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Article

Optimizing Rice Yield and Heat Stress Resilience Through Nitrogen Top Dressing Before Panicle Emergence

by
Shafiqullah Aryan
1,2,*,
Gulbuddin Gulab
2,
Safiullah Habibi
3,*,
Tayebullah Zahid
2,
Zabihullah Safi
2,
Nasratullah Habibi
1,
Abdul Basir Mahmoodzada
4,
Mohammad Wasif Amin
1,
Ijaz Ahmad Samsor
1 and
Kenji Erie
1
1
Department of International Agricultural Development, Graduate School of Agriculture, Tokyo University of Agriculture, Tokyo 156-8502, Japan
2
Faculty of Agriculture, Nangarhar University, Jalalabad 2601, Afghanistan
3
Faculty of Agriculture, Tokyo University of Agriculture and Technology (TUAT), Saiwai-cho 3-5-8, Fuchu-shi, Tokyo 183-8509, Japan
4
Faculty of Engineering Geology and Mines, Jowzjan University, Sheberghan 1901, Afghanistan
*
Authors to whom correspondence should be addressed.
Nitrogen 2025, 6(2), 40; https://doi.org/10.3390/nitrogen6020040
Submission received: 10 March 2025 / Revised: 16 May 2025 / Accepted: 22 May 2025 / Published: 29 May 2025
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Abstract

The increased frequency of extreme heat stress events due to climate change is adversely impacting rice yield. Nitrogen (N) is an essential element in the synthesis of chlorophyll in rice, contributing substantially to the achievement of spikelet fertility and addressing the high yields. Two experiments were conducted in Japan and Afghanistan in 2020 and 2022, respectively, utilizing IR64 and Nipponbare (NPB) varieties to elucidate the efficacy of N top-dressing on spikelet fertility and yield of rice under heat stress conditions. In experiment I, the treatments involved were based on N application before panicle emergence in pots, including (1) control (fertilized at the tillering stage), (2) control + N topdressing, (3) heat stress (fertilized at the tillering stage), and (4) heat stress + N topdressing. Experiment II consisted of (1) control (basal dressing at the tillering stage) and (2) control + N topdressing, which was conducted under field conditions. Results showed that N application significantly (p < 0.05) increased SPAD values and spikelet fertility rates in both experiments. A positive correlation (range; r = 0.83–0.98) was observed between enhanced SPAD values and spikelet fertility rates in IR64 and NPB rice varieties under both ambient and heat stress conditions. Moreover, there were notable increases in photosynthetic rate (7.4% to 52.6%) and leaf transpiration. N top dressing significantly (p < 0.05) increased the panicle length, panicle weight, number of secondary branches/panicle, filled grain/panicle, total spikelets/panicle, and yield/plant. However, there was no significant difference in the number of primary branches per panicle and 1000-grain weight. In addition, the number of unfilled grains/panicle decreased from 5.5 to 49.7% with N top dressing in both experiments. Applying N as a top dressing improved the spikelet fertility percentage and other yield components, resulting in a high yield/plant.

1. Introduction

The global population is experiencing rapid growth and is expected to reach 8.5 billion by 2030, 9.7 billion by 2050, and 10.4 billion by the year 2100 [1]. To provide sustenance for the projected 9.1 billion people in 2050, there is a need for a substantial 70% increase in current food production [2]. Asia dominates rice production, contributing 89.9% of global production, surpassing other continents. The Americas follow with 4.9%, Africa with 4.7%, and Europe with 0.5% sequentially [3]. Of total rice production, 85% is allocated for human consumption, compared to wheat at 72% and maize at 19%, contributing to 21% of global per capita energy intake and 15% of per capita protein consumption [4]. In Afghanistan, rice is the second staple food [5], with an average annual milled rice consumption rate of 16.7 kg/person [6]. However, the production rate is 440 thousand metric tons (MT), while the total consumption rate is 655 thousand MT [7]. Consequently, a deficit gap of 215 thousand MT exists between the production and consumption rates. The required rice is imported from neighboring countries to cater to the demand.
Climate change poses a substantial threat to worldwide food security. Among the many factors contributing to climate change, increasing temperatures present severe risks to human development [8,9,10] and food production [11]. Global temperatures have increased by around 1 °C (likely between 0.8 °C and 1.2 °C) to 2017 and are expected to reach 1.5 °C by around 2040 [12]. Exceeding critical temperature thresholds not only shortens the growth period of rice crops [13] but also increases spikelet sterility (unfilled grain), resulting in low-quality grain and crop yield [14,15,16,17,18,19,20,21]. Rice exhibits varied responses to high temperatures at different growth stages. Particularly, heat stress during the seedling stage shows a relatively higher tolerance [16] than during the flowering stage [22,23,24]. In Afghanistan, the critical high temperature during the rice reproductive stage exceeds 35 °C [25]. Thus, the average yield of paddy rice in 2010 was 2.8 t/ha, markedly below the consumption rate [5].
Nitrogen (N) is the most essential nutrient for rice, promoting increased plant height, larger leaf size, more panicles, and improved overall yield [26]. To ensure high-yielding and quality crops while promoting environmental sustainability, it is essential to balance N application with the appropriate supply of other crucial nutrients like phosphorus (P) and potassium (K) [27]. In contrast, 10–60% of improperly applied N in rice fields may be lost through ammonia volatilization, a major pathway of N loss [28]. Excessive N utilization in high rice cultivation areas, like China, can lead to pollution, risking soil, water, and air quality [29] and low nutrient use efficiency [30]. Hence, new strategies need to be adopted to optimize the amount and timing of resource utilization, considering the response of rice in terms of spikelet fertility rate and physiological attributes under heat stress conditions.
A higher rate of N application substantially increases the SPAD (Soil and Plant Development Analysis) value, an indicator of chlorophyll content. [31,32] and is the best approach to monitoring N level in rice leaves [31]. Conversely, a low soil N content reduces SPAD values, decreasing rice yield [33]. The SPAD value indicates the greenness of chlorophyll pigment as reflected by light in the leaves. At the booting stage of rice, the substantial application of N at rates of 225 kg ha−1 and 300 kg ha−1 significantly enhances photosynthesis rates compared to the heading and maturity stages [34]. Elevated chlorophyll content is essential to achieve a high photosynthesis rate, as it plays a crucial role in the absorption, transmission, and transformation of light energy during photosynthesis [35]. Since N is a primary component of chlorophyll content, adequate N in the soil is ensured [36]. Nevertheless, the absence of N for crop growth diminishes photosynthesis efficiency and hinders the synthesis of carbohydrates, leading to reduced yield [37].
An optimized N fertilizer application increases spikelet number, panicle length, and the number of secondary branches, which are all major indicators of enhanced spikelets [38]. However, excessive N input may result in inadequate grain filling of spikelets at the panicle base, ultimately causing a decline in spikelet fertility [39]. Hence, it is necessary to understand the proper amount of N application and its best utilization time for increasing spikelet fertility, particularly under heat-stress conditions.
This study aimed to explore the response of rice to N application prior to panicle emergence, focusing on physiological traits and spikelet fertility rates under both glasshouse and field conditions. To achieve this, we examined SPAD values, physiological characteristics, and spikelet fertility in two types of rice genotypes, Indica and Japonica, each subjected to varying levels of N application in Japan and Afghanistan. Therefore, the study proposed that nitrogen application before panicle emergence benefits rice crops facing heat stress and enhances spikelet fertility rates, ultimately leading to higher yields.

