Synergistic, additive and antagonistic impacts of drought and herbivory on Pinus sylvestris: leaf, tissue and whole-plant responses and recovery

Herbivory legacy modifies leaf economic spectrum and drought tolerance in two tree species

The concurring effect of herbivory by wild ungulates and drought events is experiencing a notable increase in Mediterranean and temperate forests. While many studies have addressed the influence of drought on plant susceptibility to herbivory, it appears crucial to comprehend the impact of prolonged browsing on the physiological response of plants to increasing water deficit. To this end, we analyzed the effect of long-term recurrent herbivory by ungulates on physiological, biochemical, anatomical and morphological variables of Ilex aquifolium and Fagus sylvatica saplings during the growing seasons of 2018 and 2019 in a mixed sub-Mediterranean forest. We compared plants growing within an exclosure fence since 2006 (unbrowsed) with plants growing outside (browsed) that were also fenced during the study to investigate herbivory legacy. Twelve years of herbivory pressure modified significantly plant functional performance. Independently of the species, browsed plants showed higher root-to-shoot ratio, stem cross-sectional area-to-leaf area ratio, predawn leaf water potential, leaf nitrogen concentration and leaf gas exchange rates than unbrowsed plants. Moreover, browsed plants had lower leaf bulk modulus of elasticity, and higher osmotic potential at full turgor and turgor loss point. Thus, herbivory modified the leaf economic spectrum towards a more resource-acquisitive and less water stress tolerant type. These results suggest that, once browsing has subsided, plants continue to reflect some legacy effects that make them more vulnerable to further abiotic and biotic stresses, which has implications for forest regeneration.

Responses of oak seedlings to increased herbivory and drought: a possible trade-off?

BACKGROUND AND AIMS

Anthropogenic disturbances are causing a co-occurring increase in biotic (ungulate herbivory) and abiotic (drought) stressors, threatening plant reproduction in oak-dominated ecosystems. However, could herbivory compensate for the adverse impact of drought by reducing evapotranspiration? Thus, we investigated the isolated and joint effects of herbivory and drought on oak seedlings of two contrasting Mediterranean species that differ in leaf habit and drought resistance.

METHODS

California oak seedlings from the evergreen, and more drought-resistant, Quercus agrifolia and the deciduous Q. lobata (n = 387) were assigned to a fully crossed factorial design with herbivory and drought as stress factors. Seedlings were assigned in a glasshouse to three to four clipping levels simulating herbivory and three to four watering levels, depending on the species. We measured survival, growth and leaf attributes (chlorophyll, secondary metabolites, leaf area and weight) once a month (May-September) and harvested above- and below-ground biomass at the end of the growing season.

KEY RESULTS

For both oak species, simulated herbivory enhanced seedling survival during severe drought or delayed its adverse effects, probably due to reduced transpiration resulting from herbivory-induced leaf area reduction and compensatory root growth. Seedlings from the deciduous, and less drought-resistant species benefited from herbivory at lower levels of water stress, suggesting different response across species. We also found complex interactions between herbivory and drought on their impact on leaf attributes. In contrast to chlorophyll content which was not affected by herbivory, anthocyanins increased with herbivory - although water stress reduced differences in anthocyanins due to herbivory.

CONCLUSIONS

Herbivory seems to allow Mediterranean oak seedlings to withstand summer drought, potentially alleviating a key bottleneck in the oak recruitment process. Our study highlights the need to consider ontogenetic stages and species-specific traits in understanding complex relationships between herbivory and drought stressors for the persistence and restoration of multi-species oak savannas.

Volatile compounds in citrus in adaptation to water deficit and to herbivory by Diaphorina citri: How the secondary metabolism of the plant is modulated under concurrent stresses. A review

