Next Article in Journal
DSW-YOLO-Based Green Pepper Detection Method Under Complex Environments
Previous Article in Journal
Genetic Dissection of the Powdery Mildew Resistance in a Cultivated Emmer Wheat Accession
Previous Article in Special Issue
Changes of Catalase and Peroxidase Activity and Expression Under Cold Stress in Prunus persica Cultivars with Different Cold Tolerances
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Agronomy
Volume 15
Issue 4
10.3390/agronomy15040982
agronomy-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress

by
Beibei Qi
1,2,
Simin Cheng
1,
Youjin Song
3,
Chao Wu
2,* and
Meng Yang
1,*
1
Guangxi Key Laboratory of Agro-Environment and Agric-Products Safety, College of Agriculture, Guangxi University, Nanning 530004, China
2
Guangxi Key Laboratory of Functional Phytochemicals Research and Utilization, Guangxi Institute of Botany, Chinese Academy of Sciences, Guilin 541006, China
3
College of Agronomy, Nanjing Agricultural University, Nanjing 210095, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(4), 982; https://doi.org/10.3390/agronomy15040982
Submission received: 21 February 2025 / Revised: 10 April 2025 / Accepted: 15 April 2025 / Published: 18 April 2025
(This article belongs to the Special Issue Plant Ecophysiology Under Anthropogenic and Natural Stresses)
Browse Figures
Versions Notes

Abstract

:
Heat stress during the flowering stage induces a remarkable decrease in rice spikelet fertility, mainly due to poor pollination manifesting as insufficient pollen deposited on the stigma. It is hypothesized that stigma exsertion, which confers a pollination advantage, may enhance pollen reception and improve female reproductive success under heat stress. The present study aimed to investigate the role of stigma exsertion in spikelet fertility under nocturnal heat. Four rice cultivars exhibiting distinct heat tolerance and twenty rice cultivars with varying degrees of stigma exsertion were grown and subjected to high nighttime temperature treatment at anthesis, in 2023 and 2019, respectively. Heat-tolerant rice cultivars had a relatively low percentage of spikelets with exserted stigmas, and vice versa. Under nocturnal heat stress, rice cultivars exhibiting higher stigma exsertion showed significantly greater reductions in spikelet fertility compared to lower stigma exsertion cultivars. The spikelet fertility of rice cultivars with a higher degree of stigma exsertion was reduced more seriously than that of cultivars with a lower degree of stigma exsertion. Rice spikelet fertility positively correlated with the percentage of hidden stigmas, and exogenous substance-induced increased stigma exsertion led to reduced spikelet fertility under nocturnal heat. These results indicate that a hidden stigma contributes to higher spikelet fertility, while increased stigma exsertion aggravates spikelet sterility in rice cultivars under nocturnal heat conditions. It is proposed that hidden stigmas could serve as a novel breeding trait for sustaining rice spikelet fertility against nocturnal heat stress.

1. Introduction

Global mean surface temperatures have risen steadily, accompanied by a more frequent occurrence of extreme heat events, within the context of global warming. Notably, the phenomenon of global warming exhibits diurnal asymmetry [1], characterized by a more pronounced increase in nighttime temperatures compared to daytime temperatures [2]. During the mid-summer of 2022, extensive regions in southern China experienced unprecedented heat events, with minimum nighttime temperatures exceeding 30 °C. Projections from climate modeling studies indicate that by the 2090s, the annual frequency of such nocturnal heat events could increase by up to 67.4%, coupled with a near doubling of the average temperature from 20.4 °C in the 2010s to 39.7 °C in the 2090s [3]. Consequently, heat stress, especially nocturnal heat, has attracted considerable attention across diverse scientific disciplines [4,5,6].
Heat stress poses a significant adverse impact on agricultural productivity [7,8]. Regardless of carbon dioxide fertilization, effective adaptation, and genetic improvement, each 1 °C increment in global mean temperature averagely reduces global yields of maize, wheat, and soybean by 7.4%, 6.0%, and 3.1%, respectively [9]. Noteworthy, an 1 °C increment in the nighttime temperature and daytime temperature can precipitate up to 7% and 6% losses in rice grain yields when the optimum temperatures are exceeded [10]. As a globally essential staple crop, rice plants are particularly vulnerable to heat stress, especially during the critical flowering stage [11]. The optimum nighttime temperature range for robust rice growth is approximately 22 °C [10], and remarkable reductions in spikelet fertility were observed when nighttime temperature exceeded 30 °C during the flowering stage [4,12]. Hot nights have thus been regarded as invisible agricultural natural disasters.
The mechanisms of nocturnal heat effects on rice spikelet fertility have attracted great attention and are emphasized as a key area for understanding heat responses. Previous studies on the heat’s effects on rice during the flowering stage have predominantly focused on daytime temperature regimes [13,14,15]. It is acknowledged that pollination failure is the key issue underlying heat-induced lower spikelet fertility at flowering [16]. The pollination process depends on the quantity and quality of pollens released from the anther and successfully deposited on the stigmas [17]. The mechanisms underlying heat-induced pollination failure have been extensively explored in terms of the behaviors of male floral organs [18], such as anther dehiscence and pollen vigor, but comparatively less attention has been paid to the female floral organ—the stigma [19], which serves as the recipient of pollen grains and the locus of pollen germination.
Stigma exsertion, a phenomenon wherein the stigma protrudes from the palea and lemma of the spikelet after floret opening, plays important roles in pollination and fertilization in rice plants [20]. The pollination advantages of the exserted stigmas lie in (i) an enlarged spatial foundation for pollination [21], (ii) an extended pollination period [11], and (iii) a potential pollen population effect evoked by increased pollen density [17]. It is postulated that stigma exsertion increases pollen count on the stigma and boosts pollen germination rates, potentially alleviating heat-induced injury to spikelet fertility [11]. However, according to our previous observations across five rice cultivars, high stigma exsertion impaired spikelet fertility under high daytime temperatures [20], which diminishes the viability of both exserted stigmas and pollens, thereby constraining the pollination benefits of exserted stigmas.
In contrast with daytime heat stress, nocturnal heat exhibits limited direct impact on stigma pollination, which occurs at flowering time usually during the forenoon. Thus, the roles of the exserted stigma in relation to spikelet fertility seem to diverge under asymmetric heat. The impact of nocturnal heat on the pollination benefits conferred by exserted stigmas is relatively gentle compared to that of daytime heat stress. The pollination benefits associated with exserted stigma are more prone to be realized under nocturnal heat than under daytime heat. We hypothesized that high stigma exsertion confers advantages in boosting spikelet fertility under nocturnal heat conditions. The objective is to validate the role of stigma exsertion on spikelet fertility in rice exposure to nocturnal heat stress, across various rice cultivars. The findings will contribute to novel insights for mitigating injury on pollination and spikelet fertility caused by global nighttime warming at flowering in rice plants.

