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Chloroplast is one of the most sensitive organelles to heat stress in plants. In chloroplasts, various proteases affect photosynthesis by degrading proteins under stress conditions. Tomato Lutescent2 (SlL2), a chloroplast zinc metalloprotease, was previously reported to alter chloroplast development and delay fruit ripening. However, its enzyme activity and roles in plant response to abiotic stress are still unclear. Here, we confirmed that the SlL2 protein which localized on thylakoid membrane was an ATP-independent hydrolase, and SlL2 gene responded to heat stress. Phenotype analysis showed that SlL2 plays a negative role in the heat-response mechanism. Under heat stress, the transgenic plants overexpressing SlL2 (OE) grew worse than the wild type (WT), as reflected by their decreased membrane stability, osmotic-regulating substance, and antioxidative enzyme activities, as well as increased reactive oxygen species (ROS) accumulation. By contrast, l2 mutant line showed the opposite phenotype and corresponding physiological indices under heat stress. In addition, overexpression of SlL2 decreased the photosynthetic activities, especially photosystem II. Moreover, SlL2 was found to interact with chloroplast-located chaperone protein SlCDJ1, decreasing its content under heat stress. These results indicate that SlL2 reduces the thermotolerance of tomato by reducing the content of SlCDJ1. [ABSTRACT FROM AUTHOR]

Global warming is predicted to be increased in the upcoming years, resulting in frequent heatwaves or hot days worldwide, which can seriously affect crop growth and productivity. The responses of heat stress to several photophysiological and biochemical traits in three tomato cultivars were investigated in a pot experiment, and the heat tolerance capability of these cultivars was evaluated based on the investigated traits. The experiment was followed by a factorial completely randomized design, and the factors were (i) tomato cultivars (BARI Hybrid Tomato-5, BARI Tomato-14, and BARI Tomato-15) and (ii) heat stress (control and heat). The plants of three tomato cultivars were exposed to short-term heat stress (four days at 38/25 ◦C day/night temperature) at the flowering stage. The measured traits such as dry mass, leaf greenness (SPAD), maximum photochemical efficiency of photosystem II (Fv/Fm), photosynthetic rate (A), stomatal conductance (gs), transpiration rate (E), leaf chlorophyll, and carotenoid content were significantly declined, while the catalase and ascorbate peroxidase activities were increased by heat stress in all three tomato cultivars except BARI Tomato-15, which showed unaltered gs, E, and carotenoids. The percent reduction (over control) in SPAD, Fv/Fm, A, total chlorophyll, and total carotenoids was significantly lower (11, 06, 25, 34, and 19%, respectively), whereas the percent increase in catalase and ascorbate peroxidase activities was substantially higher (70 and 72%, respectively) in BARI Tomato-15 than in other cultivars. Based on the measured physiological and biochemical traits, the cultivar BARI Tomato-15 showed better heat tolerance than the other cultivars. [ABSTRACT FROM AUTHOR]

The increasing occurrence of heatwaves has intensified temperature stress on terrestrial vegetation. Here, we investigate how two contrasting isoprene-emitting tropical species, Ficus benjamina and Pachira aquatica , cope with heat stress and assess the role of internal plant carbon sources for isoprene biosynthesis in relation to thermotolerance. To our knowledge, this is the first study to report isoprene emissions from P. aquatica. We exposed plants to two levels of heat stress and determined the temperature response curves for isoprene and photosynthesis. To assess the use of internal C sources in isoprene biosynthesis, plants were fed with 13C position-labelled pyruvate. F. benjamina was more heat tolerant with higher constitutive isoprene emissions and stronger acclimation to higher temperatures than P. aquatica , which showed higher induced isoprene emissions at elevated temperatures. Under heat stress, both isoprene emissions and the proportion of cytosolic pyruvate allocated into isoprene synthesis increased. This represents a mechanism that P. aquatica , and to a lesser extent F. benjamina , has adopted as an immediate response to sudden increase in heat stress. However, in the long run under prolonged heat, the species with constitutive emissions (F. benjamina) was better adapted, indicating that plants that invest more carbon into protective emissions of biogenic volatile organic compounds tend to suffer less from heat stress. [ABSTRACT FROM AUTHOR]

High temperature poses a severe extortion to productivity of many crops like wheat. Therefore, well documented roles of brassinosteroid (BR) and silicon (Si) in terms of abiotic stress tolerance, the current study was designed to evaluate the response of wheat (Triticum aestivum L. Var. PBW-343) to 24-epibrassinolide (EBL) mediated by silicon grown under high temperature stress. At 10- and 12-day stage after sowing, the seedlings were administered Si (0.8 mM) through the sand, and the plants at 20, 22, or 24 days after sowing (DAS) were given EBL (0.01μM) through foliage. Plants were treated to high-temperature stress (35/28 or 40/35 °C), for 24 h with 12-h photoperiod in plant growth chamber at 25- and 26-day stage of growth. High temperatures cause significant reduction in growth performance and photosynthesis-related attributes at 35 days after sowing. However, antioxidant enzymes and proline content also augmented substantially with increasing temperature. BR and Si enhanced antioxidant activity and proline content, which was earlier increased by the high temperature. It is established that interaction of EBL and Si considerably improved the growth features, photosynthetic efficacy, and several biochemical traits under high-temperature stress through elevated antioxidant system and osmoprotectant. [ABSTRACT FROM AUTHOR]

This study aimed to phenotype young tomato (Solanum lycopersicum L.) plants for heat tolerance by measuring F v/ F m after short-term heat treatments in climate chambers and selected sensitive (low F v/ F m) and tolerant (high F v/ F m) cultivars to investigate their in-field performance. Twenty-eight genotypes were phenotyped at 40 : 28°C for 2 days in climate chambers. A second screening (four high F v/ F m and four low F v/ F m genotypes) was conducted for 4 days at 38 : 28°C, followed by 5 days' recovery (26 : 20°C). The tolerant genotypes maintained high net photosynthesis (P N) and increased stomatal conductance (g s) at 38°C, allowing better leaf cooling. Sensitive genotypes had lower F v/ F m and P N at 38°C, and g s increased less than in the tolerant group, reducing leaf cooling. Under controlled conditions, all eight genotypes had the same plant size and pollen viability, but after heat stress, plant size and pollen viability reduced dramatically in the sensitive group. Two tolerant and two sensitive genotypes were grown in the field during a heat wave (38 : 26°C). Tolerant genotypes accumulated more biomass, had a lower heat injury index and higher fruit yield. To our knowledge, this is the first time screening for heat tolerance by F v/ F m in climate chambers was verified by a field trial under natural heat stress. The differences after heat stress in controlled environments were comparable to those in yield between tolerant and sensitive groups under heat stress in the field. The results suggest that F v/ F m is effective for early detection of heat tolerance, and screening seedlings for heat sensitivity can speed crop improvement. Heat stress is limiting tomato production globally. Tolerant genotypes screened for heat stress using chl a fluorescence had better growth under controlled conditions and maintained high fruit production in the field. These results may help downscale laboratory-based high-throughput phenotyping techniques used to develop heat-tolerant tomato varieties. [ABSTRACT FROM AUTHOR]

Heat stress damages the photosynthetic capacity and ultimate crop yield; the genetic basis of which is scarcely known. Studies were conducted during the years 2014 and 2015 to understand the physiological basis of heat tolerance in seven mungbean [Vigna radiata (L.) Wilczek] varieties by monitoring changes in hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents, photosynthetic pigments and gas exchange attributes in the trifoliate leaf in proximity to the pods as well as nutrient partitioning to the pods and developing seed by applying heat stress at the flowering stage. After 15 days of stress, the data for physiological and biochemical attributes were recorded while the data for seed yield and quality were taken at maturity. Heat stress caused a marked accumulation of H2O2 and MDA whilst decreasing net photosynthesis, stomatal conductance and water use efficiency, chlorophyll a and b, and total chlorophylls in the sensitive varieties, while increased carotenoids contents in the tolerant varieties. Seed yield per plant and 1000 seed weight were affected due to heat stress, leading to more reduced harvest index in the sensitive varieties. Heat stress also hampered the partitioning of nutrients such as nitrogen, sulfur and calcium to the developing seed. Interrelationships of different parameters revealed that heat stress damaged the leaf chlorophyll and leaf gas exchange due to enhanced production of H2O2 and MDA, while tolerant varieties (NM-2011 and NM-2006) exhibited better gas exchange properties and produced higher seed yield and harvest index in both the years. Seed yield was directly correlated to the partitioning of nutrients especially nitrogen, sulfur and calcium under heat stress. In conclusion, despite significant differences in the varieties, the yield loss in mungbean is associated to heat stress mediated oxidative stress on the leaf photosynthesis and reduced assimilate partitioning to the pod development and seed filling. Thus, selection based on greater rate of photosynthesis of the proximal leaf may be helpful for fetching better mungbean yield in relatively warmer environments. [ABSTRACT FROM AUTHOR]