Citrus plants are grown in diverse regions of the world, from subtropical to semi-arid and humid tropical areas. Through mechanisms essential for their survival, they adapt to the environmental conditions to which they are subjected. Although there is vast literature on adaptation of citrus plants to individual stresses, plant responses to interaction among different types of stresses have not been clearly examined. Abiotic or biotic stresses, or a combination of these stresses, result in reorganization of plant energy resources for defense, whether it be for resistance, tolerance, or prevention of stress. Plants generally respond to these stress factors through production of secondary metabolites, such as volatile compounds, derived from different biosynthesis and degradation pathways, which are released through distinct routes. Volatile compounds vary among plant species, meeting the specific needs of the plant. Simultaneous exposure to the stress factors of water deficit and herbivory leads to responses such as qualitative and quantitative changes in the emission of secondary metabolites, and compounds may accumulate within the leaves or predispose the plant to more quickly respond to the stress brought about by the herbivore. The genetic makeup of citrus plants can contribute to a better response to stress factors; however, studies on the emission of volatile compounds in different citrus genotypes under simultaneous stresses are limited. This review examines the effects of abiotic stress due to water deficit and biotic stress due to herbivory by Diaphorina citri in citrus plants and examines their connection with volatile compounds. A summary is made of advances in knowledge regarding the performance of volatile compounds in plant defense against both stress factors, as well as the interaction between them and possible findings in citrus plants. In addition, throughout this review, we focus on how genetic variation of the citrus species is correlated with production of volatile compounds to improve stress tolerance.

'Against all floods': plant adaptation to flooding stress and combined abiotic stresses

Current climate change brings with it a higher frequency of environmental stresses, which occur in combination rather than individually leading to massive crop losses worldwide. In addition to, for example, drought stress (low water availability), also flooding (excessive water) can threaten the plant, causing, among others, an energy crisis due to hypoxia, which is responded to by extensive transcriptional, metabolic and growth-related adaptations. While signalling during flooding is relatively well understood, at least in model plants, the molecular mechanisms of combinatorial flooding stress responses, for example, flooding simultaneously with salinity, temperature stress and heavy metal stress or sequentially with drought stress, remain elusive. This represents a significant gap in knowledge due to the fact that dually stressed plants often show unique responses at multiple levels not observed under single stress. In this review, we (i) consider possible effects of stress combinations from a theoretical point of view, (ii) summarize the current state of knowledge on signal transduction under single flooding stress, (iii) describe plant adaptation responses to flooding stress combined with four other abiotic stresses and (iv) propose molecular components of combinatorial flooding (hypoxia) stress adaptation based on their reported dual roles in multiple stresses. This way, more future emphasis may be placed on deciphering molecular mechanisms of combinatorial flooding stress adaptation, thereby potentially stimulating development of molecular tools to improve plant resilience towards multi-stress scenarios.

Rapid and low-cost screening for single and combined effects of drought and heat stress on the morpho-physiological traits of African eggplant (Solanum aethiopicum) germplasm

Drought and heat are two stresses that often occur together and may pose significant risks to crops in future climates. However, the combined effects of these two stressors have received less attention than single-stressor investigations. This study used a rapid and straightforward phenotyping method to quantify the variation in 128 African eggplant genotype responses to drought, heat, and the combined effects of heat and drought at the seedling stage. The study found that the morphophysiological traits varied significantly among the 128 eggplants, highlighting variation in response to abiotic stresses. Broad-sense heritability was high (> 0.60) for chlorophyll content, plant biomass and performance index, electrolyte leakage, and total leaf area. Positive and significant relationships existed between biomass and photosynthetic parameters, but a negative association existed between electrolyte leakage and morpho-physiological traits. The plants underwent more significant stress when drought and heat stress were imposed concurrently than under single stresses, with the impact of drought on the plants being more detrimental than heat. There were antagonistic effects on the morphophysiology of the eggplants when heat and drought stress were applied together. Resilient genotypes such as RV100503, RV100501, JAMBA, LOC3, RV100164, RV100169, LOC 3, RV100483, GH5155, RV100430, GH1087, GH1087*, RV100388, RV100387, RV100391 maintained high relative water content, low electrolyte leakage, high Fv/Fm ratio and performance index, and increased biomass production under abiotic stress conditions. The antagonistic interactions between heat and drought observed here may be retained or enhanced during several stress combinations typical of plants' environments and must be factored into efforts to develop climate change-resilient crops. This paper demonstrates improvised climate chambers for high throughput, reliable, rapid, and cost-effective screening for heat and drought and combined stress tolerance in plants.