2. Materials and Methods

2.1. Experiment I: Nocturnal Heat Effects on Spikelet Fertility in Contrasting Rice Cultivars in Plant Growth Chambers

During the 2023 rice-growing season, a pot experiment was meticulously carried out at the Guangxi Institute of Botany, Chinese Academy of Science, Guilin city, Guangxi Province, China (25°4′ N, 110°18′ E). Four distinct rice cultivars were purposefully selected, including two high-quality inbred cultivars, Huanghuazhan (HHZ) and Yuzhenxiang (YZX), and two red rice cultivars, Guihong1 (GH1) and Hongnuo2 (HN2). According to our identification and previous results [22], the rice cultivars HHZ and HN2 demonstrated heat tolerance, while the rice cultivars YZX and GH1 were heat-susceptible, as quantified by spikelet fertility under heat conditions.
After the breaking of dormancy at 45 °C for 6 h, rice seeds were presoaked in tap water for a duration of 12 h. Subsequently, the seeds were transferred to a constant-temperature incubator (model DHP-360, Saidelisi Technology Co., Ltd., Tianjin, China) and maintained at a temperature of 37 °C to expedite the germination. At the three-leaf growth stage, three seedlings of each rice cultivar were transplanted into 12.7-L plastic pots, which had a height of 28.0 cm, a top diameter of 26.0 cm, and a base diameter of 22.0 cm. Each pot was filled with a homogeneous mixture of 12.0 kg of red soil and 6.0 g of compound fertilizer (N:P2O5:K2O ratio of 15%:15%:15%, Fuling compound fertilizer, Sinochem Chongqing Fuling Chemical Co., Ltd., Chongqing, China). The potted rice plants were randomly arranged under natural ambient conditions, with four replicates for each cultivar. Seedlings were thinned to two uniform and healthy plants per pot. The main tiller of each plant was labeled for easy identification and subsequent sampling. Two days after the thinning operation, a topdressing of 1.0 g of urea was applied to each pot for promoting tillering.
The rice plants were cultivated under natural ambient conditions, with regulated irrigation maintaining a water table depth of approximately 2.0 cm. To minimize positional effects, each pot was manually rotated 90 degrees clockwise at seven-day intervals. At anthesis, the cultivated pots of specific cultivars were transferred into temperature-controlled plant growth chambers (model RGLC-P500-D3, Hefei Dascate Biotechnology Co., Ltd., Hefei City, China). The YZX, HN2, HHZ, and GH1 plants initiated heading on September 3rd, 19th, 22nd, and 27th, respectively, and were subsequently relocated in chronological order of their heading dates. These plants were then subjected to a continuous exposure of temperature treatments for a period of seven days. After temperature treatments were completed, the plants were removed from the chambers and allowed to resume growing under normal and natural conditions. Stringent control measures were implemented to manage pests, diseases, avian interference, rodent damage, and weed growth.
The experimental design incorporated two temperature treatments. The high nighttime temperature (HNT) treatment involved the imposition of a high temperature from 19:00 to 07:00, and the control (CK) treatment maintained a favorable temperature for rice plant growth throughout the entire day. The temperatures were set at 32 °C from 07:00 to 19:00 and 22 °C from 19:00 to 07:00 for CK treatment, and were maintained at 32 °C from 07:00 to 19:00 and 30 °C from 19:00 to 07:00 for HNT treatment (Table 1), according to a systematic integration of accumulated research evidence [23]. The relative humidity (%) in the plant growth chambers was set at 60%. A mercury thermometer was placed inside of the greenhouse to monitor the temperature, which remained stable at the targeted temperatures during specific periods under certain temperature treatments.

