation, Suwa, Japan). The root length, root surface area, root number, root diameter, and root volume data were measured using the WinRHIZO Pro 2013 software [24]. Measurements for each treatment were conducted with three replicates.

2.5. Statistical Analysis

Microsoft Excel 2016 and Origin 2024 Learning Edition were used for data collation and visualization, and R 4.4.2 software was used to perform analysis of variance. Statistical analysis was performed using one-way analysis of variance (ANOVA). The significant difference is represented by the letter mark. The group of the same letter indicates that there is no significant difference between the mean values, and the group of different letters indicates that there is a significant difference between the mean values (p < 0.05).

3. Results

3.1. Seedling Quality and Root Morphology

The plant height, the root dry weight of BT(26) and BT(30) seedlings at 10 d, the plant height of BT(30), and the root dry weight of BT(26) at 15 d were significantly increased compared with CK (

Table 1. Effects of different types of pot-mat trays (BT) on rice seedling quality. The same letter are not significantly different at p < 0.05.

Seedling Age	Seedling Tray	Plant Height (cm)	Leaf Age	Shoot Dry Weight (mg/100 Plants)	Root Dry Weight (mg/100 Plants)
5 d	CK	7.74 ± 0.13 b	1.88 ± 0.04 a	501.83 ± 43.52 a	280.17 ± 9.63 a
	BT(26)	9.30 ± 0.17 a	1.91 ± 0.03 a	542.50 ± 24.07 a	293.00 ± 18.63 a
	BT(30)	8.89 ± 0.22 a	1.93 ± 0.04 a	554.67 ± 14.27 a	286.33 ± 11.85 a
10 d	CK	9.23 ± 0.35 b	2.31 ± 0.05 a	723.50 ± 17.53 a	322.83 ± 12.08 b
	BT(26)	10.31 ± 0.20 a	2.45 ± 0.03 a	739.50 ± 47.22 a	387.17 ± 14.49 a
	BT(30)	10.37 ± 0.20 a	2.43 ± 0.09 a	725.50 ± 3.67 a	369.33 ± 13.82 a
15 d	CK	11.03 ± 0.26 b	2.85 ± 0.04 a	889.34 ± 33.28 a	334.00 ± 12.27 b
	BT(26)	11.75 ± 0.26 ab	2.77 ± 0.05 a	981.50 ± 36.72 a	426.17 ± 15.95 a
	BT(30)	12.08 ± 0.24 a	2.77 ± 0.07 a	929.50 ± 34.78 a	376.15 ± 14.07 b

Note: different lowercase letters indicate that the difference between treatments is significant at the 5% level (p < 0.05); the same applies below.

). At 15 d after emergence, the plant height of BT(30) seedlings was significantly increased by 9.52% compared with CK. The biomass was increased at 15 d, and the root dry weight for BT(26) seedlings was significantly increased by 27.59%. The shoot dry weight of BT(26) and BT(30) seedlings had a slight increasing trend compared with CK, with increases of 27.59% and 12.62%, respectively. No significant difference in the leaf age of CK, BT(26), and BT(30) seedlings was detected. In comparison with the CK, BT(26) was more conducive to dry matter accumulation by the seedlings.

The root length, root surface area, root number, root volume, and root dry weight of BT(26) were significantly increased compared with CK, which were 17.75%, 21.72%, 19.07%, 13.99%, and 23.06%, respectively (Figure 3). The root diameter in the BT(26) and BT(30) seedlings increased by 2.43% and 1.60%, respectively, but the differences were not significant compared with CK. The root length, root surface area, root number, root volume, and root dry weight of BT(30) seedlings were higher than those of CK, which were 11.31%, 15.17%, 13.89%, 9.98%, and 12.62%, respectively.

3.2. Plant Height Uniformity and Root Entwining Force

The plant height uniformity and root entwining force were closely related to the quality of rice seedlings. The uniformity of plant height and root entwining force of BT(26) were significantly increased compared with CK, which were 5.95% and 14.28%, respectively (Figure 4). The plant height uniformity and root entwining force in the BT(30) seedlings were slightly higher than those of CK, which were 3.43% and 10.21%, respectively. Thus, raising rice seedlings in BTs improved the plant height uniformity and root entwining force of the seedlings.

3.3. Mechanical Transplantation Quality

The plant height ratio of BT(30) in 5–12 cm and the missing hill rate of BT(26) and BT(30) were significantly decreased compared with CK (Figure 5). The plant height ratios of CK, BT(26), and BT(30) in the 5–12 cm and 12–16 cm ranges were 31.25%, 25.56%, and 13.33% and 65.00%, 70.00%, and 65.56%, respectively. The number of CK seedlings with a plant height of 5–12 cm was significantly higher than that of BT(30) by 134.38%. The ratio of BT(26) and BT(30) seedlings in 12–16 cm tended to increase compared with CK, and the difference was not significant. The number of holes with two to five plants per hole treated with BT(26) and BT(30) was higher by 16.75% and 18.05%, respectively, compared with the CK, showing a greater increasing trend. The missing hill rate of seedlings raised in the two BTs was significantly lower than that of the CK (53.15%).