Phytotoxicity of trihalomethanes and trichloroacetic acid on Vigna radiata and Allium cepa plant models

Disinfection by-products (DBPs) are a concern due to their presence in chlorinated wastewater, sewage treatment plant discharge, and surface water, and their potential for environmental toxicity. Despite some attention to their ecotoxicity, little is known about the phytotoxicity of DBPs. This study aimed to evaluate the individual and combined phytotoxicity of four trihalomethanes (THMs: trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and tribromomethane (TBM) and their mixture (THM4)), and trichloroacetic acid (TCAA) using genotoxic and cytotoxic assays. The analysis included seed germination tests using Vigna radiata and root growth tests, mitosis studies, oxidative stress response, chromosomal aberrations (CA), and DNA laddering using Allium cepa. The results showed a progressive increase in root growth inhibition for both plant species as the concentration of DBPs increased. High concentrations of mixtures of four THMs resulted in significant (p < 0.05) antagonistic interactions. The effective concentration (EC50) value for V. radiata was 5655, 3145, 2690, 1465, 3570, and 725 mg/L for TCM, BDCM, DBCM, TBM, THM4, and TCAA, respectively. For A. cepa, the EC50 for the same contaminants was 700, 400, 350, 250, 450, and 105 mg/L, respectively. DBP cytotoxicity was observed through CAs, including C-metaphase, unseparated anaphase, lagging chromosome, sticky metaphase, and bridging. Mitotic depression (MD) increased with dose, reaching up to 54.4% for TCAA (50-500 mg/L). The electrophoresis assay showed DNA fragmentation and shearing, suggesting genotoxicity for some DBPs. The order of phytotoxicity for the tested DBPs was TCAA > TBM > DBCM > BDCM > THM4 > TCM. These findings underscore the need for further research on the phytotoxicity of DBPs, especially given their common use in agricultural practices such as irrigation and the use of sludge as manure.

Non-target ROIMCR LC-MS analysis of the disruptive effects of TBT over time on the lipidomics of Daphnia magna

INTRODUCTION

This study has investigated the temporal disruptive effects of tributyltin (TBT) on lipid homeostasis in Daphnia magna. To achieve this, the study used Liquid Chromatography-Mass Spectrometry (LC-MS) analysis to analyze biological samples of Daphnia magna treated with TBT over time. The resulting data sets were multivariate and three-way, and were modeled using bilinear and trilinear non-negative factor decomposition chemometric methods. These methods allowed for the identification of specific patterns in the data and provided insight into the effects of TBT on lipid homeostasis in Daphnia magna.

OBJECTIVES

Investigation of how are the changes in the lipid concentrations of Daphnia magna pools when they were exposed with TBT and over time using non-targeted LC-MS and advanced chemometric analysis.

METHODS

The simultaneous analysis of LC-MS data sets of Daphnia magna samples under different experimental conditions (TBT dose and time) were analyzed using the ROIMCR method, which allows the resolution of the elution and mass spectra profiles of a large number of endogenous lipids. Changes obtained in the peak areas of the elution profiles of these lipids caused by the dose of TBT treatment and the time after its exposure are analyzed by principal component analysis, multivariate curve resolution-alternative least square, two-way ANOVA and ANOVA-simultaneous component analysis.

RESULTS

87 lipids were identified. Some of these lipids are proposed as Daphnia magna lipidomic biomarkers of the effects produced by the two considered factors (time and dose) and by their interaction. A reproducible multiplicative effect between these two factors is confirmed and the optimal approach to model this dataset resulted to be the application of the trilinear factor decomposition model.

CONCLUSION

The proposed non-targeted LC-MS lipidomics approach resulted to be a powerful tool to investigate the effects of the two factors on the Daphnia magna lipidome using chemometric methods based on bilinear and trilinear factor decomposition models, according to the type of interaction between the design factors.

