The ectomycorrhizal fungus Paxillus involutus positively modulates Castanea sativa Miller (var. Marsol) responses to heat and drought co-exposure.

Castanea sativa Miller, a high-valuable crop for Mediterranean countries, is facing frequent and prolonged periods of heat and drought, severely affecting chestnut production. Aiming to tackle this problem, this study unraveled the influence of mycorrhizal association with the fungi Paxillus involutus (Batsch) on young chestnut plants' responses to combined heat (42 °C; 4 h/day) and drought (no irrigation until soil moisture reached 25%) over 21 days of stress exposure. Heat stress had no harmful effects on growth, photosynthesis, nor induced oxidative stress in either mycorrhizal (MR) or non-mycorrhizal (NMR) chestnut plants. However, drought (alone or combined) reduced the growth of NMR plants, affecting water content, leaf production, and foliar area, while also hampering net CO2 assimilation and carbon relations. The mycorrhizal association, however, mitigated the detrimental effects of both stresses, resulting in less susceptibility and fewer growth limitations in MR chestnut plants, which were capable of ensuring a proper carbon flow. Evaluation of the oxidative metabolism revealed increased lipid peroxidation and hydrogen peroxide levels in NMR plants under water scarcity, supporting their higher susceptibility to stress. Conversely, MR plants activated defense mechanisms by accumulating antioxidant metabolites (ascorbate, proline and glutathione), preventing oxidative damage, especially under the combined stress. Overall, drought was the most detrimental condition for chestnut growth, with heat exacerbating stress susceptibility. Moreover, mycorrhizal association with P. involutus substantially alleviated these effects by improving growth, water relations, photosynthesis, and activating defense mechanisms. Thus, this research highlights mycorrhization's potential to enhance C. sativa resilience against climate change, especially at early developmental stages.

Enhancing tomato plants' tolerance to combined heat and salt stress - The role of arbuscular mycorrhizae and biochar.

The Mediterranean basin is highly susceptible to climate change, with soil salinization and the increase in average temperatures being two of the main factors affecting crop productivity in this region. Following our previous studies on describing the detrimental effects of heat and salt stress co-exposure on tomato plants, this study aimed to understand if substrate supplementation with a combination of arbuscular mycorrhizal fungi (AMF) and biochar could mitigate the negative consequences of these stresses. Upon 21 days of exposure, stressed tomato plants grown under supplemented substrates showed increased tolerance to heat (42 °C for 4 h/day), salt (100 mM NaCl), and their combination, presenting increased biomass and flowering rate. The beneficial effects of AMF and biochar were associated with a better ionic balance (i.e. lower sodium accumulation and higher uptake of calcium and magnesium) and increased photosynthetic efficiency. Indeed, these plants presented higher chlorophyll content and improved CO2 assimilation rates. Biochemical data further supported that tomato plants grown with AMF and biochar were capable of efficiently modulating their defence pathways, evidenced by the accumulation of proline, ascorbate, and glutathione, coupled with a lower dependency on energy-costly enzymatic antioxidant players. In summary, the obtained data strongly point towards a beneficial role of combined AMF and biochar as sustainable tools to improve plant growth and development under a climate change scenario, where soil salinization and heat peaks often occur together.

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.

Are tomato plants co-exposed to heat and salinity able to ensure a proper carbon metabolism? - An insight into the photosynthetic hub.

Abiotic stress combinations, such as high temperatures and soil/water salinization, severely threaten crop productivity worldwide. In this work, an integrative insight into the photosynthetic metabolism of tomato plants subjected to salt (100 mM NaCl) and/or heat (42 °C; 4 h/day) was performed. After three weeks, the stress combination led to more severe consequences on growth and photosynthetic pigments than the individual stresses. Regarding the photochemical efficiency, transcript accumulation and protein content of major actors (CP47 and D1) were depleted in all stressed plants, although the overall photochemical yield was not negatively affected under the co-exposure. Gas-exchange studies revealed to be mostly affected by salt (single or combined), which harshly compromised carbon assimilation. Additionally, transcript levels of stress-responsive genes (e.g., HsfA1 and NHX2) were differentially modulated by the single and combined treatments, suggesting the activation of stress-signature responses. Overall, by gathering an insightful overview of the main regulatory hub of photosynthesis, we show that the impacts on the carbon metabolism coming from the combination of heat and salinity, two major conditioners of crop yields, were not harsher than those of single stresses, indicating that the growth impairment might be attributed to a proficient distribution of resources towards defense mechanisms.