The root surface area, root volume of BT(26) and root length, surface area, number of roots, root volume, root diameter, and root dry weight of BT(30) were significantly increased compared with CK (

Table 2. Effects of different types of pot-mat trays (BT) on the root morphology of rice seedlings after mechanical transplanting. The same letter are not significantly different at p < 0.05.

Seedling Tray	Root Length (cm/100 Plants)	Root Surface Area (cm2/100 Plants)	Number of Roots	Root Volume (cm3/100 Plants)	Root Diameter (mm)	Root Dry Weight (mg/100 Plants)
CK	1633.79 ± 128.23 b	126.86 ± 11.72 b	8289.41 ± 679.48 b	0.78 ± 0.08 b	0.25 ± 0.01 b	46.96 ± 4.60 b
BT(26)	2048.85 ± 65.22 ab	170.47 ± 4.20 a	9814.19 ± 731.43 ab	1.13 ± 0.02 a	0.26 ± 0.01 ab	58.54 ± 4.21 ab
BT(30)	2279.82 ± 164.29 a	192.61 ± 12.01 a	13,026.56 ± 1153.91 a	1.30 ± 0.08 a	0.27 ± 0.01 a	69.12 ± 1.50 a

). The root length in the BT(30) was retained and significantly increased by 39.54%. Then, the number of roots, root diameter, and root dry weight in BT(30) were significantly increased by 57.15%, 9.67%, and 47.21%, respectively. The root surface area and root volume in BT(26) and BT(30) were significantly increased by 34.38% and 51.83%, 44.07% and 65.51%, respectively. The root length, root number, root diameter, and root dry weight of BT(26) seedlings still retained more after transplanting, which were 25.40%, 18.39%, 7.35%, and 24.68%, respectively.

The losses in root surface area and root volume of BT(30) were significantly decreased compared with CK by 8.29% and 13.96%, respectively (

Table 3. Root loss of different types of pot-mat trays (BT) after mechanical transplanting. The same letter are not significantly different at p < 0.05.

Seedling Tray	Root Length (%)	Root Surface Area (%)	Root Volume (%)
CK	80.39 ± 1.03 a	79.36 ± 1.77 a	78.42 ± 1.88 a
BT(26)	79.01 ± 1.03 a	77.08 ± 1.36 ab	72.58 ± 0.97 ab
BT(30)	75.28 ± 1.85 a	72.80 ± 1.24 b	67.20 ± 3.01 b

). The loss in root surface area and root volume of BT(26) and the root lengths of BT(26) and BT(30) were relatively smaller, reduced by 2.71% and 8.29%, and 1.59% and 6.21%, respectively. The losses in root length, root surface area, and root volume of BT(30) were lower than that of BT(26), with no significant difference between the pot-mat trays.

4. Discussion

In agricultural development in China, mechanical transplantation of rice seedlings is crucial for the overall mechanization of rice production [4,25]. Cultivation of strong seedlings and improvement in the quality of planting are vital to ensure high yields [26]. The mechanical transplanting seeding rate of densely sown seedlings is high, and thus, the quality of seedlings is readily reduced and is accompanied by the problem of a short suitable planting period [7,11,27,28]. However, after properly increasing the seeding rate, the number of trays required per unit area can be effectively reduced and reduce the missing hill rate [9,11]. In production, some measures such as shortening the period of raising seedlings and topdressing application of chemical nitrogen fertilizer are frequently used to maintain high seedling quality and to address the problem of the short suitable planting period for densely sown seedlings [13,28,29]. The seedling tray was an important part of rice machine transplanting technology. Improving the structure of the seedling tray may be an effective way to enhance the quality of seedlings and the effect of mechanical transplanting [25]. In this study, the results showed that the plant height of seedlings raised in BT(30) and the root dry weight of seedlings raised in BT(26) were significantly higher than those of seedlings raised in traditional flat trays (CK) under a high seeding rate (Table 1). This shows that under a high seeding rate, BTs are beneficial for maintaining relatively high seedling quality compared with CK. The results are consistent with previous findings [19]. This is because BTs divide the root growth area of seedlings, and the competition for nutrients among seedlings is reduced, resulting in higher seedling quality of BTs seedlings than that of CK seedlings. High seedling quality lays a sound foundation for the regreening and tillering of seedlings after mechanical transplanting [30]. In the present experiment, compared with the CK, the uniformity of the seedling height of BT(26) was significantly increased (Figure 4). Using BTs to raise seedlings enables the cultivation of machine-transplanted seedlings with an upper blanket and lower pot; the lower root system is coiled in the pot, which reduces the competition between high-density seedlings [19]. Thus, the nutrients obtained by the seedling population are more uniform, and the uniformity of plant height of the seedlings is improved.