Drought impairs herbivore-induced volatile terpene emissions by ponderosa pine but not through constraints on newly assimilated carbon

Volatile terpenes serve multiple biological roles including tree resistance against herbivores. The increased frequency and severity of drought stress observed in forests across the globe may hinder trees from producing defense-related volatiles in response to biotic stress. To assess how drought-induced physiological stress alters volatile emissions alone and in combination with a biotic challenge, we monitored pre-dawn water potential, gas-exchange, needle terpene concentrations and terpene volatile emissions of ponderosa pine (Pinus ponderosa) saplings during three periods of drought and in response to simulated herbivory via methyl jasmonate application. Although 3-, 6- and 7-week drought treatments reduced net photosynthetic rates by 20, 89 and 105%, respectively, the magnitude of volatile fluxes remained generally resistant to drought. Herbivore-induced emissions, however, exhibited threshold-like behavior; saplings were unable to induce emissions above constitutive levels when pre-dawn water potentials were below the approximate zero-assimilation point. By comparing compositional shifts in emissions to needle terpene concentrations, we found evidence that drought effects on constitutive and herbivore-induced volatile flux and composition are primarily via constraints on the de novo fraction, suggesting that reduced photosynthesis during drought limits the carbon substrate available for de novo volatile synthesis. However, results from a subsequent 13CO2 pulse-chase labeling experiment then confirmed that both constitutive (<3% labeled) and herbivore-induced (<8% labeled) de novo emissions from ponderosa pine are synthesized predominantly from older carbon sources with little contribution from new photosynthates. Taken together, we provide evidence that in ponderosa pine, drought does not constrain herbivore-induced de novo emissions through substrate limitation via reduced photosynthesis, but rather through more sophisticated molecular and/or biophysical mechanisms that manifest as saplings reach the zero-assimilation point. These results highlight the importance of considering drought severity when assessing impacts on the herbivore-induced response and suggest that drought-altered volatile metabolism constrains induced emissions once a physiological threshold is surpassed.

Synergistic effects of methyl jasmonate treatment and propagation method on Norway spruce resistance against a bark-feeding insect

Utilizing plants with enhanced resistance traits is gaining interest in plant protection. Two strategies are especially promising for increasing resistance against a forest insect pest, the pine weevil (Hylobius abietis): exogenous application of the plant defense hormone methyl jasmonate (MeJA), and production of plants through the clonal propagation method somatic embryogenesis (SE). Here, we quantified and compared the separate and combined effects of SE and MeJA on Norway spruce resistance to pine weevil damage. Plants produced via SE (emblings) and nursery seedlings (containerized and bare-root), were treated (or not) with MeJA and exposed to pine weevils in the field (followed for 3 years) and in the lab (with a non-choice experiment). Firstly, we found that SE and MeJA independently decreased pine weevil damage to Norway spruce plants in the field by 32-33% and 53-59%, respectively, compared to untreated containerized and bare-root seedlings. Secondly, SE and MeJA together reduced damage to an even greater extent, with treated emblings receiving 86-87% less damage when compared to either untreated containerized or bare-root seedlings in the field, and by 48% in the lab. Moreover, MeJA-treated emblings experienced 98% lower mortality than untreated containerized seedlings, and this high level of survival was similar to that experienced by treated bare-root seedlings. These positive effects on survival remained for MeJA-treated emblings across the 3-year experimental period. We conclude that SE and MeJA have the potential to work synergistically to improve plants' ability to resist damage, and can thus confer a strong plant protection advantage. The mechanisms underlying these responses merit further examination.

Separate and combined effects of elevated pCO2 and temperature on the branching reef corals Acropora digitifera and Montipora digitata

Ocean acidification (OA) and warming (OW) are major global threats to coral reef ecosystems; however, studies on their combined effects (OA + OW) are scarce. Therefore, we evaluated the effects of OA, OW, and OA + OW in the branching reef corals Acropora digitifera and Montipora digitata, which have been found to respond differently to environmental changes. Our results indicate that OW has a greater impact on A. digitifera and M. digitata than OA and that the former species is more vulnerable to OW than the latter. OW was the main stressor for increased mortality and decreased calcification in the OA + OW group, and the effect of OA + OW was additive in both species. Our findings suggest that the relative abundance and cover of M. digitata are expected to increase whereas those of A. digitifera may decrease in the near future in Okinawa.