2. Materials and Methods

2.1. Cultivation and Heat Treatment

Two experiments were carried out in 2020 (experiment I) at the Tokyo University of Agriculture (Setagaya Campus, Tokyo, Japan) and in 2022 (experiment II) at Nangarhar University Faculty of Agriculture (NUFA), Afghanistan. The coordinates for the Tokyo University of Agriculture are latitude 35.6411° N, longitude 139.6321° E, and an altitude of 60 m, while the coordinates for the NUFA are latitude 34.4773° N, longitude 70.3672° E, and an altitude of 595 m. Japan and Afghanistan were selected for research on rice cultivation under climate change-induced heat stress. Japan reflects a temperate region with advanced farming systems, while Afghanistan represents semi-arid to arid conditions with limited resources and high exposure to extreme heat and water scarcity (Figure 1). This contrast enabled observation of varied heat stress responses in rice varieties and evaluation of trait performance across different environments. These diverse settings enhance the general applicability of our findings and support the development of targeted breeding approaches and farming practices for specific areas. This research employed two distinct genotypes, namely Nipponbare (NPB, Japonica type) and IRRI-64 (IR64, Indica type) rice varieties. Based on our 2019 experiment results, we selected NPB (which showed heat sensitivity) and IR64 (which displayed moderate heat tolerance during flowering) for this study to determine if nitrogen application can mitigate heat stress effects in these varieties. Both experiments involved treating the rice seeds with 0.1% Benlate fungicide for 24 h, followed by a two-day pre-germination process under running water.
In experiment I, three seedlings per pot and one seedling per hill were transplanted into Wagner’s 1/2000a pots and positioned under natural conditions. This study contained four treatments: (1) application of 25 kg ha−1 chemical fertilizer at the tillering stage (control), and the composition of the nutrients were N2:P2O5:K2O (21%:17.5%:50%); (2) control + 25 kg N ha−1 (urea used, containing 21% N) was applied directly in the soil 1.5 weeks before the panicle emergence, in this study used as a top dressing for treatment 1 (control + TD); (3) application of 25 kg ha−1 chemical fertilizer at the tillering stage (the composition fertilizer was same as treatment 1) and the pots were placed under heat stress at glasshouse (HS); and (4) HS + 25 kg N ha−1 (urea used, containing 21% N) applied 1.5 weeks before the panicle emergence as a top dressing (HS + TD). The plants were subjected to the HS one day before panicle emergence in the glasshouse until harvesting. A thermo recorder (TR-72WB; ThermoWorks, American Fork, UT, USA) was positioned 1 m above the soil surface for temperature recording, and the data were collected hourly. As the effects of top dressing occur primarily during the flowering stage, we measured the average daytime and nighttime conditions for both ambient and control setups (Table 1). The initial soil pH and electrical conductivity (EC) were assessed following the protocols outlined by the Ministry of Agriculture, Forestry, and Fishery (MAFF) of Japan [40]. The three soil samples were taken with a depth of 0–30 cm from the different parts of upland soil provided by the company. Twenty grams of soil samples were crushed and sieved with a 0.5 mm particle size. Each sample received 50 mL of ultra-pure water and was placed in a shaker (Bio shaker, BR-15 LF, Tokyo, Japan) set at 180 rpm at 25 °C for 30 min. After allowing the soil particles to settle for 1 h, soil pH was measured using a pH meter (HORIBA Scientific, LAQUAact, Japan). Following pH measurement, an additional 50 mL of ultra-pure water was added to the same samples and shaken for 30 min. EC was measured using an EC meter (HIROBA, cond meter, Japan). For N content measurement, 20 mg of soil sample was weighed, and N content was determined using an Automatic Highly Sensitive NC Analyzer (SUMIGRAPH, NCH-22F, Tokyo, Japan). As a result, the initial soil pH was 6.4, electrical conductivity (EC) at 12.39 ms/m, and N content at 0.19% were recorded.
In experiment II, the same two varieties used in experiment I were assessed in a 2 m × 3 m concrete plot freshly filled with clay loam soil to a depth of 1.2 m. One rice seedling (21 days old) per hill was transplanted into the concrete plot on the first week of June 2022 and harvested in October 2022, with a 15 cm × 30 cm spacing between plants and rows, respectively. In this experiment, two treatments were applied under field conditions, involving (1) a local dose of chemical fertilizer (urea 46%, 25 kg N ha−1) at the tillering stage (control) and (2) control + the same dose of N fertilization 1.5 weeks before panicle emergence as a top-dressing (control + TD). Each treatment was performed in three blocks, with four replications for each block. In addition, three soil samples were taken with a 0–30 cm depth for the initial soil test. The samples were sent to the Shesham Bagh Regional Station for measuring the pH, EC, and N content. The procedure followed as per [41]. Initial soil pH was 7.9, EC was 1.09 (dS/m), and available N (kg ha−1) was 203.0. Irrigation was performed as required from cultivation up to one week before harvesting. Weed management was conducted through hand weeding twice during the growing season. For detailed information on the natural water availability (precipitation) and water losses, we examined the growing degree days (GDD). Figure 1A illustrates the experimental location and digital elevation model (DEM), accumulative rain (mm) (Figure 1B), and accumulative reference evapotranspiration (mm) (Figure 1C) throughout experiment II. The GDD are used on the x-axis in Figure 1B and calculated as follows:
GDD = ∑ T max +   T min 2   -   T base
Tmax and Tmin are the daily maximum and minimum temperature values (°C), respectively [42], and Tbase is the base temperature considered as described by [43].
Nitrogen 06 00040 g001
Figure 1. This figure illustrates the geographic representation of the experiment II site locations, and the digital elevation model (DEM) (A), which depicts the temperature gradient based on elevation and area. It includes the minimum, average, and maximum temperatures (B,C), the cumulative rainfall and reference evapotranspiration (ETo) during the growing season of rice. The data are sourced from a global provider [44].
Figure 1. This figure illustrates the geographic representation of the experiment II site locations, and the digital elevation model (DEM) (A), which depicts the temperature gradient based on elevation and area. It includes the minimum, average, and maximum temperatures (B,C), the cumulative rainfall and reference evapotranspiration (ETo) during the growing season of rice. The data are sourced from a global provider [44].
Nitrogen 06 00040 g001

