Coupling Effects of Optimized Planting Density and Variety Selection in Improving the Yield, Nutrient Accumulation, and Remobilization of Sweet Maize in Southeast China

Abstract

Low planting density and lack of density-tolerant varieties are the critical factors limiting the yield of sweet maize in Southeast China. However, there is relatively limited information about the interaction effects of plant density and variety on sweet maize in Southeast China. A two-year (2021–2022) field experiment was conducted with two sweet maize varieties (MT6855 and XMT10) and three plant densities (D1: 45,000 plants ha⁻¹, D2: 60,000 plants ha⁻¹, and D3: 75,000 plants ha⁻¹). The results showed that plant density and variety had significant interaction effects on sweet maize yield and most traits. Increasing plant density significantly increased the fresh ear yield of MT6855, while it did not affect the XMT10 variety. The increase in fresh ear yield for MT6855 under D2 treatment was 14.9% and 14.0% higher than that under D1 treatment in 2021 and 2022, respectively. Meanwhile, increasing plant density significantly increased the bare plant rate and decreased the number of grains per ear of XMT10, while no sustainable changes were observed in MT6855. Moreover, we observed significantly increased pre-silking dry matter, N, P, and K accumulation across different densities in both varieties. In contrast, during the post-silking stage, the increasing plant density significantly improved the accumulation of dry matter, N, P, and K, only in MT6855 but not in XMT10. Meanwhile, harvest index, dry matter remobilization, and leaf N, P, and K remobilization of MT6855 were significantly higher than those of XMT10. Increasing plant density significantly increased N, P, and K partial factor productivity of MT6855 but did not remarkably affect those of XMT10. In addition, fresh ear yield of sweet maize was significantly and positively correlated with pre-silking, post-silking, and total N and P accumulation but had no significant relationship with leaf K remobilization across the two varieties. These results suggest that MT6855 is a density-tolerant variety of sweet maize, and optimizing planting density with a density-tolerant variety can improve the accumulation and remobilization of dry matter and nutrients, thereby improving the fresh ear yield and nutrient use efficiency of sweet maize.

1. Introduction

Sweet maize is one of the important food crops grown extensively in temperate, tropical, and sub-tropical regions of the world. The planting area of sweet maize in China has reached 530,000 ha, becoming the world’s largest producer of sweet maize [1]. Sweet maize kernels are rich in protein, soluble sugar, vitamin C, folic acid, and other nutrients required by the human body. It has become a famous green and healthy food for consumers, and the demand is increasing [2]. Further improving the yield per unit area is an urgent problem to be solved in the current sweet maize production, which needs to rely on cultivation and breeding practices.

Optimizing planting density is a vital cultivation management practice to achieve a high yield of sweet maize [3]. The optimal planting density of maize in the United States has increased by an average annual rate of 700 plants per hectare over the past four decades [4]. However, the optimum planting density of spring maize in Northeast China increased by an average annual value of 175 plants per hectare [5]. Unlike grain maize, sweet maize variety breeders have been more concerned about the sweetness and taste, resulting in the planting density almost not changing [6,7]. South China is one of the main producing areas of sweet maize; the average planting density in South China is 51,300 plants ha⁻¹. We find that local farmers’ planting density of sweet maize is only 42,700 plants ha⁻¹ in Fujian Province, much lower than grain maize in Northwest China, Huang-huai-hai, and Northeast China [8]. In addition, through breeding, density-tolerant sweet maize varieties can increase density and yield [9]. However, there are significant differences in the optimal planting density of different regions and varieties [10,11]. Therefore, studying the synergistic optimization of sweet maize varieties and planting density in Southeast China is very important, but there is currently barely any relevant research.

The yield of maize is not only the photosynthetic products after silking directly transported to the ear but also the remobilization of photosynthetic products from vegetive organs [12]. Maize yield is closely related to maize plant population dry matter accumulation and remobilization [13,14]. Planting density, variety, and soil nutrient conditions all affect the accumulation and distribution of maize biomass [15,16]. Increasing planting density increases the dry matter accumulation and yield of grain maize per unit area in the Northwest China region, but, when exceeding the appropriate density, the maize yield no longer increases or even decreases [17]. Increasing plant density decreases maize dry matter per plant and can increase the yield through coordinating the growth of individuals and populations of maize [18,19]. Some researchers believed that the optimal planting density was lower for maize variety with higher single plant biomass, while maize variety with lower single plant biomass could achieve higher harvest index and yield per unit area under high density [20,21]. In addition, high-yield maize varieties have a stronger ability to transport and distribute dry matter from vegetative organs to grains [22]. Increasing planting density significantly increases maize stems’ dry matter transport rate, but a high dry matter transport rate may increase the risk of lodging and yield reduction [23]. Therefore, it is significant to clarify dry matter accumulation and remobilization characteristics of high-density and high-yield sweet maize varieties in Southeast China, but there is still little relevant information.

Nitrogen (N), phosphorus (P), and potassium (K) are the three essential nutrients for maize growth and development, playing an important role in maize yield formation [24,25,26]. The yield of maize is significantly positively correlated with the accumulation and remobilization of N, P, and K nutrients. Understanding the nutrient accumulation and remobilization status during the critical growth period of maize can achieve precise fertilization and improve fertilizer absorption and utilization efficiency [12,27]. Studies have shown that increasing plant density can significantly increase the nutrient uptake and partial productivity of N, P, and K [28]. However, increasing planting density could further reduce the post-anthesis N accumulation of grain maize, resulting in yield reduction [29]. This may be because high density could accelerate the senescence of maize leaves, reduce the concentration of organs N, P, and K, and then decrease the accumulation of N, P, and K after anthesis [30]. Some studies demonstrated that the super-high-yield maize varieties mainly rely on the absorption of nutrients after the silking stage, and the contribution of nutrient transport was decreased with the increase in yield [31]. However, some researchers have believed that, the higher the yield of maize with the more significant amount of nutrient transport, the more the stay-green variety inhibits the amount of nutrient transport in maize and leads to reduced yield under high plant density [23,32]. Contrary to common grain maize, the harvest period of sweet maize is the R3 stage (fresh eating stage), and the post-silking growth of sweet maize is shorter and different from grain maize. However, the effects of different varieties and planting densities on dry matter accumulation, nutrient accumulation, and distribution of sweet maize in Southeast China are rarely reported.

We hypothesized that optimizing planting density and variety can improve sweet maize population accumulation and remobilization of dry matter and nutrients and improve the nutrient absorption and utilization efficiency and yield of sweet maize. Therefore, the objective of this experiment was to study the effects of planting density on fresh ear yield, dry matter accumulation and distribution, nutrient accumulation, and remobilization under different varieties of sweet maize to investigate the relationship between sweet maize fresh ear yield and dry matter and nutrient accumulation and remobilization and thus to provide the scientific basis for high-density and high-yield cultivation techniques of sweet maize in Southeast China.

2. Materials and Methods

2.1. Site Description

The field experiment was conducted at Yuanfeng Farm in Minqing County of Fuzhou City, Fujian Province, Southeast China (26°14′ N, 118°75′ E) in 2021 and 2022. This region has a subtropical humid monsoon climate; the total precipitation was 175.9 mm and 230.0 mm, and the effective accumulation temperature was 1516.7 °C and 1532.8 °C during sweet maize growing season in 2021 and 2022, respectively (Figure 1). The soil was classified as loamy sand; the soil chemical characteristics of 20 cm layer depth before the experiment were as follows: 22.9 g kg⁻¹ organic matter, 81.3 mg kg⁻¹ alkali hydrolyzed nitrogen, 191.8 mg kg⁻¹ available phosphate, 278.9 mg kg⁻¹ available potassium, and pH 5.15.

