Agronomic and Physiological Performances of High-Quality Indica Rice under Moderate and High-Nitrogen Conditions in Southern China

Abstract

High-quality (i.e., higher appearance and eating quality) rice (Oryza sativa L.) is being increasingly and widely planted in China with the improvement of people’s living standards and the achievement of rice breeding efforts in recent years. However, the agronomic and physiological performances of high-quality indica rice (HQIR) under different nitrogen (N) application conditions in southern China are little known. Two-year consecutive field experiments were conducted with two HQIR and two ordinary-quality indica rice (OQIR) varieties under moderate and high-N application rates, with yield and yield components, biomass, N uptake, and their related traits, being investigated. We found that grain yields of HQIR were slightly decreased, but grain yields of OQIR were significantly increased by 7.0–9.6% under a high N rate, compared with a moderate N rate within two years. Thereby, OQIR produced a 5.7–14.7% and 18.7–25.6% higher grain yield than HQIR under moderate and high N rates, respectively. The different responses of grain yield to N application rates were mainly due to a decreased grain setting rate in HQIR and increased spikelets m⁻² in OQIR under a high N rate. Furthermore, a high N rate significantly reduced pre-anthesis AE (apparent exportation of pre-anthesis stem and leaf blade dry matter) and improved the grain-leaf area ratio, while it did not increase post-anthesis dry matter, compared with a moderate N rate in HQIR, which might result in carbon-metabolic deterioration, an imbalance of the source–sink relationship and, subsequently, a lower supply of carbohydrate in panicle. Our results suggest that a moderate N rate (165 kg N ha⁻¹) is beneficial for the HQIR varieties to balance the maximum grain yield and high quality in southern China.

1. Introduction

Rice (Oryza sativa L.) is a vital staple food for more than 60% of the population and contributes nearly 40% of the people’s total calorie intake in China [1]. Over the past several decades, the high-yielding traits were always regarded as the principal targets for rice breeding and selecting to meet the population growth of China [2]. For instance, in China, with the development of hybrid rice in 1976 and “super” rice in 1996, the newly released rice varieties, which have a higher canopy photosynthesis and larger sink size, showed about a 10% higher yield potential than their check varieties [1,2,3]. Correspondingly, the national total rice yield and average rice yield have increased by 3.09 Mt year⁻¹ and 0.127 t ha⁻¹ year⁻¹ from 1976–1995, and 1.26 Mt year⁻¹ and 0.039 t ha⁻¹ year⁻¹ from 1996–2020, respectively, in despite of the national total rice production area showing a decreasing trend (FAOSTAT, 2020) [4]. These achievements have contributed much to national food self-sufficiency and food security.

Presently, with the development of social economy and the continuous improvement of living standards, the consumer demand for rice food has shifted from quantity to high quality in China [5,6]. However, there has long been a contradiction in achieving high yield and superior quality simultaneously for rice breeders [7,8]. Most high-yielding hybrid indica rice varieties generally show poor rice quality, such as a higher chalkiness degree, lower cooking and eating quality [9,10]. The reasons for this outcome are that most rice yield- or quality-related traits are quantitative, and yield and quality are generally negatively correlated with each other [11,12,13]. Hence, it is difficult to develop new elite rice varieties with both high yield and superior quality using traditional breeding approaches [13,14]. Fortunately, based on technological innovation of rice breeding (i.e., molecular marker-assisted breeding/molecular design breeding), improving the quality of high-yielding rice varieties has been increasingly highlighted in breeding efforts, and more and more attention has been paid on high-quality rice breeding [9,13,14]. So far, over 50% of varieties released by province and state have reached the national and ministerial standards of high-quality rice since 2017, which greatly accelerates the popularizing rate of high-quality varieties in Chinas’ rice production [15]. Particularly, in late-season rice (grown from July to November), of the double-rice cropping system of southern China, more than 80% of the late rice varieties planted by farmers have high quality as the optimal temperature in the late-season, which is conducive to the formation of rice quality.

In comparison with the ordinary-quality rice, the high-quality rice varieties usually have a lower yield potential but higher price per unit yield and overall benefits [6,8]. As we all know, nitrogen (N) input plays a vital role in rice production and has significant effects on rice yield and quality. To obtain more returns, however, rice farmers unrealistically applied a large amount of nitrogen (N) fertilizer to realize higher yield, milling and appearance quality of high-quality rice varieties [16,17,18,19]. Many studies indicated that the overuse of N fertilizer was prevalent in the rice production of China, subsequently leading to rice lodging, increased pests and diseases, low N-use efficiency, and higher environmental costs [20,21,22]. In fact, potential yields of most high-yielding rice varieties do not depend on greater N fertilizer input under moderate and high-soil-fertility conditions [23,24]. On the other hand, excessive N fertilizer input may increase grain–protein content and alter starch properties, resulting in a remarkable decline in cooking and eating quality for high-quality rice [17,18,19,25,26]. Previous studies showed that optimizing N management, such as reducing the total N and/or late-stage N application rate [18,22], and split application of N [27,28], is beneficial to balance rice yield and quality. Most results indicated that the rational N application rate of simultaneously obtaining high yield and superior quality varied with the rice varieties and was 180–270 kg N ha⁻¹ for high-quality japonica rice (HQJR) [18,19,25], but such rational N application rate was 120–165 kg N ha⁻¹ for high-quality indica rice (HQIR) [29,30,31], which is close to the N application rate of producing maximum yield in ordinary-quality indica rice (OQIR) [24,32].