2.2. Physiological Attributes and SPAD Value

In this investigation, the physiological impact of heat stress and N response was assessed by examining key parameters such as photosynthesis (Pn), stomatal conductance (gs), transpiration rate (T), and leaf temperature (LT) at the flowering stage. Data were collected using the LCi-SD Portable Photosynthesis System (ADC Bioscientific, Hoddesdon, UK), with the leaf placed in the chamber for 1 min for parameter readings. Replications of all plants were measured under both heat stress and control conditions. The SPAD was estimated in the leaves using a SPAD meter (SPAD-502Plus; Spectrum Technologies, Aurora, IL). The SPAD meter records the difference in transmittance of red light (650 nm) and infrared light (940 nm) through the leaf tissue. This differential light transmittance reflects the leaf’s absorbance characteristics, which are influenced by chlorophyll and other pigments. As such, SPAD values provide an indirect, non-destructive estimate of relative chlorophyll content and, in some contexts, may correlate with leaf nitrogen content. However, this correlation is not universal and depends on developmental stage and environmental conditions. Nevertheless, SPAD readings are assumed to be an indirect indicator of chlorophyll content, light absorbance, and so “leaf greenness”. The main tiller’s flag leaf was used to record the SPAD value. Additionally, the spikelet opening time for both rice varieties was recorded both before and after 12:00 p.m.

2.3. Spikelet Fertility

All selected plants from each treatment were marked on their panicles to assess spikelet fertility. After reaching full maturity, the panicles were allowed to air dry at room temperature for two weeks. Subsequently, the spikelets were manually examined by gently pressing them with fingers to identify filled grains. The term spikelet fertility rate here means the number of grains set or matured seed. Spikelet fertility percentage was calculated using the following formula:
Spikelet fertility % = (filled grains/total number of spikelets per spike) × 100

2.4. Yield and Its Components

At the physiological maturity stage, rice plants were harvested to measure various yield components. Recorded parameters included the count of filled and unfilled grains per panicle, panicle weight, panicle length, the number of primary and secondary branches per panicle, total spikelets per panicle, 1000-grain weight, spikelet fertility percentage, and yield per plant for experiment I. However, in experiment II, the yield was measured based on kg per meter square.

2.5. Statistical Analysis

The data underwent analysis through analysis of variance (ANOVA) in experiment I, two independent samples t-test in experiment II, Pearson correlation, and correlation matrix analysis using R 3.6.2 statistical software. Means were assessed using Tukey’s test at the 0.05 significance level.

3. Results

3.1. Effects of High Temperature on Rice at the Anthesis Stage

In experiment I, the temperatures recorded during the rice flowering stage varied between control and heat stress conditions. According to the daytime mean temperatures shown in Table 1, the mean temperature was 4.9 °C higher for NPB and 5.6 °C higher for IR64 under heat stress compared to the control conditions. The air temperature, precipitation rate, and reference evapotranspiration rate for experiment II are illustrated in Figure 1B,C. However, both rice genotypes, NPB and IR64, flowered at the end of August and during September 2022, respectively.
The timing of rice spikelet opening is crucial for early morning pollination before temperatures reach a critical point. In this study, we quantified the number of open flowers before and after 12:00 p.m. The results revealed that the Indica-type rice exhibited a high percentage of flower opening before noon, while Japonica rice initiated late flower opening, continuing until 5–6 p.m. Specifically, the IR64 rice variety had 93.5% of its flowers open before 12:00 p.m., whereas NPB had only 18.1% flowering before noon, with the majority of flowers opening after this time.