2.2. Experiment II: Exploration of Stigma Exsertion on Spikelet Fertility Under Nocturnal Heat Using Diverse Rice Cultivars in the Greenhouse

During the 2019 rice-growing season, pot experiments were conducted at the white horse base of Nanjing Agricultural University, Nanjing city, Jiangsu Province, China (31°36′ N, 119°10′ E). A total of twenty rice cultivars exhibiting varying degrees of stigma exsertion (Table 2) according to our previous investigation were evaluated. The seedling establishments followed the processes described in experiment I. The plants were cultivated under natural ambient conditions and were then transferred into two temperature-controlled greenhouses (4.0 m length × 4.0 m width × 4.5 m height) one day before anthesis.
The greenhouses were equipped with a sophisticated climate control system (Nanjing Quanyou electronic technology Co., Ltd., Nanjing City, China). They were furnished with a central air conditioner (Refrigeration Copeland Compressor, Copeland, TX, USA), plant growth lights (Philips, Greenpower LED, Eindhoven, The Netherlands), ventilators, and sensors for monitoring relative humidity and air temperature. Two temperature treatments, HNT (30 °C during 07:00~19:00; 30 °C during 19:00~07:00) and CK (30 °C during 07:00~19:00; 24 °C during 19:00~07:00), were imposed (Table 1). The relative humidity (%) within the greenhouses was set at 70%. The plants were continuously exposed to temperature treatments for seven days. Air temperature and relative humidity were recorded using a standalone sensor (HOBO, H08-003-02, Onset Computer Corporation, Bourne, MA, USA). The recorded daily mean, maximum, and minimum temperatures were 27.2 °C, 31.0 °C, and 23.4 °C under CK treatment, and 30.5 °C, 31.4 °C, and 29.2 °C under HNT treatment, respectively. The mean relative humidity was 71.8% and 73.2% under CK and HNT treatments, respectively.

2.3. Experiment III: Verification of Roles of Stigma Exsertion on Spikelet Fertility Under Nocturnal Heat via Exogenous Substances Application

Two exogenous substances, gibberellin (GA3) and paclobutrazol (PAC) (Sigma-Aldrich, St. Louis, MO, USA), were employed to manipulate stigma exsertion in rice cultivars in the greenhouse during 2019. To prepare the solutions, 60 mg of GA3 and 60 mg of PAC were dissolved in 20 mL of ethanol, respectively, followed by dilution to a final volume of 1.0 L with double-distilled H2O. The solution was mixed thoroughly after addition of two drops of 0.01% (v/v) Tween®20 (Sigma-Aldrich, St. Louis, MO, USA) as surfactant. The GA3 and PAC solutions were then sprayed onto the rice panicles under the CK and HNT treatments, with a dosage of 20 mL per plant, respectively. The exogenous substances were applied twice, first at 09:00 on the day before temperature treatments and then again on the second day following the temperature treatment. During the application process, the treated plants were temporarily removed from the greenhouse to avoid drifting, and then were moved back into the greenhouse when exogenous application was completed. For the control plants, under both the CK and HNT treatments, the same volumes of double-distilled H2O containing the same concentration of Tween®20 were applied to ensure a valid comparison.

2.4. Determination of Stigma Exsertion and Spikelet Fertility

The samplings were repeated four times in the plant growth chamber and three times in the greenhouse. To determine the degree of stigma exsertion, the main tillers were harvested 15 days after heading from individual plants grown in different pots for each replication. The spikelets were categorized into three groups: spikelets with dual exserted stigmas (with two stigmas protruded from both sides of a spikelet), single exserted stigma (with a single stigma protruded from one side of a spikelet), and non-exserted stigma (with no stigma visible outside the spikelet). The spikelets were then tested by pressure between the forefinger and thumb. Spikelets containing a kernel were assigned as fertile, while those without were deemed sterile. The numbers of both fertile and sterile spikelets were recorded for each group. The percentage of spikelets with a single exserted stigma was calculated as the ratio of spikelets with a single exserted stigma to the total number of spikelets × 100%. The fertility of spikelets with a single exserted stigma was defined as the ratio of fertile spikelets with a single exserted stigma to total spikelets with a single exserted stigma × 100%. Similar calculations were performed for spikelets with dual exserted stigmas, those with non-exserted stigma, and for the entire panicles [20].

2.5. Data Analysis

An analysis of variance (ANOVA) was conducted using Statistix 9.0 (a software product of Analytical Software, Tallahassee, FL, USA). The results of the ANOVA indicated significant effects of temperature treatments, cultivars, and their interaction on stigma exsertion and spikelet fertility at significance levels of p < 0.05 and p < 0.01, respectively. To further analyze these differences, the least significant difference (LSD) test was performed at a significance level of p < 0.05. The means obtained from the analysis were used to estimate the relationships between the percentages of spikelets with exserted and hidden stigmas and spikelet fertility under the CK and HNT treatments. Figures were produced using the ggplot2 package in R statistical software (ver. 4.4.2 Analytical Software, Tallahassee, FL, USA).

3. Results

3.1. Variations in Spikelet Fertility Among Rice Cultivars with Different Stigma Exsertion

Substantial differences were discerned in both the percentage and fertility of spikelets with exserted or hidden stigmas among the various rice cultivars and between temperature treatments, except for spikelets with dual exserted stigmas, in both 2019 and 2023. Significant interactions between cultivar and treatment were observed for fertility of spikelets with exserted or hidden stigmas (except spikelets with single exserted stigmas in 2019) and percentage of spikelets with single exserted stigmas, but not for percentage of spikelets with dual exserted stigmas, in the two experimental years (Table 3). When rice cultivars were subjected to temperature treatments within plant growth chambers in 2023, heat-tolerant rice cultivars (HHZ, HN2) displayed a comparatively lower degree of stigma exsertion in contrast to heat-susceptible cultivars (GH1, YZX), especially under HNT treatment (Table 4). Under greenhouse conditions in 2019, the combined results of twenty rice cultivars indicated that the fertility of spikelets with a relatively low degree of stigma exsertion (stigma exsertion percentage < 50%) was markedly higher than that of spikelets with a high degree of stigma exsertion (stigma exsertion percentage > 50%), particularly under HNT treatment (Figure 1).