However, the abovementioned studies focused more on the effect of N management on balancing rice yield and quality. The agronomic and physiological responses of yield formation of HQIR varieties to N application rates are not detailed. In this study, therefore, two-year field experiments were carried out with two HQIR and two OQIR varieties under moderate and high N rates. The objectives of this study were to explore the grain yield performance of two types of rice varieties with contrasting quality under moderate- and high-N conditions, and to further reveal the responses of yield attributes and related physiological parameters to N application rates. The results may provide an insight into the fundamentals of balancing the high yield and superior quality of HQIR by adopting a reasonable N management.

2. Materials and Methods

2.1. Plant Materials

The tested rice varieties included Yexiangyoulisi (YXY), Wanxiangyou-982 (WXY), Jiyou T025 (JY) and Keyou-5 (KY), which are all hybrid late indica rice. The YXY and WXY are high-quality indica rice (HQIR) varieties, while JY and KY are ordinary-quality indica rice (OQIR) varieties; the related key quality properties are shown in

Table 1. The related key-quality properties of tested varieties.

Type	Variety	Length-Width Ratio of the Grain	Percentage of Chalky Grains (%)	Chalkiness Degree (%)	Gel Consistency (mm)	Amylose Content (%)
HQIR	YXY	4.0	5.0	0.9	69	13.9
	WXY	3.9	9.0	1.8	73	15.0
OQIR	JY	3.3	23.0	4.8	30	25.9
	KY	2.9	33.0	8.2	36	18.5

Note: Data obtained from China rice data center, https://www.ricedata.cn/variety/index.htm (accessed on 11 May 2023).

. In addition, the YXY and JY were first released in 2017, and WXY and KY were first released in 2019.

2.2. Experimental Design

Field experiments were conducted in Jiangxi Shanggao Rice Science and Technology Backyard in 2020 and 2021, which is located in Sixi Township, Shanggao County, Jiangxi Province, China (115°06′ E, 28°20′ N, 38 m altitude). This area has a subtropical monsoon climate with an annual average temperature of 17.6 °C and rainfall of 1733 mm. The soil during the two experimental years were clay loam. The basic physicochemical properties of the upper 20 cm of the soil in 2020 and 2021 are shown in

Table 2. The basic physicochemical properties of the upper 20 cm of the experimental soil in 2020 and 2021.

Year	H2O Extractable pH	K2Cr2O7-H2SO4 Oxidizable Organic Matter (g kg−1)	Total Kjeldahl N Content (g kg−1)	NaOH Hydrolyzable N (mg kg−1)	HCl-H2SO4 Extractable P (mg kg−1)	NH4OAc Extractable K (mg kg−1)
2020	5.5	29.5	2.0	166.1	18.7	65.1
2021	5.6	34.9	2.1	181.6	19.3	76.3

. Climate parameters, including daily average temperature and solar radiation, were collected during the growth period from sowing to maturity from an automatic weather station (Vantage Pro 2, Davis instruments Corp., Hayward, CA, USA) located near the experimental site (

Table 3. Daily average temperature and solar radiation during the late-rice growing seasons in Shanggao County, Jiangxi Province, China in 2020 and 2021.

Month	Temperature (°C)		Solar Radiation (MJ M−2)
	2020	2021	2020	2021
June	27.2	27.5	16.3	15.8
July	31.5	30.1	23.7	23.1
August	30.9	30.6	22.7	22.8
September	23.7	27.9	15.3	19.1
October	19.4	17.0	9.2	8.3

).

The experiments were all arranged in a split-plot design with N application rates as the main plots and rice varieties as the subplots. The experiments were replicated three times with a main plot size of 120 m² and subplot size of 30 m². Each main plot was separated by a ridge, with plastic film inserted into the soil to a depth of 0.3 m, to minimize leakage and nutrient loss. According to our previous study, grain yield of twenty HQIR varieties significantly increased from 105 kg N ha⁻¹ to 165 kg N ha⁻¹, while most varieties showed no increase or even decrease in grain yield from 165 kg N ha⁻¹ to 225 kg N ha⁻¹ [31]. Therefore, in this study, 165 kg N ha⁻¹ (moderate N rate) and 225 kg N ha⁻¹ (high N rate) were selected to further explore the agronomic and physiological responses of HQIR varieties to N application rates. For both moderate and high N rates, urea was used as the N fertilizer and was split-applied with 50% as basal (1 day before transplanting), 30% at early tillering (7 days after transplanting), and 20% at panicle initiation.

Pre-germinated seeds were sown on 27 June in 2020 and 24 June in 2021. The 25-day-old seedlings were transplanted at a hill spacing of 25 cm × 14 cm with two seedlings per hill. The heading date of four rice varieties was on 8 September to 11 September in both years, and physiological maturity date was 26 October in 2020 and 16 October in 2021. The amount and method of P and K fertilizer were consistent during the two years; rice plants received 105 kg ha⁻¹ P₂O₅ and 180 kg ha⁻¹ K₂O. The P was applied as basal, while the K was split equally at basal and panicle initiation. The basal fertilizers were incorporated to field with a rotary cultivator before transplanting, while the top-dressing fertilizers were broadcasted on the field surface by hand. The regime for water management was in the sequence of flooding, midseason drainage, reflooding, and moist intermittent irrigation. Weeds, pests, and diseases were intensively controlled using chemical treatments.