2.2. Experiment Design and Field Management

The field experiment was arranged in a split-plot design with three replicates; the main plots were assigned with two sweet maize varieties, including Mintian6855 (MT6855) and Xiameitian10 (XMT10), and subplots were subjected to three planting densities with 45,000 plants ha⁻¹ (D1), 60,000 plants ha⁻¹ (D2), and 75,000 plants ha⁻¹ (D3). Each subplot was 28.8 m² and remained unchanged in 2021 and 2022. The planting pattern was adopted for one ridge and two rows with a ridge width of 1.2 m. In addition, fertilizers were applied as follows: 180 kg ha⁻¹ N, 45 kg ha⁻¹ P₂O₅, and 180 kg ha⁻¹ K₂O. Further, 60 kg ha⁻¹ N, 45 kg ha⁻¹ P₂O₅, and 90 kg ha⁻¹ K₂O were applied as base fertilizer before transplanting, 60 kg ha⁻¹ N were applied at V6 stage, and then 60 kg ha⁻¹ N and 90 kg ha⁻¹ K₂O were applied at V12 stage. Field experiments in 2021 and 2022 were sown on 13 August and 12 August, respectively, and transplanted on 28 August and 22 August, respectively. MT6855 was harvested on 29 October and 25 October, respectively, and XMT10 was on 3 November and 1 November, respectively. The plots were kept free of weeds, insects, and diseases with chemicals based on standard practices. Furrow irrigation was carried out within 2 h after transplanting to ensure the survival rate of sweet maize seedlings and performed at V11 and silking stage if/when no rainfall; maize plants were irrigated with 50–100 mm once.

2.3. Sampling and Measurements

2.3.1. Fresh Ear Yield and Ear Characteristic

At the sweet maize fresh eating stage (R3 stage), female ears were harvested from 30 consecutive plants from two rows in the middle of each plot. The fresh weight of harvested female ears, including cob and bract, was weighed to record fresh ear yield, and the number of bare plants was counted to calculate the bare plant rate. Subsequently, the bracts of the maize ears were stripped off to determine ear traits: ear weight (fresh weight), ear length, bare tip length, ear diameter, 100-grain weight, and grains per ear.

2.3.2. Dry Matter and Nutrient Concentration Determination

Three representative maize plants were sampled from each plot at silking stage (R1 stage) and fresh eating stage. The aboveground parts of the maize plant were divided into stems (including sheathes and male ears), leaves, and female ears. Each part of the plant was dried at 105 °C for 30 min, dried to a constant weight at 70 °C, then weighed and ground into powder to determine mineral nutrient concentration.

Each maize organ sample was boiled with H₂SO₄-H₂O₂. Then, the N and P concentration was determined by a continuous flow analyzer (AutoAnalyzer-3, SEAL Co. Ltd., Berlin, Germany), and the K concentration was determined by a flame spectrophotometer [33].

2.3.3. Parameter Calculation

The following parameters were calculated according to previous studies [30,33].

Harvest index (%) = Fresh ear yield/dry matter at fresh eating stage × 100

Post-silking dry matter accumulation (kg ha⁻¹) = Dry matter weight at fresh eating stage − dry matter weight at silking stage

Stem (leaf) dry matter remobilization amount (kg ha⁻¹) = Stem (leaf) dry matter weight at silking stage − Stem (leaf) dry matter weight at fresh eating stage

The contribution rate of stem (leaf) dry matter remobilization to ear (%) = Stem (leaf) dry matter remobilization amount/ear dry matter weight at the fresh eating stage) × 100

Organ N (P and K) accumulation amount (kg ha⁻¹) = Organ N (P and K) concentration × organ dry matter weight

N (P and K) harvest index (%) = Ear N (P and K) accumulation amount/total N (P and K) accumulation amount at fresh eating stage) × 100

Post-silking N (P and K) accumulation (kg ha⁻¹) = Plant total N (P and K) accumulation amount at fresh eating stage − Plant total N (P and K) accumulation amount at silking stage

N (P and K) remobilization amount (kg ha⁻¹) = Vegetative organ N (P and K) accumulation amount at silking stage − Vegetative organ N (P and K) accumulation amount at fresh eating stage

The contribution rate of N (P and K) remobilization to ear (%) = Vegetative organ N (P and K) remobilization amount/ear accumulation amount at fresh eating stage) × 100

N (P and K) partial factor productivity (PFP, kg kg⁻¹) = Fresh ear yield/N (P and K) application amount

N (P and K) uptake efficiency (UPE, kg kg⁻¹) = Plant total N (P and K) accumulation amount at fresh eating stage/N (P and K) application amount

N (P and K) utilization efficiency (UE, kg kg⁻¹) = Fresh ear yield/plant total N (P and K) accumulation amount at fresh eating stage

2.4. Statistical Analysis

All data were subjected to ANOVA in the General Linear Model module of SPSS (ver. 18.0, SPSS, Chicago, IL, USA). Multiple comparisons among different treatments were undertaken with Duncan’s test at the 0.05 probability level (p < 0.05).

3. Results

3.1. Effects of Different Plant Densities and Varieties on Sweet Maize Fresh Ear Yield and Ear Characteristics

Plant density and variety had significant interaction effects on sweet maize yield and yield components (

Table 1. Fresh ear yield, ears ha⁻¹, bare plant rate, 100-grain weight, and grains per ear for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	Fresh Ear Yield	Ears	Bare Plant Rate	100-Grain Weight	Grains per Ear
			(t ha−1)	(ha−1)	(%)	(g)
2021	MT6855	D1	14.1 ± 0.1 b	45,000 ± 0 c	0.0 ± 0.0 c	35.7 ± 1.6 a	505.1 ± 31.5 abc
		D2	16.2 ± 1.3 a	60,000 ± 0 b	0.0 ± 0.0 c	34.0 ± 1.1 ab	477.5 ± 6.6 bcd
		D3	17.1 ± 0.8 a	72,500 ± 2500 a	3.3 ± 3.3 bc	32.8 ± 0.4 b	463.8 ± 26.9 cd
	XMT10	D1	14.0 ± 1.0 b	44,500 ± 866 c	1.1 ± 1.9 bc	29.0 ± 0.9 c	542.2 ± 26.1 a
		D2	13.6 ± 0.2 bc	54,000 ± 0 b	10.0 ± 0.0 b	24.9 ± 0.8 d	518.5 ± 24.0 ab
		D3	11.7 ± 2.0 c	57,500 ± 8660 b	23.3 ± 11.5 a	24.5 ± 0.8 d	447.6 ± 38.9 d
2022	MT6855	D1	17.1 ± 0.3 b	45,000 ± 0 d	0.0 ± 0.0 c	36.0 ± 1.3 a	615.6 ± 6.9 a
		D2	19.5 ± 0.9 a	60,000 ± 0 b	0.0 ± 0.0 c	31.8 ± 1.7 b	588.2 ± 9.0 a
		D3	20.8 ± 1.7 a	75,000 ± 0 a	0.0 ± 0.0 c	28.9 ± 1.1 b	542.2 ± 25.5 b
	XMT10	D1	16.1 ± 1.2 b	43,500 ± 1500 d	3.3 ± 3.3 c	30.9 ± 2.3 b	582.3 ± 20.0 a
		D2	15.5 ± 1.4 b	52,667 ± 2309 c	12.2 ± 3.8 b	28.3 ± 3.6 b	491.4 ± 32.7 c
		D3	14.9 ± 1.2 b	60,833 ± 3819 b	18.9 ± 5.1 a	24.2 ± 2.5 c	441.1 ± 25.2 d
Source of variation
Density (D)			ns	**	**	**	**
Variety (V)			**	**	**	**	**
Year (Y)			**	ns	ns	ns	**
D × V			**	**	**	ns	*
D × Y			ns	ns	ns	ns	ns
V × Y			ns	ns	ns	**	**
D × V × Y			ns	ns	ns	ns	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). Increasing plant density significantly increased the fresh ear yield of the MT6855 variety in 2021 and 2022, and fresh ear yield under D2 treatment was 14.9% and 14.0% higher than that under D1 treatment in 2021 and 2022, respectively. However, there were no significant differences among different plant densities of the XMT10 variety. Ear number per hectare of two sweet maize varieties was increased markedly with increasing plant density, but 100-grain weight and grains per ear were decreased. Increasing plant density significantly increased bare plant rate and significantly decreased grains per ear of XMT10. However, no remarkable change existed in MT6855 (except grains per ear under D3 treatment in 2022 was decreased). Grains per ear of MT6855 under D2 and D3 treatment averaged 5.0% and 10.1% lower than that of D1 treatment, and grains per ear of XMT10 under D2 and D3 treatment averaged 10.0% and 20.9% lower than that of D1 treatment, respectively.