3.2. Correlation Between Spikelet Fertility and Stigma Exsertion

The percentages of spikelets with exserted and hidden stigmas exhibited negative and positive correlations, respectively, with the spikelet fertility of the entire panicle, irrespective of the temperature treatments. Specifically, under CK and HNT treatments, the percentage of spikelets with dual/single exserted stigmas demonstrated significant negative correlations with the spikelet fertility of the whole panicle at the significance levels of p < 0.05 and p < 0.01, respectively. Correspondingly, the percentage of spikelets with hidden stigmas displayed significant positive correlations with the spikelet fertility of the whole panicle at the significance levels of p < 0.05 and p < 0.01 under the CK and HNT treatments, respectively (Figure 2).

3.3. Manipulating Stigma Exsertion Alters Spikelet Fertility

The stigmatic exsertion of rice cultivars exhibited slight alteration under HNT treatment; however, they were substantially affected by exogenous chemical treatments. The proportions of spikelets displaying dual or single exserted stigmas were augmented after the application of GA3 and PAC within both CK and HNT treatments, with more pronounced increments being observed in the case of PAC treatment (Figure 3). In contrast, the fertility rates of these panicles were diminished following the application of GA3 and PAC under HNT treatment, and the reduction was statistically significant in the context of PAC treatment (Figure 4).

4. Discussion

Spikelet fertility has been widely acknowledged as a crucial metric for evaluating heat tolerance in rice cultivars. Cultivar variations in rice spikelet fertility were commonly observed under heat stress during the flowering stage [24]. In previous individual studies, significant alterations in spikelet fertility [25] or stigma exsertion [26] under daytime heat had been reported. In the current study, a remarkable finding was that spikelet fertility exhibited significant variations among contrasting rice cultivars with different stigma exsertion when exposed to nocturnal heat (Table 3). It has been well established that heat-induced reduction in spikelet fertility is primarily attributed to pollen failure, mainly due to male sterility during the flowering stage [11]. However, the roles of the stigma, which served as the receptor of pollen grains and the site of pollen germination [19], had received less attention under high-temperature regimes. Herein, particular attention was dedicated to exploring the relationships between spikelet fertility and stigma exsertion.
Under normal conditions, stigma exsertion enhanced pollination efficiency and significantly contributed to the breeding programs and seed yields in hybrid rice production [27]. However, it had long been inconclusive if stigma exsertion interacted with spikelet fertility in rice plants under heat conditions, especially under nocturnal heat. In tomato plants, it had been proposed that stigma exsertion should not be regarded as a criterion for heat identification [28]. Heat treatment promoted stigma exsertion in tomato plants [29]; however, the tomato stigma extended beyond the anthers, causing a loss of self-pollination advantage. In rice plants, the exserted stigmas are mostly positioned below the anthers, where they can readily receive pollen scattered from the anthers. However, hybrid rice cultivars tended to inherit the paternal stigma exsertion trait to a certain extent and exhibited higher percentage of stigma exsertion compared to inbred rice cultivars [11], yet they were more heat-susceptible in most cases [24].
Our previous study reported that an enclosed stigma contributed to higher rice spikelet fertility, using five rice cultivars subjected to daytime heat [20]. As a follow-up supplementary investigation using diverse rice cultivars, comprehensive evidence was gathered for elucidating the roles of stigma exsertion in relation to spikelet fertility under nocturnal heat. Firstly, the degree of stigma exsertion in heat-tolerant rice cultivars was overall lower than that in heat-susceptible cultivars, according to the performances of four rice cultivars exhibiting distinct heat tolerance in 2023 (Table 4). Secondly, the spikelet fertility of rice cultivars with a higher degree of stigma exsertion (stigma exsertion percentage > 50%) was reduced more drastically than that of cultivars with a lower degree of stigma exsertion (stigma exsertion percentage < 50%) under HNT treatment, as supported by the results of twenty rice cultivars of different stigma exsertions in 2019 (Figure 1). Thirdly, a more negative correlation was observed between rice spikelet fertility and percentage of stigma exsertion under HNT treatment (Figure 2). Fourthly, exogenous application of GA3 or PAC led to an augmented stigma exsertion (Figure 3) accompanied by a decreased spikelet fertility under HNT treatment (Figure 4).
The regulatory techniques for manipulating stigma exsertion were well established, such as exogenous GA3 and PAC (a GA3 biosynthesis inhibitor) applications, which increase or decrease stigma exsertion rates, respectively. Stigma exsertion in rice was increased by exogenous GA3 treatment in both the current study (Figure 3) and our previous research [20]. Similarly, increased stigma exsertion by GA3 application was previously reported in tomato plants [30]. However, in this study, PAC application unexpectedly increased stigma exsertion to some extent (Figure 3), which contradicted prior findings that PAC application reduced stigma exsertion in tomato under heat conditions [30]. This discrepancy may be attributed to differences in application methodologies. Nevertheless, our validations collectively support that lower stigma exsertion contributes to higher spikelet fertility, whereas an increase in stigma exsertion exacerbates spikelet sterility in rice cultivars under nocturnal heat. Noteworthy, both the present investigation and our prior research [20] consistently demonstrate that an enclosed stigma is beneficial for higher spikelet fertility in rice cultivars under heat stress, regardless of daytime heat or nocturnal heat, although the pollination advantages of exserted stigmas varied under asymmetric heat.
Previously, a higher pollen density was observed in exserted stigmas compared to hidden stigmas, where pollination failure was prone to occur due to insufficient pollen deposition [20]. However, under daytime heat stress, the low vitality of exserted stigmas and captured pollens, resulting from direct heat exposure, restricted the pollination advantage of exserted stigmas [11]. A similar pattern was observed under nocturnal heat, with more pollen grains deposited on exserted stigmas compared to hidden stigmas, yet weaker pollen vigor was observed in exserted stigmas relative to hidden types. As discussed above, it was speculated that nocturnal heat likely restrains the pollination advantage of exserted stigmas in a manner analogous to daytime heat, and hidden stigmas might confer some protection against the adverse effects of heat stress, such as diminished pollen/stigma viability and impaired pollination.
Heat-tolerant germplasms are valuable for breeding programs aiming to develop crops that better withstand high temperatures, using their natural heat resistance genes to create more climate-resilient plants [31]. The observed variations in stigma exsertion between heat-tolerant and heat-susceptible cultivars under nocturnal heat might imply that heat tolerance is associated with the capacity to maintain a more favorable stigma exsertion pattern. This could potentially serve as a morphological marker for screening heat-tolerant genotypes against asymmetric heat. The higher fertility of spikelets with a low degree of stigma exsertion, especially under nocturnal heat condition, suggests that a certain level of stigma protection or a more optimized pistil–anther spatial arrangement within the spikelet might contribute to improved reproductive success. In rice plants, stigma exsertion was controlled by polygenes, and quantitative trait loci controlling stigma exsertion such as qDSE1, qDSE8, etc. [32] has been newly identified and could be used in quantitative trait loci pyramiding strategies for producing heat-tolerant germplasms.