2.3. Sampling and Measurements

2.3.1. Yield and Yield Components

At maturity, ten hills on a diagonal from the center of each subplot were sampled to determine the aboveground total biomass and yield components. The sampled plants were separated into straw, filled grains, unfilled grains, and rachis. Panicle number was recorded from the 10 hills and the panicles were hand threshed. Filled grains were separated from unfilled grains by submerging them in tap water. All filled and unfilled grains were air-dried and then were oven-dried at 70 °C to a constant weight. The number of filled and unfilled grains were calculated using an automatic seed counter (DC-3, Zhengzhou, China). Grain yield was determined from 5 m² in each plot, and yields were then adjusted to the standard moisture content of 0.135 g H₂O g^–1 fresh weight.

2.3.2. Biomass and Related Properties

At the heading stage, ten hills, excluding two borders, were sampled from each subplot. The rice plants were separated into leaf blades, stems (including leaf sheaths) and panicles, and then were oven-dried at 70 °C to a constant weight to determine the aboveground total dry weight. At maturity, the total biomass, including straw, filled grains, unfilled grains, and rachis, were determined from the abovementioned Section 2.3.1. Apparent exportation of the pre-anthesis stem and leaf-blade dry matter (pre-anthesis AE), contribution of pre-anthesis AE, post-anthesis dry matter (pre-anthesis DM), contribution of post-anthesis DM, and harvest index were calculated by the following formulas.

Pre-anthesis AE = stem and leaf blade dry weight at heading − stem and leaf blade dry weight at maturity.

(1)

Contribution of pre-anthesis AE = (pre-anthesis AE/grain yield) × 100

(2)

Pre-anthesis DM = biomass at maturity − biomass at heading

(3)

Contribution of post-anthesis DM = (Pre-anthesis DM/grain yield) × 100

(4)

Harvest index = filled spikelet weight/aboveground total biomass) × 100

(5)

2.3.3. Leaf-Area Index (LAI) and Grain–Leaf Ratio

Green leaves from six hills at heading stages were measured with a leaf-area meter (LI-3000C, LI-COR, Lincoln, NE, USA). LAI was calculated as leaf area/the specific land area. Grain–leaf ratio was calculated as spikelets m⁻² (filled grains m⁻² or grain weight m⁻²)/leaf area m⁻².

2.3.4. Radiation Use Efficiency (RUE) and Net Photosynthetic Rate (Pn)

Canopy light interception was measured between 11:00 h and 13:00 h at middle-tillering, panicle initiation, booting, heading, 15 days after heading (HD15), and maturity, using the SunScan Canopy Analysis System (Delta-T Devices Ltd., Burwell, Cambridge, UK). The measuring methods and computing methods were referenced by Zhang et al. [33]. Briefly intercepting radiation during the whole growing season was the summation of intercepted radiation during each growth period. RUE (aboveground total biomass/intercepted radiation during the whole growing season) was calculated.

The Pn was measured on five flag leaves for each subplot at heading, 15 days after heading (HD15) and 30 days after heading (HD30), using a portable photosynthesis system (LI-6400, Li-Cor, Lincoln, NE, USA). The measurements were collected between 9:00 and 11:00 when photosynthetic active radiation above the canopy was 1000–1200 μmol m⁻² s⁻¹. Meanwhile, a light intensity of 1200 μmol m^–2 s^–1, a leaf temperature of 30 °C, a constant CO₂ concentration of 380 μmol mol^–1, and a relative humidity of 70% were set up in the sample chamber. Moreover, a chlorophyll meter (SPAD-502, Minolta Camera Co., Ltd., Osaka, Japan) was used to measure the SPAD value for the corresponding leaves. For the four varieties, YXY and JY have nearly identical leaf-blade thickness (370 mg cm⁻²), but are lower than WXY and KY (410 mg cm⁻²)

2.3.5. Plant N uptake and N use efficiency

The aboveground plants from heading and maturity stages were ground to a fine powder to determine plant N concentration using a fully automatic Kjeldahl apparatus (Kjeltec 8400, FOSS Analytical A/S, Hilleroed, Denmark). N uptake, post-anthesis N uptake, pre-anthesis N exportation and N-use efficiency for grain production (NUE_g) were calculated by the following formulas.

N uptake = N concentration × dry matter weight.

(6)

Post-anthesis N uptake = N uptake at maturity − N uptake at heading

(7)

Pre-anthesis N exportation = N uptake of stem and leaf at heading − N uptake of stem and leaf at maturity.

(8)

NUE_g = grain yield/N uptake at maturity.

(9)

2.4. Statistical Analysis

Because the two-year experiments were carried out in adjacent fields and the climate parameters (such as temperature and solar radiation) were comparable in September of 2020 and 2021, crop data were separately analyzed within each year using the analysis of variance (ANOVA) in Statistix 8.0 (Analytical software, Tallahassee, FL, USA). In each year, means of related parameters of four varieties (including two HQIR and two OQIR varieties) under each N application rate and means of related parameters of two types (HQIR and OQIR) under two N application rates were examined with the least significant (LSD) test at the 5% probability level. Graphs were drawn using Origin 2018 (OriginLab Corp, Northampton, MA, USA).

3. Results

3.1. Temperature and Solar Radiation during Growing Season

The daily average temperature and solar radiation were similar between 2020 and 2021 among June, July, and August, but they were different among September and October (Table 3). The daily average temperature and solar radiation increased to 4.2 °C and 3.8 MJ M⁻² in September, while decreasing to 2.4 °C and 0.9 MJ M⁻² in October in 2021, respectively, compared with 2020. Moreover, from the historical data of 2019, we found that the daily average temperature and solar radiation in September were also higher (3.7 °C and 3.3 MJ M⁻²) than these of 2020.

3.2. Grain Yield and Its Components

N application rate and variety had significant effects on grain yield and its components, except for the effect of N application rate on grain weight in each year; interaction effects between N application rate and variety were observed on spikelets m⁻², grain setting rate, grain yield in each year and grain weight in 2021(

Table 4. Grain yield and its components of HQIR and OQIR varieties under moderate and high N rates.

