Heat stress in grapevine: the pros and cons of acclimation

Stilbene production as part of drought adaptation mechanisms in cultivated grapevine (Vitis vinifera L.) roots modulates antioxidant status

Stilbenes, naturally occurring polyphenolic secondary metabolites, play a pivotal role in adaptation of various plant species to biotic and abiotic factors. Recently, increased attention has been directed toward their potential to enhance plant stress tolerance. We evaluated drought tolerance of three grapevine varieties grown with different levels of water deficit. Throughout, we studied physiological mechanisms associated with drought stress tolerance, particularly stilbene accumulation in root tissues, using HPLC. Additionally, we explored the possible relationship between antioxidant potential and stilbene accumulation in response to water deficit. The results underscore the detrimental impact of water deficit on grapevine growth, water status, and membrane stability index, while revealing varying tolerance among the studied genotypes. Notably, Syrah variety had superior drought tolerance, compared to Razegui and Muscat d'Italie grapes. Under severe water deficit, Syrah exhibited a substantial increase in levels of stilbenic compounds, such as t-resveratrol, t-piceatannol, t-ɛ-viniferin, and t-piceid, in root tissues compared to other genotypes. This increase was positively correlated with total antioxidant activity (TAA), emphasizing the active role of resveratrol and its derivatives in total antioxidant potential. This demonstratres the potential involvement of resveratrol and its derivatives in enhancing antioxidant status of the drought-tolerant Syrah grape variety. Our findings suggest that these stilbenes may function as valuable markers in grapevine breeding programs, offering novel insights for the sustainable cultivation of grapevines in water-limited environments.

Selection of Reference Genes and HSP17.9A Expression Profiling in Heat-Stressed Grapevine Varieties

BACKGROUND

"Touriga Franca" (TF) and "Touriga Nacional" (TN) are grapevine varieties cultivated in the 'Douro Superior' subregion (Northern Portugal) that experience stressful environmental conditions during the summer.

OBJECTIVES

Aiming to profile the expression of stress-responsive genes by quantitative real-time PCR (qPCR) in TF and TN plants growing naturally, three candidate reference genes were first tested under controlled conditions.

METHODS

To simulate a summer's day, TF and TN in vitro plants were exposed to 32 °C-3 h (heat acclimation) and 42 °C-1 h (severe heat stress, HS) followed by two recovery periods (32 °C-3 h and 24 °C-24 h). Leaf samples were collected at the end of each phase. Control plants were kept at 24 °C.

RESULTS

Among the candidate reference genes, the UBC and VAG pair showed the highest stability. The suitability of these genes for qPCR was validated by heat shock protein 17.9A (HSP17.9A) gene profiling. The HSP17.9A expression was up-regulated in both varieties and all experimental phases except in TF control plants. TN showed the highest HSP17.9A relative expression ratio after severe HS.

CONCLUSIONS

TN responded faster than TF to the induced heat shocks. The UBC, VAG, and HSP17.9A genes revealed to be suitable for further qPCR assays in TF and TN grapevine varieties.

UV light and adaptive divergence of leaf physiology, anatomy, and ultrastructure drive heat stress tolerance in genetically distant grapevines

The genetic basis of plant response to light and heat stresses had been unveiled, and different molecular mechanisms of leaf cell homeostasis to keep high physiological performances were recognized in grapevine varieties. However, the ability to develop heat stress tolerance strategies must be further elucidated since the morpho-anatomical and physiological traits involved may vary with genotype × environment combination, stress intensity, and duration. A 3-year experiment was conducted on potted plants of Sardinian red grapevine cultivars Cannonau (syn. Grenache) and Carignano (syn. Carignan), exposed to prolonged heat stress inside a UV-blocking greenhouse, either submitted to low daily UV-B doses of 4.63 kJ m-2 d-1 (+UV) or to 0 kJ m-2 d-1 (-UV), and compared to a control (C) exposed to solar radiation (4.05 kJ m-2 d-1 average UV-B dose). Irrigation was supplied to avoid water stress, and canopy light and thermal microclimate were monitored continuously. Heat stress exceeded one-third of the duration inside the greenhouse and 6% in C. In vivo spectroscopy, including leaf reflectance and fluorescence, allowed for characterizing different patterns of leaf traits and metabolites involved in oxidative stress protection. Cannonau showed lower stomatal conductance under C (200 mmol m-2 s-1) but more than twice the values inside the greenhouse (400 to 900 mmol m-2 s-1), where water use efficiency was reduced similarly in both varieties. Under severe heat stress and -UV, Cannonau showed a sharper decrease in primary photochemical activity and higher leaf pigment reflectance indexes and leaf mass area. UV-B increased the leaf pigments, especially in Carignano, and different leaf cell regulatory traits to prevent oxidative damage were observed in leaf cross-sections. Heat stress induced chloroplast swelling, plastoglobule diffusion, and the accumulation of secretion deposits in both varieties, aggravated in Cannonau -UV by cell vacuolation, membrane dilation, and diffused leaf blade spot swelling. Conversely, in Carignano UV-B, cell wall barriers and calcium oxalate crystals proliferated in mesophyll cells. These responses suggest an adaptive divergence among cultivars to prolonged heat stress and UV-B light. Further research on grapevine biodiversity, heat, and UV-B light interactions may give new insights on the extent of stress tolerance to improve viticulture adaptive strategies in climate change hotspots.

Cytogenetic and Molecular Effects of Kaolin's Foliar Application in Grapevine (Vitis vinifera L.) under Summer's Stressful Growing Conditions

Grapevine varieties from "Douro Superior" (NE Portugal) experience high temperatures, solar radiation, and water deficit during the summer. This summer's stressful growing conditions induce nucleic acids, lipids, and protein oxidation, which cause cellular, physiological, molecular, and biochemical changes. Cell cycle anomalies, mitosis delay, or cell death may occur at the cellular level, leading to reduced plant productivity. However, the foliar application of kaolin (KL) can mitigate the impact of abiotic stress by decreasing leaf temperature and enhancing antioxidant defence. Hence, this study hypothesised that KL-treated grapevine plants growing in NE Portugal would reveal, under summer stressful growing conditions, higher progression and stability of the leaf mitotic cell cycle than the untreated (control) plants. KL was applied after veraison for two years. Leaves, sampled 3 and 5 weeks later, were cytogenetically, molecularly, and biochemically analysed. Globally, integrating these multidisciplinary data confirmed the decreased leaf temperature and enhanced antioxidant defence of the KL-treated plants, accompanied by an improved regularity and completion of the leaf cell cycle relative to the control plants. Nevertheless, the KL efficacy was significantly influenced by the sampling date and/or variety. In sum, the achieved results confirmed the hypothesis initially proposed.

Canopy Architecture and Sun Exposure Influence Berry Cluster-Water Relations in the Grapevine Variety Muscat of Alexandria

Climate-change-related increases in the frequency and intensity of heatwaves affect viticulture, leading to losses in yield and grape quality. We assessed whether canopy-architecture manipulation mitigates the effects of summer stress in a Mediterranean vineyard. The Vitis vinifera L variety Muscat of Alexandria plants were monitored during 2019-2020. Two canopy shoot-positioning treatments were applied: vertical shoot positioning (VSP) and modulated shoot positioning (MSP). In MSP, the west-side upper foliage was released to promote partial shoot leaning, shading the clusters. Clusters were sampled at pea size (PS), veraison (VER), and full maturation (FM). Measurements included rachis anatomy and hydraulic conductance (Kh) and aquaporins (AQP) and stress-related genes expression in cluster tissues. The results show significant seasonal and interannual differences in Kh and vascular anatomy. At VER, the Kh of the rachis and rachis+pedicel and the xylem diameter decreased but were unaffected by treatments. The phloem-xylem ratio was either increased (2019) or reduced (2020) in MSP compared to VSP. Most AQPs were down-regulated at FM in pedicels and up-regulated at VER in pulp. A potential maturation shift in MSP was observed and confirmed by the up-regulation of several stress-related genes in all tissues. The study pinpoints the role of canopy architecture in berry-water relations and stress response during ripening.

The role of soil temperature in mediterranean vineyards in a climate change context

The wine sector faces important challenges related to sustainability issues and the impact of climate change. More frequent extreme climate conditions (high temperatures coupled with severe drought periods) have become a matter of concern for the wine sector of typically dry and warm regions, such as the Mediterranean European countries. Soil is a natural resource crucial to sustaining the equilibrium of ecosystems, economic growth and people's prosperity worldwide. In viticulture, soils have a great influence on crop performance (growth, yield and berry composition) and wine quality, as the soil is a central component of the terroir. Soil temperature (ST) affects multiple physical, chemical and biological processes occurring in the soil as well as in plants growing on it. Moreover, the impact of ST is stronger in row crops such as grapevine, since it favors soil exposition to radiation and favors evapotranspiration. The role of ST on crop performance remains poorly described, especially under more extreme climatic conditions. Therefore, a better understanding of the impact of ST in vineyards (vine plants, weeds, microbiota) can help to better manage and predict vineyards' performance, plant-soil relations and soil microbiome under more extreme climate conditions. In addition, soil and plant thermal data can be integrated into Decision Support Systems (DSS) to support vineyard management. In this paper, the role of ST in Mediterranean vineyards is reviewed namely in terms of its effect on vines' ecophysiological and agronomical performance and its relation with soil properties and soil management strategies. The potential use of imaging approaches, e.g. thermography, is discussed as an alternative or complementary tool to assess ST and vertical canopy temperature profiles/gradients in vineyards. Soil management strategies to mitigate the negative impact of climate change, optimize ST variation and crop thermal microclimate (leaf and berry) are proposed and discussed, with emphasis on Mediterranean systems.

Dynamic Regulation of Grapevine's microRNAs in Response to Mycorrhizal Symbiosis and High Temperature

MicroRNAs (miRNAs) are non-coding small RNAs that play crucial roles in plant development and stress responses and can regulate plant interactions with beneficial soil microorganisms such as arbuscular mycorrhizal fungi (AMF). To determine if root inoculation with distinct AMF species affected miRNA expression in grapevines subjected to high temperatures, RNA-seq was conducted in leaves of grapevines inoculated with either Rhizoglomus irregulare or Funneliformis mosseae and exposed to a high-temperature treatment (HTT) of 40 °C for 4 h per day for one week. Our results showed that mycorrhizal inoculation resulted in a better plant physiological response to HTT. Amongst the 195 identified miRNAs, 83 were considered isomiRs, suggesting that isomiRs can be biologically functional in plants. The number of differentially expressed miRNAs between temperatures was higher in mycorrhizal (28) than in non-inoculated plants (17). Several miR396 family members, which target homeobox-leucine zipper proteins, were only upregulated by HTT in mycorrhizal plants. Predicted targets of HTT-induced miRNAs in mycorrhizal plants queried to STRING DB formed networks for Cox complex, and growth and stress-related transcription factors such as SQUAMOSA promoter-binding-like-proteins, homeobox-leucine zipper proteins and auxin receptors. A further cluster related to DNA polymerase was found in R. irregulare inoculated plants. The results presented herein provide new insights into miRNA regulation in mycorrhizal grapevines under heat stress and can be the basis for functional studies of plant-AMF-stress interactions.

Global transcriptome and gene co-expression network analyses reveal regulatory and non-additive effects of drought and heat stress in grapevine

Despite frequent co-occurrence of drought and heat stress, the molecular mechanisms governing plant responses to these stresses in combination have not often been studied. This is particularly evident in non-model, perennial plants. We conducted large scale physiological and transcriptome analyses to identify genes and pathways associated with grapevine response to drought and/or heat stress during stress progression and recovery. We identified gene clusters with expression correlated to leaf temperature and water stress and five hub genes for the combined stress co-expression network. Several differentially expressed genes were common to the individual and combined stresses, but the majority were unique to the individual or combined stress treatments. These included heat-shock proteins, mitogen-activated kinases, sugar metabolizing enzymes, and transcription factors, while phenylpropanoid biosynthesis and histone modifying genes were unique to the combined stress treatment. Following physiological recovery, differentially expressed genes were found only in plants under heat stress, both alone and combined with drought. Taken collectively, our results suggest that the effect of the combined stress on physiology and gene expression is more severe than that of individual stresses, but not simply additive, and that epigenetic chromatin modifications may play an important role in grapevine responses to combined drought and heat stress.

Phenotyping methods to assess heat stress resilience in grapevine

Global warming has become an issue in recent years in viticulture, as increasing temperatures have a negative impact on grapevine (Vitis vinifera) production and on wine quality. Phenotyping for grapevine response to heat stress is, therefore, important to understand thermotolerance mechanisms, with the aim of improving field management strategies or developing more resilient varieties. Nonetheless, the choice of the phenotypic traits to be investigated is not trivial and depends mainly on the objectives of the study, but also on the number of samples and on the availability of instrumentation. Moreover, the grapevine literature reports few studies related to thermotolerance, generally assessing physiological responses, which highlights the need for more holistic approaches. In this context, the present review offers an overview of target traits that are commonly investigated in plant thermotolerance studies, with a special focus on grapevine, and of methods that can be employed to evaluate those traits. With the final goal of providing useful tools and references for future studies on grapevine heat stress resilience, advantages and limitations of each method are highlighted, and the available or possible implementations are described. In this way, the reader is guided in the choice of the best approaches in terms of speed, complexity, range of application, sensitivity, and specificity.

Developmental Regulation of Transcription in Touriga Nacional Berries under Deficit Irrigation

Grapevine (Vitis vinifera L.) is one of the most economically important crops worldwide, especially due to the economic relevance of wine production. Abiotic stress, such as drought, may contribute to low yield, shifts in quality, and important economic loss. The predicted climate change phenomena point to warmer and dryer Mediterranean environmental conditions; as such, it is paramount to study the effects of abiotic stress on grapevine performance. Deficit irrigation systems are applied to optimize water use efficiency without compromising berry quality. In this research, the effect of two deficit irrigation strategies, sustained deficit irrigation (SDI) and regulated deficit irrigation (RDI), in the grape berry were assessed. The effects of different levels of drought were monitored in Touriga Nacional at key stages of berry development (pea size, véraison, and full maturation) through RNA-Seq transcriptome analysis and by specific differentially expressed genes (DEGs) monitoring through RT-qPCR. Handy datasets were obtained by bioinformatics analysis of raw RNA-Seq results. The dominant proportion of transcripts was mostly regulated by development, with véraison showing more upregulated transcripts. Results showed that primary metabolism is the functional category more severely affected under water stress. Almost all DEGs selected for RT-qPCR were significantly upregulated in full maturation and showed the highest variability at véraison and the lowest gene expression values in the pea size stage.

Mediterranean viticulture in the context of climate change

The exposure of viticulture to climate change and extreme weather conditions makes the winemaking sector particularly vulnerable, being one of its major challenges in the current century. While grapevine is considered a highly tolerant crop to several abiotic stresses, Mediterranean areas are frequently affected by adverse environmental factors, namely water scarcity, heat and high irradiance, and are especially vulnerable to climate change. Due to the high socio-economic value of this sector in Europe, the study of adaptation strategies to mitigate the negative climate change impacts are of main importance for its sustainability and competitiveness. Adaptation strategies include all the set of actions and processes that can be performed in response to climate change. It is crucial to improve agronomic strategies to offset the loss of productivity and likely changes in production and fruit quality. It is important to look for new insights concerning response mechanisms to these stresses to advance with more effective and precise measures. These measures should be adjusted to local terroirs and regional climate change projections for the sustainable development of the winemaking sector. This review describes the direct climate change impacts (on phenology, physiology, yield and berry quality), risks, and uncertainties for Mediterranean viticulture, as well as a set of canopy, soil and water management practices that winegrowers can use to adapt their vines to warmer and drier conditions.

The application of ozonated water rearranges the Vitis vinifera L. leaf and berry transcriptomes eliciting defence and antioxidant responses

Ozonated water has become an innovative, environmentally friendly tool for controlling the development of fungal diseases in the vineyard or during grape postharvest conservation. However, little information is currently available on the effects of ozonated water sprayings on the grapevine physiology and metabolism. Using the microvine model, we studied the transcriptomic response of leaf and fruit organs to this treatment. The response to ozone was observed to be organ and developmental stage-dependent, with a decrease of the number of DEGs (differentially expressed genes) in the fruit from the onset of ripening to later stages. The most highly up-regulated gene families were heat-shock proteins and chaperones. Other up-regulated genes were involved in oxidative stress homeostasis such as those of the ascorbate-glutathione cycle and glutathione S-transferases. In contrast, genes related to cell wall development and secondary metabolites (carotenoids, terpenoids, phenylpropanoids / flavonoids) were generally down-regulated after ozone treatment, mainly in the early stage of fruit ripening. This down-regulation may indicate a possible carbon competition favouring the re-establishment and maintenance of the redox homeostasis rather than the synthesis of secondary metabolites at the beginning of ripening, the most ozone responsive developmental stage.

Kaolin Application Modulates Grapevine Photochemistry and Defence Responses in Distinct Mediterranean-Type Climate Vineyards

At a local scale, kaolin particle-film technology is considered a short-term adaptation strategy to mitigate the adverse effects of global warming on viticulture. This study aims to evaluate kaolin application effects on photochemistry and related defence responses of Touriga Franca (TF) and Touriga Nacional (TN) grapevines planted at two Portuguese winegrowing regions (Douro and Alentejo) over two summer seasons (2017 and 2018). For this purpose, chlorophyll a fluorescence transient analysis, leaf temperature, foliar metabolites, and the expression of genes related to heat stress (VvHSP70) and stress tolerance (VvWRKY18) were analysed. Kaolin application had an inhibitory effect on VvHSP70 expression, reinforcing its protective role against heat stress. However, VvWRKY18 gene expression and foliar metabolites accumulation revealed lower gene expression in TN-treated leaves and higher in TF at Alentejo, while lipid peroxidation levels decreased in both treated varieties and regions. The positive kaolin effect on the performance index parameter (PIABS) increased at ripening, mainly in TN, suggesting that stress responses can differ among varieties, depending on the initial acclimation to kaolin treatment. Moreover, changes on chlorophyll fluorescence transient analysis were more pronounced at the Douro site in 2017, indicating higher stress severity and impacts at this site, which boosted kaolin efficiency in alleviating summer stress. Under applied contexts, kaolin application can be considered a promising practice to minimise summer stress impacts in grapevines grown in Mediterranean-like climate regions.

Mechanisms underlying photosynthetic acclimation to high temperature are different between Vitis vinifera cv. Syrah and Grenache

Photosynthesis acclimation to high temperature differs among and within species. Grapevine intra-specific variation in photosynthetic acclimation to elevated temperature has been scarcely assessed. Our objectives were to (i) evaluate the mechanisms underlying long-term acclimation of photosynthesis to elevated temperature in grapevine, and (ii) determine whether these responses are similar among two varieties. A warming experiment with well irrigated Grenache and Syrah field-grown plants was performed during two growing seasons comparing plants exposed at ambient temperature (control) with plants in open-top chambers (heating) that increased mean air temperature between 1.5 and 3.6°C. Photosynthetic acclimation was assessed through the response of net assimilation (An), Rubisco carboxylation rate (Vcmax) and electron transport rate (Jmax), at leaf temperatures from 20 to 40°C. Our results evidenced different mechanisms for photosynthetic acclimation to elevated temperature. Compared with control, Grenache heated increased An, maintaining higher Vcmax and Jmax at temperatures above 35°C. By contrast, Syrah heated and control presented similar values of An, Vcmax and Jmax, evidencing an adjustment of photosynthesis without increasing C assimilation. Both varieties increased the optimum temperature for An, but to a lesser extent when growth temperature was higher. Our study provides evidence that grapevine varieties present different acclimation mechanisms to expected warming.

Leaf Monoterpene Emission Limits Photosynthetic Downregulation under Heat Stress in Field-Grown Grapevine

Rising temperature is among the most remarkably stressful phenomena induced by global climate changes with negative impacts on crop productivity and quality. It has been previously shown that volatiles belonging to the isoprenoid family can confer protection against abiotic stresses. In this work, two Vitis vinifera cv. 'Chardonnay' clones (SMA130 and INRA809) differing due to a mutation (S272P) of the DXS gene encoding for 1-deoxy-D-xylulose-5-phosphate (the first dedicated enzyme of the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway) and involved in the regulation of isoprenoids biosynthesis were investigated in field trials and laboratory experiments. Leaf monoterpene emission, chlorophyll fluorescence and gas-exchange measurements were assessed over three seasons at different phenological stages and either carried out in in vivo or controlled conditions under contrasting temperatures. A significant (p < 0.001) increase in leaf monoterpene emission was observed in INRA809 when plants were experiencing high temperatures and over two experiments, while no differences were recorded for SMA130. Significant variation was observed for the rate of leaf CO₂ assimilation under heat stress, with INRA809 maintaining higher photosynthetic rates and stomatal conductance values than SMA130 (p = 0.003) when leaf temperature increased above 30 °C. At the same time, the maximum photochemical quantum yield of PSII (F_v/F_m) was affected by heat stress in the non-emitting clone (SMA130), while the INRA809 showed a significant resilience of PSII under elevated temperature conditions. Consistent data were recorded between field seasons and temperature treatments in controlled environment conditions, suggesting a strong influence of monoterpene emission on heat tolerance under high temperatures. This work provides further insights on the photoprotective role of isoprenoids in heat-stressed Vitis vinifera, and additional studies should focus on unraveling the mechanisms underlying heat tolerance on the monoterpene-emitter grapevine clone.

Selecting Aragonez Genotypes Able to Outplay Climate Change-Driven Abiotic Stress

High temperatures and extreme drought are increasingly more frequent in Portugal, which represents a strong threat to viticulture in certain regions of the country. These multifactorial abiotic stresses are threatening viticultural areas worldwide, and the problem can hardly be overcome only by changing cultural practices. This scenario has raised a major challenge for plant scientists to find ways to adapt existing varieties to the new conditions without loss of their characteristic flavors, yield, and associated varietal character of wines. Aragonez (syn. Tempranillo) is one such variety, widely cultivated in Portugal and Spain, with specific characteristics associated with terroir. In this context, insight into intravarietal variability to enable its exploitation for selection becomes an important tool to mitigate the effect of multifactorial stresses driven by climate changes. The present work describes an innovative selection approach: selection for abiotic stress tolerance, measured by the leaf temperature of clones under environmental conditions of drought and extreme heat. This evaluation was complemented with values of yield and quality characteristics of the must (pH, acidity, °Brix, and anthocyanins). The application of this methodology was done in an experimental population of 255 clones of Tempranillo for 3 years. The genotypes were then ranked according to their level of tolerance to abiotic stress without loss of yield/quality. To understand the differences at the transcription level that could account for such variability, several of the most tolerant and most sensitive genotypes were analyzed for key genes using reverse transcriptase-quantitative polymerase chain reaction. The results enabled the selection of a group of genotypes with increased tolerance to stress, in relation to the average of the variety, which maintained the typical must quality of Aragonez. In parallel, several transcripts previously acknowledged as markers for abiotic stress tolerance were identified in several clones and are possible targets for plant breeding and genetic modification and/or to develop screening procedures to select genotypes better adapted to the abiotic stress driven by climate change.

Response of Mycorrhizal 'Touriga Nacional' Variety Grapevines to High Temperatures Measured by Calorespirometry and Near-Infrared Spectroscopy

Heat stress negatively affects several physiological and biochemical processes in grapevine plants. In this work, two new methods, calorespirometry, which has been used to determine temperature adaptation in plants, and near-infrared (NIR) spectroscopy, which has been used to determine several grapevine-related traits and to discriminate among varieties, were tested to evaluate grapevine response to high temperatures. 'Touriga Nacional' variety grapevines, inoculated or not with Rhizoglomus irregulare or Funneliformis mosseae, were used in this study. Calorespirometric parameters and NIR spectra, as well as other parameters commonly used to assess heat injury in plants, were measured before and after high temperature exposure. Growth rate and substrate carbon conversion efficiency, calculated from calorespirometric measurements, and stomatal conductance, were the most sensitive parameters for discriminating among high temperature responses of control and inoculated grapevines. The results revealed that, although this vine variety can adapt its physiology to temperatures up to 40 °C, inoculation with R. irregulare could additionally help to sustain its growth, especially after heat shocks. Therefore, the combination of calorespirometry together with gas exchange measurements is a promising strategy for screening grapevine heat tolerance under controlled conditions and has high potential to be implemented in initial phases of plant breeding programs.

What Is the Impact of Heatwaves on European Viticulture? A Modelling Assessment

Extreme heat events or heatwaves can be particularly harmful to grapevines, posing a major challenge to winegrowers in Europe. The present study is focused on the application of the crop model STICS to assess the potential impacts of heatwaves over some of the most renowned winemaking regions in Europe. For this purpose, STICS was applied to grapevines, using high-resolution weather, soil and terrain datasets from 1986 to 2015. To assess the impact of heatwaves, the weather dataset was artificially modified, generating periods with anomalously high temperatures (+5 °C), at specific onset dates and with specific episode durations (from five to nine days). The model was then run with this modified weather dataset, and the results were compared to the original unmodified runs. The results show that heatwaves can have a very strong impact on grapevine yields. However, these impacts strongly depend on the onset dates and duration of the heatwaves. The highest negative impacts may result in a decrease in the yield by up to −35% in some regions. The results show that regions with a peak vulnerability on 1 August will be more negatively impacted than other regions. Furthermore, the geographical representation of yield reduction hints at a latitudinal gradient in the heatwave impact, indicating stronger reductions in the cooler regions of Central Europe than in the warmer regions of Southern Europe. Despite some uncertainties inherent to the current modelling assessment, the present study highlights the negative impacts of heatwaves on viticultural yields in Europe, which is critical information for stakeholders within the winemaking sector for planning suitable adaptation measures.

Heat stress tolerance assayed in four wine-producing grapevine varieties using a cytogenetic approach

The degree of tolerance to heat stress (HS) differs among grapevine varieties. HS affects the duration and the regularity of the cell cycle in plants. The cytogenetic studies in grapevine are scarce, and the consequences of HS in the mitosis are barely known. This work intends to evaluate the consequences of induced HS in the mitotic cell cycle and chromosomes of four wine-producing varieties: Touriga Franca (TF), Touriga Nacional (TN), Rabigato and Viosinho using a cytogenetic approach. HS (1h at 42 ºC) was induced in plants of the four grapevine varieties that grew in vitro for 11 months. Plants of the same varieties and with equal age were used as control (maintained in vitro at 25 ºC). Three plants per variety and treatment (control and HS) were analysed. After HS, root-tips were collected in all plants and immediately fixed to be used for the preparation of mitotic chromosome spreads. In total, 6,116 root-tip cells were scored. Among the 5,973 dividing cells, 24.33% showed different types of irregularities in all mitotic phases (prophase, metaphase, anaphase, and telophase). After HS, the mitotic index (MI) decreased in the varieties TF and Viosinho, and increased in TN and Rabigato, relatively to the control. However, the average values of MI did not show statistically significant differences (p ˃ 0.05) among varieties, treatments and for the variety x treatment interaction. The percentage of dividing cells with anomalies (%DCA) increased after HS in all varieties relatively to the control. The average values of %DCA presented statistically significant differences (p < 0.05) only between treatments. As far as we know, this work constitutes the first cytogenetic evaluation of the HS effects in the mitotic cell cycle and chromosomes of grapevine using meristematic cells of root-tips. TN has been considered tolerant to various abiotic stresses (drought and excessive heat and light) based on other methodologies. TF and Viosinho have been referred as tolerant to abiotic stress without deeper studies available, and till the development of our investigation, the sensibility of Rabigato was unknown. Among the varieties analysed in this work, Rabigato revealed to be the less tolerant to HS. This research can be useful for selection of grapevine clones more tolerant to HS for commercialization and for the improvement of the economic sustainability.

Grapevine trunk diseases under thermal and water stresses

Heat and water stresses, individually or combined, affect both the plant (development, physiology, and production) and the pathogens (growth, morphology, dissemination, distribution, and virulence). The grapevine response to combined abiotic and biotic stresses is complex and cannot be inferred from the response to each single stress. Several factors might impact the response and the recovery of the grapevine, such as the intensity, duration, and timing of the stresses. In the heat/water stress-GTDs-grapevine interaction, the nature of the pathogens, and the host, i.e., the nature of the rootstock, the cultivar and the clone, has a great importance. This review highlights the lack of studies investigating the response to combined stresses, in particular molecular studies, and the misreading of the relationship between rootstock and scion in the relationship GTDs/abiotic stresses. Grapevine trunk diseases (GTDs) are one of the biggest threats to vineyard sustainability in the next 30 years. Although many treatments and practices are used to manage GTDs, there has been an increase in the prevalence of these diseases due to several factors such as vineyard intensification, aging vineyards, or nursery practices. The ban of efficient treatments, i.e., sodium arsenite, carbendazim, and benomyl, in the early 2000s may be partly responsible for the fast spread of these diseases. However, GTD-associated fungi can act as endophytes for several years on, or inside the vine until the appearance of the first symptoms. This prompted several researchers to hypothesise that abiotic conditions, especially thermal and water stresses, were involved in the initiation of GTD symptoms. Unfortunately, the frequency of these abiotic conditions occurring is likely to increase according to the recent consensus scenario of climate change, especially in wine-growing areas. In this article, following a review on the impact of combined thermal and water stresses on grapevine physiology, we will examine (1) how this combination of stresses might influence the lifestyle of GTD pathogens, (2) learnings from grapevine field experiments and modelling aiming at studying biotic and abiotic stresses, and (3) what mechanistic concepts can be used to explain how these stresses might affect the grapevine plant status.

Linking monoterpenes and abiotic stress resistance in grapevines

Rising temperatures and ozone levels are among the most striking stressful phenomena of global climate changes, and they threaten plants that are unable to react rapidly and efficiently. Generic responses of plants to stresses include the production of excess reactive oxygen species (ROS). Excessive ROS accumulation can lead to extensive oxidation of important components such as nucleic acids, proteins and lipids which can further exacerbate ROS accumulation leading to programmed cell death. Although most studies on plant antioxidants have focused on non-volatile compounds, volatiles belonging to the isoprenoid family have been implicated in the protection against abiotic stresses, in particular thermal and oxidative stress whose frequency and extent is being exacerbated by ongoing global change and anthropogenic pollution. Historically, research has focused on isoprene, demonstrating that isoprene-emitting plants are more tolerant to ozone exposure and heat stress, reducing ROS accumulation. Yet, evidence is being compiled that shows other volatile isoprenoids may be involved in plant responses against abiotic stresses. Grapevines are not isoprene emitters but some varieties produce other volatile isoprenoids such as monoterpenes. We investigated photosynthesis and emission of volatile organic compounds upon heat stress in two Vitis vinifera cv. ‘Chardonnay’ clones differing only for a mutation in the DXS gene (2-C-methyl-D-erythritol 4-phosphate (MEP) pathway), regulating volatile isoprenoid biosynthesis. We showed that the mutation led to a strong increase in monoterpene emission upon heat stress. At the same time, maximum photochemical quantum yield (Fv/Fm ratio) of PSII was affected by the stress in the non-emitting clone while the monoterpene emitter showed a significant resilience, thus indicating a possible antioxidant role of monoterpenes in grapevine. Future mechanistic studies should focus on unveiling the actual mechanism responsible for such findings.

Cutting the Gordian Knot of abiotic stress in grapevine: From the test tube to climate change adaptation

In Mediterranean climate areas, the available scenarios for climate change suggest an increase in the frequency of heat waves and severe drought in summer. Grapevine (Vitis vinifera L.) is a traditional Mediterranean species and is the most valuable fruit crop in the world. Currently, viticulture must adjust to impending climate changes that are already pushing vine-growers toward the use of irrigation, with the concomitant losses in wine quality, and researchers to study tolerance to stress in existing genotypes. The viticulture and winemaking worlds are in demand to understand the physiological potential of the available genotypes to respond to climate changes. In this review, we will focus on the cross-talk between common abiotic stresses that currently affect grapevine productivity and that are prone to affect it deeper in the future. We will discuss results obtained under three experimental stress conditions and that call for specific responses: (1) acclimatization of in vitro plantlets, (2) stress combinations in controlled conditions for research purposes, (3) extreme events in the field that, driven by climate changes, are pushing Mediterranean species to the limit. The different levels of tolerance to stress put in evidence by the plasticity of phenotypic and genotypic response mechanisms, will be addressed. This information is relevant to understand varietal adaptation to impending climate changes and to assist vine growers in choosing genotypes and viticulture practices.

Grape berry flavonoids: a review of their biochemical responses to high and extreme high temperatures

Climate change scenarios predict an increase in average temperatures and in the frequency, intensity, and length of extreme temperature events in many wine regions around the world. In already warm and hot regions, such changes may compromise grape growing and the production of high quality wine as high temperature has been found to affect berry composition critically. Most recent studies focusing on the sole effect of temperature, separated from light and water, on grape berry composition found that high temperature affects a wide range of metabolites, and in particular flavonoids-key compounds for berry and wine quality. A decrease in total anthocyanins is reported in most cases, and appears to be directly associated with high temperature. Changes in anthocyanin composition, and flavonol and proanthocyanidin responses are however less consistent, and reflect the complexity of the underlying biosynthetic pathways and diversity of experimental treatments that have been used in these studies. This review examines the impact of high temperature on the biosynthesis, accumulation, and degradation of flavonoids, and attempts to reconcile the diversity of responses in relation to the latest understanding of flavonoid chemistry and molecular regulation.

Effects of heat stress in the leaf mitotic cell cycle and chromosomes of four wine-producing grapevine varieties

Grapevine varieties respond differentially to heat stress (HS). HS ultimately reduces the photosynthesis and respiratory performance. However, the HS effects in the leaf nuclei and mitotic cells of grapevine are barely known. This work intends to evaluate the HS effects in the leaf mitotic cell cycle and chromosomes of four wine-producing varieties: Touriga Franca (TF), Touriga Nacional (TN), Rabigato, and Viosinho. In vitro plants with 11 months were used in a stepwise acclimation and recovery (SAR) experimental setup comprising different phases: heat acclimation period (3 h-32 °C), extreme HS (1 h-42 °C), and two recovery periods (3 h-32 °C and 24 h-25 °C), and compared to control plants (maintained in vitro at 25 °C). At the end of each SAR phase, leaves were collected, fixed, and used for cell suspensions and chromosome preparations. Normal and abnormal interphase and mitotic cells were observed, scored, and statistically analyzed in all varieties and treatments (control and SAR phases). Different types of chromosomal anomalies in all mitotic phases, treatments, and varieties were found. In all varieties, the percentage of dividing cells with anomalies (%DCA) after extreme HS increased relative to control. TF and Viosinho were considered the most tolerant to HS. TF showed a gradual MI reduction from heat acclimation to HS and the lowest %DCA after HS and 24 h of recovery. Only Viosinho reached the control values after the long recovery period. Extrapolating these data to the field, we hypothesize that during consecutive hot summer days, the grapevine plants will not have time or capacity to recover from the mitotic anomalies caused by high temperatures.

Physiological and agronomical responses to environmental fluctuations of two Portuguese grapevine varieties during three field seasons

Extensive agricultural losses are attributed to heat, often combined with drought. These abiotic stresses occur in the field simultaneously, namely in areas with Mediterranean climate, where grapevine traditionally grows. The available scenarios for climate change suggest an increase in the frequency of heat waves and severe drought events in summer, also affecting the South of Portugal. In this work we monitored several production-related parameters and evaluated the state of the oxidative stress response apparatus of two grapevine varieties, Touriga Nacional (TN) and Trincadeira (TR), with and without irrigation, during three field seasons (2010 to 2012). Overall, results point to a high correlation of most yield and stress-associated parameters with the specific characteristics of each variety and to each season rather than the irrigation treatments. In the season with the driest winter, 2012, the lack of irrigation significantly affected yield in TR, while in the two other seasons the impact of the irrigation regime was much lower. In 2012, the yield of TN was affected by environmental conditions of the previous season. The irrigation treatments significantly affected berry size rather than quality.

Design of a Custom RT-qPCR Array for Assignment of Abiotic Stress Tolerance in Traditional Portuguese Grapevine Varieties

Widespread agricultural losses attributed to drought, often combined with high temperatures, frequently occur in the field, namely in Mediterranean climate areas, where the existing scenarios for climate change indicate an increase in the frequency of heat waves and severe drought events in summer. Grapevine (Vitis vinifera L.) is the most cultivated fruit species in the world and the most valuable one and is a traditional Mediterranean species. Currently, viticulture must adjust to impending climate changes that are already pushing vine-growers toward the use of ancient and resilient varieties. Portugal is very rich in grapevine biodiversity, however, currently, 90% of the total producing area is planted with only 16 varieties. There is a pressing need to understand the existing genetic diversity and the physiological potential of the varieties/genotypes available to be able to respond to climate changes. With the above scenario in mind, an assembly of 65 differentially expresses genes (DEGs) previously identified as responsive to abiotic stresses in two well studied genotypes, 'Touriga Nacional' and 'Trincadeira,' was designed to scan the gene expression of leaf samples from 10 traditional Portuguese varieties growing in two regions with distinct environmental conditions. Forty-five of those DEGs proved to be associated to "abiotic stress" and were chosen to build a custom qPCR array to identify uncharacterized genotypes as sensitive or tolerant to abiotic stress. According to the experimental set-up behind the array design these DEGs can also be used as indicators of the main abiotic stress that the plant is subjected and responding to (drought, heat, or excess light).

Validation of Reference Genes for Quantitative Real-Time PCR during Bicolor Tepal Development in Asiatic Hybrid Lilies (Lilium spp.)

Quantitative real-time PCR (qRT-PCR) is a reliable and high-throughput technique for gene expression studies, but its accuracy depends on the expression stability of reference genes. To date, several reliable reference gene identifications have been reported in Lilium spp., but none has been obtained for lily tepals at different developmental stages. In this study, ten candidate reference genes were selected and evaluated for their expression stability in Lilium 'Tiny Padhye' during the process of bicolor tepal development. The expression stability of these candidates was evaluated by three software programs (geNorm, NormFinder, and BestKeeper) and the comparative ΔCt method, and comprehensive stability rankings were generated by RefFinder. As a result, TIP41-like family gene (TIP41) and actin (ACT) were the best combination of reference genes for tepals at different developmental stages; TIP41 and F-box family gene (F-box) for tepals under shading treatment; ACT, actin11 (ACT11), and elongation factor 1-α (EF1-α) for different tissues; and ACT, TIP41, and ACT11 for all samples. The selected optimal reference genes were further verified by analyzing the expression levels of flavonoid 3'-hydroxylase (LhF3'H) and anthocyanidin 3-O-glucosyltransfersae (LhUFGT) in tepals at different developmental stages. This study provides useful information for gene expression characterization in lilies under different experimental conditions, and can serve as a basis for similar research in other closely related species.

Transcriptomic comparison between two Vitis vinifera L. varieties (Trincadeira and Touriga Nacional) in abiotic stress conditions

BACKGROUND

Predicted climate changes announce an increase of extreme environmental conditions including drought and excessive heat and light in classical viticultural regions. Thus, understanding how grapevine responds to these conditions and how different genotypes can adapt, is crucial for informed decisions on accurate viticultural actions. Global transcriptome analyses are useful for this purpose as the response to these abiotic stresses involves the interplay of complex and diverse cascades of physiological, cellular and molecular events. The main goal of the present work was to evaluate the response to diverse imposed abiotic stresses at the transcriptome level and to compare the response of two grapevine varieties with contrasting physiological trends, Trincadeira (TR) and Touriga Nacional (TN).

RESULTS

Leaf transcriptomic response upon heat, high light and drought treatments in growth room controlled conditions, as well as full irrigation and non-irrigation treatments in the field, was compared in TR and TN using GrapeGene GeneChips®. Breakdown of metabolism in response to all treatments was evidenced by the functional annotation of down-regulated genes. However, circa 30 % of the detected stress-responsive genes are still annotated as «Unknown» function. Selected differentially expressed genes from the GrapeGene GeneChip® were analysed by RT-qPCR in leaves of growth room plants under the combination of individual stresses and of field plants, in both varieties. The transcriptomic results correlated better with those obtained after each individual stress than with the results of plants from field conditions.

CONCLUSIONS

From the transcriptomic comparison between the two Portuguese grapevine varieties Trincadeira and Touriga Nacional under abiotic stress main conclusions can be drawn: 1. A different level of tolerance to stress is evidenced by a lower transcriptome reprogramming in TN than in TR. Interestingly, this lack of response in TN associates with its higher adaptation to extreme conditions including environmental conditions in a changing climate; 2. A complex interplay between stress transcriptional cascades is evidenced by antagonistic and, in lower frequency, synergistic effects on gene expression when several stresses are imposed together; 3. The grapevine responses to stress under controlled conditions are not fully extrapolated to the complex vineyard scenario and should be cautiously considered for agronomic management decision purposes.

Field-Grown Grapevine Berries Use Carotenoids and the Associated Xanthophyll Cycles to Acclimate to UV Exposure Differentially in High and Low Light (Shade) Conditions

Light quantity and quality modulate grapevine development and influence berry metabolic processes. Here we studied light as an information signal for developing and ripening grape berries. A Vitis vinifera Sauvignon Blanc field experiment was used to identify the impacts of UVB on core metabolic processes in the berries under both high light (HL) and low light (LL) microclimates. The primary objective was therefore to identify UVB-specific responses on berry processes and metabolites and distinguish them from those responses elicited by variations in light incidence. Canopy manipulation at the bunch zone via early leaf removal, combined with UVB-excluding acrylic sheets installed over the bunch zones resulted in four bunch microclimates: (1) HL (control); (2) LL (control); (3) HL with UVB attenuation and (4) LL with UVB attenuation. Metabolite profiles of three berry developmental stages showed predictable changes to known UV-responsive compound classes in a typical UV acclimation (versus UV damage) response. Interestingly, the berries employed carotenoids and the associated xanthophyll cycles to acclimate to UV exposure and the berry responses differed between HL and LL conditions, particularly in the developmental stages where berries are still photosynthetically active. The developmental stage of the berries was an important factor to consider in interpreting the data. The green berries responded to the different exposure and/or UVB attenuation signals with metabolites that indicate that the berries actively managed its metabolism in relation to the exposure levels, displaying metabolic plasticity in the photosynthesis-related metabolites. Core processes such as photosynthesis, photo-inhibition and acclimation were maintained by differentially modulating metabolites under the four treatments. Ripe berries also responded metabolically to the light quality and quantity, but mostly formed compounds (volatiles and polyphenols) that have direct antioxidant and/or "sunscreening" abilities. The data presented for the green berries and those for the ripe berries conform to what is known for UVB and/or light stress in young, active leaves and older, senescing tissues respectively and provide scope for further evaluation of the sink/source status of fruits in relation to photosignalling and/or stress management.

Changes in the protective mechanism of photosystem II and molecular regulation in response to high temperature stress in grapevines

The response to high temperature stress, which influences the growth and development of grapes, varies between laboratory conditions and ambient growth conditions, and is poorly understood. In the present study, we investigated the effects of high temperature on grapevines (Vitis vinifera L. × Vitis labrusca L.) grown under artificial and ambient conditions. A temperature of 35 °C did not alter Photosystem II (PS II) activity and the expression of some heat-shock protein (HSPs) genes. These changes were, however, observed at 45 °C under artificial conditions, as well as when the ambient natural temperature was greater than 40 °C. Interestingly, these changes corresponded to shifts in PS II activity and HSPs expression. The protective mechanism of PS II was induced by temperatures greater than 40 °C. These data indicating that the expression of HSFA2, GLOS1 and some heat-shock protein (sHSPs) genes were more sensitive to the heat stress. Unlike the Kyoho grapevines, the Jumeigui grapevines showed rapid and dramatically deterioration in PS II activity and the expression of some heat response genes and HSP21, indicating that the Jumeigui grapevines could not counter the heat stress. These were some differences in PSII activity and the expression of heat response genes between the two cultivated conditions could be attributed to other environmental factors, inherent plant vigor, and the adaptation mechanism.

Differential physiological response of the grapevine varieties Touriga Nacional and Trincadeira to combined heat, drought and light stresses

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 H2 O2 , 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.