The timing of heat waves has multiyear effects on milkweed and its insect community

Extreme heat events are becoming more frequent and intense as climate variability increases, and these events inherently vary in their timing. We predicted that the timing of a heat wave would determine its consequences for insect communities owing to temporal variation in the susceptibility of host plants to heat stress. We subjected common milkweed (Asclepias syriaca) plants to in-field experimental heat waves to investigate how the timing of heat waves, both seasonally and relative to a biotic stressor (experimental herbivory), affected their ecological consequences. We found that heat waves had multiyear, timing-specific effects on plant-insect communities. Early-season heat waves led to greater and more persistent effects on plants and herbivore communities than late-season heat waves. Heat waves following experimental herbivory had reduced consequences. Our results show that extreme climate events can have complex, lasting ecological effects beyond the year of the event-and that timing is key to understanding those effects.

Impacts of Dutch elm disease-causing fungi on foliage photosynthetic characteristics and volatiles in Ulmus species with different pathogen resistance

Global warming affects the abiotic and biotic growth environment of plants, including the spread of fungal diseases such as Dutch elm disease (DED). Dutch elm disease-resistance of different Ulmus species varies, but how this is reflected in leaf-level physiological pathogen responses has not been investigated. We studied the impacts of mechanical injury alone and mechanical injury plus inoculation with the DED-causing pathogens Ophiostoma novo-ulmi subsp. novo-ulmi and O. novo-ulmi subsp. americana on Ulmus glabra, a more vulnerable species, and U. laevis, a more resistant species. Plant stress responses were evaluated for 12 days after stress application by monitoring leaf net CO2 assimilation rate (A), stomatal conductance (gs), ratio of ambient to intercellular CO2 concentration (Ca/Ci) and intrinsic water-use efficiency (A/gs), and by measuring biogenic volatile (VOC) release by plant leaves. In U. glabra and U. laevis, A was not affected by time, stressors or their interaction. Only in U. glabra, gs and Ca/Ci decreased in time, yet recovered by the end of the experiment. Although the emission compositions were affected in both species, the stress treatments enhanced VOC emission rates only in U. laevis. In this species, mechanical injury especially when combined with the pathogens increased the emission of lipoxygenase pathway volatiles and dimethylallyl diphosphate and geranyl diphosphate pathway volatiles. In conclusion, the more resistant species U. laevis had a more stable photosynthesis, but stronger pathogen-elicited volatile response, especially after inoculation by O. novo-ulmi subsp. novo-ulmi. Thus, stronger activation of defenses might underlay higher DED-resistance in this species.

Differential tolerance of the woody invasive Hakea sericea to drought and terminal heat stress

Drought and extreme temperatures are likely to be more common and intense in the Mediterranean region as a consequence of climate change. Both stresses usually arise together in the field, but our understanding of their joint influence on the performance of invasive alien species (IAS) is limited. Thus, the main objective of the present study is to fill this gap by analyzing the individual and combined effects of drought and terminal heat stress on the leaf physiology, biochemistry and growth of Hakea sericea Schrader, one of the most problematic IAS in the Mediterranean-type ecosystems. In this study, 1-year-old plants of H. sericea were exposed to four treatments under controlled conditions: control (CT), drought (DS), terminal heat stress (Ht), and combined Ht and DS (DHt). The DS treatment alone caused a marked reduction in shoot biomass, net photosynthetic (A) rate and stomatal conductance, while increasing the proline content, as compared with CT plants. In turn, the Ht treatment promoted the accumulation of malondialdehyde but hastened the decline in all gas exchange parameters, and also decreased leaf photosynthetic pigments, carotenoids, proline and relative water contents Exposure of H. sericea plants to the combined DHt exacerbated the impacts of Ht, which was accompanied by significant decreases in net photosynthetic and transpiration rates, and intrinsic water-use efficiency. Principal component analysis clearly separated the DHt from the other treatments and revealed similarities between DS and CT treatment. These findings suggest that xerothermic weather conditions might modify the fitness, competitive ability, resilience and spread of this IAS, thereby providing opportunities for its control.