Year	N Rate	Variety	Panicles (m−2)	Spikelets (Panicle−1)	Spikelets (×103 m−2)	Grain Setting Rate (%)	Grain Weight (mg)	Grain Yield (t ha−1)
2020	Moderate	YXY	311.2 a	146.70 b	45.6 c	73.9 a	19.8 d	7.5 c
	N	WXY	298.8 b	145.0 b	43.3 d	72.7 a	23.4 a	7.6 c
		MeanHQIR	305.0 B	145.9 B	44.5 C	73.3 A	21.6 B	7.6 C
		KY	283.0 c	172.8 a	48.9 a	74.4 a	21.5 c	8.6 a
		JY	279.6 c	170.2 a	47.6 b	70.2 b	22.5 b	8.3 b
		MeanOQIR	281.3 D	171.5 A	48.2 B	72.3 AB	22.0 A	8.5 B
	High N	YXY	327.7 a	140.3 b	46.0 c	70.0 b	19.7 d	7.4 c
		WXY	315.9 b	137.7 b	43.5 d	67.6 c	23.1 a	7.5 c
		MeanHQIR	321.8 A	139.0 C	44.7 C	68.8 C	21.4 B	7.4 C
		KY	300.1 c	170.1 a	51.0 a	73.7 a	21.7 c	9.2 a
		JY	293.5 c	169.2 a	49.7 b	69.7 b	22.6 b	8.9 b
		MeanOQIR	296.8 C	169.7 A	50.4 A	71.7 B	22.2 A	9.0 A
2021	Moderate	YXY	301.0 a	146.2 b	43.0 b	82.0 a	20.7 c	8.5 b
	N	WXY	286.4 b	142.7 b	40.9 c	81.6 a	23.7 a	8.8 b
		MeanHQIR	293.7 B	144.4 B	42.4 C	81.8 A	22.2 B	8.6 C
		KY	281.7 b	164.7 a	46.4 a	82.2 a	23.2 b	9.4 a
		JY	280.3 b	163.2 a	45.7 a	82.6 a	23.5 a	9.3 a
		MeanOQIR	281.0 C	163.9 A	46.1 B	82.4 A	23.4 A	9.4 B
	High N	YXY	325.5 a	136.1 b	44.3 c	77.4 b	20.6 c	8.2 b
		WXY	306.0 b	133.9 b	41.0 d	77.0 b	23.4 b	8.4 b
		MeanHQIR	315.7 A	135.0 C	42.6 C	77.2 B	21.0 B	8.3 C
		KY	301.0 b,c	163.1 a	49.1 a	82.1 a	23.4 b	10.3 a
		JY	293.6 c	160.2 a	47.0 b	81.3 a	23.7 a	10.1 a
		MeanOQIR	297.3 B	161.7 A	48.1 A	81.7 A	23.6 A	10.2 A
Analysis of variance
2020	N rate		**	*	**	**	ns	*
	Variety		***	***	***	***	***	***
	N rate × Variety		ns	ns	***	**	ns	**
2021	N rate		*	*	*	*	ns	*
	Variety		***	***	***	**	***	***
	N rate × Variety		ns	ns	***	*	**	***

Note: Within a column, different lowercase letters between varieties in the same N rate indicate significant differences at the p < 0.05 level; different uppercase letters between varietal types across two N rates indicate significant differences at the p < 0.05 level. *, ** and *** represent significant differences at the p < 0.05, p < 0.01 and p < 0.001 probability levels, respectively; ns means no significance.

). Compared to moderate N rate, grain yields of HQIR were slightly decreased, while grain yields of OQIR were significantly increased by 7.0–9.6% under a high N rate within 2020 and 2021. A high N rate significantly increased panicles m⁻² of HQIR and OQIR, but decreased spikelets per panicle of HQIR, which resulted in significantly increased in spikelets m⁻² of OQIR. In addition, a high N rate also led to a marked decline in the grain setting rate of HQIR, while having no effect for OQIR.

OQIR produced a higher grain yield of 5.7–14.7% under a moderate N rate and 18.7–25.6% under a high N rate than HQIR within two years. Although panicles m⁻² of OQIR were significantly lower than HQIR, OQIR had significantly higher spikelets per panicle and spikelets m⁻² than HQIR under two N rates in each year. However, a significantly higher grain setting rate was only observed in OQIR, than that in HQIR under a high N rate. Moreover, OQIR had a higher grain weight than YXY (one of the varieties of HQIR), but a lower grain weight than WXY.

3.3. Biomass Production

Significant effects on biomass at heading and maturity stages, pre-anthesis AE and post-anthesis DM and its contribution to grain yield, as well as harvest index, were observed among N application rate, variety, and their interaction in each year, except for the interaction effect on the contribution of post-anthesis DM to grain yield (

Table 5. Biomass production and its contribution to grain yield of HQIR and OQIR varieties under moderate and high N rates.