The developed root system and high physiological activity of seedlings were conducive to rapid regreening after transplanting, which was an important index to measure the robustness of seedlings [19,31]. The better entwining ability of roots improved the efficiency and quality of mechanical transplanting [8,11]. However, the root system of traditional flat tray seedlings showed staggered growth and was easily damaged during mechanized transplanting [19,32]. In this study, the root entwining force of the BT-raised seedlings increased by 14.28% and 10.21% compared with that of the CK, respectively, and the difference between BT(26) and CK was significant (Figure 4). Higher root entwining force was more conducive to the formation of a carpet structure suitable for mechanical transplanting. The root system of rice seedlings supplied nutrients and water for the growth of shoots, and the surface area of the root system affected the nutrient absorption efficiency [33,34]. The present results showed that the root dry weight and morphology of BT-raised seedlings were higher than those of CK seedlings following the division of the root growth area in the BTs. Among them, the root length, root surface area, root number, root volume, and root dry weight of BT(26) seedlings were significantly increased (Figure 3). The characteristics of bowls and blankets are more conducive to cultivating seedlings with well-developed root systems compared with flat trays, which was also the main reason for the increase in root entwining force.

The quality of machine transplantation was one of the main factors affecting rice yield [22]. The uniformity of the amount of rice seedlings taken by machine transplanting was conducive to ensuring consistent growth of seedlings in the field [5]. The unreasonable seeding rate would affect the seedlings per hole and the missing hill rate of mechanical transplanting and thus impact rice yield [22]. Increasing the seeding rate of rice in the process of mechanical transplanting can effectively reduce the rate of missing hills [22,35]. In this study, BTs increased the proportion of seedlings with a plant height of 12–16 cm, and the proportion of holes with two to five seedlings increased after transplanting, which effectively increased the seedling uniformity (Figure 5). The increase in the proportion of 12–16 cm seedlings and the proportion of two to five seedlings per hole is beneficial to improve the consistency of seedling growth in the field after transplanting. Raising the seedlings in BTs further reduced the missing hill rate of mechanical transplanting, which was 53.15% significantly lower than that of the CK, and effectively reduced the frequency of subsequent manual replacement of seedlings, which was consistent with previous studies [5,11]. The degree of plant injury during transplanting determines the length of the seedling stage, and root damage will affect the growth of the aboveground parts [36].

Minimization of damage is beneficial for early and rapid growth after transplanting and reduces the duration of the entire growth period [34,37]. The present results showed that the root surface area and root volume in BT-raised seedlings were significantly higher than those of the CK seedlings after transplanting (Table 2). The relative loss in root length, root surface area, and root volume of BT-raised seedlings was lower than that of the CK seedlings (Table 3). This is caused by the fact that when the transplanter takes the CK seedlings, the machine tears the roots of the seedlings, which will cause the roots of the seedlings to break and damage [20]. The number of bowls of BTs corresponds to the number of seedlings taken by the transplanter. The transplanter accurately takes seedlings from each bowl each time, thus reducing the root damage of BTs seedlings. Thus, BTs effectively protected the roots of the rice seedlings and enabled rapid regreening after mechanical transplanting. The root system of young seedlings is able to recover more quickly after suffering damage [17,19]. In addition, raising seedlings in BTs is not only beneficial to the root growth of the seedlings but also has the effect of stabilizing and increasing grain yield in production [19].

Dense-seeding machine transplanting could greatly improve production efficiency without excessively reducing seedling quality, but it also has potential limitations [11]. The seedling rate of seeds decreased under a higher sowing rate, which did not conform to the concept of hybrid rice seedling raising with a low amount of precision sowing [38]. The plant height of seedlings decreased after dense sowing, which made the quality requirements for field transplanting more stringent. Additionally, a longer seedling age is not conducive to dense seedling sowing pick-up by needle in machine transplanting operation [39]. In production, the connection interval should be determined by the transplanting period and the sowing date to avoid affecting the quality of transplanting [40]. Although pot-mat tray seedling raising improved the quality of dense seedlings, the number of groups in the tray was still large. The ventilation and light transmittance of the seedling shed and the management level of water and fertilizer were more demanding [22]. In future rice production, the pot-mat dense seedling technology needs to be continuously updated, supported, and improved by theoretical data.

5. Conclusions

The findings of this study indicate that utilizing a pot-mat tray to cultivate rice seedlings under a high seeding density is conducive to mechanical transplantation. The root growth area of the seedlings was separated by pot-mat seedling raising, which improved the seedling quality, plant height uniformity, and root winding force of the dense seedlings. Under the dense sowing rate, the missing hill rate was significantly reduced, and more roots in the rice seedlings were retained, while the proportion of holes with two to five seedlings increased after mechanical transplanting. The pot-mat tray was more conducive to the growth of dense-seeded rice seedlings and promoted the quality of mechanical transplanting.