Plant density had considerable effects on sweet maize ear characteristics, and ear weight, ear length, ear diameter, and grain number per row ear of both varieties were decreased significantly with increasing plant density in 2021 and 2022; however, bare tip length was increased significantly (

Table 2. Ear weight, ear length, bare tip length, ear diameter, grain row number, and grain number per row of sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	Ear Weight	Ear Length	Bare Tip Length	Ear Diameter	Grain Row Number	Grain Number per Row
			(g)	(cm)	(cm)	(mm)
2021	MT6855	D1	232.2 ± 18.7 ab	18.8 ± 0.8 a	2.2 ± 0.2 b	47.3 ± 2.7 a	14.5 ± 0.8 a	34.8 ± 1.5 bc
		D2	207.6 ± 5.7 bc	18.1 ± 0.2 ab	2.8 ± 0.2 a	47.7 ± 0.4 a	14.8 ± 0.2 a	32.3 ± 0.5 c
		D3	187.1 ± 11.1 c	17.8 ± 0.4 ab	3.3 ± 0.5 a	46.6 ± 1.3 a	14.4 ± 0.5 a	32.2 ± 1.2 c
	XMT10	D1	242.4 ± 19.8 a	18.7 ± 0.8 a	1.7 ± 0.5 b	47.3 ± 1.1 a	13.7 ± 0.8 a	39.7 ± 0.6 a
		D2	212.5 ± 11.7 bc	16.8 ± 0.8 b	3.1 ± 0.2 a	46.0 ± 0.4 a	14.3 ± 0.1 a	36.4 ± 2.0 b
		D3	156.8 ± 19.7 d	16.7 ± 1.1 b	3.3 ± 0.3 a	43.1 ± 1.2 b	13.9 ± 0.4 a	32.4 ± 1.8 c
2022	MT6855	D1	285.3 ± 4.4 a	19.9 ± 0.2 a	1.1 ± 0.2 d	50.1 ± 2.0 a	15.8 ± 0.3 a	39.0 ± 0.5 a
		D2	243.9 ± 6.9 b	18.8 ± 0.1 ab	1.8 ± 0.1 c	48.7 ± 0.7 ab	15.9 ± 0.1 a	36.9 ± 0.5 ab
		D3	210.8 ± 7.6 cd	17.8 ± 0.2 bc	2.1 ± 0.1 bc	47.1 ± 0.7 bc	15.5 ± 0.5 a	34.9 ± 0.8 b
	XMT10	D1	287.6 ± 15.2 a	19.9 ± 0.6 a	2.6 ± 0.3 ab	49.9 ± 0.4 a	15.4 ± 0.3 a	37.8 ± 1.2 a
		D2	225.8 ± 14.5 bc	18.6 ± 0.6 b	2.9 ± 0.6 a	47.1 ± 0.5 bc	14.3 ± 0.6 b	34.5 ± 2.4 b
		D3	191.9 ± 17.2 d	17.3 ± 1.4 c	3.1 ± 0.7 a	45.2 ± 1.2 c	14.3 ± 0.2 b	30.9 ± 2.1 c
Source of variation
Density (D)			**	**	**	**	ns	**
Variety (V)			ns	*	**	**	**	ns
Year (Y)			**	**	**	**	**	*
D × V			*	ns	ns	ns	ns	*
D × Y			ns	ns	ns	ns	ns	ns
V × Y			ns	ns	*	ns	ns	**
D × V × Y			ns	ns	ns	ns	ns	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). There were significant interaction effects of plant density and variety on sweet maize ear weight and grain number per row. Ear weight and grain number per row of MT6855 under D2 treatment and D1 treatment were similar; however, those of XMT10 under D2 treatment were significantly lower than those under D1 treatment. In general, increasing plant density had smaller induction effects on ear traits of MT6855 than those of XMT10 (Table 2).

3.2. Effects of Different Plant Densities and Varieties on Sweet Maize Dry Matter Accumulation and Remobilization

Plant density and variety had interactive effects on dry matter accumulation and remobilization of sweet maize (