On Sandy, Boron-Poor Soils, Liming Induced Severe Boron Deficiency and Drastically Reduced the Dry Matter Yield of Young Olive Trees.

In the northeast of Portugal, like in many parts of the world, most soils are acidic, which may hamper crop productivity. This study presents the findings of a factorial experiment on olive (Olea europaea L.) involving three factors: (i) soil type [schist (Sch) and granite (Gra)]; (ii) cultivars [Cobrançosa (Cob) and Arbequina (Arb)]; and (iii) fertilizer treatments [liming (CaCO3) plus magnesium (Mg) (LMg), phosphorus (P) application (+P), boron (B) application (+B), all fertilizing materials combined (Con+), and an untreated control (Con-)]. Dry matter yield (DMY) did not show significant differences between cultivars, but plants grown in schist soil exhibited significantly higher biomass compared to those in granite soil. Among the treatments, +B and Con+ resulted in the highest DMY (50.8 and 47.2 g pot-1, respectively), followed by +P (34.3 g pot-1) and Con- (28.6 g pot-1). Treatment LMg yielded significantly lower values (15.6 g pot-1) than Con-. LMg raised the pH above 7 (7.36), leading to a severe B deficiency. Although Con+ also raised the pH above 7 (7.48), it ranked among the most productive treatments for providing B. Therefore, when applying lime to B-poor sandy soils, moderate rates are advised to avoid inducing a B deficiency. Additionally, it seems prudent to apply B after lime application.

Different LED light intensity and quality change perennial ryegrass (Lolium perenne L.) physiological and growth responses and water and energy consumption.

Light intensity and spectral composition highly affect plant physiology, growth, and development. According to growing conditions, each species and/or cultivar has an optimum light intensity to drive photosynthesis, and different light spectra trigger photosynthetic responses and regulate plant development differently. For the maintenance of natural sports pitches, namely professional football competitions, turf quality is a key condition. Due to the architecture of most football stadiums, the lawns receive low intensities of natural light, so supplementary artificial lighting above the turf is required. The use of light-emitting diodes (LEDs) can have a higher cost-benefit ratio than traditional high-pressure sodium lamps. The continuous emission spectrum, combined with high spectral selectivity and adjustable optical power, can be used to optimize plant growth and development. Thus, perennial ryegrass (Lolium perenne L.) plants, commonly used for lawns, were primarily grown at three different intensities (200, 300, and 400 µmol m-2 s-1) of cool white light. Despite the higher water and energy consumption, 400 µmol m-2 s-1 maximizes the plant's efficiency, with higher photosynthetic rates and foliar pigment concentration, and more foliar soluble sugars and aboveground biomass accumulation. Then, it was evaluated the perennial ryegrass (Double and Capri cultivars) response to different spectral compositions [100% cool white (W), 80% Red:20% Blue (R80:B20), 90% Red:10% Blue (R90:B10), and 65% Red:15% Green:20% Blue (R65:G15:B20)] at 400 µmol m-2 s-1. Both cultivars exhibited similar responses to light treatments. In general, W contributed to the better photosynthetic performance and R90:B10 to the worst one. Water consumption and aboveground biomass were equal in all light treatments. R80:B20 allows energy savings of 24.3% in relation to the W treatment, showing a good compromise between physiological performance and energy consumption.

Olive Yield and Physicochemical Properties of Olives and Oil in Response to Nutrient Application under Rainfed Conditions.