5. Conclusions

The spikelets with a low degree of stigma exsertion exhibited higher spikelet fertility compared with the spikelets with a high degree of stigma exsertion among and within rice cultivar exposure to nocturnal heat. Increased stigma exsertion induced by exogenous substance resulted in reduced spikelet fertility under nocturnal heat. Positive correlation was identified between rice spikelet fertility and percentage of hidden stigmas under nocturnal heat regimes. The multiple experiments supplied evidence that hidden stigmas contribute to higher spikelet fertility, and increased stigma exsertion exacerbates spikelet sterility in rice cultivars under nocturnal heat. It was proposed that strategic phenotyping of rice germplasms with low stigma exertion could offer novel breeding potential to counter warming night, an invisible disaster under global warming.

Author Contributions

C.W. designed the experiments; B.Q. and Y.S. performed the experiments; B.Q., S.C., C.W. and M.Y. analyzed the data and wrote the manuscript; B.Q., C.W. and M.Y. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (grant nos. 32160504, 32360528, 32001471), the Guangxi Natural Science Foundation (grants no. 2020GXNSFAA297027) and the Bagui Young Top-notch Talents Project.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets presented in this study are included in the main text.

Acknowledgments

We are deeply grateful to Yanfeng Ding and Ganghua Li (Nanjing Agricultural University) for providing access to their experimental facilities and technical support. We acknowledge the anonymous reviewers for their constructive comments, which greatly improved the quality of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Jiang, S.; Wang, K.; Mao, Y. Rapid local urbanization around most meteorological stations explain the observed daily asymmetric warming rates across China from 1985 to 2017. J. Clim. 2020, 33, 9045–9061. [Google Scholar] [CrossRef]
  2. Cox, D.T.C.; Maclean, I.M.D.; Gardner, A.S.; Gaston, K.J. Global variation in diurnal asymmetry in temperature, cloud cover, specific humidity and precipitation and its association with leaf area index. Glob. Change Biol. 2020, 26, 7099–7111. [Google Scholar] [CrossRef] [PubMed]
  3. He, C.; Kim, H.; Hashizume, M.; Lee, W.; Honda, Y.; Kim, S.E.; Kinney, P.L.; Schneider, A.; Zhang, Y.; Zhu, Y.; et al. The effects of night-time warming on mortality burden under future climate change scenarios: A modelling study. Lancet Planet. Health 2022, 6, e648–e657. [Google Scholar] [CrossRef] [PubMed]
  4. Sakai, H.; Cheng, W.; Chen, C.P.; Hasegawa, T. Short-term high nighttime temperatures pose an emerging risk to rice grain failure. Agric. For. Meteorol. 2022, 314, 108779. [Google Scholar] [CrossRef]
  5. Peng, S.; Piao, S.; Ciais, P.; Myneni, R.B.; Chen, A.; Chevallier, F.; Dolman, A.J.; Janssens, I.A.; Peñuelas, J.; Zhang, G.; et al. Asymmetric effects of daytime and night-time warming on Northern Hemisphere vegetation. Nature 2013, 501, 88–92. [Google Scholar] [CrossRef]
  6. Xiong, Z.; Zheng, F.; Wu, C.; Tang, H.; Xiong, D.; Cui, K.; Peng, S.; Huang, J. Nitrogen supply mitigates temperature stress effects on rice photosynthetic nitrogen use efficiency and water relations. Plants 2025, 14, 961. [Google Scholar] [CrossRef]
  7. Sadok, W.; Jagadish, S.V.K. The hidden costs of nighttime warming on yields. Trends Plant Sci. 2020, 25, 644–651. [Google Scholar] [CrossRef]
  8. Cabusora, C.C. Developing climate-resilient crops: Adaptation to abiotic stress-affected areas. Technol. Agron. 2024, 4, e005. [Google Scholar] [CrossRef]
  9. Zhao, C.; Liu, B.; Piao, S.; Wang, X.; Lobell, D.B.; Huang, Y.; Huang, M.; Yao, Y.; Bassu, S.; Ciais, P.; et al. Temperature increase reduces global yields of major crops in four independent estimates. Proc. Natl. Acad. Sci. USA 2017, 114, 9326–9331. [Google Scholar] [CrossRef]
  10. Su, Q.; Rohila, J.S.; Ranganathan, S.; Karthikeyan, R. Rice yield and quality in response to daytime and nighttime temperature increase–A meta-analysis perspective. Sci. Total Environ. 2023, 898, 165256. [Google Scholar] [CrossRef]
  11. Qi, B.; Wu, C. Potential roles of stigma exsertion on spikelet fertility in rice (Oryza sativa L.) under heat stress. Front. Plant Sci. 2022, 13, 983070. [Google Scholar] [CrossRef]