Year	N Rate	Variety	Biomass (g m−2)		Pre-Anthesis AE (g m−2)	Contribution of Pre-Anthesis AE (%)	Post-Anthesis DM (g m−2)	Contribution of Post-Anthesis DM (%)
			Heading	Maturity
2020	Moderate	YXY	1020.4 a	1401.3 c	261.2 a	34.8 a	380.8 d	50.7 c
	N	WXY	1017.1 a	1423.8 b	248.6 b	32.6 b	406.6 c	53.2 c
		MeanHQIR	1018.8 B	1412.5 D	254.9 A	33.7 A	393.7 C	52.0 C
		KY	926.2 c	1449.4 b	216.3 d	25.1 d	523.2 a	60.6 a
		JY	943.9 b	1424.1 a	234.2 c	28.1 c	480.2 b	57.6 b
		MeanOQIR	935.0 D	1436.7 C	225.3 B	26.6 C	501.7 B	59.1 A
	High N	YXY	1097.1 a	1488.9 c	223.2 ab	30.4 a	391.8 d	53.3 d
		WXY	1090.9 a	1504.6 b	208.6 c	28.0 b	413.8 c	55.5 c
		MeanHQIR	1094.0 A	1496.8 B	215.9 B	29.2 B	402.8 C	54.4 B
		KY	980.9 c	1553.2 a	215.5 bc	23.5 d	572.3 a	62.3 a
		JY	1018.9 b	1547.4 a	229.4 a	25.8 c	528.5 b	59.4 b
		MeanOQIR	999.9 C	1550.3 A	222.5 B	24.6 D	550.4 A	60.8 A
2021	Moderate	YXY	1019.0 a	1424.8 d	309.2 a	36.5 a	405.8 d	47.9 d
	N	WXY	1012.8 a	1452.8 c	277.3 b	31.6 b	440.0 c	50.1 c
		MeanHQIR	1015.9 B	1438.8 D	293.3 A	34.0 A	422.9 C	49.0 D
		KY	903.2 c	1473.1 b	230.2 d	24.6 c	569.9 a	60.8 a
		JY	949.5 b	1501.2 a	243.4 c	26.1 c	551.7 b	59.1 b
		MeanOQIR	926.4 D	1487.1 C	236.8 C	25.3 C	560.8 B	60.0 B
	High N	YXY	1089.9 a	1501.1 c	272.3 a	33.3 a	411.2 d	50.3 b
		WXY	1072.5 b	1520.6 b	253.9 b	30.1 b	448.1 c	53.1 b
		MeanHQIR	1081.2 A	1510.8 B	263.1 B	31.7 B	429.6 C	51.7 C
		KY	929.9 d	1575.8 a	222.4 d	21.6 d	645.9 a	62.8 a
		JY	961.6 c	1583.5 a	235.9 c	23.4 c	621.9 b	61.7 a
		MeanOQIR	945.7 C	1579.6 A	229.1 C	22.5 D	633.9 A	62.3 A
Analysis of variance
2020	N rate		***	***	**	**	**	*
	Variety		***	***	**	***	***	***
	N rate × Variety		***	***	***	*	***	ns
2021	N rate		***	***	**		**	*
	Variety		***	***	***	**	***	***
	N rate × Variety		***	***	***	***	***	ns

Note: Within a column, different lowercase letters between varieties in the same N rate indicate significant differences at the p < 0.05 level; different uppercase letters between varietal types across two N rates indicate significant differences at the p < 0.05 level. *, ** and *** represent significant differences at the p < 0.05, p < 0.01 and p < 0.001 probability levels, respectively; ns means no significance.

and Figure 1). Compared to a moderate N rate, a high N rate significantly increased the biomass of HQIR and OQIR at heading and maturity stages, post-anthesis DM of OQIR, and contribution of post-anthesis DM to grain yield of HQIR and OQIR (excluding 2020) in each year. On the other hand, a high N rate markedly reduced pre-anthesis AE and the harvest index of HQIR, and the contribution of pre-anthesis AE to grain of HQIR and OQIR.

At the heading stage, HQIR produced more biomass than OQIR under moderate and high N rates in each year. On the contrary, OQIR had more biomass at the maturity stage and subsequently showed significantly higher post-anthesis DM and its contribution to grain yield than OQIR. However, in comparison with OQIR, higher pre-anthesis AE (excluding a high N rate in 2020) and its contribution to grain yield were observed in HQIR under two N rates. Moreover, OQIR showed a higher harvest index than HQIR under two N application rates in each year.

3.4. Net Photosynthetic Rate (Pn), RUE, and Sink-Source Relationship

Compared to a moderate N rate, a high N rate improved SPAD values of HQIR and OQIR at HD, HD15d and HD30d, excluding SPAD values of HQIR at HD and of OQIR at HD15d in 2020 (

Table 6. SPAD value and net photosynthetic rate (Pn) of HQIR and OQIR varieties under moderate and high N rates.

Year	N Rate	Variety	SPAD Value			Pn (μmol CO2 m−2 s−1)
			HD	HD15d	HD30d	HD	HD15d	HD30d
2020	Moderate	YXY	34.9 d	30.9 d	21.0 b	18.0 c	15.4 b	12.2 a
	N	WXY	36.0 c	32.1 c	21.9 a	18.1 c	16.0 b	11.5 b
		MeanHQIR	35.4 C	31.5 C	21.5 B	18.0 B	15.7 C	11.8 A
		KY	39.1 a	33.0 b	16.6 d	23.3 a	16.8 a	10.3 c
		JY	38.0 b	34.2 a	18.8 c	22.7 b	17.0 a	10.6 c
		MeanOQIR	38.5 B	33.6 A	17.7 D	23.0 A	16.9 B	10.5 C
	High N	YXY	35.6 d	31.9 c	23.4 b	18.4 c	17.6 b	12.4 a
		WXY	37.9 c	33.8 b	27.0 a	18.8 c	18.0 b	12.0 b
		MeanHQIR	36.8 C	32.8 B	25.2 A	18.6 B	17.8 B	12.2 A
		KY	42.0 a	34.9 a	17.6 d	25.5 a	20.5 a	11.0 d
		JY	39.4 b	33.8 b	19.8 c	21.8 b	18.0 b	11.3 c
		MeanOQIR	40.7 A	34.4 A	18.7 C	23.7 A	19.2 A	11.2 B
2021	Moderate	YXY	33.8 c	29.5 c	21.4 b	17.8 b	15.1 c	12.0 a
	N	WXY	36.4 b	33.6 a	24.5 a	18.3 b	15.9 b	11.5 b
		MeanHQIR	35.1 D	31.5 C	22.9 B	18.0 C	15.5 C	11.8 B
		KY	39.3 a	32.2 b	16.0 d	23.0 a	16.0 b	10.0 c
		JY	38.6 a	34.3 a	16.7 c	22.9 a	16.9 a	10.0 c
		MeanOQIR	39.0 B	33.2 B	16.3 D	230 B	16.5 AB	10.0 D
	High N	YXY	35.9 c	32.8 c	22.6 b	18.2 d	16.3 c	12.3 a
		WXY	38.3 b	34.2 b	27.4 a	18.7 c	16.0 c	11.9 b
		MeanHQIR	37.1 C	33.5 B	25.0 A	18.5 C	16.2 BC	12.1 A
		KY	41.8 a	33.8 b	17.0 d	25.3 a	17.3 a	10.6 d
		JY	42.2 a	36.6 a	18.9 c	24.4 b	17.0 b	11.1 c
		MeanOQIR	42.0 A	35.2 A	17.9 C	24.9 A	17.2 A	10.8 C

Note: Within a column, different lowercase letters between varieties in the same N rate indicate significant differences at the p < 0.05 level; different uppercase letters between varietal types across two N rates indicate significant differences at the p < 0.05 level.

). Meanwhile, a high N rate significantly enhanced the Pn of HQIR and OQIR at HD15d in 2020 and HD30d in 2021, and of OQIR at HD30d in 2020 and HD in 2021. At HD and HD15d, significant higher SPAD values and the Pn of OQIR were found than those of HQIR, while OQIR showed lower SPAD values and Pn than HQIR at HD30d in each year.

A high N rate significantly increased intercepted radiation and RUE of HQIR and OQIR, compared to moderate N rate in each year. OQIR showed a markedly higher intercepted radiation, but lower RUE than HQIR under two N rates (Figure 2).

In comparison with a moderate N rate, a high N rate significantly increased the LAI of HQIR and OQIR at the heading, but significantly decreased the spikelets–leaf ratio, filled grains–leaf ratio and grain–weight–leaf ratio of HQIR in each year (

Table 7. LAI and grain–leaf area ratio of HQIR and OQIR varieties under moderate and high N rates.

N Rate	Variety	2020				2021
		LAI	Spikelets-Leaf Area Ratio (cm−2)	Filled Grains-Leaf Area Ratio (cm−2)	Grain Weight-Leaf Area Ratio (mg cm−2)	LAI	Spikelets-Leaf Area Ratio (cm−2)	Filled Grains-Leaf Area Ratio (cm−2)	Grain Weight-Leaf Area Ratio (mg cm−2)
Moderate	YXY	7.1 a	0.64 b	0.48 c	9.4 c	7.1 a	0.62 b	0.51 b	10.5 c
N	WXY	7.0 a	0.62 c	0.45 d	10.5 b	6.9 a	0.59 b	0.48 c	11.3 b
	MeanHQIR	7.0 B	0.63 B	0.46 B	10.0 B	7.0 B	0.60 B	0.49 B	10.9 B
	KY	6.5 b	0.75 a	0.56 a	12.0 a	6.5 b	0.73 a	0.60 a	14.1 a
	JY	6.4 b	0.75 a	0.52 b	11.8 a	6.4 b	0.72 a	0.59 a	13.9 a
	MeanOQIR	6.4 D	0.75 A	0.54 A	11.9 A	6.4 D	0.72 A	0.60 A	14.0 A
High N	YXY	7.7 a	0.60 b	0.42 c	8.2 c	7.6 a	0.58 b	0.45 c	9.3 c
	WXY	7.4 b	0.59 b	0.40 d	9.2 b	7.4 b	0.55 b	0.42 d	10.0 b
	MeanHQIR	7.5 A	0.59 C	0.41 C	8.7 C	7.5 A	0.57 C	0.44 C	9.6 C
	KY	6.8 c	0.76 a	0.56 a	12.1 a	6.7 c	0.73 a	0.61 a	14.2 a
	JY	6.6 d	0.76 a	0.53 b	11.9 a	6.5 d	0.72 a	0.59 b	14.0 a
	MeanOQIR	6.7 C	0.76 A	0.54 A	12.0 A	6.6 C	0.73 A	0.60 A	14.1 A

Note: Within a column, different lowercase letters between varieties in the same N rate indicate significant differences at the p < 0.05 level; different uppercase letters between varietal types across two N rates indicate significant differences at the p < 0.05 level.

). Furthermore, HQIR had a higher LAI and lower spikelets–leaf ratio, filled grains–leaf ratio and grain–weight–leaf ratio than OQIR under two N rates.

3.5. Plant N Uptake and N Use Efficiency

Compared with a moderate N rate, a high N rate significantly increased the N uptake of HQIR by 23.7–27.1% at the heading and 10.6–20.7% at maturity, and of OQIR by 12.8–19.1% at the heading and 12.1–19.1% at maturity within 2020 and 2021 (

Table 8. Aboveground plant N uptake and NUEg of HQIR and OQIR varieties under moderate and high N rates.