The effects of mineral fertilizers on the physicochemical properties of olives and oil under rainfed conditions is scarce. In this three-year study, the results of a nitrogen (N), phosphorus (P), potassium (K) and boron (B) fertilization trial carried out in a young rainfed olive grove and arranged as a nutrient omission trial are reported. The control consisted of the application of N, P, K and B (NPKB) and four other treatments corresponded to the removal of one of them (N0, P0, K0 and B0). Olive yield and several variables associated with the physicochemical properties of olives and oil were evaluated. The NPKB treatment increased olive yield compared to the treatment that did not receive N (N0). Although dependent on the climate conditions of the crop season, the NPKB treatment increased fruit weight and the pulp/pit ratio and its fruits tended to accumulate more oil than K0. However, the phenolics concentrations on fruits and oil tended to be lower. All olive oil samples were classified in the "extra virgin" category and all showed a decrease in its stability between 3 and 15 months of storage, regardless of treatment, especially in N0, P0 and B0 treatments. The results of the sensorial analysis indicate that all the oils fell into the medium fruitiness and greenly-fruity category. Only in P0 and B0 were defects detected, namely muddy sediment. Thus, this study seems to indicate the importance of N application, but also a balanced nutrient application and that further studies are needed, given the difficulty in finding clear trends in the response of measured variables to fertilizer treatments.

Uncovering the effects of kaolin on balancing berry phytohormones and quality attributes of Vitis vinifera grown in warm-temperate climate regions.

BACKGROUND: The application of kaolin particle film is considered a short-term strategy against several environmental stresses in areas with a Mediterranean-like climate. However, it is known that temperature fluctuations and water availability over the season can jeopardize kaolin efficiency in many Mediterranean crops. Hence, this study aims to evaluate the effects of kaolin foliar application on berry phytohormones, antioxidant defence, and oenological parameters at veraison and harvest stages of Touriga-Franca (TF) and Touriga-Nacional (TN) grapevines in two growing seasons (2017 and 2018). The 2017 growing season was considered the driest (-147.1 dryness index) and the warmest (2705 °C growing degree days) of the study. RESULTS: In 2017, TF kaolin-treated berries showed lower salicylic acid (-26.6% compared with unsprayed vines) and abscisic acid (ABA) (-10.5%) accumulation at veraison, whereas salicylic acid increased up to 28.8% at harvest. In a less hot season, TN and TF kaolin-treated grapevines showed a twofold in ABA content and a threefold increase in the indole-3-acetic acid content at veraison and lower ABA levels (83.8%) compared with unsprayed vines at harvest. Treated berries showed a decreased sugar content, without compromising malic and tartaric acid levels, and reactive oxygen species accumulation throughout berry ripening. CONCLUSION: The results suggest kaolin exerts a delaying effect in triggering ripening-related processes under severe summer stress conditions. Treated berries responded with improved antioxidant defence and phytohormone balance, showing significant interactions between kaolin treatment, variety, and developmental stage in both assessed years. © 2021 Society of Chemical Industry.

Grey and Black Anti-Hail Nets Ameliorated Apple (Malus × domestica Borkh. cv. Golden Delicious) Physiology under Mediterranean Climate.

The use of anti-hail nets on orchards changes the microclimate underneath the net. This might be of great importance in apple growing regions characterized by high radiation levels and hot and dry climates during the summer season. But, depending on the net colour and on the local climatic conditions, the shade promoted triggers different responses by the trees. Grey and black anti-hail nets were applied in an apple orchard (cv. 'Golden Delicious') located in Northeast Portugal. Under the nets a lower concentration of glomalin related-soil proteins was observed, along with an improvement on trees water status, stomatal conductance, net photosynthetic rate, total chlorophylls, N, Mg, Fe and Cu concentrations, as well as an increase in mean fruit weight. The major difference between nets was on the photosynthetic efficiency, being higher on black net in sunny days, while grey net performed better under cloudy conditions. The use of netting systems proved to be effective in improving "Golden Delicious" apple trees performance under a Mediterranean climate, mainly when the radiation reaching the plants surpass the tree saturation point for photosynthesis. Therefore, these findings anticipate solutions for current and forecasted negative effects of climate change.