Highlights • Photosynthesis, stomatal conductance and leaf photosynthetic pigments decreased. • Chlorophyll fluorescence parameters were affected by high daytime temperatures. • High temperatures caused oxidative stress. • Accumulation of proline reduced lipid peroxidation. • The physiological responses induced by high temperatures of bean plants are genotype-dependent. Abstract Two separated experiments were conducted to study the effect of various exposure periods of heat stress and the interaction of four common bean cultivars at different daytime temperatures. In the first experiment, 'Cerinza' plants were subjected to four periods (0, 5, 10, 20 d) at 40○C. In the second experiment, 4 cultivars of common bean ('Cerinza', 'Bachue', 'Bacata' and 'Bianca') were exposed to four different temperatures for 20 d. The results from the first experiment showed that the photosynthesis rate decreased by ≈94% (16.2 vs. 1.1 m−2 s-1, respectively), while proline content increased by ≈100% (22 vs. 44 μmol g-1FW) in 'Cerinza' plants exposed to high temperature for 20 d compared to control plants. In the second experiment, photosynthesis and leaf photosynthetic pigments (chlorophyll and carotenoids) also diminished (≈45% and ≈40%, respectively) when the daytime temperature changed from 25 to 40○C in all cultivars. Proline and malondialdehyde were enhanced at 40○C, mainly, in 'Cerinza', 'Bachue', and 'Bacata'. 'Bianca' plants registered a high-stress susceptibility index and low-stress tolerance index scores at 30, 35 and 40 °C. Based on these results, 'Cerinza', 'Bacata' and 'Bachue' cultivars can be considered moderately tolerant to long periods of heat stress and could adapt well to periods of high daytime temperatures in Andean areas. [ABSTRACT FROM AUTHOR]

Highlights • Elevated CO2 moderates pre-anthesis heat stress effects on net CO2 assimilation rates. • Growth under elevated CO2 increases stem water soluble carbohydrates (WSC) in wheat. • Greater WSC pool does not result in greater remobilisation into the grain under heat. • Elevated CO2 may moderate some effects of heat stress. • A moderating effect is dependent on heat stress timing and season. Abstract Increasing atmospheric CO 2 concentration [CO 2 ] drives the rise in global temperatures, with predictions of an increased frequency of heat waves (short periods of high temperatures). Both, CO 2 and high temperature, have profound effects on wheat growth and productivity. We tested whether elevated [CO 2 ] (eCO 2 ) has a potential to ameliorate the effects of simulated heat waves (HT) on wheat in a dryland cropping system. Wheat was field-grown at the Australian Grains Free Air CO 2 Enrichment (AGFACE) facility under ambient [CO 2 ] (∼390 ppm) or eCO 2 (∼550 ppm) for two growing seasons, one with ample water supply and one of severe drought. Using heated chambers, heat waves (3-day periods of high temperatures) were imposed at critical growth stages before anthesis (HT1) or post-anthesis (HT2, HT3). Gas exchange, chlorophyll content and concentration of nitrogen (N) in mainstem flag leaves, as well as concentrations of stem water-soluble carbohydrates (WSC) in mainstems were monitored throughout the season. Yield, biomass and thousand kernel weights (TKW) were measured at maturity. Elevated [CO 2 ] moderated the effect on net CO 2 assimilation rates of pre-anthesis (HT1), but not of post-anthesis heat waves (HT2, HT3). Growth under eCO 2 increased stem WSC both, with and without experimental heat waves, but remobilisation decreased significantly under heat indicating that a greater WSC pool does not necessarily translate into greater remobilisation into the grain. Grain yield (g m−2) was greater under eCO 2 and especially pre-anthesis heat stress decreased grain yield in the wetter season, and this decrease was stronger under eCO 2 (up to 20%) than under aCO 2 (up to 10%). Grain N decreased under eCO 2 , but less so under heat stress. We conclude that eCO 2 may moderate some effects of heat stress in wheat but such effects strongly depend on seasonal conditions and timing of heat stress. [ABSTRACT FROM AUTHOR]

Potato is an important staple food with increasing popularity worldwide. Elevated temperatures significantly impair tuber yield and quality. Breeding heat‐tolerant cultivars is therefore an urgent need to ensure sustainable potato production in the future. An integrated approach combining physiology, biochemistry, and molecular biology was undertaken to contribute to a better understanding of heat effects on source‐ (leaves) and sink‐organs (tubers) in a heat‐susceptible cultivar. An experimental set‐up was designed allowing tissue‐specific heat application. Elevated day and night (29°C/27°C) temperatures impaired photosynthesis and assimilate production. Biomass allocation shifted away from tubers towards leaves indicating reduced sink strength of developing tubers. Reduced sink strength of tubers was paralleled by decreased sucrose synthase activity and expression under elevated temperatures. Heat‐mediated inhibition of tuber growth coincided with a decreased expression of the phloem‐mobile tuberization signal SP6A in leaves. SP6A expression and photosynthesis were also affected, when only the belowground space was heated, and leaves were kept under control conditions. By contrast, the negative effects on tuber metabolism were attenuated, when only the shoot was subjected to elevated temperatures. This, together with transcriptional changes discussed, indicated a bidirectional communication between leaves and tubers to adjust the source capacity and/or sink strength to environmental conditions. Here, we performed a comprehensive study to analyse changes in potato plants under elevated temperatures. A special experimental set‐up allowed us to separately apply temperature stress to either above‐ or belowground tissues and thereby to distinguish between source and sink signals. Our results show that elevated temperatures impair photosynthetic assimilate production and partitioning towards tubers together with a decreased expression of the mobile tuberization signal. Our data led us to conclude that there is a bidirectional signalling between source leaves and sink organs (tubers), and candidate genes are discussed which may be involved in the metabolic readjustment. [ABSTRACT FROM AUTHOR]

High temperature has adverse effects on rice growth by inhibiting the flag leaf photosynthetic and antioxidant capacity, which can be alleviated by various exogenous chemicals such as spermidine (Spd). However, the role of Spd in conferring heat tolerance in rice is not well documented. Conventional japonica rice varieties Wuyunjing 24 and Ningjing 3 were treated with high temperatures at 37.5/27.0°C (day/night) and foliar sprayed with 1 mmol L-1 Spd after flowering. Results showed activities of superoxide dismutase (SOD) and peroxidase (POD) activities were deceased during high temperature treatment and eventually lead to the malondialdehyde (MDA) accumulation. Exogenous Spd significantly increased both SOD and POD activities at the later stage of high-temperature treatment, and reduced MDA accumulation were identified in both rice varieties. Application of Spd further increased the amount of soluble sugars during high temperature stress and that maintained the osmotic equilibrium of rice leaves. Spd significantly increased photosystem II (ΦPSII), photosynthetic electron transport rate (ETR), variable fluorescence/maximum fluorescence ratio (Fv'/Fm'), stomatal conductance and the photochemical reaction of light energy ratio (Pr), and ultimately improved the photosynthetic and transpiration rate during high temperature stress. In conclusion, exogenous Spd can effectively alleviate the adverse consequences of high temperature and could be further applied to provide strategies in mitigating the challenges of global warming-induced yield loss and other possible relevant issues. [ABSTRACT FROM AUTHOR]

Heat stress frequently imposes a strong negative impact on vegetative and reproductive development of plants leading to severe yield losses. Wheat, a major temperate crop, is more prone to suffer from increased temperatures than most other major crops. With heat waves becoming more intense and frequent, as a consequence of global warming, a decrease in wheat yield is highly expected. Here, we examined the impact of a short-term (48 h) heat stress on wheat imposed during reproduction at the pollen mitosis stage both, at the physiological and molecular level. We analyzed two sets of summer wheat germplasms from Australia (Kukri, Drysdale, Gladius, and RAC875) and Europe (Epos, Cornetto, Granny, and Chamsin). Heat stress strongly affected gas exchange parameters leading to reduced photosynthetic and transpiration rates in the European cultivars. These effects were less pronounced in Australian cultivars. Pollen viability was also reduced in all European cultivars. At the transcriptional level, the largest group of heat shock factor genes (type A HSFs), which trigger molecular responses as a result of environmental stimuli, showed small variations in gene expression levels in Australian wheat cultivars. In contrast, HSFs in European cultivars, including Epos and Granny, were strongly downregulated and partly even silenced, while the high-yielding variety Chamsin displayed a strong upregulation of type A HSFs. In conclusion, Australian cultivars are well adapted to moderate heat stress compared to European summer wheat. The latter strongly react after heat stress application by downregulating photosynthesis and transpiration rates as well as differentially regulating HSFs gene expression pattern. [ABSTRACT FROM AUTHOR]

Comparative studies of photosynthetic physiology were carried out to evaluate high light and temperature stresses in reproductive (receptacles) and vegetative (blades) tissues of fertile thalli of Hizikia fusiformis from Nanao Island, China. Receptacles had much higher rates of photosynthesis than blades, and temperature sensitivity was much higher for photosynthesis than for respiration. Both receptacles and blades survived extended exposure to 1500 μmol photons m-2 s-1; however, above 30°C, photosynthetic activity declined rapidly. Prolonged exposure to high temperature sharply depressed photosynthesis, and extended exposure to 40°C was more detrimental to photosynthesis than exposure to 30°C. Moreover, photosynthesis of receptacles was more susceptible to heat than photosynthesis in blades. Following 15 min at 40°C, receptacle photosynthesis declined to 28% of initial value and recovered to only 60% of initial value within 1 h in recovery conditions. However, after 30 min at 40°C, receptacle photosynthesis did not recover within 4 h, suggesting that the photosynthetic apparatus had suffered irreversible injury. Respiration was not affected by high temperature. Our study shows that high temperature but not high light stress might hamper sexual reproduction in H. fusiformis. [ABSTRACT FROM AUTHOR]

Heat stress during grain filling substantially decreases wheat productivity; thus, to ensure food security, heat tolerance in wheat needs to be developed. In this study, we evaluated the effect of heat priming applied during the stem-elongation stage, booting and anthesis, followed by 5 days of severe heat stress (a 7.86°C rise in temperature) during the grain-filling stage on physiological activities and grain yield of winter wheat in pot experiments during the 2015-2017 growing seasons using the winter wheat cultivars Yangmai 18 (a vernal type) and Yannong 19 (a facultative type). Compared with the damage observed in non-primed plants, heat priming during the stem-elongation stage and booting significantly prevented the grain-yield damage caused by heat stress during grain filling. Heat-primed plants displayed higher sucrose contents and sucrosephosphate activity in leaves and greater above-ground dry matter than non-primed plants. Priming during stem elongation and booting led to increased photosynthetic capacity, stomatal conductance and chlorophyll contents in comparison with nonpriming. Improved tolerance to heat stress due to the enhanced activities of antioxidant enzymes superoxide dismutase and peroxidase and reductions in reactive oxygen species and malondialdehyde production was observed in primed plants compared with non-primed plants of both cultivars. The positive effect of heat priming on the response to heat stress during grain filling was more pronounced in plants primed at the booting stage than in those primed at the stem-elongation or anthesis stage. Moreover, the vernal-type Yangmai 18 benefited more from heat priming than did Yannong 19, as evidenced by its higher productivity. We conclude that heat priming during early reproductive-stage growth can improve post-anthesis heat tolerance in winter wheat. [ABSTRACT FROM AUTHOR]

In many biomes, plants are subject to heatwaves, potentially causing irreversible damage to the photosynthetic apparatus. Field surveys have documented global, temperature-dependent patterns in photosynthetic heat tolerance (PHT); however, it remains unclear if these patterns reflect acclimation in PHT or inherent differences among species adapted to contrasting habitats. To address these unknowns, we quantified seasonal variations in Tcrit (high temperature where minimal chlorophyll-a fluorescence rises rapidly, reflecting disruption to photosystem II) in 62 species native to 6 sites from 5 thermally contrasting biomes across Australia. Tcrit and leaf fatty acid (FA) composition (important for membrane stability) were quantified in three temperaturecontrolled glasshouses in 20 of those species. Tcrit was greatest at hot field sites and acclimated seasonally (summer > winter, increasing on average 0.34 °C per °C increase in growth temperature). The glasshouse study showed that Tcrit was inherently higher in species from warmer habitats (increasing 0.16 °C per °C increase in origin annual mean maximum temperature) and acclimated to increasing growth temperature (0.24 °C °C-1). Variations in Tcrit were positively correlated with the relative abundance of saturated FAs, with FAs accounting for 40% of Tcrit variation. These results highlight the importance of both plastic adjustments and inherent differences determining contemporary continent-wide patterns in PHT. [ABSTRACT FROM AUTHOR]

Abiotic stresses such as drought, heat or salinity are major causes of yield loss worldwide. Recent studies have revealed that the acclimation of plants to a combination of different environmental stresses is unique and therefore cannot be directly deduced from studying the response of plants to each of the different stresses applied individually. The efficient detoxification of reactive oxygen species (ROS) is thought to play a key role in enhancing the tolerance of plants to abiotic stresses. Here, we report on the role of melatonin in the protection of the photosynthetic apparatus through the increase in ROS detoxification in tomato plants grown under the combination of salinity and heat, two of the most common abiotic stresses known to act jointly. Plants treated with exogenous melatonin showed a different modulation in the expression on some antioxidant-related genes and their related enzymes. More specifically, ascorbate peroxidase, glutathione reductase, glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase (APX, GR, GPX and Ph-GPX, resepctively) showed an antagonistic regulation as compared to plants that did not receive melatonin. This translated into a better antioxidant capacity and to a lesser ROS accumulation under stress combination. The performance of the photosynthesis parameters and the photosystems was also increased in plants treated with exogenous melatonin under the combination of salinity and heat. In accordance with these findings, tomato plants treated with melatonin were found to grow better under stress combination that the non-treated ones. Our study highlights the important role that exogenous melatonin plays in the acclimation of plants to a combination of two different abiotic stresses, and how this compound can specifically regulate oxidative stress-related genes and enzymes to increase plant tolerance. [ABSTRACT FROM AUTHOR]

Indirect effects of high temperature through increased vapor pressure deficit (VPD) are vital but often ignored in climate impact studies. We investigated the direct ( via heat) and indirect ( via VPD) effects of a post-anthesis applied high temperature episode on biochemical and diffusional components of photosynthesis in two wheat cultivars. Plants were exposed to a five-day episode of hot humid (HH: 36 °C; 1.96 kPa) or hot dry (HD: 36 °C; 3.92 kPa) climate, including normal climate (NC: 24 °C; 1.49 kPa VPD) as control. The capacity and sensitivity of different photosynthetic components that included diffusional and biochemical processes was investigated both during the stress and after six-days of recovery under NC. The results showed that, regardless of VPD, photosynthetic capacity under high temperature was largely impaired by biochemical limitations while the diffusional limitations were relatively insignificant. The processes involved in CO 2 -use ( i.e. in vivo carboxylation efficiency and V c max ) presented higher sensitivity than the processes involved in light-use (PSII efficiency, quantum yield and chlorophyll content index). Maximum photosynthetic capacity under high temperature was primarily restricted by limitations to RuBP-regeneration. Effects of higher VPD through diffusional limitations (stomatal and mesophyll conductance) were potentially insignificant. The effects of VPD through changes in transpiration were observed under HH, where significantly higher g s and increase in g s (and transpiration) over time due to acclimation led to drought, because the irrigation amounts per day remained same over the stress period. Resultantly, the photosynthesis under HH was co-limited by both RuBP-carboxylation and RuBP-regeneration. Irrespective of VPD, the after-effects of heat-wave were either non-existent or potentially insignificant, as most of the photosynthetic and related parameters recovered to their respective control. The results indicated that the effect of VPD through changes in soil moisture dynamics is an important confounding variable to consider in climate-impact studies. Higher sensitivity of CO 2 -use suggested that even moderately high temperature-episodes might limit photosynthetic capacity and hence crop productivity, thus reiterating the need to develop crop cultivars with greater tolerance to high temperatures. [ABSTRACT FROM AUTHOR]

Climate change is increasing the frequency of heat waves accompanied by drought episodes. These challenges are increasing in the Mediterranean basin, where Olea europaea L. has an important ecological and economic role. Olive breeding programs have been focused on highly productive cultivars, while ancient cultivars may present higher tolerance to drought and heat resilience. Therefore, it is important to select traditional cultivars that may give reliable performances under the emerging climate change scenarios. In the present work, the differential physiological response of economically important traditional Portuguese olive cultivars, Cobrançosa, Cordovil de Castelo Branco (C.C. Branco), and Cordovil de Serpa (C. Serpa), to drought combined with heat are evaluated. Stress treatment had lowest impacts on water status in Cobrançosa. Also, this cultivar was less affected regarding pigments content, maximum and effective quantum yield of photosystem II (F v /F m and Φ PSII ) and exhibited higher ability to trigger an antioxidant response. C.C. Branco was the most sensitive cultivar in response to pigments (carotenoids), F v /F m and Φ PSII , and cell membrane stability. Principal component analysis suggested that Cobrançosa has high potential to withstand climate change events, particularly drought combined with heat episodes, followed by C. Serpa and C.C. Branco. [ABSTRACT FROM AUTHOR]

Photosystem II (PSII) is sensitive to heat stress which can overproduce reactive oxygen species (ROS). To investigate the underlying mechanism of exogenously-supplied trehalose to improve photosynthesis and heat tolerance, in this study, we initially determined the antioxidants to get rid of ROS and examined the effects of trehalose pretreatment on D1 protein. Our results show that heat (40°C) stress decreased PSII activity and D1 protein content. However, trehalose pretreatment enhanced PSII activity and the content of D1 protein during heat stress. In the presence of streptomycin, an inhibitor of D1 protein synthesis, the maximal photochemical efficiency of PSII (Fv/Fm) in trehalose pretreated seedlings was enhanced. Hydrogen peroxide content and malondialdehyde content in trehalose pretreated seedlings were less than those in control plants under heat conditions. Consistently, the ratio of reduced ascorbate to dehydroascorbic acid, the activities of catalase (CAT) and ascorbate peroxidase (APX) and the gene expressions of antioxidant enzymes including superoxide dismutase, CAT, peroxidase and APX were increased in trehalose pretreated plants compared to control seedlings. These results suggest that exogenously applied trehalose may play a key role in the recovery of decreased synthesis of D1 protein and protect it from heat-induced photoinhibition in wheat (Triticum aestivum L.) by decreasing the production of ROS and reducing membrane lipid peroxidation via the change of antioxidants including APX, CAT and reduced ascorbate and by increasing gene transcript level of related antioxidant enzymes during heat stress. [ABSTRACT FROM AUTHOR]

Heat stress is one of the major abiotic factors limiting the growth of cool-season grass species during summer season. The objectives of this study were to assess genetic variations in the transcript levels of selected genes in fine fescue cultivars differing in heat tolerance, and to identify single nucleotide polymorphism (SNP) markers associated with candidate genes related to heat tolerance. Plants of 26 cultivars of five fine fescue species (Festuca spp.) were subjected to heat stress (38/33 °C, day/night temperature) in controlled environmental growth chambers. Physiological analysis including leaf chlorophyll content, photochemical efficiency, and electrolyte leakage demonstrated significant genetic variations in heat tolerance among fine fescue cultivars. The transcript levels of selected genes involved in photosynthesis (RuBisCO activase, Photosystem II CP47 reaction center protein), carbohydrate metabolism (Sucrose synthase), energy production (ATP synthase), growth regulation (Actin), oxidative response (Catalase), and stress protection (Heat shock protein 90) were positively correlated with the physiological traits for heat tolerance. SNP markers for those candidate genes exhibited heterozygosity, which could also separate heat-sensitive and heat-tolerant cultivars into clusters. The development of SNP markers for candidate genes in heat tolerance may allow marker-assisted breeding for the development of new heat-tolerant cultivars in fine fescue and other cool-season grass species. [ABSTRACT FROM AUTHOR]

Stressful episodic weather is likely to affect the C balance of trees as the climate changes, potentially altering survival. However, the role of elevated CO2 concentration ([CO2]) in tolerating off-season episodic extremes is not clear. We tested for interactive effects of elevated CO2 and springtime heat stress on photosynthesis for seven genotypes of Eucalyptus camaldulensis Dehnh. var. camaldulensis, representing its widespread distribution across south-eastern Australia. We grew clonal material under glasshouse conditions of ambient (aCO2; 400 parts per million (ppm)) or elevated (eCO2; 640 ppm) [CO2], and air temperatures of 25 : 17°C (day : night), and measured the electron transport rate in PSII (ETR), stomatal conductance to water vapour (gs) and net CO2 assimilation (A). Measurements were made before, during and after a four-day temperature excursion of 35 : 27°C. ETR and A were ~17% higher for plants grown in eCO2 than in aCO2. Photosynthesis remained stable for plants in eCO2 during the heatwave. Based on the effect size ratio (eCO2 : aCO2), gs and ETR were temporarily affected more by the heatwave than A. A reduction in ETR in eCO2 was the only lasting effect of the heatwave. There were no significant differences among genotypes. Correlations between photosynthesis and climate of origin differed for plants grown in aCO2 compared with eCO2, suggesting potential complex and multiple control points on photosynthesis. [ABSTRACT FROM AUTHOR]

Heat stress is a major abiotic stress posing a serious threat to plants. Heat-responsive mechanisms in plants are complicated and fine-tuned. Heat signaling transduction and photosynthesis are highly sensitive. Therefore, a thorough understanding of the molecular mechanism in heat stressed-signaling transduction and photosynthesis is necessary to protect crop yield. Current high-throughput proteomics investigations provide more useful information for underlying heat-responsive signaling pathways and photosynthesis modulation in plants. Several signaling components, such as guanosine triphosphate (GTP)-binding protein, nucleoside diphosphate kinase, annexin, and brassinosteroid-insensitive I-kinase domain interacting protein 114, were proposed to be important in heat signaling transduction. Moreover, diverse protein patterns of photosynthetic proteins imply that the modulations of stomatal CO2 exchange, photosystem II, Calvin cycle, ATP synthesis, and chlorophyll biosynthesis are crucial for plant heat tolerance. [ABSTRACT FROM AUTHOR]

Thermal stress during reproductive development and grain-filling phases is a serious threat to the quality and productivity of grain legumes. The optimum temperature range for grain legume crops is 10 - 36°C, above which severe losses in grain yield can occur. Various climatic models have simulated that the temperature near the earth's surface will increase (by up to 4°C) by the end of this century, which will intensify the chances of heat stress in crop plants. The magnitude of damage or injury posed by a high-temperature stress mainly depends on the defence response of the crop and the specific growth stage of the crop at the time of exposure to the high temperature. Heat stress affects grain development in grain legumes because it disintegrates the tapetum layer, which reduces nutrient supply to microspores leading to premature anther dehiscence; hampers the synthesis and distribution of carbohydrates to grain, curtailing the grain-filling duration leading to low grain weight; induces poor pod development and fractured embryos; all of which ultimately reduce grain yield. The most prominent effects of heat stress include a substantial reduction in net photosynthetic rate, disintegration of photosynthetic apparatus and increased leaf senescence. To curb the catastrophic effect of heat stress, it is important to improve heat tolerance in grain legumes through improved breeding and genetic engineering tools and crop management strategies. In this review, we discuss the impact of heat stress on leaf senescence, photosynthetic machinery, assimilate translocation, water relations, grain quality and development processes. Furthermore, innovative breeding, genetic, molecular and management strategies are discussed to improve the tolerance against heat stress in grain legumes. [ABSTRACT FROM AUTHOR]

Increasing temperature and consequent changes in climate adversely affect plant growth and development, resulting in catastrophic loss of wheat productivity. For each degree rise in temperature, wheat production is estimated to reduce by 6%. A detailed overview of morpho-physiological responses of wheat to heat stress may help formulating appropriate strategies for heat-stressed wheat yield improvement. Additionally, searching for possible management strategies may increase productivity and sustainability of growing wheat. The major findings from this review are as follows: (1) heat stress significantly reduces seed germination and seedling growth, cell turgidity, and plant water-use efficiency; (2) at a cellular level, heat stress disturbs cellular functions through generating excessive reactive oxygen species, leading to oxidative stress; (3) the major responses of wheat to heat stress include the enhancement of leaf senescence, reduction of photosynthesis, deactivation of photosynthetic enzymes, and generation of oxidative damages to the chloroplasts; (4) heat stress also reduces grain number and size by affecting grain setting, assimilate translocation and duration and growth rate of grains; (5) effective approaches for managing heat stress in wheat include screening available germplasm under field trials and/or employing marker-assisted selection, application of exogenous protectants to seeds or plants, mapping quantitative trait locus conferring heat resistance and breeding; (6) a well-integrated genetic and agronomic management option may enhance wheat tolerance to heat. However, the success of applying various techniques of heat stress management requires greater understanding of heat tolerance features, molecular cloning, and characterization of genes. The overall success of the complex plant heat stress management depends on the concerted efforts of crop modelers, molecular biologists, and plant physiologists. [ABSTRACT FROM AUTHOR]

High temperature above the optimum has a drastic effect on the stomatal activity and gaseous exchange which ultimately affect the yield and yield attributing traits. In this study, the Net photosynthetic rate (An), stomatal conductance (gs), transpiration rate (E), and intrinsic water use efficiency (iWT) on flag leaves of 100 recombinant inbred lines of bread wheat derived from the cross of WH1021 (heat-tolerant) and WH711 (heat-sensitive) were evaluated. Recombinant inbred lines and both parents were assessed for various gas attributes under two sowing dates i.e., normal (17.11.2015) and late sowing (27.12.2015). The temperature persisted normal from December, 2015 to February, 2016 and there was a consistent rise in temperature in the last week of March. Measurements were made at anthesis and 10 days after anthesis. Average An, E, gs and iWT decreased as the temperature increase. A contrasting difference was observed in timely sown and late sown at 10 days after anthesis for photosynthetic rate and transpiration rate. Decrease in stomatal conductance and water use efficiency was found with delayed sowing and high temperature. There results may prove useful in the selection of physiologically efficient heat-tolerant genotypes with higher rate of photosynthesis. [ABSTRACT FROM AUTHOR]

Since potato production has been expanded into warmer regions, breeding heat-tolerant potato varieties has also been considered among the top priorities in most breeding programs in recent years. Identification of traits related to heat tolerance in potato is crucial for selection of heat-tolerant genotypes. The objective of this study was to evaluate the responses of 17 potato genotypes to high temperature stress for identifying some candidate traits associated with heat tolerance. Haulm dry weight (HDW), leaf area index (LAI), photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), SPAD value, and mean tuber weight (MTW) of potato genotypes at control conditions were significantly and positively correlated with tuber yield of genotypes grown under high temperature conditions, whereas canopy temperature (CT) was negatively associated with tuber yield. Classification of potato genotypes based on heat tolerance was done by principal component analysis with yield-correlated traits. The classification results showed high similarities with the yield performance of genotypes grown under high temperature conditions. The HDW, LAI, Pn, Gs, Tr, CT, and SPAD of potato genotypes grown under normal conditions might be useful traits to screen for heat-tolerant genotypes. [ABSTRACT FROM AUTHOR]

Temperature and the frequency and intensity of heat waves are predicted to increase throughout the 21st century. Germinant seedlings are expected to be particularly vulnerable to heat stress because they are in the boundary layer close to the soil surface where intense heating occurs in open habitats. We quantified leaf thermotolerance and whole-plant physiological responses to heat stress in first-year germinant seedlings in two populations each of Pinus ponderosa P. and C. Lawson (PIPO) and Pseudotsuga menziesii (Mirb.) Franco (PSME) from climates with contrasting precipitation and temperature regimes. Thermotolerance of detached needles was evaluated using chlorophyll fluorescence (FV/FM, FO) and electrolyte leakage. PSME was more heat tolerant than PIPO according to both independent assessments of thermotolerance. Following exposure of whole seedlings to a simulated heat wave at 45 °C for 1 h in a growth chamber, we monitored FV/FM, photosynthesis, stomatal conductance, non-structural carbohydrates (NSCs) and carbon isotope ratios (δ13C) for 14 days. Heat treatment induced significant reductions in FV/FM in both species and a transient reduction in photosynthetic gas exchange only in PIPO 1 day after treatment. Heat treatment induced an increase in glucose + fructose concurrent with a decrease in starch in both species, whereas total NSC and sucrose were not affected by heat treatment. The negative relationship between glucose + fructose and starch observed in treated plants may be due to the conversion of starch to glucose + fructose to aid recovery from heat-induced damage. Populations from drier sites displayed greater δ13C values than those from wetter sites, consistent with higher intrinsic water-use efficiency and drought resistance of populations from drier climates. Thermotolerance and heat stress responses appeared to be phenotypically plastic and representative of the environment in which plants were grown, whereas intrinsic water-use efficiency appeared to reflect ecotypic differentiation and the climate of origin. [ABSTRACT FROM AUTHOR]

A field experiment was conducted during winter (rabi) season of 2014-15 at IARI, New Delhi to evaluate the response of wheat varieties to heat stress. Three wheat varieties (HD 2967, WR 544 and HD 2985) were grown under ambient and elevated temperature (1.9 to 3.4 °C more than ambient during crop season) condition. Results indicated that the variety WR 544 is highly resistant to heat stress as it exhibited stability of leaf area index (LAI), photosynthesis (Pn), grain yield, harvest index (HI) and test weight under elevated temperature. However, HD 2967 recorded more yield but WR 544 performs better in elevated temperature conditions. [ABSTRACT FROM AUTHOR]

Epigenetic modifications are implicated in plant adaptations to abiotic stresses. Exposure of plants to one stress can induce resistance to other stresses, a process termed cross-adaptation, which is not well understood. In this study, we aimed to unravel the epigenetic basis of elevated heat-tolerance in cold-acclimated Brassica rapa by conducting a genome-wide DNA methylation analysis of leaves from control (CK) and cold-acclimated (CA) plants. We found that both methylation and demethylation occurred during cold acclimation. Two significantly altered pathways, malate dehydrogenase activity and carbon fixation, and 1562 differentially methylated genes, including BramMDH1, BraKAT2, BraSHM4, and Bra4CL2, were identified in CA plants. Genetic validation and treatment of B. rapa with 5-aza-2-deoxycytidine (Aza) suggested that promoter demethylation of four candidate genes increased their transcriptional activities. Physiological analysis suggested that elevated heat-tolerance and high growth rate were closely related to increases in organic acids and photosynthesis, respectively. Functional analyses demonstrated that the candidate gene BramMDH1 (mMDH: mitochondrial malate dehydrogenase) directly enhances organic acids and photosynthesis to increase heattolerance and growth rate in Arabidopsis. However, Aza-treated B. rapa, which also has elevated BramMDH1 levels, did not exhibit enhanced heat-tolerance. We therefore suggest that DNA demethylation alone is not sufficient to increase heat-tolerance. This study demonstrates that altered DNA methylation contributes to cross-adaptation. [ABSTRACT FROM AUTHOR]

Short heat waves during grain filling can reduce grain size and consequently yield in wheat (Triticum aestivum L.). Grain weight responses to heat represent the net outcome of reduced photosynthesis, increased mobilisation of stem reserves (water-soluble carbohydrates, WSC) and accelerated senescence in the grain. To compare their relative roles in grain weight responses under heat, these characteristics were monitored in nine wheat genotypes subjected to a brief heat stress at early grain filling (37°C maximum for 3 days at 10 days after anthesis). Compared with the five tolerant varieties, the four susceptible varieties showed greater heat-triggered reductions in final grain weight, grain filling duration, flag leaf chla and chlb content, stem WSC and PSII functionality (Fv/Fm). Despite the potential for reductions in sugar supply to the developing grains, there was little effect of heat on grain filling rate, suggesting that grain size effects of heat may have instead been driven by premature senescence in the grain. Extreme senescence responses potentially masked stem WSC contributions to grain weight stability. Based on these findings, limiting heat-triggered senescence in the grain may provide an appropriate focus for improving heat tolerance in wheat. [ABSTRACT FROM AUTHOR]

Extreme temperature and drought stress are major environmental factors limiting agriculture worldwide. A comprehensive understanding of plant behavior under different environmental conditions can be gained through experiments and through the application of biophysical crop models. This study presents a field experiment conducted with bean exposed to heat and drought stress. Based on an experimental data collection a crop model was set up, calibrated and validated. Hereby, the two different photosynthesis model approaches already implemented in the model, a simple empirical (the Goudriaan and van Laar or GvL model) and a biochemical photosynthesis model approach (the Farquhar-Ball-Collatz or FBC model), were tested. Both photosynthesis model approaches performed adequately under no stress conditions. Under heat stress conditions, yield was underestimated by both models. However, the FBC model performed better than the simpler photosynthesis model approach of the GvL model. The FBC crop model was able to predict the soil water dynamics, the plant growth and the stomatal conductance. [ABSTRACT FROM AUTHOR]

Photosynthesis is one of the most thermo-sensitive processes in plants. Although the severity of heat stress could be attenuated by grafting approach, the primary damaged site of photosynthesis system under heat stress and the regulatory mechanism of rootstock-mediated heat tolerance are poorly understood. In the current study, cucumber plants grafted onto their own roots and heat-tolerant luffa roots were exposed to root-zone heat (25/40°C) and aerial heat (40/25°C) individually and in combination (40/40°C) to understand the response of photosynthetic process by investigating energy absorption and distribution, electron transport in photosystem (PS) II and I, and CO2 assimilation. According to the results, root-zone heat stress inhibited photosynthesis mainly through decreasing Rubisco activity, while aerial heat stress mainly through inhibiting PSII acceptor side. The imbalance in light absorption and utilization resulted in accumulation of reactive oxygen species that caused damage to photosynthetic apparatus, forming a vicious cycle. On the contrary, grafting cucumber onto heattolerant luffa rootstock alleviated heat-induced photosynthetic inhibition and oxidative stress by maintaining higher root vitality, HSP70 accumulation, and antioxidant potential. [ABSTRACT FROM AUTHOR]

Heat shock is considered an abiotic stress for plant growth, but the effects of heat shock on physiological responses of cucumber plant leaves with and without downy mildew disease are still not clear. In this study, cucumber seedlings were exposed to heat shock in greenhouses, and the responses of photosynthetic properties, carbohydrate metabolism, antioxidant enzyme activity, osmolytes, and disease severity index of leaves with or without the downy mildew disease were measured. Results showed that heat shock significantly decreased the net photosynthetic rate, actual photochemical efficiency, photochemical quenching coefficient, and starch content. Heat shock caused an increase in the stomatal conductance, transpiration rate, antioxidant enzyme activities, total soluble sugar content, sucrose content, soluble protein content and proline content for both healthy leaves and downy mildew infected leaves. These results demonstrate that heat shock activated the transpiration pathway to protect the photosystem from damage due to excess energy in cucumber leaves. Potential resistance mechanisms of plants exposed to heat stress may involve higher osmotic regulation capacity related to an increase of total accumulations of soluble sugar, proline and soluble protein, as well as higher antioxidant enzymes activity in stressed leaves. Heat shock reduced downy mildew disease severity index by more than 50%, and clearly alleviated downy mildew development in the greenhouses. These findings indicate that cucumber may have a complex physiological change to resist short-term heat shock, and suppress the development of the downy mildew disease. [ABSTRACT FROM AUTHOR]

Long-term moderate heat is often experienced by plants and will become even more common in the future due to global warming. However, the responses of plants to this stress have not been characterised. In the present study, growth between Arabidopsis thaliana and its relative Arabis paniculata upon long-term exposure to moderate heat was compared. It was found that the latter was more tolerant than the former, and the patterns of physiological and biochemical responses of both plants presented two phases. The early phase involved no significant visible morphological and physiological changes. It occurred during the first third of the heat treatment and was extended when the stress was attenuated. During the later phase, the plants died or were damaged. Heat shock proteins were dramatically induced at the early phase and gradually decreased at the later phase in A. thaliana . By contrast, the levels were induced and maintained in A. paniculata . Profiling of membrane lipids found that the two plants exhibited opposite patterns of lipid remodelling at the early phase: A. paniculata synthesised phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol and showed a rapid decrease in the degree of lipid unsaturation, while A. thaliana degraded its lipids at the early phase and showed an accelerated degradation at the later phase. These biochemical adjustments during the early phase could favor the thermotolerance of A. paniculata . These results suggest that this species could thus be a model for the study of resistance to long-term moderate heat, through a strategy by which plants can adapt to long-term moderate heat. [ABSTRACT FROM AUTHOR]

Heat waves that trigger severe droughts are predicted to increase globally; however, we lack an understanding of how trees respond to the combined change of extreme temperatures and water availability. Here, we studied the impacts of two consecutive heat waves as well as post-stress recovery in young Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) and Robinia pseudoacacia L. (black locust) growing under controlled conditions. Responses were compared under water supply close to the long-term average and under reduced irrigation to represent drought. Exposure to high temperatures (+10 °C above ambient) and vapour pressure deficit strongly affected the trees in terms of water relations, photosynthesis and growth. Douglas-fir used water resources conservatively, and transpiration decreased in response to mild soil water limitation. In black locust, heat stress led to pronounced tree water deficits (stem diameter shrinkage), accompanied by leaf shedding to alleviate stress on the hydraulic system. The importance of water availability during the heat waves became further apparent by a concurrent decline in photosynthesis and stomatal conductance with increasing leaf temperatures in both species, reaching the lowest rates in the heat-drought treatments. Stress severity determined both the speed and the amount of recovery. Upon release of stress, photosynthesis recovered rapidly in drought-treated black locust, while it remained below control rates in heat (t = -2.4, P < 0.05) and heat-drought stressed trees (t = 2.96, P < 0.05). In Douglas-fir, photosynthesis recovered quickly, while water-use efficiency increased in heat-drought trees because stomatal conductance remained reduced (t = -2.92, P < 0.05). Moreover, Douglas-fir was able to compensate for stem-growth reductions following heat (-40%) and heat-drought stress (-68%), but most likely at the expense of storage and other growth processes. Our results highlight the importance of studying heat waves alongside changes in water availability. They further suggest that we should look beyond the actual stress event to identify lagged effects and acclimation processes that may determine tree resilience in the long term. [ABSTRACT FROM AUTHOR]

Worldwide, extensive agricultural losses are attributed to drought, often in combination with heat in Mediterranean climate regions, where grapevine traditionally grows. The available scenarios for climate change suggest increases in aridity in these regions. Under natural conditions plants are affected by a combination of stresses, triggering synergistic or antagonistic physiological, metabolic or transcriptomic responses unique to the combination. However the study of such stresses in a controlled environment can elucidate important mechanisms by allowing the separation of the effects of individual stresses. To gather those effects, cuttings of two grapevine varieties, Touriga Nacional ( TN) and Trincadeira ( TR), were grown under controlled conditions and subjected to three abiotic stresses (drought - WS, heat - HS and high light - LS) individually and in combination two-by-two ( WSHS, WSLS, HSLS) or all three ( WSHSLS). Photosynthesis, water status, contents of H2O2, abscisic acid and metabolites of the ascorbate-glutathione cycle were measured in the leaves. Common and distinct response features were identified in the different stress combinations. Photosynthesis was not hindered in TN by LS, while even individual stresses severely affect photosynthesis in TR. Abscisic acid may be implicated in grapevine osmotic responses since it is correlated with tolerance parameters, especially in combined stresses involving drought. Overall, the responses to drought-including treatments were clearly distinct to those without drought. From the specific behaviours of the varieties, it can be concluded that TN shows a higher capacity for heat dissipation and for withstanding high light intensities, indicating better adjustment to warm conditions, provided that water supply is plentiful. [ABSTRACT FROM AUTHOR]

The effects of high temperature and the polyamine spermine on the physiological and biochemical status of garden pea plants were investigated. The plants were preliminary treated with 1 mM spermine and 24 h later were subjected for 48 h to conditions with daily temperature up to 38°C. High temperature stress caused more than 20% decrease of leaf pigments content and significant suppression of net photosynthesis rate. An enhanced level of lipid peroxidation was observed in leaves suggesting that oxidative stress occurred. A decrease in the content of free proline, total phenolics, and hydrogen peroxide accompanied by an increase of the activity of catalase, superoxide dismutase, and guaiacol peroxidase was established in plants subjected to high temperature. The harmful physiological effects of high temperature were alleviated by spraying the plants with spermine. The preliminary application of spermine retarded leaf pigment loss and maintained photosynthetic rate and antioxidant enzyme activities, as well as content of non-enzymatic antioxidants. [ABSTRACT FROM AUTHOR]

In changing climatic conditions, heat stress caused by high temperature poses a serious threat to rice cultivation. A multiple organizational analysis at physiological, biochemical, and molecular levels is required to fully understand the impact of elevated temperature in rice. This study was aimed at deciphering the elevated temperature response in 11 popular and mega rice cultivars widely grown in India. Physiological and biochemical traits specifically membrane thermostability (MTS), antioxidants, and photosynthesis were studied at vegetative and reproductive phases, which were used to establish a correlation with grain yield under stress. Several useful traits in different genotypes were identified, which will be an important resource to develop high temperature-tolerant rice cultivars. Interestingly, Nagina22 emerged as the best performer in terms of yield as well as expression of physiological and biochemical traits at elevated temperature. It showed lesser relative injury, lesser reduction in chlorophyll content, increased super oxide dismutase, catalase and peroxidase activities, lesser reduction in net photosynthetic rate (PN), high transpiration rate (E), and other photosynthetic/fluorescence parameters contributing to least reduction in spikelet fertility and grain yield at elevated temperature. Furthermore, expression of 14 genes including heat shock transcription factors and heat shock proteins was analyzed in Nagina22 (tolerant) and Vandana (susceptible) at flowering phase, strengthening the fact that N22 performed better at molecular level also during elevated temperature. This study shows that elevated temperature response is complex and involves multiple biological processes that are needed to be characterized to address the challenges of extreme conditions of future climate. [ABSTRACT FROM AUTHOR]

Heat waves already have a large impact on crops and are predicted to become more intense and more frequent in the future. In this study, heat waves were imposed on soybean using infrared heating technology in a fully open-air field experiment. Five separate heat waves were applied to field-grown soybean ( Glycine max) in central Illinois, three in 2010 and two in 2011. Thirty years of historical weather data from Illinois were analyzed to determine the length and intensity of a regionally realistic heat wave resulting in experimental heat wave treatments during which day and night canopy temperatures were elevated 6 °C above ambient for 3 days. Heat waves were applied during early or late reproductive stages to determine whether and when heat waves had an impact on carbon metabolism and seed yield. By the third day of each heat wave, net photosynthesis ( A), specific leaf weight ( SLW), and leaf total nonstructural carbohydrate concentration ( TNC) were decreased, while leaf oxidative stress was increased. However, A, SLW, TNC, and measures of oxidative stress were no different than the control ca. 12 h after the heat waves ended, indicating rapid physiological recovery from the high-temperature stress. That end of season seed yield was reduced (~10%) only when heat waves were applied during early pod developmental stages indicates the yield loss had more to do with direct impacts of the heat waves on reproductive process than on photosynthesis. Soybean was unable to mitigate yield loss after heat waves given during late reproductive stages. This study shows that short high-temperature stress events that reduce photosynthesis and increase oxidative stress resulted in significant losses to soybean production in the Midwest, U.S. The study also suggests that to mitigate heat wave-induced yield loss, soybean needs improved reproductive and photosynthetic tolerance to high but increasingly common temperatures. [ABSTRACT FROM AUTHOR]

Thermotolerance acclimation of photosystem II to heat and drought is well documented, but studies demonstrating developmental impacts on heat tolerance in field-grown plants are limited. Consequently, climatic variables, estimated canopy temperature, predawn leaf water potential (Ψ PD), and the temperature responses of maximum quantum yield of photosystem II ( Fv/ Fm), variable fluorescence ( Fv/ F0), quantum yield of electron transport (φEο) and efficiency of PSI electron acceptor reduction ( REο/ ABS) were characterized for Gossypium hirsutum at three sample times during the growing season (21 June, 2 July and 18 July 2013) under well-watered conditions. The temperature decreasing a given photosynthetic parameter 15% from the optimum is referred to as T15 and served as a standardized measure of heat tolerance. Ambient and estimated canopy temperatures were well within the optimal range for cotton throughout the sample period, and leaves were verified well watered using Ψ PD measurements. However, T15 varied with sample date (highest on July 2 for all parameters), being 2 °C ( Fv/ F0) to 5.5 °C (φEο) higher on July 2 relative to June 21, despite optimal temperature conditions and predawn leaf water potential on all sample dates. These findings suggest that even under optimum temperature conditions and water availability, heat tolerance could be influenced by plant developmental stage. [ABSTRACT FROM AUTHOR]

The role of a proposed micro-fog system in regulating greenhouse environments and enhancing tomato (Solanum lycopersicum L.) productivity during summer season was studied. Experiments were carried out in a multi-span glass greenhouse, which was divided into two identical compartments involving different environments: (1) without environment control and (2) with a micro-fog system operating when the air vapor pressure deficit (VPD) of greenhouse was higher than 0.5 KPa. The micro-fog system effectively alleviated heat stress and evaporative demand in the greenhouse during summer season. The physiologically favourable environment maintained by micro-fog treatment significantly enhanced elongation of leaf and stem, which contributed to a substantial elevation of final leaf area and shoot biomass. These improvements in physiological and morphological traits resulted in around 12.3% increase of marketable tomato yield per plant. Relative growth rate (RGR) of micro-fog treatment was also significantly higher than control plants, which was mainly determined by the substantial elevation in net assimilation rate (NAR), and to a lesser extent caused by leaf area ratio (LAR). Measurement of leaf gas exchange parameters also demonstrated that micro-fog treatment significantly enhanced leaf photosynthesis capacity. Taken together, manipulation of VPD in greenhouses by micro-fog systems effectively enhanced tomato growth and productivity via improving photosynthesis during summer season. [ABSTRACT FROM AUTHOR]

The application of brassinosteroids (BRs) has been reported to alleviate heat stress. This study investigated the effect of 7,8-dihydro-8α-20-hydroxyecdysone (DHECD)-a BR mimic-by comparison with 24-epibrassinolide (EBR) on the changes in photosynthetic performance, lipid peroxidation, and rice seed set. The results demonstrated that 10 M EBR and 10 M DHECD had the best actions to counteract the lethal heat temperature of 47 °C for 2 h indicated by a reduction in the number of wilted leaves and an increase in the relative water content and leaf greenness. Moreover, plants treated with EBR or DHECD were exposed to high day/night temperatures of 40/30 °C for 7 days. EBR-treated and DHECD-treated plants showed a high shoot fresh weight, leaf area, chlorophyll content, and carotenoid content. High temperature significantly decreased the leaf net CO assimilation rate as well as increased lipid peroxidation. The application of EBR and DHECD maintained the high level of the net CO assimilation rate by increasing the stomatal conductance and photochemical quenching. On the other hand, EBR and DHECD decreased the intracellular CO content and non-photochemical quenching leading to enhance photosynthesis under heat stress. EBR-treated and DHECD-treated plants significantly reduced their malondialdehyde and hydrogen peroxide contents as well as increasing their total soluble sugar contents. Moreover, BR treatments increased the filled seed of rice. This study confirmed that DHECD-a BR mimic-has activities of heat stress alleviation similar to EBR. [ABSTRACT FROM AUTHOR]

Heat stress is a major limiting factor of grapevine production and quality. Acclimation and recovery are essential to ensure plant survival, and the recovery mechanisms can be independent of the heat response mechanisms. An experimental set up with and without acclimation to heat followed by recovery [stepwise acclimation and recovery ( SAR) and stepwise recovery ( SR), respectively] was applied to two grapevine varieties, Touriga Nacional ( TN), and Trincadeira ( TR), with different tolerance to abiotic stress. Major differences were found between leaves of SAR and SR, especially after recovery; in SAR, almost all parameters returned to basal levels while in SR they remained altered. Acclimation led to a swifter and short-term antioxidative response, affecting the plant to a lesser extent than SR. Significant differences were found among varieties: upon stress, TN significantly increased ascorbate and glutathione reduction levels, boosting the cell's redox-buffering capacity, while TR needed to synthesize both metabolites, its response being insufficient to keep the redox state at working levels. TR was affected by stress for a longer period and the up-regulation pattern of antioxidative stress genes was more obvious. In TN, heat shock proteins were significantly induced, but the canonical heat-stress gene signature was not evident probably because no shutdown of the housekeeping metabolism was needed. [ABSTRACT FROM AUTHOR]

Bulliform cells are large, thin-walled and highly vacuolated cells, and play an important role in controlling leaf rolling in response to drought and high temperature. However, the molecular mechanisms regulating bulliform cell development have not been well documented. Here, we report isolation and characterisation of a rice leaf-rolling mutant, named shallot-like 2 ( sll2). The sll2 plants exhibit adaxially rolled leaves, starting from the sixth leaf stage, accompanied by increased photosynthesis and reduced plant height and tiller number. Histological analyses showed shrinkage of bulliform cells, resulting in inward-curved leaves. The mutant is recessive and revertible at a rate of 9%. The leaf rolling is caused by a T- DNA insertion. Cloning of the insertion using TAIL- PCR revealed that the T- DNA was inserted in the promoter region of LOC_Os07 g38664. Unexpectedly, the enhanced expression of LOC_Os07 g38664 by the 35S enhancer in the T- DNA is not responsible for the leaf rolling phenotype. Further, the enhancer also exerted a long-distance effect, including up-regulation of several bulliform cell-related genes. sll2 suppressed the outward leaf rolling of oul1 in the sll2oul1 double mutant. We conclude that leaf rolling in sll2 could be a result of the combined effect of multi-genes, implying a complex network in regulation of bulliform cell development. [ABSTRACT FROM AUTHOR]

We examined the effects of foliar application of various nitrogen (urea) concentrations on gas-exchange and chlorophyll (Chl) fluorescence characteristics in bean plants treated by heat stress (42/30°C, day/night temperatures). Heat stress caused reductions in contents of Chl a, Chl b, and in maximum photochemical efficiency of PSII by 13, 20, and 27%, respectively, regardless of the N treatment. However, N fertilization caused significant increases in these parameters, especially at higher N concentrations. The net photosynthetic rate and stomatal conductance were enhanced by 32, 60, and 69% and by 25, 88, and 100% due to addition of 5, 10, and 15 mM N, respectively. However, gas-exchange parameters were reduced by 24% due to heat stress. N fertilization alleviated adverse effects of heat stress. [ABSTRACT FROM AUTHOR]

The chlorophyll fluorescence parameter Fv/Fm reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three‐tiered approach of phenotyping by Fv/Fm to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on Fv/Fm value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North‐Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to Fv/Fm. The high Fv/Fm group maintained significantly higher total chlorophyll and net photosynthetic rate (PN) than the low group, accompanied by higher stomatal conductance (gs), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in PN between the groups was not caused by differences in PSII capacity or gs as the variation in Fv/Fm and intracellular CO2 (Ci) was non‐significant under the given heat stress. This study validated that our three‐tiered approach of phenotyping by Fv/Fm performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis. [ABSTRACT FROM AUTHOR]

Targeting of the heat stress (HS, 40 °C) to shoots, roots or whole plants substantially affects Arabidopsis physiological responses. Effective stress targeting was proved by determination of the expression of HS markers, HsfA2 and HSA32 , which were quickly stimulated in the targeted organ(s), but remained low in non-stressed tissues for at least 2 h. When shoots or whole plants were subjected to HS, a transient decrease in abscisic acid, accompanied by a small increase in active cytokinin levels, was observed in leaves, consistent with stimulation of transpiration, the main cooling mechanism in leaves. HS application targeted to part of plant resulted in a rapid stimulation of expression of components of cytokinin signaling pathway (especially of receptor genes) in the non-exposed tissues, which indicated fast inter-organ communication. Under all HS treatments, shoot apices responded by transient elevation of active cytokinin contents and stimulation of transcription of genes involved in photosynthesis and carbohydrate metabolism. Duration of this stimulation was negatively correlated with stress strength. The impact of targeted HS on the expression of 63 selected genes, including those coding regulatory 14-3-3 proteins, was compared. Stimulation of GRF9 ( GRF14μ ) in stressed organs after 2–6 h may be associated with plant stress adaptation. [ABSTRACT FROM AUTHOR]

Experiments to explore physiological and biochemical differences of the effects of heat stress in ten wheat ( Triticum aestivum L.) cultivars have been performed. Based on the response of photosynthesis rates, cell membrane lipid peroxide concentrations and grain yield to heat, six cultivars were clustered as heat-tolerant (cv. ‘579′, cv. ‘810′, cv. ‘1110′, cv. Terice, cv. Taifun and cv. Vinjett) and four as heat-sensitive (cv. ‘490′, cv. ‘633′, cv. ‘1039′ and cv. ‘1159′). Higher rates of photosynthetic carbon- and light-use were accompanied by lower damage to cell membranes in leaves of tolerant compared to sensitive cultivars under heat stress. The tolerant cv. ‘810′ and the sensitive cv. ‘1039′ were selected for further proteome analysis of leaves. Proteins related to photosynthesis, glycolysis, stress defence, heat shock and ATP production were differently expressed in leaves of the tolerant and sensitive cultivar under heat stress in relation to the corresponding control. The abundance of proteins related to signal transduction, heat shock, photosynthesis, and antioxidants increased, while the abundance of proteins related to nitrogen metabolism decreased in the tolerant cv. ‘810′ under heat stress as compared to the control. Collectively, the results indicate that primarily changes in both the amount and activities of enzymes involved in photosynthesis and antioxidant activities in leaves contributed to higher heat tolerance in the cv. ‘810′ compared to the heat sensitive cv. ‘1039′. [ABSTRACT FROM AUTHOR]

Climate changes increasingly threaten plant growth and productivity. Such changes are complex and involve multiple environmental factors, including rising CO2 levels and climate extreme events. As the molecular and physiological mechanisms underlying plant responses to realistic future climate extreme conditions are still poorly understood, a multiple organizational level analysis (i.e. eco -physiological, biochemical, and transcriptional) was performed, using Arabidopsis exposed to incremental heat wave and water deficit under ambient and elevated CO2. The climate extreme resulted in biomass reduction, photosynthesis inhibition, and considerable increases in stress parameters. Photosynthesis was a major target as demonstrated at the physiological and transcriptional levels. In contrast, the climate extreme treatment induced a protective effect on oxidative membrane damage, most likely as a result of strongly increased lipophilic antioxidants and membrane -protecting enzymes. Elevated CO2 significantly mitigated the negative impact of a combined heat and drought, as apparent in biomass reduction, photosynthesis inhibition, chlorophyll fluorescence decline, H2O2 production, and protein oxidation. Analysis of enzymatic and molecular antioxidants revealed that the stress -mitigating CO2 effect operates through up-regulation of antioxidant defense metabolism, as well as by reduced photorespiration resulting in lowered oxidative pressure. Therefore, exposure to future climate extreme episodes will negatively impact plant growth and production, but elevated CO2 is likely to mitigate this effect. [ABSTRACT FROM AUTHOR]

Physiological responses from sensitive (S156) and resistant (R123) genotypes of ozone bioindicator, snap bean, were investigated after exposing the plants to cumulative, phytotoxic ozone amounts. Daily course of gas-exchange parameters showed delayed stomatal response in S156 leaves to environmental changes comparing to the response of R123 leaves. Potential photosynthetic quantum conversion, Stern-Volmer nonphotochemical quenching (NPQ), and maximum photochemical efficiency of PSII (F/F) values changed differently in the two genotypes between the first and last measuring days. We concluded that the higher ozone sensitivity originated at least partly from inferior regenerating and/or antioxidant capacity. Experimental protocol proved to be determinant on chlorophyll fluorescence parameters: F/F and NPQ declined at midday, and only the sensitive leaves showed a slight increase in NPQ between 12 h and 16 h. We explained these results by moderately high temperatures and shade-adapted state of our experimental plants under substantial ozone stress. On the base of temperature dependence of minimal fluorescence yield (F), critical temperature proved to be higher than 32.7°C for Phaseolus vulgaris under these conditions. We found a strong linear correlation between NPQ and nonphotochemical quenching of F, indicating that NPQ was determined mostly by energy-dependent quenching (q). The q is the light-harvesting complex located component of NPQ and depends on the amount of zeaxanthin molecules bound in PSII proteins. Thus, difference between daily courses of NPQ in the two genotypes was probably due to different ways of utilization of the zeaxanthin pool under the interactive effect of ozone and moderate heat stress. [ABSTRACT FROM AUTHOR]

Leafy radish ( Raphanus sativus L. var. oleiformis Pers.), a temperate crop, is usually heat-sensitive. Line 9911-9 is a heat-tolerant line bred for summer vegetables in Taiwan. Understanding the heat tolerance mechanism of Line 9911-9 may facilitate future breeding programs. We conducted a comparative study on growing plants of heat-tolerant Line 9911-9, moderately tolerant Line 9611, and heat-sensitive ‘Taichung No. 1’ at 40/35 °C and 25/20 °C to elucidate the heat tolerance mechanism of leafy radish. After 28-day of acclimation, the maintenance of net photosynthetic rate (Pn) and chlorophyll fluorescence ( F v / F m ) accompanying a double increase of stomatal conductance ( g s ) and transpiration rate ( E ) in Line 9911-9 revealed that the heat-tolerant line had adjusted physiologically to be more efficient in heat dispersing. Morphologically, Line 9911-9 developed more xylem vessels, an enlarged stomatal aperture by 5-fold, and an increased stomatal opening to 93%, which contributed 6-fold increase in the occupancy of total stomata aperture on leaf without altering stomata density under high temperature stress. Furthermore, the occupancy was correlated with g s and E with a coefficient of 0.87 and 0.98, respectively, confirming that stomatal factors play a crucial role in heat tolerance in leafy radish. Regarding the heat shock response, Line 9911-9 could have sustained over 9 days at 40/35 °C, whereas Line 9611 was maintained over 3 days, with ‘Taichung No. 1’ for only 1 day. Line 9911-9 was able to maintain its cellular membrane stability by inducing the activities of superoxide dismutase (SOD) and catalase (CAT) to scavenge reactive oxygen species (ROS) within the first 24 h and detoxify ROS unbalance under high temperature stress. These results indicated that Line 9911-9 was able to lower the non-stomatal limitation effect on Pn. A high correlation ( R 2 = 0.89) between cotyledon relative injury at 50 °C for 30 min in 21 leafy radish accessions and their yield reduction ratio under high temperature stress suggests that the CMT of cotyledon may be used as a reliable index for selecting heat-tolerant leafy radish. [ABSTRACT FROM AUTHOR]