Table 3. Dry matter accumulation, harvest index, dry matter remobilization amount, and contribution rate of dry matter remobilization to ear for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	Dry Matter Accumulation			Harvest Index (%)	Dry Matter Remobilization Amount			Contribution Rate of Dry Matter Remobilization to Ear
			Pre-Silking	Post-Silking	Total		Stem	Leaf	Total	Stem	Leaf	Total
			(t ha−1)	(t ha−1)	(t ha−1)		(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)	(%)	(%)
2021	MT6855	D1	3.98 ± 0.08 e	2.31 ± 0.06 d	6.29 ± 0.08 d	52.1 ± 0.7 a	184.2 ± 26.1 b	37.0 ± 6.5 ab	221.2 ± 31.0 a	5.6 ± 0.8 a	1.1 ± 0.2 a	6.8 ± 1.0 a
		D2	4.75 ± 0.07 c	2.74 ± 0.03 c	7.49 ± 0.05 c	51.9 ± 0.5 a	299.9 ± 31.1 a	69.2 ± 36.1 a	369.1 ± 65.6 a	7.7 ± 0.7 a	1.8 ± 0.9 a	9.5 ± 1.5 a
		D3	5.46 ± 0.03 a	2.85 ± 0.07 c	8.30 ± 0.09 b	48.2 ± 1.3 ab	321.5 ± 22.9 a	36.5 ± 38.6 ab	358.0 ± 39.0 a	8.0 ± 0.6 a	0.9 ± 1.0 a	9.0 ± 1.2 a
	XMT10	D1	4.32 ± 0.06 d	2.98 ± 0.15 bc	7.31 ± 0.21 c	44.5 ± 2.9 b	−77.1 ± 47.0 c	−74.0 ± 84.1 ab	−151.1 ± 127.6 b	−2.4 ± 1.6 b	−2.4 ± 2.7 ab	−4.9 ± 4.1 b
		D2	4.95 ± 0.05 b	3.31 ± 0.17 a	8.27 ± 0.20 b	38.3 ± 0.9 c	−149.5 ± 35.8 cd	−33.8 ± 159.8 ab	−183.3 ± 143.7 bc	−4.7 ± 1.2 b	−1.1 ± 5.1 ab	−5.8 ± 4.6 b
		D3	5.44 ± 0.02 a	3.17 ± 0.22 ab	8.61 ± 0.24 a	29.2 ± 4.3 d	−219.0 ± 69.6 d	−99.6 ± 34.8 b	−318.6 ± 41.7 c	−9.0 ± 3.6 c	−4.0 ± 1.3 b	−13.0 ± 3.3 c
2022	MT6855	D1	4.90 ± 0.06 f	3.19 ± 0.26 e	8.10 ± 0.25 e	52.9 ± 1.2 a	56.4 ± 3.2 c	224.4 ± 79.0 a	280.8 ± 77.0 b	1.3 ± 0.1 b	5.3 ± 2.1 a	6.6 ± 2.1 b
		D2	6.30 ± 0.14 b	3.85 ± 0.12 cd	10.15 ± 0.17 b	52.7 ± 0.3 a	230.3 ± 66.4 b	301.2 ± 102.9 a	531.5 ± 38.7 a	4.3 ± 1.2 a	5.6 ± 2.0 a	9.9 ± 0.9 a
		D3	7.24 ± 0.21 a	4.80 ± 0.07 a	12.04 ± 0.19 a	53.3 ± 1.5 a	348.3 ± 64.6 a	263.6 ± 43.0 a	611.9 ± 30.0 a	5.4 ± 1.1 a	4.1 ± 0.6 a	9.5 ± 0.5 a
	XMT10	D1	5.27 ± 0.12 e	3.54 ± 0.10 d	8.80 ± 0.21 d	46.6 ± 1.6 b	8.3 ± 14.6 c	4.6 ± 76.6 b	12.9 ± 62.7 c	0.2 ± 0.3 b	0.2 ± 1.9 b	0.3 ± 1.5 c
		D2	5.65 ± 0.08 d	4.03 ± 0.30 c	9.68 ± 0.22 c	39.6 ± 1.1 c	−215.5 ± 46.6 d	−55.6 ± 46.1 b	−271.1 ± 33.3 d	−5.6 ± 1.3 c	−1.4 ± 1.2 b	−7.1 ± 0.9 d
		D3	5.98 ± 0.11 c	4.45 ± 0.07 b	10.43 ± 0.08 b	36.5 ± 2.3 d	−306.3 ± 57.2 e	−80.8 ± 49.8 b	−387.0 ± 8.6 e	−8.1 ± 1.8 d	−2.1 ± 1.3 b	−10.2 ± 0.8 e
Source of variation
Density (D)			**	**	**	**	ns	ns	ns	**	ns	**
Variety (V)			**	**	ns	**	**	**	**	**	**	**
Year (Y)			**	**	**	**	*	**	**	*	**	ns
D × V			**	**	**	**	**	ns	**	**	ns	**
D × Y			*	**	**	**	ns	ns	ns	ns	ns	ns
V × Y			**	**	**	ns	ns	**	**	**	ns	ns
D × V × Y			**	ns	**	ns	**	ns	**	ns	ns	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). Increasing plant density significantly increased pre-silking and total dry matter accumulation of two sweet maize varieties in 2021 and 2022. Notably, increasing plant density significantly improved post-silking dry weight accumulation of MT6855 in both years; however it did not significantly increase post-silking dry weight accumulation of XMT10 (Table 3). With the increase in plant density, the harvest index of MT6855 was not changed markedly in both years; however, that of XMT10 was decreased significantly. In addition, the harvest index of MT6855 was average, 51.9% across different densities, and was significantly higher than that of XMT10. The harvest index of MT6855 was 35.9% and 29.5% higher than that of XMT10 in 2021 and 2022, respectively. Stem and total dry matter remobilization of MT6855 were increased significantly with increasing plant density; however, those of XMT10 were decreased dramatically in both years. Further, plant density and variety had a similarly considerable effect on the contribution rate of stem and total dry matter remobilization to ear, especially in 2022. Stem, leaf, and total dry matter remobilization of MT6855 were higher than those of XMT10. The contribution rate of vegetive organ dry matter remobilization to ear was similar to the response of dry matter remobilization to plant density and variety. Notably, the stem, leaf, and total dry matter remobilization and contribution rates of vegetive organ dry matter remobilization to the ear of MT6855 were positive, but those of XMT10 were negative (Table 3).

3.3. Effects of Different Plant Densities and Varieties on Sweet Maize Nutrients Accumulation and Remobilization

There were significant interaction effects between planting density and variety on pre-silking, post-silking, total N, P, K accumulation, and N, P, and K harvest index of sweet maize, except for post-silking K accumulation (

Table 4. N, P, and K accumulation and harvest index for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	N Accumulation			N Harvest Index	P Accumulation			P Harvest Index	K Accumulation			K Harvest Index
			Pre-Silking	Post-Silking	Total		Pre-Silking	Post-Silking	Total		Pre-Silking	Post-Silking	Total
			(kg ha−1)	(kg ha−1)	(kg ha−1)		(kg ha−1)	(kg ha−1)	(kg ha−1)		(kg ha−1)	(kg ha−1)	(kg ha−1)
2021	MT6855	D1	83.6 ± 3.5 c	31.5 ± 3.8 c	115.1 ± 1.3 d	50.3 ± 1.5 a	10.3 ± 0.3 c	6.5 ± 0.3 b	16.8 ± 0.6 c	68.0 ± 0.4 a	122.9 ± 1.9 d	16.1 ± 1.7 abc	138.9 ± 3.0 d	26.0 ± 0.5 a
		D2	95.4 ± 2.9 b	38.4 ± 1.8 abc	133.8 ± 1.1 c	50.0 ± 2.2 a	12.3 ± 0.3 b	7.7 ± 0.5 ab	20.0 ± 0.4 b	65.5 ± 1.3 ab	151.5 ± 3.7 b	15.4 ± 2.3 bc	166.9 ± 1.9 b	25.8 ± 0.2 a
		D3	98.2 ± 2.3 ab	44.6 ± 2.1 a	142.8 ± 1.6 ab	48.9 ± 0.2 a	14.0 ± 0.7 a	8.6 ± 1.0 a	22.6 ± 0.9 a	63.5 ± 1.0 b	166.9 ± 1.7 a	10.4 ± 2.5 d	177.3 ± 1.5 a	26.0 ± 1.6 a
	XMT10	D1	93.4 ± 4.4 b	41.9 ± 6.0 ab	135.3 ± 7.6 bc	45.0 ± 3.2 b	10.7 ± 0.6 c	8.7 ± 0.7 a	19.3 ± 0.9 b	55.8 ± 2.9 c	122.3 ± 1.9 d	19.9 ± 1.2 a	142.2 ± 2.6 d	24.9 ± 1.3 a
		D2	103.3 ± 1.4 a	43.6 ± 3.5 ab	146.9 ± 4.6 a	40.4 ± 1.3 c	11.6 ± 0.3 b	8.7 ± 0.5 a	20.4 ± 0.5 b	49.9 ± 1.1 d	141.9 ± 4.2 c	19.4 ± 2.3 ab	161.3 ± 4.1 c	21.3 ± 1.2 b
		D3	103.4 ± 1.7 a	36.7 ± 4.9 bc	140.1 ± 5.8 abc	32.7 ± 3.0 d	12.2 ± 0.2 b	6.7 ± 1.0 b	18.9 ± 1.2 b	40.2 ± 3.6 e	155.9 ± 2.9 b	12.1 ± 2.7 cd	168.0 ± 0.7 b	16.1 ± 1.9 c
2022	MT6855	D1	101.2 ± 2.2 d	44.6 ± 4.2 b	145.8 ± 6.3 de	55.6 ± 2.0 a	15.5 ± 0.6 cd	13.1 ± 1.7 b	28.7 ± 2.4 b	65.2 ± 2.0 b	148.3 ± 2.0 d	25.0 ± 2.3 a	173.3 ± 4.3 c	24.8 ± 1.8 a
		D2	127.4 ± 3.3 b	54.9 ± 3.5 a	182.3 ± 4.6 b	57.1 ± 0.6 a	20.1 ± 1.4 b	15.8 ± 1.9 a	35.9 ± 2.6 a	65.1 ± 1.4 b	197.9 ± 5.9 b	15.1 ± 2.0 b	213.1 ± 6.0 b	25.5 ± 1.3 a
		D3	151.9 ± 6.6 a	70.9 ± 4.5 a	222.9 ± 2.4 a	56.9 ± 4.5 a	23.1 ± 0.5 a	12.7 ± 0.8 b	35.8 ± 1.3 a	67.9 ± 0.6 a	235.3 ± 2.6 a	15.0 ± 7.4 b	250.3 ± 10.1 a	26.4 ± 0.7 a
	XMT10	D1	98.5 ± 3.9 d	45.0 ± 6.0 b	143.6 ± 3.3 e	49.4 ± 2.8 b	14.7 ± 0.6 d	11.4 ± 0.8 b	26.1 ± 0.7 b	61.7 ± 1.7 c	125.7 ± 3.6 e	13.0 ± 4.7 bc	138.7 ± 4.7 e	24.9 ± 1.1 a
		D2	107.9 ± 2.6 c	44.3 ± 3.4 b	152.2 ± 3.8 cd	47.1 ± 1.4 b	15.3 ± 0.1 cd	11.6 ± 0.4 b	26.9 ± 0.4 b	58.2 ± 0.6 d	150.7 ± 1.9 d	12.1 ± 1.5 bc	162.8 ± 2.7 d	19.7 ± 0.5 b
		D3	113.7 ± 0.7 c	45.3 ± 4.2 b	159.0 ± 3.9 c	45.5 ± 3.0 b	16.5 ± 0.4 c	10.8 ± 0.9 b	27.3 ± 1.2 b	57.9 ± 1.2 d	158.7 ± 3.3 c	6.2 ± 4.9 c	164.8 ± 1.9 cd	18.2 ± 0.5 b
Source of variation
Density (D)			**	**	**	**	**	*	**	**	**	**	**	**
Variety (V)			**	**	**	**	**	**	**	**	**	*	**	**
Year (Y)			**	**	**	**	**	**	**	**	**	ns	**	ns
D × V			**	**	**	**	**	*	**	**	**	ns	**	**
D × Y			**	*	**	*	**	ns	ns	**	**	ns	**	ns
V × Y			**	**	**	ns	**	**	**	**	**	**	**	ns
D × V × Y			**	ns	**	ns	**	*	ns	ns	**	ns	**	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). Increasing plant density increased pre-silking and total N, P, and K accumulation of sweet maize across both varieties in two years, except for the total P accumulation of XMT10. Increasing plant density significantly increased post-silking N and P accumulation of MT6855; however, it did not increase that of XMT10. Increasing plant density did not affect the N, P, and K harvest index values of MT6855 but significantly decreased those of XMT10. Moreover, the N, P, and K harvest index values of MT6855 were higher than those of XMT10 (

