Effect of long-term heat stress on grain yield, pollen grain viability and germinability in bread wheat (Triticum aestivum L.) under field conditions

Effect of high temperature on pollen grains and yield in economically important crops: a review

MAIN CONCLUSION

This review explores how climate change affects plant reproductive structures and causes significant yield loss, and discusses the effect of high temperatures on pollen viability, tube length, and germination percentage. Climate change-induced extreme heat and drought increasingly threaten plant growth and development, significantly impacting sexual reproduction. Heat and drought stress can disrupt key stages of plant sexual reproduction, including flowering time, gametophyte development, pollination, and seed formation, leading to infertility and substantial yield reductions in crops. A key consequence is compromised agricultural productivity and heightened food insecurity. The productivity in terms of crop yield is reduced due to a direct correlation between phenology and climate change. The reproductive organs of a plant and other parameters that define good fertility of a species are all affected by the increasing temperatures during their vegetative and reproductive phases of growth and development. This review dissects the detrimental effects of high temperatures on pollen grain viability, germination, and morphology, directly translating to yield reductions in major crops. It underscores the critical role of pollen viability and germination studies as potential tools for identifying heat-tolerant genotypes crucial for future food security. We delve into the intricate details of high-temperature stress's impact on pollen across various developmental stages, emphasizing the paramount importance of pollen studies as a criterion for heat tolerance in economically important crops within the context of climate change.

Impact of High-Temperature Stress on Maize Seed Setting: Cellular and Molecular Insights of Thermotolerance

Global warming poses a significant threat to crop production and food security, with maize (Zay mays L.) particularly vulnerable to high-temperature stress (HTS). This review explores the detrimental impacts of elevated temperatures on maize development across various growth stages, analyzed within the source-sink framework, with a particular focus on seed setting and yield reduction. It provides a broad analysis of maize cellular and molecular responses to HTS, highlighting the key roles of plant hormone abscisic acid (ABA) signaling, calcium signaling, chloroplast, and the DNA damage repair (DDR) system in maize. HTS disrupts ABA signaling pathways, impairing stomatal regulation and reducing water-use efficiency, while calcium signaling orchestrates stress responses by activating heat shock proteins and other protective mechanisms. Chloroplasts, as central to photosynthesis, are particularly sensitive to HTS, often exhibiting photosystem II damage and chlorophyll degradation. Recent studies also highlight the significance of the DDR system, with genes like ZmRAD51C playing crucial roles in maintaining genomic stability during reproductive organ development. DNA damage under HTS conditions emerges as a key factor contributing to reduced seed set, although the precise molecular mechanisms remain to be fully elucidated. Furthermore, the review examines cutting-edge genetic improvement strategies, aimed at developing thermotolerant maize cultivars. These recent research advances underscore the need for further investigation into the molecular basis of thermotolerance and open the door for future advancements in breeding thermotolerant crops.

Impacts of increased temperatures on floral rewards and pollinator interactions: a meta-analysis

Flowering plants produce pollinator rewards such as nectar and pollen, whose quantity and quality usually depend on the whole-plant state under specific environmental conditions. Increasing aridity and temperature linked to climate change may force plants to allocate fewer resources to these traits, potentially disrupting plant-pollinator interactions. In this study, for the first time, both quantitative review (vote-counting procedure) and meta-analytic approach were used to assess the implications of increased temperatures linked to global warming on floral rewards, including nectar (sugar concentration, content, and volume) and pollen (germination and viability), as well as on pollinator visits. Furthermore, we explored whether observed effects of warming are related either to temperature range, plant type (wild vs crop), or study approach (greenhouse vs field experiments). We also assessed the correlations between elevated temperatures and the characteristics that were affected by the temperature range. The results of the vote-counting technique showed that higher temperatures led to a decrease in floral rewards but did not affect the number of pollinator visits. Concurrently, meta-analysis detected adverse effects of warming on pollen germination and viability. Warming effects depended on the plant type for pollen germination and viability, on study approach for nectar sugar concentration and pollen germination, and on temperature range for pollen germination and pollinator visits. Additionally, we found that pollen germination and pollinator visits significantly decreased as temperature range increased. Our results showed that global warming affects floral rewards in both wild and crop plants, providing insights into the effects of changing climatic conditions on plant-pollinator interactions and pollination services.