  12. Jagadish, S.V.K.; Murty, M.V.R.; Quick, W.P. Rice responses to rising temperatures-challenges, perspectives and future directions. Plant Cell Environ. 2015, 38, 1686–1698. [Google Scholar] [CrossRef] [PubMed]
  13. Wang, W.; Cui, K.; Hu, Q.; Wu, C.; Li, G.; Huang, J.; Peng, S. Response of spikelet water status to high temperature and its relationship with heat tolerance in rice. Crop J. 2021, 9, 1344–1356. [Google Scholar] [CrossRef]
  14. Yu, J.; Du, T.; Zhang, P.; Ma, Z.; Chen, X.; Cao, J.; Li, H.; Li, T.; Zhu, Y.; Xu, F.; et al. Impacts of high temperatures on the growth and development of rice and measures for heat tolerance regulation: A review. Agronomy 2024, 14, 2811. [Google Scholar] [CrossRef]
  15. Zhou, H.; Chen, X.; Li, M.; Shi, C.; Jiang, M. Simulation model for assessing high-temperature stress on rice. Agronomy 2024, 14, 900. [Google Scholar] [CrossRef]
  16. Wu, C.; Cui, K.; Tang, S.; Li, G.; Wang, S.; Fahad, S.; Nie, L.; Huang, J.; Peng, S.; Ding, Y. Intensified pollination and fertilization ameliorate heat injury in rice (Oryza sativa L.) during the flowering stage. Field Crops Res. 2020, 252, 107795. [Google Scholar] [CrossRef]
  17. Zhang, C.; Tateishi, N.; Tanabe, K. Pollen density on the stigma affects endogenous gibberellin metabolism, seed and fruit set, and fruit quality in Pyrus pyrifolia. J. Exp. Bot. 2010, 61, 4291–4302. [Google Scholar] [CrossRef]
  18. Shi, W.; Li, X.; Schmidt, R.C.; Struik, P.C.; Yin, X.; Jagadish, S.V.K. Pollen germination and in vivo fertilization in response to high temperature during flowering in hybrid and inbred rice. Plant Cell Environ. 2018, 41, 1287–1297. [Google Scholar] [CrossRef]
  19. Wang, Y.; Impa, S.M.; Sunkar, R.; Jagadish, S.V.K. The neglected other half-role of the pistil in plant heat stress responses. Plant Cell Environ. 2021, 44, 2200–2210. [Google Scholar] [CrossRef]
  20. Wu, C.; Cui, K.; Hu, Q.; Wang, W.; Nie, L.; Huang, J.; Peng, S. Enclosed stigma contributes to higher spikelet fertility for rice (Oryza sativa L.) subjected to heat stress. Crop J. 2019, 7, 335–349. [Google Scholar] [CrossRef]
  21. Molano-Flores, B.; Faivre, A.E. Sexual differences in Lobelia spicata populations: Floral morphometrics, stigma pollen loads, and pollen tube growth. Plant Species Biol. 2014, 30, 202–211. [Google Scholar] [CrossRef]
  22. Cao, Z.Z.; Zhao, Q.; Huang, F.D.; Wei, K.S.; Zaidi, S.H.R.; Zhou, W.J.; Cheng, F.M. Effects of high temperature at anthesis on spikelet fertility and grain weight in relation to floral positions within a panicle of rice (Oryza sativa L.). Crop Pasture Sci. 2015, 66, 922–929. [Google Scholar] [CrossRef]
  23. Sarsu, F. Screening protocols for heat tolerance in rice at the seedling and reproductive stages. In Pre-Field Screening Protocols for Heat-Tolerant Mutants in Rice; Springer International Publishing: Cham, Switzerland, 2018; pp. 9–24. [Google Scholar]
  24. Xiao, B.; Nan, B.; Zhang, F. Identification of heat tolerance of rice test varieties in the middle and lower reaches of the Yangtze river. China Rice 2022, 28, 21–26. [Google Scholar] [CrossRef]
  25. Rezaul, I.M.; Feng, B.; Chen, T.; Fu, W.; Zhang, C.; Tao, L.; Fu, G. Abscisic acid prevents pollen abortion under high-temperature stress by mediating sugar metabolism in rice spikelets. Physiol. Plant. 2019, 165, 644–663. [Google Scholar] [CrossRef] [PubMed]
  26. Ramesh, R.; Ramesh, T.; Rao, P.R.; Shankar, V.G.; Bhave, M.H.V. Study the effect of high temperature stress on pollen viability, stigma exertion, sterility in rice. J. Pharmacogn. Phytochem. 2017, 6, 2016–2024. [Google Scholar]
  27. Guo, H.; Liang, Y.; Lv, J.; Su, X.; Ren, G.; Gao, F. Research progress on the trait of stigma exsertion in rice. Plants 2024, 13, 3404. [Google Scholar] [CrossRef]
  28. Lohar, D.P.; Peat, W.E. Floral characteristics of heat-tolerant and heat-sensitive tomato (Lycopersicon esculentum Mill.) cultivars at high temperature. Sci. Hortic. 1998, 73, 53–60. [Google Scholar] [CrossRef]
  29. Pan, C.; Yang, D.; Zhao, X.; Jiao, C.; Yan, Y.; Lamin-Samu, A.T.; Wang, Q.; Xu, X.; Fei, Z.; Lu, G. Tomato stigma exsertion induced by high temperature is associated with the jasmonate signalling pathway. Plant Cell Environ. 2018, 42, 1205–1221. [Google Scholar] [CrossRef]
  30. Yan, W.; Changtian, P.; Jie, W.; Li, Q.; Tao, Z.; Gang, L. Effects of gibberellin on tomato stigma exsertion and hormone related gene expression under moderate heat stress. J. Zhejiang Univ. (Agric. Life Sci.) 2015, 41, 449–457. [Google Scholar] [CrossRef]