Table 5. N remobilization amount and contribution rate of N remobilization to ear for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	N Remobilization Amount			Contribution Rate of N Remobilization to Ear
			Stem	Leaf	Total	Stem	Leaf	Total
			(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)	(%)	(%)
2021	MT6855	D1	6.9 ± 0.3 b	2.2 ± 1.8 a	9.1 ± 1.9 ab	12.0 ± 0.5 a	3.8 ± 3.1 a	15.7 ± 3.5 a
		D2	7.8 ± 1.1 ab	2.1 ± 1.2 a	10.0 ± 0.9 a	11.7 ± 1.7 a	3.2 ± 1.8 a	15.0 ± 1.9 a
		D3	4.9 ± 0.3 c	1.2 ± 1.7 a	6.2 ± 1.4 b	7.0 ± 0.4 b	1.8 ± 2.5 a	8.8 ± 2.1 b
	XMT10	D1	7.6 ± 1.2 ab	0.6 ± 2.4 a	8.2 ± 1.2 ab	12.7 ± 2.9 a	0.8 ± 3.7 a	13.5 ± 1.1 ab
		D2	9.1 ± 1.0 a	−1.6 ± 3.3 a	7.4 ± 3.0 ab	15.3 ± 1.8 a	−2.8 ± 5.6 a	12.5 ± 5.1 ab
		D3	6.5 ± 1.3 bc	−6.6 ± 0.9 b	−0.1 ± 1.2 c	14.2 ± 3.0 a	−14.7 ± 3.3 b	−0.5 ± 2.7 c
2022	MT6855	D1	6.9 ± 1.9 b	12.5 ± 2.0 a	19.4 ± 1.4 b	8.4 ± 1.8 c	15.5 ± 3.2 a	23.9 ± 1.4 ab
		D2	14.2 ± 1.9 a	13.4 ± 2.1 a	27.6 ± 3.3 a	13.6 ± 2.0 bc	12.9 ± 2.0 a	26.5 ± 3.4 a
		D3	17.8 ± 1.8 a	15.5 ± 2.5 a	33.3 ± 3.9 a	14.1 ± 1.3 b	12.2 ± 1.1 a	26.3 ± 1.7 a
	XMT10	D1	14.8 ± 2.6 a	−3.3 ± 2.2 b	11.5 ± 4.4 c	20.9 ± 2.9 a	−4.7 ± 3.0 b	16.2 ± 5.9 b
		D2	15.9 ± 4.1 a	−1.4 ± 1.8 b	14.5 ± 5.9 bc	22.1 ± 5.5 a	−2.0 ± 2.6 b	20.2 ± 8.0 ab
		D3	16.1 ± 2.8 a	−1.9 ± 2.1 b	14.2 ± 4.2 bc	22.2 ± 2.2 a	−2.8 ± 3.0 b	19.4 ± 4.5 ab
Source of variation
Density (D)			**	ns	ns	ns	**	*
Variety (V)			**	**	**	**	**	**
Year (Y)			**	**	**	**	**	**
D × V			*	ns	*	ns	ns	ns
D × Y			**	**	**	ns	*	**
V × Y			ns	**	**	**	**	ns
D × V × Y			*	ns	ns	*	**	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

).

Planting density and variety had significant interaction effects on N remobilization amount and the contribution rate of N remobilization to ear of sweet maize (Table 5,