Abiotic and Herbivory Combined Stress in Tomato: Additive, Synergic and Antagonistic Effects and Within-Plant Phenotypic Plasticity

Background: Drought, N deficiency and herbivory are considered the most important stressors caused by climate change in the agro- and eco-systems and varied in space and time shaping highly dynamic and heterogeneous stressful environments. This study aims to evaluate the tomato morpho-physiological and metabolic responses to combined abiotic and herbivory at different within-plant spatial levels and temporal scales. Methods: Leaf-level morphological, gas exchange traits and volatile organic compounds (VOCs) profiles were measured in tomato plants exposed to N deficiency and drought, Tuta absoluta larvae and their combination. Additive, synergistic or antagonistic effects of the single stress when combined were also evaluated. Morpho-physiological traits and VOCs profile were also measured on leaves located at three different positions along the shoot axes. Results: The combination of the abiotic and biotic stress has been more harmful than single stress with antagonistic and synergistic but non-additive effects for the morpho-physiological and VOCs tomato responses, respectively. Combined stress also determined a high within-plant phenotypic plasticity of the morpho-physiological responses. Conclusions: These results suggested that the combined stress in tomato determined a “new stress state” and a higher within-plant phenotypic plasticity which could permit an efficient use of the growth and defense resources in the heterogeneous and multiple stressful environmental conditions.

Responses to simultaneous anthropogenic and biological stressors were mixed in an experimental saltmarsh ecosystem

Coastal ecosystems are essential for absorbing and bouncing back from the impacts of climate change, yet accelerating climate change is causing anthropogenically-derived stressors in these ecosystems to grow. The effects of stressors are more difficult to foresee when they act simultaneously, however, predicting these effects is critical for understanding ecological change. Spartina alterniflora (Spartina), a foundational saltmarsh plant key to coastal resilience, is subject to biological stress such as herbivory, as well as anthropogenic stress such as chemical pollution. Using saltmarsh mesocosms as a model system in a fully factorial experiment, we tested whether the effects of herbivory and two chemicals (oil and dispersant) were mediated or magnified in combination. Spartina responded to stressors asynchronously; ecophysiology responded negatively to oil and herbivores in the first 2-3 weeks of the experiment, whereas biomass responded negatively to oil and herbivores cumulatively throughout the experiment. We generally found mixed multi-stressor effects, with slightly more antagonistic effects compared to either synergistic or additive effects, despite significant reductions in Spartina biomass and growth from both chemical and herbivore treatments. We also observed an indirect positive effect of oil on Spartina, via a direct negative effect on insect herbivores. Our findings suggest that multi-stressor effects in our model system, 1) are mixed but can be antagonistic more often than expected, a finding contrary to previous assumptions of primarily synergistic effects, 2) can vary in duration, 3) can be difficult to discern a priori, and 4) can lead to ecological surprises through indirect effects with implications for coastal resilience. This leads us to conclude that understanding the simultaneous effects of multiple stressors is critical for predicting foundation-species persistence, discerning ecosystem resilience, and managing and mitigating impacts on ecosystem services.

Comparing Exogenous Methods to Induce Plant-Resistance Against a Bark-Feeding Insect

Exogenous application of the plant hormone methyl jasmonate (MeJA) can trigger induced plant defenses against herbivores, and has been shown to provide protection against insect herbivory in conifer seedlings. Other methods, such as mechanical damage to seedlings, can also induce plant defenses, yet few have been compared to MeJA and most studies lack subsequent herbivory feeding tests. We conducted two lab experiments to: (1) compare the efficacy of MeJA to mechanical damage treatments that could also induce seedling resistance, (2) examine if subsequent insect damage differs depending on the time since induction treatments occurred, and (3) assess if these induction methods affect plant growth. We compared Scots pine (Pinus sylvestris) seedlings sprayed with MeJA (10 or 15 mM) to seedlings subjected to four different mechanical bark damage treatments (two different bark wound sizes, needle-piercing damage, root damage) and previous pine weevil (Hylobius abietis) damage as a reference treatment. The seedlings were exposed to pine weevils 12 or 32 days after treatments (early and late exposure, hereafter), and resistance was measured as the amount of damage received by plants. At early exposure, seedlings treated with needle-piercing damage received significantly more subsequent pine weevil feeding damage than those treated with MeJA. Seedlings treated with MeJA and needle-piercing damage received 84% less and 250% more pine weevil feeding, respectively, relative to control seedlings. The other treatments did not differ statistically from control or MeJA in terms of subsequent pine weevil damage. For the late exposure group, plants in all induction treatments tended to receive less pine weevil feeding (yet this was not statistically significant) compared to control seedlings. On the other hand, MeJA significantly slowed down seedling growth relative to control and all other induction treatments. Overall, the mechanical damage treatments appeared to have no or variable effects on seedling resistance. One of the treatments, needle-piercing damage, actually increased pine weevil feeding at early exposure. These results therefore suggest that mechanical damage shows little potential as a plant protection measure to reduce feeding by a bark-chewing insect.