3.2. Response of N Top Dressing to Heat Stress, SPAD Value, and Spikelet Fertility

The utilization of N before panicle emergence significantly impacted the SPAD value in experiments I and II under heat stress and control conditions. However, both experiments observed notable variations among rice genotypes, as illustrated in Figure 2A,C. In experiment I, in the NPB variety, the highest SPAD value was recorded under control conditions with N top-dressing (in control + TD treatment), followed by the Heat stress + TD treatment. The application of N increased the SPAD value in leaves by 23.3% in NPB and 17.5% in IR64 under heat stress + TD compared to the heat-stressed treatment. Conversely, there was a substantial difference in NPB and a significant (p < 0.05) difference in the IR64 rice variety under control and control + TD treatments. When considering the percentage difference, there was a 7.5% increase in SPAD value for NPB and an 18.3% increase for IR64 genotypes with N fertilizer application before panicle emergence, as shown in Figure 2C. Based on the overall physical appearance of the plants, the crop appears significantly healthier with N top dressing compared to the control treatment. A visual comparison of the heat stress and heat stress + TD treatments is presented in Figure 4 from experiment I.
The primary factors influencing spikelet fertility, a key attribute of this study, exhibited significant improvement with applying N as a top dressing. In experiment I, the control + TD and heat stress + TD treatments effectively increased the spikelet fertility percentage in rice varieties under heat stress and control conditions, as depicted in Figure 2B. In addition, the control treatment resulted in a lower spikelet fertility rate compared to control + TD under control conditions. In the NPB rice variety, there was a 2.1% difference with the application of N under control conditions, and a 7.9% difference was observed under heat-stress conditions. In comparison to the NPB variety, a significant (p < 0.05) difference was recorded for the spikelet fertility of the IR64 variety. The percentage difference between heat stress and heat stress + TD was 13.5% and 23.3%, respectively. In experiment II, a significant (p < 0.05) difference in spikelet fertility was observed in both rice varieties under the control and control + TD treatments, as depicted in Figure 2D. The percentage difference for the NPB rice variety was 5.2%, while for the IR64 variety, it was 8.2%.

3.3. Relationship of SPAD Value with Spikelet Fertility

A positive correlation was observed between the increment in SPAD values and the increase in spikelet fertility percentage for both NPB and IR64 rice varieties under both heat stress and control conditions in experiment I. This trend was also observed in experiment II, which was conducted under field conditions for the same rice varieties. A significant increase in SPAD values led to a notable rise in spikelet fertility in the heat stress + TD treatment compared to the heat stress treatment in both NPB and IR64 varieties. Additionally, in experiment I, IR64 demonstrated a higher percentage of spikelet fertility compared to NPB despite having a similar SPAD value, as depicted in Figure 3A,B. Similar trends were observed for control and control + TD treatments under control conditions in experiment I. However, in experiment II, the NPB variety exhibited a higher spikelet fertility percentage with top dressing (control + TD treatment) under field conditions than the IR64 variety (Figure 3C). This pattern aligns with the results observed in the control condition treatments of experiment I. Experiment I showed higher spikelet fertility compared to experiment II, as the temperature rate under the field condition in Afghanistan was higher than in Japan. Also, the NPB variety showed a higher spikelet fertility rate compared to the IR64.

3.4. Effects of High Temperature on Physiological Attributes and the Response of N Top Dressing

The key determinants of enhancing rice tolerance under heat stress conditions are its physiological characteristics. Notably, applying N fertilizer before panicle emergence significantly (p < 0.05) improved the photosynthetic rate under control and heat stress conditions. Statistical analysis revealed a significant difference (p < 0.05) among treatments, with the highest photosynthetic rates observed in the following sequence: control + TD, control, heat stress + TD, and heat stress for both NPB and IR64 varieties. In the case of the NPB variety, N application resulted in a 52.6% increase in photosynthetic rate under heat stress, while for IR64, the increase was 16.6% compared to the heat stress treatment. Similarly, under control conditions, N application led to a 7.4% increase for the NPB variety and a 20.2% increase for IR64 compared to the control group plants (Table 2).
Stomatal conductance rates aligned with photosynthetic rates in both rice genotypes. The control + TD and control treatments exhibited the highest stomatal conductance rates, surpassing those observed under heat stress conditions. The application of N demonstrated a significant difference compared to heat stress treatments for both rice genotypes under heat stress conditions. Furthermore, leaf temperature was higher in both heat stress treatments for NPB and IR64 rice varieties compared to control conditions. Notably, N application resulted in lower leaf temperatures under heat stress compared to heat stress treatment without N application, although this difference was not statistically significant (p > 0.05), as indicated in Table 2.
The application of N exerted positive effects on the transpiration rate under elevated temperature conditions for both rice genotypes, with variations among treatments. Figure 4, depicting the phenotypical appearance of the rice canopy, revealed that the heat stress + TD treatment appeared healthier and greener due to higher chlorophyll content, thereby exhibiting a higher transpiration rate than other treatments. The NPB variety displayed an 11.1% increase, and IR64 showed a 2.2% higher transpiration rate compared to the heat stress treatment under high-temperature conditions.