  31. Ndlovu, E.; Maphosa, M.; van Staden, J. Unlocking basal and acquired thermotolerance potential in tropical sorghum. Technol. Agron. 2024, 4, e026. [Google Scholar] [CrossRef]
  32. Liu, Y.; Zhang, A.; Wang, F.; Kong, D.; Li, M.; Bi, J.; Zhang, F.; Wang, J.; Luo, X.; Pan, Z.; et al. Fine mapping a quantitative trait locus, qSER-7, that controls stigma exsertion rate in rice (Oryza sativa L.). Rice 2019, 12, 46. [Google Scholar] [CrossRef] [PubMed]
Agronomy 15 00982 g001
Figure 1. Spikelet fertility of twenty rice cultivars with different stigma exsertions under temperature treatments in greenhouses during 2019. Different letters indicate significant differences among the temperature treatments for the rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment.
Figure 1. Spikelet fertility of twenty rice cultivars with different stigma exsertions under temperature treatments in greenhouses during 2019. Different letters indicate significant differences among the temperature treatments for the rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment.
Agronomy 15 00982 g001
Agronomy 15 00982 g002
Figure 2. Relationships between stigma exsertion and spikelet fertility. CK, control temperature treatment; HNT, high nighttime temperature treatment. Grey shallows indicate confidence interval.
Figure 2. Relationships between stigma exsertion and spikelet fertility. CK, control temperature treatment; HNT, high nighttime temperature treatment. Grey shallows indicate confidence interval.
Agronomy 15 00982 g002
Agronomy 15 00982 g003
Figure 3. Alterations of stigma exsertion induced by exogenous substance application under temperature treatments during 2019 in greenhouse. Different letters indicate significant differences among the exogenous chemical treatments and temperature treatments across rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment; H2O, control without chemical application; GA3, exogenous gibberellin A3 application; PAC, exogenous paclobutrazol application.
Figure 3. Alterations of stigma exsertion induced by exogenous substance application under temperature treatments during 2019 in greenhouse. Different letters indicate significant differences among the exogenous chemical treatments and temperature treatments across rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment; H2O, control without chemical application; GA3, exogenous gibberellin A3 application; PAC, exogenous paclobutrazol application.
Agronomy 15 00982 g003
Agronomy 15 00982 g004
Figure 4. Changes of spikelet fertility induced by exogenous substance application under temperature treatments during 2019 in greenhouse. Different letters indicate significant differences among the exogenous chemical treatments and temperature treatments across rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment; H2O, control without chemical application; GA3, exogenous gibberellin A3 application; PAC, exogenous paclobutrazol application.
Figure 4. Changes of spikelet fertility induced by exogenous substance application under temperature treatments during 2019 in greenhouse. Different letters indicate significant differences among the exogenous chemical treatments and temperature treatments across rice cultivars at the p < 0.05 level according to the LSD test. CK, control temperature treatment; HNT, high nighttime temperature treatment; H2O, control without chemical application; GA3, exogenous gibberellin A3 application; PAC, exogenous paclobutrazol application.
Agronomy 15 00982 g004
Table 1. Temperature settings of CK and HNT treatments.
Table 1. Temperature settings of CK and HNT treatments.
YearCK TreatmentHNT Treatment
07:00~19:0019:00~07:0007:00~19:0019:00~07:00
202332 °C22 °C32 °C30 °C
201930 °C24 °C30 °C30 °C
CK, control temperature treatment; HNT, high nighttime temperature treatment.
Table 2. Rice cultivars evaluated during 2019 in the greenhouse.
Table 2. Rice cultivars evaluated during 2019 in the greenhouse.
Rice CultivarsPercentage of Stigma ExsertionRice CultivarsPercentage of Stigma Exsertion
Zhongzheyou874.16%Wankennuo17.26%
Liangyoupeijiu72.31%Nangeng91082.43%
Yliangyou90069.90%Ninggeng72.36%
Taoyouxiangzhan68.76%Wuyungeng72.31%
Xiangliangyou90062.82%Zhendao181.88%
Zhaoyou543154.76%Huaidao50.67%