Physical map of QTL for eleven agronomic traits across fifteen environments, identification of related candidate genes, and development of KASP markers with emphasis on terminal heat stress tolerance in common wheat

KEY MESSAGE

Key message This study identified stable QTL, promising candidate genes and developed novel KASP markers for heat tolerance, providing genomic resources to assist breeding for the development of high-yielding and heat-tolerant wheat germplasm and varieties. To understand the genetic architecture of eleven agronomic traits under heat stress, we used a doubled-haploid population (177 lines) derived from a heat-sensitive cultivar (PBW343) and a heat-tolerant genotype (KSG1203). This population was evaluated under timely, late and very late sown conditions over locations and years comprising fifteen environments. Best linear unbiased estimates and a genetic map (5,710 SNPs) developed using sequencing-based genotyping were used for QTL mapping. The identified 66 QTL (20 novel) were integrated into wheat physical map (14,263.4 Mb). These QTL explained 5.3% (QDth.ccsu-4A for days to heading and QDtm.ccsu-5B for days to maturity) to 24.9% (QGfd.ccsu-7D for grain filling duration) phenotypic variation. Thirteen stable QTL explaining high phenotypic variation were recommended for marker-assisted recurrent selection (MARS) for optimum/heat stress environments. Selected QTL were validated by their presence in high-yielding doubled-haploid lines. Some QTL for 1000-grain weight (TaERF3-3B, TaFER-5B, and TaZIM-A1), grain yield (TaCol-B5), and developmental traits (TaVRT-2) were co-localized with known genes. Specific known genes for traits like abiotic/biotic stress, grain quality and yield were co-located with 26 other QTL. Furthermore, 209 differentially expressed candidate genes for heat tolerance in plants that encode 28 different proteins were identified. KASP markers for three major/stable QTL, namely QGfd.ccsu-7A for grain filling duration on chromosome 7A (timely sown), QNgs.ccsu-3A for number of grains per spike on 3A, and QDth.ccsu-7A for days to heading on 7A (late and very late sown) environments were developed for MARS focusing on the development of heat-tolerant wheat varieties/germplasm.

Unlocking genetic insights: Evaluating wheat RILs for physiobiochemical traits under terminal heat stress conditions

BACKGROUND

The increasing impacts of heat stress on wheat production due to climate change has entailed the development of heat-resilient crop varieties. To address this, two hundred recombinant inbred lines (RILs) derived from a cross between WH711/WH1021 were evaluated in a randomized block design (RBD) with two replications at CCSHAU, Hisar, during 2018-19 under heat stress and non-stress conditions. Heat stress was induced by altering the date of sowing so that the grain filling stage coincide with heat stress.

RESULTS

Heat stress adversely affects RILs performance, as illustrated by alterations in phenotypic traits. Highest coefficients of variations were recorded for TAA, CTD 1, WUE, CTD 2, Cc and A under non-stress and heat stress conditions whereas gs, WUEi and GY under non-stress and SPAD 1, SPAD 2, GY and NDVI 2 under heat-stress conditions recorded moderate estimates of coefficient of variations. CTD 2, TAA, E, WUE and A displayed a significant occurrence of both high heritability and substantial genetic advance under non-stress. Similarly, CTD 2, NDVI 2, A, WUEi, SPAD 2, gs, E, Ci, MDA and WUE exhibited high heritability with high genetic advance under heat-stress conditions.

CONCLUSIONS

Complementary and duplicate types of interactions with number of controlling genes were observed for different parameters depending on the traits and environments. RILs 41, 42, 59, 74, 75, 180 and 194 were categorized as heat tolerant RILs. Selection preferably for NDVI 1, RWC, TAA, A, E and WUEi to accumulate heat tolerance favorable alleles in the selected RILs is suggested for development of heat resilient genotypes for sustainable crop improvement. The results showed that traits such as such as NDVI, RWC, TAA, A, E, and WUEi, can be effective for developing heat-resilient wheat genotypes and ensuring sustainable crop improvement.

QTL mapping: insights into genomic regions governing component traits of yield under combined heat and drought stress in wheat

Drought and heat frequently co-occur during crop growth leading to devastating yield loss. The knowledge of the genetic loci governing component traits of yield under combined drought and heat stress is essential for enhancing the climate resilience. The present study employed a mapping population of 180 recombinant inbred lines (RILs) derived from a cross between GW322 and KAUZ to identify quantitative trait loci (QTLs) governing the component traits of yield under heat and combined stress conditions. Phenotypic evaluation was conducted across two consecutive crop seasons (2021-2022 and 2022-2023) under late sown irrigation (LSIR) and late sown restricted irrigation (LSRI) conditions at the Indian Council of Agricultural Research Institute-Indian Agricultural Research Institute (ICAR-IARI), New Delhi. Various physiological and agronomic traits of importance were measured. Genotyping was carried out with 35K SNP Axiom breeder's genotyping array. The linkage map spanned a length of 6769.45 cM, ranging from 2.28 cM/marker in 1A to 14.21 cM/marker in 5D. A total of 35 QTLs were identified across 14 chromosomes with 6B containing the highest (seven) number of QTLs. Out of 35 QTLs, 16 were major QTLs explaining the phenotypic variance greater than 10%. The study identified eight stable QTLs along with two hotspots on chromosomes 6B and 5B. Five QTLs associated with traits thousand-grain weight (TGW), normalized difference vegetation index (NDVI), and plant height (PH) were successfully validated. Candidate genes encoding antioxidant enzymes, transcription factors, and growth-related proteins were identified in the QTL regions. In silico expression analysis highlighted higher expression of transcripts TraesCS2D02G021000.1, TraesCS2D02G031000, TraesCS6A02G247900, and TraesCS6B02G421700 under stress conditions. These findings contribute to a deeper understanding of the genetic architecture underlying combined heat and drought tolerance in wheat, providing valuable insights for wheat improvement strategies under changing climatic conditions.

Magnesium fertilization reduces high-temperature damages during anthesis in spring wheat (Triticum aestivum L.) by affecting pollen viability and seed weight

Terminal heat during reproductive stages of wheat (Triticum aestivum L.) limits the productivity of the crop. Magnesium (Mg) is an essential macronutrient that is involved in many physiological and biochemical processes to affect photosynthesis and seed weight. The present study comparatively evaluated Mg applied to soil (80 kg MgSO4·7H2O ha-1) and to plant foliage (4% w/v) in improving wheat performance under terminal heat. Wheat crop was grown in two sets of treatments until the booting stage, and then one set of plants was shifted to a glasshouse (±5 °C) at the booting stage to grow until maturity in comparison to control plants kept under ambient warehouse condition. Heat stress reduced the pollen viability while foliar- and soil-applied Mg improved it by 3% and 6% under heat stress, respectively, compared to the control without Mg treatment. The 100-seed weight, spike length, and biological yield reduced by 39%, 19%, and 50% under heat stress; however, foliar and soil application increased 100-seed weight by 45% and 40%, spike length by 8% and 5%, and biological yield by 35% and 25% under heat stress, respectively. Soil Mg showed maximum SPAD chlorophyll values; however, response was statistically similar to that of foliar Mg as compared to the control without Mg supply. Membrane stability decreased (4%) due to heat stress while foliar and soil treatments improved membrane stability by 8% and 5% compared to that of the control, respectively. Thus, Mg application through soil or plant foliage can be an effective way to reduce negative impacts of terminal heat in wheat by improving pollen viability at anthesis and 100-seed weight that was attributed to increased chlorophyll contents during anthesis.

Pollen viability-based heat susceptibility index (HSIpv): A useful selection criterion for heat-tolerant genotypes in wheat

Terminal heat stress during reproductive stage in wheat (Triticum aestivum L.) causes pollen grain sterility and has a drastic impact on wheat crop production. Finding genotypes with high pollen viability under heat stress is crucial to cope with the impact of climate change through developing heat-tolerant cultivars. To assess the effect of terminal heat stress on pollen viability in a panel of spring wheat genotypes (N = 200), RCBD (randomized complete block design) field trials were conducted under normal and heat stress conditions for two consecutive years (2020-2021 and 2021-2022). Analysis of variance showed significant variation in genotypes, treatments, and genotype × treatment interaction. Fifty and 46 genotypes were categorized as heat tolerant (HSI _pv < 0.5) in the first and second year, respectively. Twelve genotypes, namely, Chenab-70, Pari-73, Pak-81, MH-21, Punjab-76, NIFA-Aman, NUWYT-63, Swabi-1, Nisnan-21, Frontana, Amin-2000, and Pirsabak-2004, were found to be heat tolerant across the years. The violin plot displayed a trend of improvement in heat tolerance (HSI _pv < 0.5) over the period of time in many modern wheat varieties. However, some modern wheat varieties released after 2001 such as Janbaz-09 (57%), Ghazi-2019 (57%), and Sindhu-16 (43%) had very low pollen viability under heat stress conditions. The results of phenotypic coefficient of variance (PCV%), genotypic coefficient of variance (GCV%), broad sense heritability (h² _bs), and genetic advance (GA) suggested the major contribution of genetic factors in controlling pollen viability trait. Higher values of h² _bs and GA under heat stress conditions suggested pollen viability as a heat tolerance trait controlled by additive genetic effects. Taken together, these results suggested pollen viability as a useful trait for selection in early generations under elevated temperatures. The genotypes identified as heat tolerant in both years can be used as genetic resources for breeding cultivars with higher pollen viability under elevated temperature conditions.

Strengthening leaf physiological functioning and grain yield formation in heat-stressed wheat through potassium application

Wheat crops are highly sensitive to high temperatures during their reproductive and grain-filling phases. We hypothesized that potassium could increase thermotolerance in wheat during grain filling by protecting cellular organelles, particularly chlorophyll, from heat injury. Two wheat genotypes, Ujala-16 (relatively heat tolerant) and Anaj-17 (relatively susceptible) were grown in pots and were submitted to 4 and 8 days of heat stress under polythene sheets 1 week after anthesis. One day before the onset of heat stress, 2% potassium (K) as K₂SO₄ was sprayed on all the plants. Flag leaves from both genotypes were collected after 4 and 8 days of heat stress. Leaf physiology changes were measured to quantify heat damage and to understand the K-induced recovery mechanism. The crop was harvested 125 days after sowing, and grain yield data were collected. Increasing duration of heat stress significantly impaired leaf physiology and grain yield of both studied wheat genotypes. Compared with control (under optimum temperature), 4 and 8 days heat-stressed plants produced 11 and 19% lesser grain yield per spike (averaged across genotypes and in the second years of study), respectively. Likewise, 4- and 8-days heat-stressed plants had 15 and 37% (averaged across genotypes and in the second years of study) lower flag leaf photosynthesis, respectively, compared with control plants. Across the genotypes, 8-days heat caused significantly more grain yield loss in Anaj-17 during the second year than in Ujala-16. Foliar K significantly restored leaf chlorophyll, Pn, Fv/Fm by reducing cellular membrane damage in the heat-stressed plants. This physiological recovery and activation of the plant defensive system by K under high-temperature stress protected the growth and grain development. For example, K_-treated plants produced 19% higher 1,000 grain weight in 8 days of heat stress (across genotypes and in the second years of study) compared with water-treated plants under the hot environment of the respective thermal regime. Our study suggests that wheat performance under terminal heat stress can be improved through the exogenous application of K.