Table 6. P remobilization amount and contribution rate of P remobilization to ear for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	P Remobilization Amount			Contribution Rate of P Remobilization to Ear
			Stem	Leaf	Total	Stem	Leaf	Total
			(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)	(%)	(%)
2021	MT6855	D1	1.9 ± 0.2 b	0.3 ± 0.1 a	2.2 ± 0.1 b	17.0 ± 0.7 abc	2.6 ± 0.6 a	19.6 ± 0.1 a
		D2	2.6 ± 0.2 a	0.0 ± 0.4 a	2.6 ± 0.7 ab	19.9 ± 2.0 a	0.2 ± 3.2 a	20.1 ± 5.3 a
		D3	3.1 ± 0.6 a	−0.1 ± 0.2 a	3.0 ± 0.5 a	21.6 ± 4.8 a	−0.6 ± 1.6 a	21.0 ± 4.0 a
	XMT10	D1	1.4 ± 0.1 b	−0.9 ± 0.3 b	0.6 ± 0.3 c	13.5 ± 0.4 c	−8.4 ± 3.0 b	5.0 ± 2.6 b
		D2	1.5 ± 0.2 b	−1.3 ± 0.2 b	0.2 ± 0.3 cd	15.1 ± 2.0 bc	−12.7 ± 1.3 b	2.4 ± 2.8 b
		D3	1.5 ± 0.2 b	−1.9 ± 0.3 c	−0.4 ± 0.2 d	19.2 ± 2.0 ab	−25.1 ± 4.6 c	−5.9 ± 2.6 c
2022	MT6855	D1	2.1 ± 0.0 d	0.1 ± 0.1 c	2.2 ± 0.1 cd	11.4 ± 1.1 d	0.5 ± 0.8 c	11.9 ± 1.6 c
		D2	3.0 ± 0.2 b	0.5 ± 0.0 b	3.5 ± 0.2 b	13.1 ± 1.6 cd	2.2 ± 0.3 b	15.3 ± 1.9 b
		D3	5.3 ± 0.5 a	2.1 ± 0.2 a	7.4 ± 0.6 a	21.9 ± 2.0 a	8.6 ± 0.3 a	30.6 ± 1.7 a
	XMT10	D1	2.5 ± 0.2 cd	−0.7 ± 0.1 d	1.8 ± 0.0 d	15.6 ± 1.5 bc	−4.4 ± 0.9 d	11.2 ± 0.6 c
		D2	2.5 ± 0.2 cd	−0.8 ± 0.2 d	1.7 ± 0.3 d	15.7 ± 1.2 bc	−4.8 ± 1.1 d	10.9 ± 2.2 c
		D3	2.7 ± 0.1 bc	−0.1 ± 0.1 c	2.7 ± 0.1 c	17.4 ± 1.4 b	−0.6 ± 0.5 c	16.8 ± 1.4 b
Source of variation
Density (D)			**	**	**	**	ns	**
Variety (V)			**	**	**	*	**	**
Year (Y)			**	**	**	*	**	**
D × V			**	**	**	ns	**	**
D × Y			**	**	**	ns	**	**
V × Y			ns	ns	ns	**	**	**
D × V × Y			**	ns	**	**	*	ns

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

and

Table 7. K remobilization amount and contribution rate of K remobilization to ear for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	K Remobilization Amount			Contribution Rate of K Remobilization to Ear
			Stem	Leaf	Total	Stem	Leaf	Total
			(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)	(%)	(%)
2021	MT6855	D1	0.3 ± 0.5 d	5.2 ± 0.5 b	5.5 ± 0.7 c	1.0 ± 1.4 d	14.4 ± 1.4 a	15.3 ± 2.3 c
		D2	4.2 ± 0.7 c	8.7 ± 1.1 a	12.9 ± 0.4 b	9.7 ± 1.7 c	20.3 ± 2.6 a	30.0 ± 0.9 b
		D3	11.1 ± 1.9 a	9.6 ± 0.4 a	20.8 ± 1.5 a	24.1 ± 3.9 a	21.0 ± 1.5 a	45.0 ± 3.5 a
	XMT10	D1	6.9 ± 0.2 b	0.0 ± 0.5 c	6.9 ± 0.2 c	19.5 ± 0.8 b	−0.1 ± 1.3 b	19.5 ± 1.7 c
		D2	6.9 ± 0.4 b	−0.2 ± 2.5 c	6.7 ± 2.3 c	20.1 ± 0.9 b	−0.7 ± 7.1 b	19.4 ± 6.4 c
		D3	7.3 ± 0.5 b	−1.2 ± 1.2 c	6.1 ± 1.6 c	27.4 ± 2.4 a	−4.6 ± 4.2 b	22.8 ± 5.9 c
2022	MT6855	D1	−0.2 ± 0.8 d	3.3 ± 0.9 b	3.0 ± 0.3 d	−0.7 ± 1.7 d	7.7 ± 2.8 a	7.1 ± 1.1 d
		D2	14.1 ± 1.2 b	7.7 ± 0.9 a	21.8 ± 2.1 b	26.0 ± 0.3 c	14.1 ± 0.7 a	40.1 ± 0.8 ab
		D3	23.8 ± 4.1 a	9.1 ± 0.4 a	32.9 ± 3.8 a	36.1 ± 6.4 b	13.8 ± 0.4 a	49.9 ± 6.0 a
	XMT10	D1	10.3 ± 0.6 c	−2.4 ± 1.6 c	7.9 ± 2.0 c	30.1 ± 2.8 bc	−6.8 ± 4.5 b	23.3 ± 7.2 c
		D2	11.1 ± 0.7 bc	−4.1 ± 1.7 c	7.0 ± 1.1 c	34.4 ± 1.3 b	−12.6 ± 5.1 b	21.9 ± 3.8 c
		D3	14.4 ± 1.4 b	−3.4 ± 1.5 c	11.0 ± 2.5 c	48.0 ± 6.1 a	−11.3 ± 4.6 b	36.7 ± 9.5 b
Source of variation
Density (D)			**	**	**	**	ns	**
Variety (V)			ns	**	**	**	**	**
Year (Y)			**	**	**	**	**	*
D × V			**	**	**	**	**	**
D × Y			**	ns	**	**	ns	*
V × Y			*	ns	**	**	ns	ns
D × V × Y			**	ns	**	*	ns	*

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). N remobilization amount and the contribution rate of N remobilization to ear of sweet maize under D1 treatment were consistent with those under D2 treatment for both varieties. Interestingly, high plant density under D3 treatment, leaf and total N remobilization amount, and the contribution rate of N remobilization to ear of MT6855 were significantly higher than those of XMT10 (Table 5).

Increasing plant density significantly increased the stem and total P remobilization amounts of MT6855 in 2021 and 2022 and significantly increased the contribution rates of stem, leaf, and total P remobilization to ear of MT6855 in 2022, although the effects of plant density on P remobilization amount and contribution rate of P remobilization to ear of XMT10 were not stable. Leaf and total P remobilization amount and contribution rate of P remobilization to ear of MT6855 were significantly higher than those of XMT10 in both years (Table 6).

Increasing plant density significantly increased the stem, leaf, and total K remobilization amounts of MT6855 in 2021 and 2022 but did not affect those of XMT10 (Table 7). The contribution rates of stem and total K remobilization to ear of two varieties were increased significantly with the increase in plant density in two years; however, there was no significant difference among different plant densities across two varieties in 2021 and 2022. Similarly, leaf and total K remobilization amount and contribution rate of K remobilization to ear of MT6855 were significantly higher than those of XMT10 in both years (Table 7).

3.4. Effects of Different Plant Densities and Varieties on Sweet Maize Nutrients Uptake and Utilization Efficiency

There were significant plant density × variety interactive effects on sweet maize N, P, and K partial factor productivity and uptake efficiency (

Table 8. N, P, and K partial factor productivity (PFP), utilization efficiency (UE), and uptake efficiency (UPE) for sweet maize varieties MT6855 and XMT10 under three plant densities in 2021 and 2022.