Year	N Rate	Variety	N Uptake at Heading (kg ha−1)	N Uptake at Maturity (kg ha−1)	Post-Anthesis N Uptake (kg ha−1)	Pre- Anthesis N Exportation (kg ha−1)	NUEg (kg kg−1)
2020	Moderate	YXY	123.5 c	149.9 c	26.4 b	44.5 b	50.2 bc
	N	WXY	122.4 c	157.9 b	35.5 a	41.8 b	48.4 c
		MeanHQIR	122.9 C	153.9 D	30.9 AB	43.2 C	49.3 B
		KY	129.8 b	164.6 a	34.8 a	55.3 a	52.4 a
		JY	134.7 a	162.7 a	28.0 b	59.4 a	51.2 ab
		MeanOQIR	132.3 B	163.7 C	31.4 A	57.3 B	51.8 A
	High N	YXY	156.2 ab	177.8 b	21.6 b	62.2 b	41.4 b
		WXY	152.0 bc	174.6 b	22.6 b	56.3 c	42.8 b
		MeanHQIR	154.1 A	176.2 B	22.2 C	59.2 B	42.1 C
		KY	151.5 c	184.5 a	33.0 a	68.4 a	49.8 a
		JY	160.4 a	184.5 a	24.1 b	69.9 a	48.2 a
		MeanOQIR	155.9 A	184.5 A	28.6 B	69.2 A	49.0 B
2021	Moderate	YXY	125.6 bc	142.6 d	17.0 c	50.9 c	59.4 b
	N	WXY	124.4 c	146.9 c	22.5 ab	47.2 d	59.8 ab
		MeanHQIR	125.0 D	144.8 D	19.7 B	49.1 C	59.6 A
		KY	128.9 ab	152.6 b	23.7 a	56.9 b	61.4 a
		JY	136.7 a	155.1 a	18.4 bc	61.2 a	60.2 ab
		MeanOQIR	132.8 C	153.8 C	21.0 B	59.0 B	60.8 A
	High N	YXY	159.6 a	172.1 b	12.5 c	64.7 bc	47.5 c
		WXY	153.9 b	167.8 c	13.9 c	59.8 c	50.3 b
		MeanHQIR	156.7 A	170.0 B	13.2 C	62.2 B	48.9 C
		KY	145.3 c	181.8 a	36.4 a	68.6 b	56.6 a
		JY	154.7 b	183.7 a	29.0 b	76.0 a	54.9 a
		MeanOQIR	150.0 B	182.8 A	32.7 A	72.3 A	55.7 B

Note: Within a column, different lowercase letters between varieties in the same N rate indicate significant differences at the p < 0.05 level; different uppercase letters between varietal types across two N rates indicate significant differences at the p < 0.05 level.

). In contrast, a high N rate significantly decreased the post-anthesis N uptake of HQIR by 18.2–38.2% in each year, and of OQIR by 9.6% in 2020 (but increased by 56.1% in 2021). However, a high N rate significantly increased the pre-anthesis N exportation of HQIR and OQIR by 26.7–39.5% and 17.7–24.2%,within two years, respectively. Finally, the significant declines in NUE_g of HQIR by 11.6–20.0% and of OQIR by 5.1–8.8% were found under a high N rate compared to a moderate N rate.

Compared with HQIR, OQIR showed a significantly higher N uptake at the heading under a moderate N rate and N uptake at maturity, under two N rates in each year. Similarly, a significantly higher post-anthesis N uptake under a high N rate and pre-anthesis N expropriation under two N rates were observed in OQIR than in HQIR. Accordingly, OQIR also had a higher NUE_g under two N rates than HQIR.

4. Discussion

4.1. Yield Responses of HQIR to N Application Rates

N is an essential nutrient for rice growth, development, and yield formation in almost all environments. Over the past several decades, most previous studies have demonstrated that the continuously increase of rice yield in China and the world is mainly attributed to the genetic improvement and N fertilizer input [23,34,35]. On further improving rice yield potential, China has made two breakthroughs in breeding for hybrid and “super” rice varieties since the semi-dwarf rice varieties were successfully bred in 1956 [1,2]. In addition, many modern high-yielding rice varieties usually have great N responsiveness and a higher lodging resistance [23]. On this account, rice farmers often apply a large amount of N fertilizers to obtain the highest grain yield of high-yielding rice varieties [20,22,24]. Ultimately, a synchronous increased tendency is observed between total rice grain yields and total N fertilizers consumption in China during the past several decades [4]. In fact, numerous studies have shown that there is a quadratic–function relationship between grain yield (per unit area yield) and N application rates in most high-yielding rice varieties [24,32,35,36]. These results suggest that higher yields of high-yielding rice varieties do not depend on much more N fertilizer inputs.

In this study, we find that different yield responses of HQIR and OQIR to moderate and high N application rates were mainly explained by the difference in spikelets m⁻² and/or grain setting rate. A high N rate significantly increased OQIR’s yield by 7.0–9.6% but had none or a slightly adverse effect on HQIR’s yield, in comparison with a moderate N rate (Table 4). The results indicate that HQIR can obtain the highest grain yield under the moderate N rate (165 kg N ha⁻¹), which is in accordance with the recommended optimum N application rates (120–165 kg N ha⁻¹) from previous reports for HQIR grown in southern China and Pakistan [29,30,31]. Meanwhile, our results also suggest that a further increase in N fertilizer input is beneficial to obtain the maximum grain yield for OQIR, compared with the moderate N rate. However, previous studies revealed that OQIR could produce the maximum grain yield under moderate N rates (120–190 kg N ha⁻¹) in the single- and double-rice cropping systems in southern China [32,37]. The difference in OQIR may result from the site-specific N management technology that was adopted in previous studies, which would significantly improve rice grain yield and N use efficiency, as well as reduce N loss [20,32]. To that end, OQIR produced 5.7–14.7% and 18.7–25.6% higher grain yield than HQIR did under moderate and high N rates within two years, respectively.

On the other hand, previous studies showed that by both increasing the N application rate and splitting the application of the N reduced percentage of chalky grains and chalkiness degree, increased the percentage of head rice [16,17,18,19,27,28]. However, such N managements had adverse effects on eating quality, such as decreasing gel consistency, peak and breakdown viscosity of head rice flour. In this study, we also found that a high N rate reduced the percentage of chalky grains, chalkiness degree, gel consistency, and peak and breakdown viscosity of head rice flour, compared with a moderate N. These results indicate that a moderate N rate has a beneficial effect on eating quality, but a slightly detrimental effect on appearance quality. Overall, a moderate N rate may obtain the maximum grain yield and better quality for HQIR varieties in our study.

4.2. Agronomical and Physiological Responses of HQIR to N Application rates

In the present study, and in previous studies, a high N rate did not increase spikelets m⁻² but led to a significant decline in the grain setting rate of HQIR [30,31]. On the contrary, when significant increases in spikelets m⁻² were observed, they had no effect on the grain setting rate of OQIR under a high N rate (Table 4). Fu et al. [36] found that a high N rate could reduce the grain setting rate of spikelets at the panicle base of “super” rice, without the increased spikelets per panicle. In general, “super” rice varieties have numerous spikelets per panicle, but they often fail to achieve their high-yield potential because of poor grain-filling of inferior spikelets, particularly under high N rates [38]. The main reason may be due to a lower partitioning of assimilates in developing inferior spikelets resulted from the low activity of enzymes involved in carbohydrate metabolism [38,39]. On the other hand, a high N rate could significantly increase plant (panicles, stems, and leaves) N concentration, which might result in the enhancement of plant N metabolism and overconsumption of carbohydrate, thus decreasing carbohydrate supply to panicles [40]. However, the spikelets per panicle of HQIR (140–150 spikelets panicle⁻¹) and OQIR (160–170 spikelets panicle⁻¹) in our study are less than that of “super” rice (more than 180 spikelets panicle⁻¹). The decreased grain setting rate of HQIR under a high N rate are probably related to the above-mentioned interpretations.

In addition, HQIR had a 6.2–16.9% higher LAI than OQIR, and the LAI of HQIR and OQIR were increased by 5.7–8.5% and 1.6–4.6%, respectively, under a high N rate, compared with a moderate N rate (Table 7). Meanwhile, we also found that a high N rate significantly decreased pre-anthesis AE in HQIR but increased post-anthesis DM in OQIR compared with a moderate N rate (Table 5); HQIR had an obviously higher canopy-intercepted radiation, but a lower RUE than OQIR under moderate and high N application rates (Figure 2). These results confirm that a higher LAI of HQIR might lead to canopy closure and result in further aggravated carbohydrate metabolism under a high N rate [19,36]. Generally, the grain–leaf area ratio, including the spikelets–leaf area ratio, filled grains–leaf area ratio, and grain weight–leaf area ratio, is an important parameter to evaluate the relationship between source and sink, and is taken as a comprehensive index to breed and select high-yielding rice varieties [41]. Thereby, it is an efficient approach to improving rice grain yield by increasing the grain–leaf area ratio [42]. In our study, grain–leaf ratios were significantly decreased in HQIR, and there was no effect in OQIR under a high N rate compared with a moderate N rate (Table 7). The results indicate that a high N rate may result in an imbalance of the source and sink relationship in HQIR. On the other hand, HQIR had a higher or close N uptake at the heading, but a lower pre-anthesis N exportation, as well as a higher Pn, at the late-grain filling stage, than OQIR under a high N rate (Table 6 and Table 8). The results suggest that the carbon and nitrogen metabolisms of HQIR could be disturbed by an excessive N fertilizer input, resulting in a lower supply of carbohydrate for panicle [43]. In comparison with 2021, grain setting rate and grain yield of HQIR and OQIR were significantly decreased in 2020, which resulted from a lower daily average temperature (4.2 °C) and daily average solar radiation (3.8 MJ m⁻²) in September 2020. In the present study, the heading dates of HQIR and OQIR varieties were ranged from 8 September to 11 September. Thus, the abnormal climates in September 2020 might reduce rice-pollen fertility, as well as assimilate accumulation, and then lead to a significant decrease in the grain setting rate and grain weight [44]. Nevertheless, the agronomical and physiological mechanism underlying the lower grain setting rate of HQIR under a high N rate, needs further studying in the future.

5. Conclusions

The present study showed that the yield responses of HQIR and OQIR to moderate and high N application rates were different, which is mainly explained by the difference in spikelets m⁻² and/or grain setting rate. Compared with a moderate N rate, a slightly decreased grain yield of HQIR was due to a decreased grain setting rate, whereas a significantly increased grain yield of OQIR was attributed to increased spikelets m⁻² under a high N rate. A high N rate reduced pre-anthesis AE and its contribution, as well as the grain–leaf area ratio of HQIR, but did not increase its post-anthesis dry matter, compared with a moderate N rate. Therefore, the mechanism underlying the lower grain setting rate of HQIR under a high N rate, might be aggravated by carbohydrate metabolism and an imbalance of the source–sink relationship. These results suggest that a moderate N rate is beneficial for the HQIR varieties to balance the maximum grain yield and high quality.