Physiological, Biochemical and Molecular Assessment of UV-A and UV-B Supplementation in Solanum lycopersicum.

Daily UV-supplementation during the plant fruiting stage of tomato (Solanum lycopersicum L.) growing indoors may produce fruits with higher nutraceutical value and better acceptance by consumers. However, it is important to ensure that the plant's performance during this stage is not compromised by the UV supplement. We studied the impact of UV-A (1 and 4 h) and UV-B (2 and 5 min) on the photosynthesis of greenhouse-grown tomato plants during the fruiting/ripening stage. After 30 d of daily irradiation, UV-B and UV-A differently interfered with the photosynthesis. UV-B induced few leaf-necrotic spots, and effects are more evidenced in the stimulation of photosynthetic/protective pigments, meaning a structural effect at the Light-Harvesting Complex. UV-A stimulated flowering/fruiting, paralleled with no visible leaf damages, and the impact on photosynthesis was mostly related to functional changes, in a dose-dependent manner. Both UV-A doses decreased the maximum quantum efficiency of photosystem II (Fv/Fm), the effective efficiency of photosystem II (ΦPSII), and gas exchange processes, including net carbon assimilation (PN). Transcripts related to Photosystem II (PSII) and RuBisCO were highly stimulated by UV supplementation (mostly UV-A), but the maintenance of the RuBisCO protein levels indicates that some protein is also degraded. Our data suggest that plants supplemented with UV-A activate adaptative mechanisms (including increased transcription of PSII peptides and RuBisCO), and any negative impacts on photosynthesis do not compromise the final carbohydrate balances and plant yield, thus becoming a profitable tool to improve precision agriculture.

Particle film technology modulates xanthophyll cycle and photochemical dynamics of grapevines grown in the Douro Valley.

Field-grown grapevines are often exposed to multiple environmental stresses, which challenges wine-growers to develop sustainable measures to sustain vine growth, yield, and quality. Under field conditions this task is demanding, due to differences in the magnitudes of stresses and associated plant responses. In this study we explored the hypothesis that kaolin-particle film application improves grapevine photoprotection through the regulation of xanthophyll cycle genes, limiting the thermal dissipation of excess energy under harsh environmental conditions. Hence, we selected two grapevine varieties, Touriga-Nacional (TN) and Touriga-Franca (TF), grown in the Douro Demarcated Region, and evaluated changes in light dissipation mechanisms, xanthophyll cycle components, and the expression of xanthophyll cycle genes during the 2017 summer season. The results showed that, from veraison to ripening, kaolin triggered the up-regulation of violaxanthin de-epoxidase (VvVDE1) and zeaxanthin epoxidase (VvZEP1) genes, indicating optimised regulation of the xanthophyll cycle. Kaolin treatment also decreased chlorophyll (Chla, Chlb, Chl(a+b)) and carotenoid (Car) accumulation under increasing summer stress conditions in both varieties and lowered the non-photochemical quenching (NPQ) of grapevines on ripening, suggesting a long-term response to summer stress. In addition, kaolin-treated grapevines showed increased Chla/Chlb and lower Chl(a+b)/Car ratios, displaying some features of high light adapted leaves. Overall, this study suggests that kaolin application enabled grapevines to benefit from fluctuating periods of summer stress by managing chlorophyll and carotenoid content and limiting down-regulation of both photochemistry and photoinhibition processes. Under Mediterranean field conditions, kaolin application can be considered an efficient method of minimising summer stress impact on grapevines.

Glyphosate-dependent effects on photosynthesis of Solanum lycopersicum L.-An ecophysiological, ultrastructural and molecular approach.

This study aimed to assess the toxicity of glyphosate (GLY; 0, 10, 20 and 30 mg kg-1) in Solanum lycopersicum L., particularly focusing on the photosynthetic metabolism. By combining ecophysiological, ultrastructural, biochemical and molecular tools, the results revealed that the exposure of tomato plants to GLY led to alterations in leaf water balance regulation [increasing stomatal conductance (gs) and decreasing water use efficiency (WUEi) at higher concentrations] and induced slight alterations in the structural integrity of cells, mainly in chloroplasts, accompanied by a loss of cell viability. Moreover, the transcriptional and biochemical control of several photosynthetic-related parameters was reduced upon GLY exposure. However, in vivo chlorophyll fluorometry and IRGA gas-exchange studies revealed that the photosynthetic yield of S. lycopersicum was not repressed by GLY. Overall, GLY impacts cellular and subcellular homeostasis (by affecting chloroplast structure, reducing photosynthetic pigments and inhibiting photosynthetic-related genes transcription), and leaf structure, but is not reducing the carbon flow on a leaf area basis. Altogether, these results suggest a trade-off effect in which GLY-induced toxicity is compensated by a higher photosynthetic activity related to GLY-induced dysfunction in gs and an increase in mesophyll thickness/density, allowing the viable leaf cells to maintain their photosynthetic capacity.

Foliar Pre-Treatment with Abscisic Acid Enhances Olive Tree Drought Adaptability.

Water is the most widely limiting factor for plants distribution, survival and agricultural productivity, their responses to drought and recovery being critical for their success and productivity. Olea europaea L. is a well-adapted species to cyclic drought events, still at considerable expense of carbon reserves and CO2 supply. To study the role of abscisic acid (ABA) as a promoter of drought adaptability, young potted olive trees subjected to three drought-recovery cycles were pre-treated with ABA. The results demonstrated that ABA pre-treatment allowed the delay of the drought effects on stomatal conductance (gs) and net photosynthesis (An), and under severe drought, permitted the reduction of the non-stomatal limitations to An and the relative water content decline, the accumulation of compatible solutes and avoid the decline of photosynthetic pigments, soluble proteins and total thiols concentrations and the accumulation of ROS. Upon rewatering, ABA-sprayed plants showed an early recovery of An. The plant ionome was also changed by the addition of ABA, with special influence on root K, N and B concentrations. The improved physiological and biochemical functions of the ABA-treated plants attenuated the drought-induced decline in biomass accumulation and potentiated root growth and whole-plant water use efficiency after successive drought-rewatering cycles. These changes are likely to be of real adaptive significance, with important implications for olive tree growth and productivity.

Olive tree physiology and chemical composition of fruits are modulated by different deficit irrigation strategies.

BACKGROUND: Cropping practices focusing on agronomic water use efficiency and their impact on quality parameters must be investigated to overcome constraints affecting olive groves. We evaluated the response of olive trees (Olea europaea, cv. 'Cobrançosa') to different water regimes: full irrigation (FI, 100% crop evapotranspiration (ETc )), and three deficit irrigation strategies (DIS) (regulated (RDI, irrigated with 80% of crop evapotranspiration (ETc ) in phases I and III of fruit growth and 10% of ETc in the pit hardening stage), and two continuous sustained strategies (SDI) - a conventional SDI (27.5% of ETc ), and low-frequency irrigation adopted by the farmer (SDIAF, 21.2% of ETc ). RESULTS: The effects of water regimes on the plant water status, photosynthetic performance, metabolite fluctuations and fruit quality parameters were evaluated. All DIS treatments enhanced leaf tissue density; RDI and SDI generally did not affect leaf water status and maintained photosynthetic machinery working properly, and the SDIAF treatment impaired olive tree physiological indicators. The DIS treatments maintained the levels of primary metabolites in leaves, but SDIAF plants showed signs of oxidative stress. Moreover, DIS treatments led to changes in the secondary metabolism, both in leaves and in fruits, with increased total phenolic compounds, ortho-diphenols, and flavonoid concentration, and higher total antioxidant capacity, as well higher oil content. Phenolic profiles showed the relevance of an early harvest in order to obtain higher oleuropein levels with associated higher health benefits. CONCLUSION: Adequate DIS are essential for sustainable olive growing, as they enhance the competitiveness of the sector in terms of olive production and associated quality parameters. © 2019 Society of Chemical Industry.

Evaluating stress responses in cowpea under drought stress.

Drought impact on plants is an increasing concern under the climate change scenario. Cowpea (Vigna unguiculata L. Walp.) is considered as one of the most tolerant legume crops to drought, being the search for the best well-adapted genotypes crucial to face the future challenges. Different approaches have been used for differentiating plant responses to drought stress. Plants of four cowpea genotypes were submitted to three watering regimens (a severe and moderate drought stress, and well-watered control) during 15 days, and several physiological, biochemical and molecular parameters were evaluated. Stressed plants revealed commonly-described drought stress characteristics, but not all assayed parameters were useful for discriminating plants with different drought severities or genotypes. The analyses which have contributed most to genotype discrimination were those related with stomatal function, and biochemical markers such as proline and anthocyanin contents. Antioxidant enzymes activities and related genes expression did not differed among genotypes or upon drought stress treatments, suggesting that scavenging enzymes are not involved in the differential ability of cowpea plants to survive under drought stress. This information will be useful to evaluate and use genetic resources, as well as design strategies for breeding cowpea resistance to drought stress.

Drought Stress Effects and Olive Tree Acclimation under a Changing Climate.

Increasing consciousness regarding the nutritional value of olive oil has enhanced the demand for this product and, consequently, the expansion of olive tree cultivation. Although it is considered a highly resilient and tolerant crop to several abiotic stresses, olive growing areas are usually affected by adverse environmental factors, namely, water scarcity, heat and high irradiance, and are especially vulnerable to climate change. In this context, it is imperative to improve agronomic strategies to offset the loss of productivity and possible changes in fruit and oil quality. To develop more efficient and precise measures, it is important to look for new insights concerning response mechanisms to drought stress. In this review, we provided an overview of the global status of olive tree ecology and relevance, as well the influence of environmental abiotic stresses in olive cultivation. Finally, we explored and analysed the deleterious effects caused by drought (e.g., water status and photosynthetic performance impairment, oxidative stress and imbalance in plant nutrition), the most critical stressor to agricultural crops in the Mediterranean region, and the main olive tree responses to withstand this stressor.

Salicylic acid increases drought adaptability of young olive trees by changes on redox status and ionome.

Different SA concentrations (10, 100 and 1000 µM) were applied in young olive trees (Olea europaea L.) subjected to drought and rewatering. Plants treated with 10 µM exhibited a close behavior to SA-starved plants. Although both 100 and 1000 µM improved the balance between ROS production and scavenging, 100 µM was more efficient. During drought, 100 µM improved ROS detoxification and scavenging by the maintenance or overaccumulation of soluble proteins. During recovery, soluble proteins return to well-watered values and increased the investment in non-enzymatic antioxidants. 100 µM was also the most effective in plant ionome regulation, improving macro and micronutrients uptake, namely P, Fe, Mn and Zn, and changing mineral allocation patterns. Therefore, 100 µM also countered the drought-induced decline in total plant biomass accumulation. The application of suitable SA concentrations is an efficient tool to improve cellular homeostasis and growth of plants subjected to recurrent drought episodes.

Responses of olive plants exposed to different irrigation treatments in combination with heat shock: physiological and molecular mechanisms during exposure and recovery.