Jingliangyouhuazhan52.52%Ninggeng50.00%
Longliangyou53435.99%Nangeng50550.00%
Nagina2228.76%Zhendao140.00%
Shanyou6315.20%Huaidao130.00%
Table 3. Combined analysis of variance across twenty rice cultivars in greenhouses during 2019, and across four rice cultivars in plant growth chamber during 2023.
Table 3. Combined analysis of variance across twenty rice cultivars in greenhouses during 2019, and across four rice cultivars in plant growth chamber during 2023.
YearSource of VariationPercentage of Spikelets with Dual Exserted Stigmas (%)Percentage of Spikelets with Single Exserted Stigmas (%)Percentage of Spikelets with Exserted Stigmas (%)Percentage of Spikelets with Hidden Stigmas (%)Fertility of Spikelets with Dual Exserted Stigmas (%)Fertility of Spikelets with Single Exserted Stigmas (%)Fertility of Spikelets with Hidden Stigmas (%)Spikelet Fertility of the Whole Panicle
2023Cultivar
(V)		34.62 **94.38 **114.5 **114.53 **0.74 ns4.19 *2.69 ns4.00 *
Treatment
(T)		1.60 ns4.33 *5.59 *5.59 *11.7 **83.94 **48.1 **73.1 **
V × T0.22 ns3.07 *2.75 ns2.75 ns1.42 ns3.84 *3.55 *4.96 **
2019Cultivar
(V)		56.69 **256.20 **292.82 **292.82 **4.08 **15.90 **35.79 **33.64 **
Treatment
(T)		0.18 ns7.96 **5.69 *5.69 *2.97 ns21.57 **12.84 **20.39 **
V × T1.18 ns2.11 **0.71 ns0.71 ns0.68 ns1.74 ns3.02 **4.57 **
*, **, and ns indicate significance at the p < 0.05, p < 0.01 levels and nonsignificance, respectively.
Table 4. Performances of stigma exsertion and spikelet fertility of four rice cultivars exhibiting distinct heat tolerances under temperature treatments during 2023 in plant growth chambers.
Table 4. Performances of stigma exsertion and spikelet fertility of four rice cultivars exhibiting distinct heat tolerances under temperature treatments during 2023 in plant growth chambers.
CultivarTreat-mentPercentage of Spikelets with Dual Exserted Stigmas (%)Percentage of Spikelets with Single Exserted Stigmas (%)Percentage of Spikelets with Exserted Stigmas (%)Percentage of Spikelets with Hidden Stigmas (%)Fertility of Spikelets with Dual Exserted Stigmas (%)Fertility of Spikelets with Single Exserted Stigmas (%)Fertility of Spikelets with Hidden Stigmas (%)Spikelet Fertility of the Whole Panicle
GH1CK10.9 ± 4.10 a35.8 ± 4.19 a46.7 ± 1.03 a53.3 ± 1.03 c81.4 ± 5.46 a81.5 ± 4.09 ab82.2 ± 2.12 ab82.0 ± 1.22 ab
HNT12.1 ± 2.18 a35.5 ± 3.66 a47.6 ± 3.74 a52.4 ± 3.74 c62.3 ± 2.04 b66.1 ± 3.05 c68.3 ± 2.17 d66.8 ± 1.99 de
HN2CK0.00 ± 0.00 c4.80 ± 1.70 c4.80 ± 1.70 c95.2 ± 1.70 a—79.8 ± 5.58 ab79.6 ± 3.89 ab79.7 ± 3.62 ab
HNT0.00 ± 0.00 c4.20 ± 1.87 c4.20 ± 1.87 c95.8 ± 1.87 a—67.7 ± 5.24 c72.2 ± 7.59 cd72.0 ± 7.43 cd
HHZCK3.70 ± 2.73 b24.7 ± 4.44 b28.4 ± 6.78 b71.6 ± 6.78 b80.4 ± 14.2 a82.8 ± 2.15 a84.3 ± 3.87 ab83.8 ± 2.28 a
HNT4.80 ± 1.57 b27.4 ± 2.18 b32.2 ± 2.49 b67.8 ± 2.49 b75.5 ± 6.87 ab76.8 ± 2.24 b78.4 ± 6.23 bc77.8 ± 4.68 bc
YZXCK3.50 ± 2.03 b25.9 ± 6.70 b29.5 ± 7.58 b70.5 ± 7.58 b83.8 ± 19.7 a84.0 ± 3.53 a86.5 ± 3.31 a85.5 ± 3.84 a
HNT5.40 ± 2.53 b36.0 ± 4.64 a41.4 ± 7.06 a58.6 ± 7.06 c61.0 ± 8.81 b64.7 ± 5.10 c66.8 ± 5.98 d66.1 ± 3.40 e
The values are shown as the means ± SDs (n = 4). Different letters indicate significant differences among the temperature treatments for the rice cultivars at the p < 0.05 level according to the LSD test. GH1, Guihong1; HN2, Hongnuo2; HHZ, Huanghuazhan; YZX, Yuzhenxiang; CK, control temperature treatment; HNT, high nighttime temperature treatment. — indicate no data observed in rice cultivar HN2, which observed no dual exserted stigmas.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Qi, B.; Cheng, S.; Song, Y.; Wu, C.; Yang, M. Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress. Agronomy 2025, 15, 982. https://doi.org/10.3390/agronomy15040982

AMA Style

Qi B, Cheng S, Song Y, Wu C, Yang M. Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress. Agronomy. 2025; 15(4):982. https://doi.org/10.3390/agronomy15040982

Chicago/Turabian Style

Qi, Beibei, Simin Cheng, Youjin Song, Chao Wu, and Meng Yang. 2025. "Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress" Agronomy 15, no. 4: 982. https://doi.org/10.3390/agronomy15040982

APA Style

Qi, B., Cheng, S., Song, Y., Wu, C., & Yang, M. (2025). Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress. Agronomy, 15(4), 982. https://doi.org/10.3390/agronomy15040982

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop