Post‐flowering night respiration and altered sink activity account for high night temperature‐induced grain yield and quality loss in rice (Oryza sativa L.)

High night temperature (HNT) is a major constraint to sustaining global rice production under future climate. Physiological and biochemical mechanisms were elucidated for HNT‐induced grain yield and quality loss in rice. Contrasting rice cultivars (N22, tolerant; Gharib, susceptible; IR64, high yielding with superior grain quality) were tested under control (23°C) and HNT (29°C) using unique field‐based tents from panicle initiation till physiological maturity. HNT affected 1000 grain weight, grain yield, grain chalk and amylose content in Gharib and IR64. HNT increased night respiration (Rn) accounted for higher carbon losses during post‐flowering phase. Gharib and IR64 recorded 16 and 9% yield reduction with a 63 and 35% increase in average post‐flowering Rn under HNT, respectively. HNT altered sugar accumulation in the rachis and spikelets across the cultivars with Gharib and IR64 recording higher sugar accumulation in the rachis. HNT reduced panicle starch content in Gharib (22%) and IR64 (11%) at physiological maturity, but not in the tolerant N22. At the enzymatic level, HNT reduced sink strength with lower cell wall invertase and sucrose synthase activity in Gharib and IR64, which affected starch accumulation in the developing grain, thereby reducing grain weight and quality. Interestingly, N22 recorded lower Rn‐mediated carbon losses and minimum impact on sink strength under HNT. Mechanistic responses identified will facilitate crop models to precisely estimate HNT‐induced damage under future warming scenarios.     

Expression Dynamics of Genes and microRNAs at Different Growth Stages and Heat Treatments in Contrasting High Temperature Responsive Rice Genotypes

The global warming-driven climate change is becoming a major challenge for rice cultivation in Asia and Africa. High-temperature stress impairs the physiology and growth of rice plant, and ultimately results in reduced grain yield. This study was aimed to decipher the physiological and molecular changes occurring during different growth stages of heat-tolerant (N22) and -susceptible (Vandana) rice cultivars under three different heat treatments. Chlorophyll content, membrane integrity, gas exchange parameters and expression of genes and miRNAs were analyzed in N22 and Vandana at seedling, vegetative, and reproductive growth stages after exposing to short and long duration of high temperature stress, and recovery. A number of genes and miRNAs showed dynamic changes in their expression patterns at different growth stages and heat treatments, highlighting the necessity to understand gene regulation before employing the genes for modification through transgenic or gene editing approaches. Predominantly N22 showed distinct and unique capability to reprogram its physiological and molecular machinery during prolonged heat stress at reproductive stage, suggesting that the dynamics in gene regulation is crucial to determine its heat tolerant ability. The study has larger implications in deploying genes for the development of heat tolerant rice cultivars through breeding, transgenic, and genome editing approaches.

     

Super-optimal temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide

Continuing increases in atmospheric carbon dioxide concentration (CO₂) will likely be accompanied by global warming. Our research objectives were (a) to determine the effects of season‐long exposure to daytime maximum/nighttime minimum temperatures of 32/22, 36/26, 40/30 and 44/34°C at ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ on reproductive processes and yield of peanut, and (b) to evaluate whether the higher photosynthetic rates and vegetative growth at elevated CO₂ will negate the detrimental effects of high temperature on reproductive processes and yield. Doubling of CO₂ increased leaf photosynthesis and seed yield by 27% and 30%, respectively, averaged across all temperatures. There were no effects of elevated CO₂ on pollen viability, seed‐set, seed number per pod, seed size, harvest index or shelling percentage. At ambient CO₂, seed yield decreased progressively by 14%, 59% and 90% as temperature increased from 32/22 to 36/26, 40/30 and 44/34°C, respectively. Similar percentage decreases in seed yield occurred at temperatures above 32/22°C at elevated CO₂ despite greater photosynthesis and vegetative growth. Decreased seed yields at high temperature were a result of lower seed‐set due to poor pollen viability, and smaller seed size due to decreased seed growth rates and decreased shelling percentages. Seed harvest index decreased from 0.41 to 0.05 as temperature increased from 32/22 to 44/34°C under both ambient and elevated CO₂. We conclude that there are no beneficial interactions between elevated CO₂ and temperature, and that seed yield of peanut will decrease under future warmer climates, particularly in regions where present temperatures are near or above optimum.     

Effects of Asymmetric Heat on Grain Quality During the Panicle Initiation Stage in Contrasting Rice Genotypes

Heat stress during the panicle initiation stage hinders the formation of rice grains. It is speculated that heat exposure during the panicle initiation stage could influence grain quality in rice. To obtain preliminary knowledge on the effects of asymmetric heat on rice grain quality during the panicle initiation stage, four rice genotypes (Shanyou63, Liangyoupeijiu, Huanghuazhan, and Nagina22) were subjected to three heat treatments, i.e., high daytime temperature (HDT; 37.9 °C/24.5 °C), high nighttime temperature (HNT; 30.9 °C/30.5 °C), the combination of high daytime and nighttime temperature (HDNT; 38.5 °C/31.0 °C), and a control (CK, 31.5 °C/24.0 °C) in temperature-controlled greenhouses for 15 days during the panicle initiation stage. The milling and appearance qualities, which are crucial for commercial value, were studied. Heat treatments significantly reduced the amounts of brown rice, milled rice, and head rice and the grain length, grain width, chalky grain amount, and grain chalkiness in the rice genotypes Liangyoupeijiu, Nagina22, and Huanghuazhan during the panicle initiation stage, and the largest reductions in grain quality were frequently observed under HDNT treatment. The milling and appearance qualities in genotype Shanyou63 were negligibly affected by heat treatments and thus were regarded as tolerant to heat, and the rice genotypes Liangyoupeijiu, Huanghuazhan, and especially Nagina22 were susceptible to heat during the panicle initiation stage. We concluded that heat stress during panicle initiation impacted the milling and appearance qualities in rice, and differences existed among rice genotypes. The underlying mechanisms of the effects of heat on rice grain quality need further study.

     

Effect of elevated CO2 and high temperature on seed-set and grain quality of rice

Hybrid vigour may help overcome the negative effects of climate change in rice. A popular rice hybrid (IR75217H), a heat-tolerant check (N22), and a mega-variety (IR64) were tested for tolerance of seed-set and grain quality to high-temperature stress at anthesis at ambient and elevated [CO(2)]. Under an ambient air temperature of 29 °C (tissue temperature 28.3 °C), elevated [CO(2)] increased vegetative and reproductive growth, including seed yield in all three genotypes. Seed-set was reduced by high temperature in all three genotypes, with the hybrid and IR64 equally affected and twice as sensitive as the tolerant cultivar N22. No interaction occurred between temperature and [CO(2)] for seed-set. The hybrid had significantly more anthesed spikelets at all temperatures than IR64 and at 29 °C this resulted in a large yield advantage. At 35 °C (tissue temperature 32.9 °C) the hybrid had a higher seed yield than IR64 due to the higher spikelet number, but at 38 °C (tissue temperature 34-35 °C) there was no yield advantage. Grain gel consistency in the hybrid and IR64 was reduced by high temperatures only at elevated [CO(2)], while the percentage of broken grains increased from 10% at 29 °C to 35% at 38 °C in the hybrid. It is concluded that seed-set of hybrids is susceptible to short episodes of high temperature during anthesis, but that at intermediate tissue temperatures of 32.9 °C higher spikelet number (yield potential) of the hybrid can compensate to some extent. If the heat tolerance from N22 or other tolerant donors could be transferred into hybrids, yield could be maintained under the higher temperatures predicted with climate change.     

Deterioration of ovary plays a key role in heat stress-induced spikelet sterility in sorghum

In sorghum (Sorghum bicolor [L.] Moench), the impact of heat stress during flowering on seed set is known, but mechanisms that lead to tolerance are not known. A diverse set of sorghum genotypes was tested under controlled environment and field conditions to ascertain the impact of heat stress on time-of-day of flowering, pollen viability, and ovarian tissue. A highly conserved early morning flowering was observed, wherein >90% of spikelets completed flowering within 30 min after dawn, both in inbreds and hybrids. A strong quantitative impact of heat stress was recorded before pollination (reduced pollen viability) and post pollination (reduced pollen tube growth and linear decline in fertility). Although viable pollen tube did reach the micropylar region, 100% spikelet sterility was recorded under 40/22°C (day/night temperatures), even in the tolerant genotype Macia. Heat stress induced significant damage to the ovarian tissue near the micropylar region, leading to highly condensed cytoplasmic contents and disintegrated nucleolus and nucleus in the susceptible genotype RTx430. Whereas, relatively less damages to ovarian cell organelles were observed in the tolerant genotype Macia under heat stress. Integrating higher tolerance in female reproductive organ will help in effective utilization of the early morning flowering mechanism to enhance sorghum productivity under current and future hotter climate.     

Abscisic acid prevents pollen abortion under high‐temperature stress by mediating sugar metabolism in rice spikelets

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near‐isogenic line) were subjected to heat stress of 40°C after pre‐foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non‐structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.     

Golgi/plastid‐type manganese superoxide dismutase involved in heat‐stress tolerance during grain filling of rice

Superoxide dismutase (SOD) is widely assumed to play a role in the detoxification of reactive oxygen species caused by environmental stresses. We found a characteristic expression of manganese SOD 1 (MSD1) in a heat‐stress‐tolerant cultivar of rice (Oryza sativa). The deduced amino acid sequence contains a signal sequence and an N‐glycosylation site. Confocal imaging analysis of rice and onion cells transiently expressing MSD1‐YFP showed MSD1‐YFP in the Golgi apparatus and plastids, indicating that MSD1 is a unique Golgi/plastid‐type SOD. To evaluate the involvement of MSD1 in heat‐stress tolerance, we generated transgenic rice plants with either constitutive high expression or suppression of MSD1. The grain quality of rice with constitutive high expression of MSD1 grown at 33/28 °C, 12/12 h, was significantly better than that of the wild type. In contrast, MSD1‐knock‐down rice was markedly susceptible to heat stress. Quantitative shotgun proteomic analysis indicated that the overexpression of MSD1 up‐regulated reactive oxygen scavenging, chaperone and quality control systems in rice grains under heat stress. We propose that the Golgi/plastid MSD1 plays an important role in adaptation to heat stress.     

Effects of high-temperature stress on microsporogenesis in heat-sensitive and heat-tolerant genotypes of Phaseolus vulgaris

High ambient temperature (32/27 °C, day/night, 12 h photoperiod) applied prior to anthesis to Phaseolus vulgaris plants results in abnormal pollen and anther development during microsporogenesis. Scanning and transmission electron microscopy were used to examine anther and pollen morphology and pollen wall architecture after heat stress was applied to two genotypes that differ with respect to yield potential under high‐temperature field conditions: one, a heat‐sensitive, Mesoamerican genotype, A55, the second, a heat‐tolerant, Andean genotype, G122. High‐temperature treatment of both genotypes was applied 1–13 d before anthesis. Under heat stress, the heat‐tolerant genotype showed anther and pollen characteristics that were generally similar to the low temperature controls. In contrast, after 9 d of heat treatment before anthesis, the anthers of the heat‐sensitive genotype were indehiscent and contained abnormal pollen. Pollen wall architecture was also affected in the 12 and 13 d treatments. In addition to the morphological changes, the heat‐sensitive genotype also experienced reduced pollen viability and reduced yield in high‐temperature experiments conducted in both the greenhouse and field.     

Geochemical signatures of benthic foraminiferal shells from a heat‐polluted shallow marine environment provide field evidence for growth and calcification under extreme warmth

Shallow marine calcifiers play an important role as marine ecosystem engineers and in the global carbon cycle. Understanding their response to warming is essential to evaluate the fate of marine ecosystems under global change scenarios. A rare opportunity to test the effect of warming acting on natural ecosystems is by investigation of heat‐polluted areas. Here, we study growth and calcification in benthic foraminifera that inhabit a thermally polluted coastal area in Israel, where they are exposed to elevated temperatures reaching up to ~42°C in summer. Live specimens of two known heat‐tolerant species Lachlanella sp. 1 and Pararotalia calcariformata were collected over a period of 1 year from two stations, representing thermally polluted and undisturbed (control) shallow hard bottom habitats. Single‐chamber element ratios of these specimens were obtained using laser ablation, and the Mg/Ca of the most recently grown final chambers were used to calculate their calcification temperatures. Our results provide the first direct field evidence that these foraminifera species not only persist at extreme warm temperatures but continue to calcify and grow. Species‐specific Mg/Ca thermometry indicates that P. calcariformata precipitate their shells at temperatures as high as 40°C and Lachlanella sp. 1 at least up to 36°C, but both species show a threshold for calcification at cold temperatures: calcification in P. calcariformata only occurred above 22°C and in Lachlanella sp. 1 above 15°C. Our observations from the heat‐polluted area indicate that under future warming scenarios, calcification in heat‐tolerant foraminifera species will not be inhibited during summer, but instead the temperature window for their calcification will be expanded throughout much of the year. The observed inhibition of calcification at low temperatures indicates that the role of heat‐tolerant foraminifera in carbonate production will most likely increase in future decades.     

Pollen germination and in vivo fertilization in response to high‐temperature during flowering in hybrid and inbred rice

High‐temperature during flowering in rice causes spikelet sterility and is a major threat to rice productivity in tropical and subtropical regions, where hybrid rice development is increasingly contributing to sustain food security. However, the sensitivity of hybrids to increasing temperature and physiological responses in terms of dynamic fertilization processes is unknown. To address these questions, several promising hybrids and inbreds were exposed to control temperature and high day‐time temperature (HDT) in Experiment 1, and hybrids having contrasting heat tolerance were selected for Experiment 2 for further physiological investigation under HDT and high‐night‐time‐temperature treatments. The day‐time temperature played a dominant role in determining spikelet fertility compared with the night‐time temperature. HDT significantly induced spikelet sterility in tested hybrids, and hybrids had higher heat susceptibility than the high‐yielding inbred varieties. Poor pollen germination was strongly associated with sterility under high‐temperature. Our novel observations capturing the series of dynamic fertilization processes demonstrated that pollen tubes not reaching the viable embryo sac was the major cause for spikelet sterility under heat exposure. Our findings highlight the urgent need to improve heat tolerance in hybrids and incorporating early‐morning flowering as a promising trait for mitigating HDT stress impact at flowering.     

Sugars,antioxidant enzymes and IAA mediate salicylic acid to prevent rice spikelet degeneration caused by heat stress

Salicylic acid (SA) can alleviate damage to rice plants induced by heat stress, but its role in preventing spikelet degeneration under high-temperature stress has not been documented. Rice plants pretreated with SA (0–50 mmol L^?1) were subjected to heat stress at 40?°C at the pollen mother cell meiosis stage for 10 days. The results indicated that there were no significant differences in grain yields and yield components among rice plants that were exogenously sprayed with SA (0–50 mmol L^?1) under natural conditions. Under heat stress, the grain yield, spikelet number per panicle and setting rate in response to SA treatments were higher than under the control (0 mmol L^?1 SA or NON-SA) treatment, especially with 1 and 10 mmol L^?1 SA. A higher grain yield, spikelet number per panicle and setting rate were recorded in these two SA treatments compared with the NON-SA treatment. During this process, soluble sugars, proline, phytohormones including ABA, GA₃, BRs, IAA, ZR and JA, and antioxidant enzymes, such as superoxide dismutase, peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX), were induced by SA. Moreover, soluble sugars, IAA, POD and APX in the spikelets with SA treatments were not only higher than with the NON-SA treatment but the changing patterns were also similar to that of the spikelet number per panicle under natural conditions and heat stress. Therefore, our results suggest that sugars, antioxidant enzymes and IAA might mediate SA to prevent spikelet degeneration caused by heat stress.     

Plant traits and functional types in response to reduced disturbance in a semi‐natural grassland

Abstract. Question: How do functional types respond to contrasting levels of herbage use in temperate and fertile grasslands? Location: Central France (3°1’E, 45°43’N), 870 m a.s.l. Methods: Community structure and the traits of dominant plant species were evaluated after 12 years of contrasted grazing and mowing regimes in a grazing trial, comparing three levels of herbage use (high, medium and low). Results and Conclusions: Of 22 measured traits (including leaf traits, shoot morphology and composition, phenology), seven were significantly affected by the herbage use treatment. A decline in herbage use reduced individual leaf mass, specific leaf area and shoot digestibility, but increased leaf C and dry matter contents. Plants were taller, produced larger seeds and flowered later under low than high herbage use. Nine plant functional response types were identified by multivariate optimization analysis; they were based on four optimal traits: leaf dry matter content, individual leaf area, mature plant height and time of flowering. In the high‐use plots, two short and early flowering types were co‐dominant, one competitive, grazing‐tolerant and moderately grazing‐avoiding, and one grazing‐avoiding but not ‐tolerant. Low‐use plots were dominated by one type, neither hardly grazing‐avoiding nor grazing‐tolerant, but strongly competitive for light.