MAIN CONCLUSION: A water-deficit period, leading to stomatal control and overexpression of protective proteins (sHSP and DHN), contributes to olive´s tolerance to later imposed stress episodes. Aquaporins modulation is important in olive recovery. Olive is traditionally cultivated in dry farming or in high water demanding irrigated orchards. The impact of climate change on these orchards remains to unveil, as heat and drought episodes are increasing in the Mediterranean region. To understand how young plants face such stress episodes, olive plants growing in pots were exposed to well-irrigated and non-irrigated treatments. Subsequently, plants from each treatment were either exposed to 40 °C for 2 h or remained under control temperature. After treatments, all plants were allowed to grow under well-irrigated conditions (recovery). Leaves were compared for photosynthesis, relative water content, mineral status, pigments, carbohydrates, cell membrane permeability, lipid peroxidation and expression of the protective proteins' dehydrin (OeDHN1), heat-shock proteins (OeHSP18.3), and aquaporins (OePIP1.1 and OePIP2.1). Non-irrigation, whilst increasing carbohydrates, reduced some photosynthetic parameters to values below the ones of the well-irrigated plants. However, when both groups of plants were exposed to heat, well-irrigated plants suffered more drastic decreases of net CO2 assimilation rate and chlorophyll b than non-irrigated plants. Overall, OeDHN1 and OeHSP18.3 expression, which was increased in non-irrigated treatment, was potentiated by heat, possibly to counteract the increase of lipid peroxidation and loss of membrane integrity. Plants recovered similarly from both irrigation and temperature treatments, and recovery was associated with increased aquaporin expression in plants exposed to one type of stress (drought or heat). These data represent an important contribution for further understanding how dry-farming olive will cope with drought and heat episodes.

Kaolin particle film modulates morphological, physiological and biochemical olive tree responses to drought and rewatering.

Regarding the foreseeing climate change is reasonable to expect harmful consequences to olive tree (Olea europaea L.), an iconic species of Mediterranean region. Thus, the selection of practices that allow a better drought resistance and recovery capacity needs the immediate attention of scientific community. This study evaluates the strategies adopted by young potted olive trees, subjected to three cycles of drought and rewatering, in the presence of a reflective clay, kaolin (KL). The results demonstrated that KL induced shade-related leaf structural changes and was effective in keeping leaf water status during the most stressful periods. In general, photosynthetic activity of sprayed plants was improved by the alleviation of drought-induced stomatal and non-stomatal limitations. Moreover, during stress imposition sprayed leaves showed reduced oxidative damages, allowing lower investment in antioxidant defences. Furthermore, sprayed plants also had lower nighttime water losses due to inferior nighttime stomatal conductance, and are able to maintain higher respiration rates. Upon rewatering, the shaded effect conferred by KL limited gas exchange restauration, but improved the plants' capacity to restore the metabolic functions. In spite of the induced physiological and biochemical changes, no significant differences were found in whole-plant water use efficiency and plant biomass accumulation, possibly by the attenuation of photosynthesis restauration during the recovery events. In conclusion, the changes induced by KL might be beneficial under severe conditions, as on realistic Mediterranean field environments.

Salicylic acid modulates olive tree physiological and growth responses to drought and rewatering events in a dose dependent manner.

The predicted accentuation of drought events highlights the importance of optimize plants capacity to tolerate drought, but also the capacity to recovery from it, especially in species, as olive tree (Olea europaea L.), that grows in particularly susceptible regions. Three different concentrations (10, 100 and 1000 µM) of salicylic acid (SA), a stress signaling phytohormone, was sprayed on 3-year-old potted olive trees subjected to three successive drought and rewatering events. Trees responses to SA application are concentration dependent, being 100 µM the most effective concentration to improve drought tolerance and recovery capacity. During drought events, this effectiveness was achieved by osmolytes accumulation, leaf water status maintenance, reduced photosynthetic systems drought-associated damages, and by optimizing shoot/root ratio. The better plant fitness during drought allowed a fast recovery of the physiological functions upon rewatering and reduced the necessity to invest in extra repair damages, allowing the regrowth. The intense abscisic acid (ABA) signal close to upper epidermis in stressed controls suggests a "memory" of the worst water status displayed by those plants. SA attenuated the limitation of total biomass accumulation imposed by drought, mainly in root system, increased water use efficiency and lead to a higher intense signal of indoleacetic acid (IAA) in leaves during recovery period. In summary, in a suitable concentration, SA demonstrate to be a promising tool to increase drought adaptability of olive trees.