Years	Hybrids	Density	N			P			K
			PFP	UE	UPE	PFP	UE	UPE	PFP	UE	UPE
			(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)	(kg kg−1)
2021	MT6855	D1	78.1 ± 0.8 b	122.1 ± 2.4 a	63.9 ± 0.7 d	715.1 ± 7.1 b	837.6 ± 24.9 a	85.4 ± 3.3 c	112.9 ± 1.1 b	101.2 ± 2.1 a	111.6 ± 2.4 d
		D2	90.2 ± 7.1 a	121.3 ± 9.3 a	74.3 ± 0.6 c	826.6 ± 65.3 a	811.0 ± 49.7 ab	101.8 ± 2.1 b	130.5 ± 10.3 a	97.3 ± 6.8 a	134.1 ± 1.5 b
		D3	95.1 ± 4.2 a	119.9 ± 5.8 a	79.3 ± 0.9 ab	871.2 ± 38.3 a	759.1 ± 61.1 abc	115.0 ± 4.5 a	137.5 ± 6.0 a	96.6 ± 4.3 a	142.4 ± 1.2 a
	XMT10	D1	77.9 ± 5.4 b	103.6 ± 1.5 b	75.2 ± 4.2 bc	713.6 ± 49.4 b	725.3 ± 28.5 bc	98.3 ± 4.4 b	112.6 ± 7.8 b	98.6 ± 6.7 a	114.2 ± 2.1 d
		D2	75.4 ± 1.2 bc	92.5 ± 1.8 bc	81.6 ± 2.6 a	690.8 ± 11.2 bc	666.5 ± 26.9 cd	103.7 ± 2.5 b	109.0 ± 1.8 bc	84.2 ± 2.3 b	129.6 ± 3.3 c
		D3	65.2 ± 11.3 c	83.5 ± 11.6 c	77.8 ± 3.2 abc	597.2 ± 103.9 c	619.1 ± 82.5 d	96.2 ± 5.9 b	94.3 ± 16.4 c	69.8 ± 11.9 c	135.0 ± 0.5 b
2022	MT6855	D1	94.9 ± 1.6 b	117.4 ± 6.6 a	81.0 ± 3.5 de	869.8 ± 14.8 b	599.2 ± 56.5 a	145.9 ± 12.1 b	137.3 ± 2.3 b	98.7 ± 4.0 b	139.2 ± 3.4 c
		D2	108.2 ± 4.8 a	106.8 ± 2.1 abc	101.3 ± 2.5 b	991.5 ± 44.4 a	544.2 ± 30.1 a	182.6 ± 13.4 a	156.5 ± 7.0 a	91.4 ± 1.9 bc	171.2 ± 4.8 b
		D3	115.6 ± 9.2 a	93.4 ± 7.7 d	123.8 ± 1.3 a	1058.6 ± 84.4 a	580.8 ± 42.5 a	182.3 ± 6.8 a	167.1 ± 13.3 a	83.3 ± 8.8 c	201.1 ± 8.1 a
	XMT10	D1	89.7 ± 6.8 b	112.4 ± 8.1 ab	79.8 ± 1.8 e	821.5 ± 62.6 b	618.4 ± 63.5 a	133.1 ± 3.6 b	129.7 ± 9.9 b	116.3 ± 6.4 a	111.5 ± 3.8 e
		D2	86.1 ± 7.8 b	101.9 ± 8.7 bcd	84.6 ± 2.1 cd	789.2 ± 71.1 b	577.3 ± 49.0 a	136.7 ± 2.1 b	124.6 ± 11.2 b	95.2 ± 7.5 bc	130.8 ± 2.2 d
		D3	83.0 ± 6.5 b	94.0 ± 6.7 cd	88.3 ± 2.2 c	760.5 ± 59.4 b	546.5 ± 37.9 a	139.2 ± 6.2 b	120.0 ± 9.4 b	90.6 ± 6.5 bc	132.4 ± 1.6 bc
Source of variation
Density (D)			ns	**	**	ns	**	**	ns	**	**
Variety (V)			**	**	**	**	**	**	**	ns	**
Year (Y)			**	ns	**	**	**	**	**	*	**
D × V			**	ns	**	**	ns	**	**	**	**
D × Y			ns	ns	**	ns	ns	ns	ns	ns	**
V × Y			ns	**	**	ns	**	**	ns	**	**
D × V × Y			ns	ns	**	ns	ns	ns	ns	ns	**

Note: MT6855, Mintian6855; XMT10, Xiameitian10. D1, D2, and D3 indicate 45,000 plants ha⁻¹, 60,000 plants ha⁻¹, and 75,000 plants ha⁻¹, respectively. Values indicate mean ± standard deviation; the same letters within column within year are not significantly different at p < 0.05; ns, not significant. * and ** indicate significances at p < 0.05 and p < 0.01, respectively.

). Increasing plant density significantly increased the N, P, and K partial factor productivity of MT6855 but did not remarkably affect those of XMT10 in 2021 and 2022, and the N, P, and K partial factor productivity values of MT6855 were average, 20.6%, 21.8%, and 21.9% higher than those of XMT10, respectively (Table 8). The sweet maize N, P, and K uptake efficiencies of two varieties were markedly improved with increasing plant density in both years, expect for P uptake efficiency of XMT10. Increasing plant density significantly decreased the utilization efficiency regarding the N, P, and K utilization efficiency of XMT10 but did not affect MT6855 (except for D3 treatment in 2022).

3.5. Relationship between Fresh Ear Yield Dry Matter, N, P, and K Accumulation and Remobilization

Fresh ear yield of MT6855 was correlated significantly positively with pre-silking (R² = 0.831, p < 0.01), post-silking (R² = 0.772, p < 0.01), and total (R² = 0.835, p < 0.01) dry matter accumulation. Fresh ear yield of XMT10 had a significantly positive correlation with post-silking (R² = 0.659, p < 0.01) and total (R² = 0.338, p < 0.05) dry matter accumulation but had no significant correlation with dry matter accumulation of pre-silking. There were significantly positive correlations between fresh ear yield and leaf (R² = 0.556, p < 0.01) and total (R² = 0.730, p < 0.01) dry matter remobilization of MT6855 but no significant correlation between fresh ear yield and organ dry matter remobilization of XMT10 (Figure 2).

Fresh ear yield of sweet maize was correlated significantly positively with pre-silking, post-silking, and total N and P accumulation, except for pre-silking N accumulation of XMT10 (Figure 3). Fresh ear yield of MT6855 had a significantly positive correlation with pre-silking (R² = 0.844, p < 0.01) and total K accumulation (R² = 0.829, p < 0.01) but had no significant correlation with pre-silking, post-silking, and total K accumulation (Figure 3). Across two sweet maize varieties, fresh ear yield was correlated significantly positively with stem, leaf, and total N and P remobilization, except for leaf N remobilization of XMT10. Fresh ear yield was correlated significantly positively with stem K remobilization but had no significant relationship with leaf K remobilization across two varieties. Moreover, fresh ear yield of MT6855 was significantly positively correlated with its total K remobilization (R² = 0.538, p < 0.01), but no significant correlation existed between fresh ear yield of XMT10 and its total K remobilization (Figure 4).

Fresh ear yield of sweet maize was correlated significantly positively with N harvest index, N, P, and K partial factor productivity, and N and P uptake efficiency across two varieties. Fresh ear yield of MT6855 was correlated significantly negatively with N, P, and K utilization efficiency; however, there were significantly positive correlations between fresh ear yield of XMT10 and N and K utilization efficiency (Figure 5).

4. Discussion

4.1. Yield and Ear Traits

Maize yield gains rely on the interaction between genotypes, environmental factors, and cultivation management, and recent work shows that the maize yield gains from genetic and density improvements are 5.9% and 7.3% [9]. Reasonably close planting can construct a good canopy structure, improve the efficiency of canopy light interception and utilization, and thus achieve high yield [34,35]. The optimal plant density of maize varies with variety and regional environmental conditions [9]. In this study, increasing the plant density did not improve the fresh ear yield of the XMT10 variety in Southeast China but significantly increased the fresh ear yield of MT6855. Moreover, maize yield was correlated significantly negatively with bare plant rate but was correlated significantly positively with grains per ear; these findings were consistent with previous studies [36]. Thus, the increase in MT6855 fresh ear yield under dene planting is a result of lower bare plant rate and less grain reduction per ear. On one hand, high planting density resulting in shading significantly decreases interception and utilization of light energy, but MT6855 has a compact plant shape with upright leaves and may intercept more light energy to improve photosynthesis and ears per hectare [37]. On the other hand, high planting density reduces grain filling and grain assimilation, leading to grain abortion. Nevertheless, MT6855 had similar grains per ear and other ear traits between D1 and D2 treatment, perhaps because MT6855 can improve maize grain filling traits and sink capacity [38,39]. In addition, increasing plant density improved fresh ear yield without decreasing marketable ears from D1 to D2 treatment.

The results of this experiment demonstrated that the optimal plant density of sweet maize MT6855 was 60,000 plants ha⁻¹; however, sweet maize XMT10 reached the maximum yield at 45,000 plants ha⁻¹; these agree with other sweet maize plant density experiments [7,40]. These results indicate that MT6855 is a density-tolerant variety of sweet maize. Previous research found that the highest optimum plant density was 105,000 plants ha⁻¹ in Northwest China [10], thereby indicating a large gap regarding optimum planting density between sweet maize and grain maize. In addition, the optimum planting density of grain maize increased by a rate of 700 plant ha⁻¹ per year in American and grain maize and obtained higher grain yield with high plant density using a newer hybrid [4,36]. Therefore, it is vital to clarify the density tolerance traits of sweet maize MT6855 variety, which can provide a basis for breeding and cultivation of density-tolerant and high-yield sweet maize to satisfy production requirements.

4.2. Dry Matter Accumulation and Remobilization

Maize yield depends on the accumulation, distribution, and remobilization of dry matter, and some studies showed that the contribution rate of aboveground dry matter accumulation to grain yield in maize was 71.4–89.1% [30,41,42]. The dry matter accumulation of the maize population represented a parabolic relationship with planting density [43]. Low plant density causes insufficient utilization of nutrients and light in a maize plant population; when plant density exceeds the appropriate density, fierce competition for nutrient and light resources leads to a decrease in maize population dry matter accumulation [44,45]. Reasonably increasing planting density can improve the dry matter accumulation of the population and achieve a maize yield increase [46]. Similarly, the results of this experiment showed that increasing planting density increased the pre-silking and total dry matter accumulation of sweet maize MT6855 and XMT10 varieties. Meanwhile, fresh ear yield of sweet corn was significantly positively correlated with post-silking and total dry matter accumulation. In addition, increasing plant density increased the post-silking dry matter accumulation of MT6855 but did not increase that of XMT10. This accords with previous studies, which revealed that maize varieties with more post-silking dry matter accumulation under high planting density achieved higher yields [23,47].

Increasing maize dry matter accumulation while transferring dry matter to grain as much as possible can increase maize yield [16]. Mazie yield was related significantly positively with dry matter remobilization [30]. For grain maize, increasing planting density can increase the dry matter remobilization contribution rate to grain, reaching 20.8–37.2% at high densities [48,49]. However, excessively high dry matter transport rates can cause premature leaf senescence and increase the risk of maize stem lodging, affecting maize yield [23,50]. There is still controversy over how much better to transfer dry matter to ear. Our results found that the contribution rate of dry matter transport to the ear of MT6855 was increased with the increase in planting density, and sweet maize fresh yield had a significant positive correlation with leaf and total dry matter remobilization in MT6855, but no significant relationship between fresh ear yield and dry matter remobilization existed for XMT10. Compared to grain maize, sweet maize is harvested at the fresh eating stage, which is equivalent to the R3 stage in grain maize. The post-silking period is short, about 20–25 days, with the leaves staying green. Although Liu et al. [31] stated that there was a decreasing trend in the total, stem, and leaf dry matter remobilization rates with increasing grain maize yield level, it is still essential to transfer vegetive organ dry matter to ears in a short time for a density-tolerant sweet maize variety because leaves staying green may suppress dry matter transport to ears, resulting in reduced maize yield. Moreover, the harvest index of grain maize reaches a limit of 52% when the grain yield is ≥10.1 t ha⁻¹, and the harvest index of high-yield maize is about 51–57% [31]. In this study, the harvest index of MT6855 was average, 51.9% across different densities, and was significantly higher than that of XMT10. The high harvest index of density-tolerant variety MT6855 is attribute to the trade-off between the increased post-silking dry matter accumulation and remobilization [51]. Thus, high vegetive organs, especially leaf dry matter remobilization and high harvest index, are the key density tolerance traits of sweet maize within the current density selection range.

4.3. Nutrient Accumulation and Remobilization

Nitrogen, phosphorus, and potassium are essential nutrients for maize growth and development, and understanding the characteristics of nutrient uptake, remobilization, and utilization is key to high yields of maize and highly efficient production [41,52]. In this study, increasing planting density significantly increased the accumulation of nitrogen, phosphorus, and potassium at the silking stage and fresh eating stage. This is consistent with previous research results [28]. A possible cause is the high shoot dry matter of the maize population under dense planting, and thus the high shoot demand for N drives the high root uptake rate of nutrients [53]. Moreover, the present study indicated that post-silking N, P, and K accumulation of MT6855 was increased significantly with plant density increasing; this may be due to the MT688 variety owning a large root system and strong root absorption nutrient capacity during post-silking, resulting from the larger lateral distribution and higher root depth, as well as that upregulating N transporter genes in the root system is conductive to maintaining high nutrient uptake under high density [54,55,56].

In the current study, fresh ear yield of sweet maize was correlated significantly positively with N and P accumulation and remobilization across two varieties and was associated significantly positively with K accumulation and remobilization of MT6855; however, there was no relationship between fresh ear yield and K accumulation and remobilization of XMT10. These results are consistent with a previous study [30] and suggest that K accumulation and remobilization may be some important traits of density tolerance for sweet maize. It is notable that K fertilizer application should be independent of fresh ear yield for the current sweet maize density-sensitive variety, for example, the XMT10 variety that is planted widely in China. Moreover, the leaf NPK remobilization of MT6855 was significantly higher than that of XMT10, which was almost negative in this study. It was also found that the nutrient remobilization of the grain maize stay-green variety was lower than that of the normal variety, and the lower nutrient transport decreased the maize yield at high density [30,57]. These suggest that leaf nutrient remobilization is conductive to density tolerance and high yield. Eventually, increasing planting density significantly improved N, P, and K partial productivity and uptake efficiency of the density-tolerant variety MT6855, but those of XMT10 were unchanged or decreased, which is consistent with previous research results [28].

This experiment cleared the interactive effects of planting density and variety on yield, dry matter, and nutrient accumulation and remobilization. However, selecting some vital density-tolerant traits regarding dry matter and nutrient accumulation and remobilization, there are morphology traits and root traits that should be paid more attention in future. In future studies, we are keen to find the maximum density for the minimum ear length acceptable for the market and the gains in yield and nutrient use.

5. Conclusions

Increasing planting density from 45,000 plants ha⁻¹ to 60,000 plants ha⁻¹ can significantly improve the fresh ear yield of sweet maize varieties of MT6855 and maintain fine ear traits. These indicated that MT6855 is a sweet maize density-tolerant variety. The dry matter, N, P, and K accumulation and N, P, and K partial factor productivity of sweet maize MT6855 were significantly increased with increasing planting density. Sweet maize MT6855 has a lower bare plant rate and higher grains per ear, harvest index, nutrients harvest index and leaf dry matter, and nutrients remobilization compared to normal variety XMT10. Therefore, density-tolerant varieties have strong dry matter and nutrient accumulation and remobilization under high density to achieve high yield and high efficiency. Based on these results, we recommend that farmers use the MT6855 variety with an optimal planting density of 60,000 plants ha⁻¹ in sweet maize production.