3.5. Effects of N Top Dressing on Yield Components and Grain Yield of Rice

Obviously, heat stress had a detrimental effect on rice yield and its components. However, the application of N fertilizer as a top dressing notably enhanced these parameters. In the first experiment, the number of panicles per plant exhibited an increase under the control + TD and heat stress + TD treatments. This increase was attributed to the influence of N on the production of new tillers (baby tillers), a variation that was significant when compared to treatments without N top dressing in both rice genotypes. Furthermore, under the field condition (in experiment II), no significant variation was observed in the number of panicles per plant between treatments, as indicated in Table 3. Both rice genotypes exhibited an increase in both panicle length and weight when N was applied before panicle emergence, with variations observed between treatments for both genotypes. The heat stress treatment particularly influenced the panicle weight and length. In experiment I, panicle length with top dressing under heat stress increased by 8.6% in NPB and 9.8% in IR64, while the difference was 1.9% (in NPB) and 8.5% (in IR64) under control conditions. In experiment II, panicle weight recorded an increase of 3.3% in NPB and 3.2% in IR64 under N top dressing compared to the control. For the panicle weight, 1.9% and 8.5% increases were observed for experiment I and 15.1% and 14.3% in experiment II for the NPB and IR64, respectively (Table 3).
The number of primary and secondary branches per panicle plays a pivotal role in achieving high rice plant yields. There was no significant difference (p < 0.05) in the number of primary branches per panicle across all treatments in both genotypes. However, improvements were still observed under heat stress and in control conditions in both experiments. Specifically, there was an increment in primary branches per panicle for IR64 (2.1%) and NPB (2.7%) under control conditions, while under heat stress, IR64 showed a 1.0% increase, and NPB exhibited a more substantial improvement with a 4.8% increase. In experiment II, there was a notable improvement of 4.2% in IR64 and 6.9% in NPB. In addition, N application before the panicle emergence stage significantly influenced the number of secondary branches, especially under heat-stress conditions. In experiment I, compared to the heat stress treatment, the heat stress + TD treatment increased the number of secondary branches per panicle in the NPB variety by 23.5%, while for the IR64 variety, the difference rate was 4.2%. In experiment II, IR64 showed a 0.9% increase, and NPB increased by 1.5% under N top dressing (Table 3).
The number of filled grains and total spikelets per panicle exhibited a significant increase (p < 0.05) in both rice genotypes under N application treatments, irrespective of heat stress or control conditions. Conversely, the number of unfilled grains per panicle was highest for the heat stress treatment among the various conditions. In the first experiment, the percentage difference indicated a substantial impact on IR64, with a 49.7% increase under heat stress + TD and a 28.9% increase under control + TD treatments compared to their respective controls. For NPB, the difference was 14.0% and 3.6% in heat stress + TD and control + TD, respectively. In the second experiment, IR64 exhibited an increase of 9.3%, and NPB showed a 5.5% increase compared to their controls. Furthermore, in line with our hypothesis, compared to their controls, the percentage of unfilled grains per panicle ranged from 8.7% to 69.0% in IR64 and NPB rice varieties under control + TD and heat stress + TD. A significant difference was observed in experiment II, with a 45.0% decrement in IR64 and 46.5% in NPB. Also, the number of total spikelets per panicle and 1000-grain weight recorded an increase in both experiments, as shown in Table 3.
The number of spikelets per panicle exhibited significant influence in both rice varieties under N top dressing. In experiment I, NPB and IR64 demonstrated a 7.3% and 23.0% increase in heat stress + TD and a 2.5% and 13.1% increase under control + TD compared to their respective controls, respectively. The second experiment had varying percentage differences, with increases of 8.9% in IR64 and 5.5% in NPB. Correspondingly, in alignment with the spikelet fertility percentage, the yield per plant experienced notable increases. In NPB, there was a 28.8% increase under heat stress + TD and a 50.2% increase under control + TD treatments. In IR64, the increase was 70.3% under heat stress + TD and 60.3% under control + TD. Furthermore, in experiment II, there were increases of 16.9% and 23.7% in yield per plant for IR64 and NPB, respectively (Table 3).

4. Discussion

Elevated temperatures experienced by rice plants during the flowering stage can cause substantial adverse impacts. High temperatures during this critical phase may diminish spikelet fertility, impeding the essential processes of pollination and fertilization and shortening the grain-filling period. Hence, this can significantly decline spikelet fertility and decrease grain yield [18,45,46,47]. The reproductive stage of rice is the most susceptible stage to high temperatures [48]. Therefore, heat can catastrophically impact and damage various attributes of rice. In addition, heat stress negatively affects physiological activities such as photosynthesis and restricts the nutrient use efficiency in rice [22,49]. N is principal among other nutrients, employing a significant influence on the yield and quality of rice [50], while insufficient levels of N can impose limitations on the growth and yield of rice [51], and yellow leaves [52]. Considering the above concerns and aiming for robust and healthy crop growth, applying a sufficient amount of N fertilizer is imperative to achieve the desired crop yield. Therefore, in this study, we suggested that the coincidence of heat stress and the rice flowering stage could potentially lead to substantial damage to the rice crop. Hence, we employed top-dressing of N fertilizer to enhance the nutritional support for rice, strengthening the crop’s defense system. According to [53,54], the early morning flowering trait in rice plants serves as a key mechanism to mitigate the adverse effects of heat stress, reducing spikelet sterility. In alignment with this finding, our study also illustrated that the flower opening time significantly influenced spikelet fertility percentages in Indica rice. The notable 75.4% difference in morning flowering between IR64 and NPB rice resulted in lower spikelet sterility for IR64, providing a strategic advantage against heat stress in the early morning compared to the NPB variety.
Research has demonstrated that elevating N rates positively impacts rice leaf SPAD readings, offering a valuable tool for determining more accurate N application rates. The SPAD meter proves reliable in analyzing N levels in rice, facilitating real-time N fertilization assessments, and enhancing overall N use efficiency [33]. The widespread application of SPAD measurements extends to monitoring rice N status and assessing N distribution within the rice canopy [32]. According to [55], optimizing the interaction of N levels can contribute to increased rice productivity and enhanced N use efficiency. Effective N management is crucial in optimizing rice yield and quality. Heat stress catastrophically decreased spikelet fertility in rice [17,56,57,58,59,60,61].
The application of N in rice substantially enhanced spikelet fertility under heat stress conditions, with obvious variations observed among different genotypes [62]. Under ambient environmental conditions, the application of N fertilizer during the early panicle stage has been demonstrated to elevate the number of spikelets [63]. Additionally, ensuring enough nutrient availability during the grain-filling stage has been shown to mitigate the occurrence of partially unfilled grains per panicle [40,64,65]. In this study, the application of N top dressing led to a significant increase in SPAD values for both rice genotypes in both experiments I and II. In experiment I, the control + TD and heat stress + TD treatments exhibited a significant (p < 0.05) enhancement in chlorophyll content compared to their respective non-top-dressed treatments (Figure 2A). Similarly, in experiment II, consistent results were observed under field conditions, demonstrating an elevation in SPAD values with N top dressing treatment compared to its control (Figure 2C). Aligned with the SPAD values, the fundamental characteristic of spikelet fertility showed a significant increase with top dressing under heat stress conditions. A notable difference ranging from 2.1% to 23.3% was evident between the IR64 and NPB varieties. Heat stress substantially reduced the percentage of spikelet fertility; however, the application of N top dressing effectively contributed to an increase in spikelet fertility rates in both experiments I and II (Figure 2B,D). There was a strong relationship between SPAD value and spikelet fertility rate with N top dressing, the relationship ranging as (r = 0.83–0.98), as shown in Figure 3 and Figure 5.
Rice plants exposed to increased temperature conditions exhibited a substantial reduction, ranging from 66.3% to 80.3%, in stomatal conductance rate within 5–7 days [66]. Heat stress significantly obstructs rice plants’ photosynthetic rate, causing a 40–60% reduction at the anthesis stage [67]. Notably, a heightened N level contributes to an enhanced photosynthesis rate, with N playing a crucial role in the CO2 assimilatory capacity in crops [68]. Ref. [69] emphasized that a greater accumulation of N before and during the ripening stage correlates with a heightened photosynthetic rate during rice ripening. Elevated temperatures serve as the primary restriction to stomatal conductance, consequently diminishing the gas exchange rate, a pivotal limiting factor in photosynthetic processes [70]. In this study, the photosynthetic rate, stomatal conductance, and transpiration rate significantly (p < 0.05) increased under N top dressing before panicle emergence. In contrast, leaf temperature was decreased. The findings of this study revealed a positive correlation between the photosynthetic rate and yield components, while a negative correlation was observed between leaf temperature and yield components, as depicted in Figure 5. Also, there was a strong relationship between the increment of photosynthesis rate and panicle weight (r = 0.96), filled grain (r = 0.96), spikelet fertility % (r = 0.85), and yield per plant (r = 0.8) (Figure 5).
The impacts of N on the length and weight of rice panicles were significant, as N applications exert influence on these attributes in rice plants. A study revealed a substantial interaction effect between N applications and N-biofertilization inoculations, specifically on the panicle length of rice plants [71]. In addition, the panicle weight demonstrated an increase with rising N levels, but this trend did not persist beyond a certain threshold. This suggests a non-linear relationship between N levels and panicle weight [72]. As per [73], the utilization of N and animal manure have been associated with enhanced tiller number, increased panicle length, elevated total spikelet number per panicle, and enhanced grain yield in paddy rice. Additionally, ref. [74] proposed that the application of N fertilizer induces an increase in cytokinin biosynthesis within plants, leading to an accumulation of cytokinin that correlates with an increased number of spikelets per panicle and branches. Moreover, ref. [40] found that the application of optimized N fertilizer resulted in elevated secondary branches per panicle and enhanced yield per plant. In our study, the application of N top dressing significantly (p < 0.05) increased the panicle weight, panicle length, secondary branches per panicle, filled grain per panicle, total spikelet per panicle, spikelet fertility %, and yield per plant. The number of secondary branches per panicle, which was strongly correlated with filled grains per panicle and total spikelets per panicle, resulted in a high yield per plant, as shown in Figure 5. These findings provide valuable insights into managing heat stress through supplying nitrogen fertilizer as a nutrient source to maximize rice yield and heat stress resilience.

5. Conclusions

The results of this study demonstrate that applying N fertilizer before panicle emergence as a top-dressing enhanced SPAD value and improved photosynthetic processes. Furthermore, it resulted in a higher number of panicles per plant and an increased number of secondary branches per panicle, which is crucial for achieving high yields. In experiment I, applying N top-dressing resulted in a 28.8% increase in rice yield under heat stress and a 50.2% increase under control conditions for the NPB variety. For IR64, the yield increment was 70.3% under heat stress and 60.3% under control conditions. However, in experiment II, both IR64 and NPB showed increases in yield by 16.9% and 23.7% under field conditions, respectively. This study suggests that applying N before panicle emergence significantly boosts rice spikelet fertility rates and yields, particularly under heat-stressed conditions. This is the best and innovative method for practically applying this technique to help farmers reduce the impacts of heat stress on rice. Future investigations should delve into varying N fertilizer rates (splits) and assess diverse rice genotypes with various degrees of heat tolerance for a comprehensive understanding of their interactions.

Author Contributions

Conceptualization, S.A., G.G., T.Z., Z.S. and M.W.A.; methodology, S.A., S.H., G.G., M.W.A., N.H., T.Z., A.B.M., I.A.S. and K.E.; software, S.H., A.B.M., T.Z., N.H., I.A.S. and S.H.; validation, S.A., G.G., T.Z. and S.H.; formal analysis, S.A., S.H., I.A.S. and Z.S.; investigation, S.H., K.E. and G.G.; resources, S.A., K.E., G.G., N.H. and M.W.A.; data curation, S.A., S.H., M.W.A. and N.H.; writing—original draft preparation, S.A., G.G. and S.H.; writing—review and editing, K.E., S.H., M.W.A. and I.A.S.; visualization, S.H., Z.S., N.H., A.B.M. and G.G.; supervision, K.E., G.G. and N.H.; project administration, S.A. and K.E.; funding acquisition, S.A. and K.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All the source data are used in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Nitrogen 06 00040 g002
Figure 2. (A,B): experiment I and (C,D): experiment II. These figures illustrate the impact of N top dressing on SPAD value and spikelet fertility percentage in Indica and Japonica rice. The effects of N top dressing on the spikelet fertility of Indica and Japonica rice are shown here. The bars in the graph show the mean value along with the standard deviation of the mean (SD). Different letters above SD and the symbol * indicate significant differences between treatments at the p < 0.05 level. ns: indicates not significant. The TD stands for the top dressing.
Figure 2. (A,B): experiment I and (C,D): experiment II. These figures illustrate the impact of N top dressing on SPAD value and spikelet fertility percentage in Indica and Japonica rice. The effects of N top dressing on the spikelet fertility of Indica and Japonica rice are shown here. The bars in the graph show the mean value along with the standard deviation of the mean (SD). Different letters above SD and the symbol * indicate significant differences between treatments at the p < 0.05 level. ns: indicates not significant. The TD stands for the top dressing.
Nitrogen 06 00040 g002
Nitrogen 06 00040 g003
Figure 3. The relationship between SPAD value and spikelet fertility was investigated under heat stress and control conditions, specifically focusing on N top dressing. In experiment I, all four treatments, control, control + TD, heat stress, and heat stress + TD, were illustrated in the (A,B) sections of the figure for NPB and IR64 rice genotypes, along with their corresponding r and p-values. However, the (C) section of the figure represents experiment II. The TD stands for the top dressing. The significance level was determined at p < 0.05, with **, and *** indicating p < 0.01, and p < 0.001, respectively.
Figure 3. The relationship between SPAD value and spikelet fertility was investigated under heat stress and control conditions, specifically focusing on N top dressing. In experiment I, all four treatments, control, control + TD, heat stress, and heat stress + TD, were illustrated in the (A,B) sections of the figure for NPB and IR64 rice genotypes, along with their corresponding r and p-values. However, the (C) section of the figure represents experiment II. The TD stands for the top dressing. The significance level was determined at p < 0.05, with **, and *** indicating p < 0.01, and p < 0.001, respectively.
Nitrogen 06 00040 g003
Nitrogen 06 00040 g004
Figure 4. A comprehensive model elucidates N application’s impact on spikelet fertility and yield in rice. The application of top-dressing N notably mitigated the consequences of heat stress, thereby influencing SPAD value, physiological characteristics, and spikelet fertility rate in rice. The red arrows indicate negative effects. The pointed red arrow highlights the physiological attributes that are affected.
Figure 4. A comprehensive model elucidates N application’s impact on spikelet fertility and yield in rice. The application of top-dressing N notably mitigated the consequences of heat stress, thereby influencing SPAD value, physiological characteristics, and spikelet fertility rate in rice. The red arrows indicate negative effects. The pointed red arrow highlights the physiological attributes that are affected.
Nitrogen 06 00040 g004
Nitrogen 06 00040 g005
Figure 5. The correlation coefficient of rice’s physiological attributes and yield component parameters under heat stress and N top dressing at the booting stage. The significance level was determined at p < 0.05, with ✩, ✩✩, and ✩✩✩ indicating p < 0.05, p < 0.01, and p < 0.001, respectively.
Figure 5. The correlation coefficient of rice’s physiological attributes and yield component parameters under heat stress and N top dressing at the booting stage. The significance level was determined at p < 0.05, with ✩, ✩✩, and ✩✩✩ indicating p < 0.05, p < 0.01, and p < 0.001, respectively.
Nitrogen 06 00040 g005
Table 1. Temperature rate in ambient and under glasshouse conditions during rice flowering stage.
Table 1. Temperature rate in ambient and under glasshouse conditions during rice flowering stage.
TreatmentMean Daytime
Temperature (°C)		Mean Nighttime
Temperature (°C)
NPBIR64NPBIR64
Ambient29.625.625.722.8
HS34.531.227.024.9
Table 2. Effects of heat stress on the physiological characteristics of Indica and Japonica rice genotypes, and the response of rice crops to N application as a top dressing before panicle emergence.
Table 2. Effects of heat stress on the physiological characteristics of Indica and Japonica rice genotypes, and the response of rice crops to N application as a top dressing before panicle emergence.
VarietyTreatmentPhotosynthetic Rate
(μmol CO2 m−2 s−1)		Stomatal Conductance
(mol CO2 m−2 s−1)		Transpiration Rate
(μmol m−2 s−1)		Leaf Temperature (°C)
NPBControl7.5 ± 0.65 a0.17 ± 0.02 a4.5 ± 1.18 a40.1 ± 1.53 ab
Control + TD8.1 ± 1.01 a0.12 ± 0.01 a4.8 ± 1.04 a40.9 ± 1.82 ab
Heat stress 1.8 ± 0.70 b0.09 ± 0.03 ab5.6 ± 1.54 a43.2 ± 1.45 a
Heat stress + TD3.8 ± 1.28 b0.11 ± 0.04 a6.3 ± 1.78 a43.0 ± 1.34 a
Significance****ns*
IR64Control7.5 ± 0.78 a0.20 ± 0.02 a2.4 ± 0.86 b36.7 ± 1.78 b
Control + TD9.4 ± 1.87 a0.21 ± 0.04 a4.1 ± 1.41 ab36.3 ± 1.75 b
Heat stress 5.5 ± 1.40 ab0.05 ± 0.04 b8.6 ± 1.15 a43.1 ± 2.13 a
Heat stress + TD6.6 ± 1.14 a0.11 ± 0.03 ab8.8 ± 1.87 a42.8 ± 2.13 a
Significance*****
The data presented in this table represent the mean values accompanied by their respective standard deviations. Distinct letters signify variations among treatments. The significance level was determined at p < 0.05, with *, **, and *** indicating p < 0.05, p < 0.01, and p < 0.001, respectively. “ns” was used to indicate no significance.
Table 3. The influence of N application on yield components and overall yield in both Indica and Japonica rice is examined in experiments I and II.
Table 3. The influence of N application on yield components and overall yield in both Indica and Japonica rice is examined in experiments I and II.
Variety TreatmentNPPPL (cm)PW (g)NPBPNSBPFGPUGPTSP1000-GW (g)YP (g)
NPBExperiment IControl6.4 ± 0.8 b20.2 ± 0.6 a1.88 ± 0.10 a8.2 ± 0.3 a15.7 ± 1.2 a63.3 ± 3.6 a7.1 ± 1.9 b69.0 ± 3.1 a54.8 ± 0.5 a21.7 ± 2.1 b
Control + TD12.8 ± 2.0 a20.5 ± 0.5 a1.78 ± 0.10 a8.6 ± 0.2 a16.1 ± 1.0 a65.6 ± 2.9 a4.2 ± 2.1 b69.8 ± 1.8 a51.0 ± 2.3 a43.7 ± 8.3 a
Heat stress 5.6 ± 0.51b17.3 ± 0.4 b0.70 ± 0.05 b7.4 ± 0.4 a9.1 ± 0.7 b21.3 ± 1.4 b29.7 ± 2.4 a51.0 ± 2.5 b48.7 ± 5.5 a5.7 ± 1.7 c
Heat stress + TD8.2 ± 1.2 ab18.8 ± 0.2 ab0.76 ± 0.06 b7.6 ± 0.2 a11.9 ± 1.1 ab24.3 ± 4.1 b27.3 ± 2.2 a51.6 ± 1.3 b44.1 ± 2.3 a8.0 ±1.9 c
Significance*******ns************ns***
Experiment IIControl27.8 ± 9.3 ns26.0 ± 1.0 ns2.5 ± 0.3 ns11.3 ± 1.1 *19.1 ± 4.0 ns97.9 ± 10.7 *19.3 ± 8.4 *116.5 ± 15.4 ns34.1 ± 4.9 *93.1 ± 16.8 *
Control TD28.1 ± 12.926.8 ± 1.32.9 ± 0.312.1 ± 1.119.4 ± 3.7103.3 ± 13.113.1 ± 4.9117.2 ± 12.737.4 ± 6.0115.2 ± 30.5
IR64Experiment IControl10.9 ± 0.9 b22.7 ± 0.4 b1.99 ± 0.13 b9.5 ± 0.2 a25.0 ± 1.8 b75.4 ± 4.6 b31.5 ± 7.7 ab106.9 ± 8.6 a46.2 ± 0.7 a77.3 ± 5.8 a
Control + TD22.0 ± 1.4 a24.7 ± 0.3 a2.54 ± 0.12 a9.6 ± 0.1 a29.3 ± 1.0 a97.2 ± 5.5 a22.3 ± 2.7 b119.5 ± 4.3 a46.4 ± 0.5 a92.0 ± 7.6 a
Heat stress 10.0 ± 1.0 b22.4 ± 0.5 b1.42 ± 0.04 c9.3 ± 0.3 a24.8 ± 1.9 b46.2 ± 2.5 c53.5 ± 6.8 a99.7 ± 5.0 ab46.0 ± 4.3 a22.9 ± 3.0 b
Heat stress + TD20.6 ± 1.7 a24.6 ± 0.5 a1.95 ± 0.10 b9.5 ± 0.1 a25.9 ± 2.2 b69.2 ± 4.3 b44.6 ± 4.4 ab113.8 ± 4.5 a52.1 ± 5.3 a35.6 ± 6.1 b
Significance*********ns*******ns***
Experiment IIControl34.8 ± 5.5 ns19.9 ± 1.6 ns2.3 ± 0.5 ns8.7 ± 0.5 ns22.2 ± 3.9 ns92.1 ± 9.6 *27.8 ± 8.2 ***119.9 ± 11.3 ns40.9 ± 6.1 ns126.9 ± 23.6 *
Control + TD34.2 ± 10.720.6 ± 1.12.6 ± 0.29.1 ± 0.822.4 ± 2.3100.6 ± 12.119.2 ± 4.3120.1 ± 10.942.9 ± 3.5148.5 ± 29.8
The data presented in this table represent the mean values accompanied by their respective standard deviations. Distinct letters signify variations among treatments. NPP: number of panicles per plant, PL: panicle length, NPBP: number of primary branches per panicle, NSBP: number of secondary branches per panicle, FGP: filled grains per panicle, UFP: unfilled grains per panicle, TSP: total number of spikelets per panicle, 1000-GW: 1000-grain weight, and YP: yield per plant. The significance level was determined at p < 0.05, with *, **, and *** indicating p < 0.05, p < 0.01, and p < 0.001, respectively. “ns” was used to indicate no significance.
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Aryan, S.; Gulab, G.; Habibi, S.; Zahid, T.; Safi, Z.; Habibi, N.; Mahmoodzada, A.B.; Amin, M.W.; Samsor, I.A.; Erie, K. Optimizing Rice Yield and Heat Stress Resilience Through Nitrogen Top Dressing Before Panicle Emergence. Nitrogen 2025, 6, 40. https://doi.org/10.3390/nitrogen6020040

AMA Style

Aryan S, Gulab G, Habibi S, Zahid T, Safi Z, Habibi N, Mahmoodzada AB, Amin MW, Samsor IA, Erie K. Optimizing Rice Yield and Heat Stress Resilience Through Nitrogen Top Dressing Before Panicle Emergence. Nitrogen. 2025; 6(2):40. https://doi.org/10.3390/nitrogen6020040

Chicago/Turabian Style

Aryan, Shafiqullah, Gulbuddin Gulab, Safiullah Habibi, Tayebullah Zahid, Zabihullah Safi, Nasratullah Habibi, Abdul Basir Mahmoodzada, Mohammad Wasif Amin, Ijaz Ahmad Samsor, and Kenji Erie. 2025. "Optimizing Rice Yield and Heat Stress Resilience Through Nitrogen Top Dressing Before Panicle Emergence" Nitrogen 6, no. 2: 40. https://doi.org/10.3390/nitrogen6020040

APA Style

Aryan, S., Gulab, G., Habibi, S., Zahid, T., Safi, Z., Habibi, N., Mahmoodzada, A. B., Amin, M. W., Samsor, I. A., & Erie, K. (2025). Optimizing Rice Yield and Heat Stress Resilience Through Nitrogen Top Dressing Before Panicle Emergence. Nitrogen, 6(2), 40. https://doi.org/10.3390/nitrogen6020040

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