Global Warming, Climate Change, and Environmental Pollution: Recipe for a Multifactorial Stress Combination Disaster

Global warming, climate change, and environmental pollution present plants with unique combinations of different abiotic and biotic stresses. Although much is known about how plants acclimate to each of these individual stresses, little is known about how they respond to a combination of many of these stress factors occurring together, namely a multifactorial stress combination. Recent studies revealed that increasing the number of different co-occurring multifactorial stress factors causes a severe decline in plant growth and survival, as well as in the microbiome biodiversity that plants depend upon. This effect should serve as a dire warning to our society and prompt us to decisively act to reduce pollutants, fight global warming, and augment the tolerance of crops to multifactorial stress combinations.

Phenolic and lipophilic metabolite adjustments in Olea europaea (olive) trees during drought stress and recovery

The frequency of combined stress events is increasing due to climate change and represents a new threat to olive (Olea europaea) culture. How olive plants modulate their profile of metabolites under multiple stressing agents remains to unveil, although several metabolites affect plants' resilience, and olive production and quality. Young olive plants were exposed to a water deficit (WD) for 30 days and then exposed to a shock of heat and high UVB-radiation (WD^HS+UVB treatment) for 2 days. Then, plants were re-watered and grown under optimal conditions (recovery) for 30 days. Leaves were collected after stress and recovery, analysed by liquid and gas chromatography, and the lipophilic and phenolic profiles were characterized. Except for the oleuropein derivatives, the qualitative metabolite profile was similar during stress and recovery. Metabolite increases or decreases in response to stress were stronger when WD was followed by WD^HS+UVB treatment. Phenolic compounds (luteolin-7-O-glucoside, quercetin-3-O-rutinoside, apigenin-7-O-glucoside, chrysoeriol-7-O-glucoside, kaempferol derivatives, oleuropein, and lucidumoside C) were the most involved after WD and WD^HS+UVB, possibly acting as reactive oxygen species (ROS) scavengers. Lipophilic compounds were more relevant during the recovery period. The catabolism of fatty acids and carbohydrates may provide the necessary energy for plant performance reestablishment, and sterols, long-chain alkanes, and terpenes metabolic pathways may be shifted for the production of compounds with a more important stress protection role. This work highlights for the first time that tolerance mechanisms activated by WD in olive plants are related to metabolite changes, that are adjusted when other stressors are overlapped (WD^HS+UVB), and also help the plants recover. This metabolites' plasticity represents an essential contribution to understanding how dry-farming olive orchards may deal with drought combined with high UV-B or heat.

Relationships between nitrogen cycling microbial community abundance and composition reveal the indirect effect of soil pH on oak decline

Tree decline is a global concern and the primary cause is often unknown. Complex interactions between fluctuations in nitrogen (N) and acidifying compounds have been proposed as factors causing nutrient imbalances and decreasing stress tolerance of oak trees. Microorganisms are crucial in regulating soil N available to plants, yet little is known about the relationships between soil N-cycling and tree health. Here, we combined high-throughput sequencing and qPCR analysis of key nitrification and denitrification genes with soil chemical analyses to characterise ammonia-oxidising bacteria (AOB), archaea (AOA) and denitrifying communities in soils associated with symptomatic (declining) and asymptomatic (apparently healthy) oak trees (Quercus robur and Q. petraea) in the United Kingdom. Asymptomatic trees were associated with a higher abundance of AOB that is driven positively by soil pH. No relationship was found between AOA abundance and tree health. However, AOA abundance was driven by lower concentrations of NH₄⁺, further supporting the idea of AOA favouring lower soil NH₄⁺ concentrations. Denitrifier abundance was influenced primarily by soil C:N ratio, and correlations with AOB regardless of tree health. These findings indicate that amelioration of soil acidification by balancing C:N may affect AOB abundance driving N transformations, reducing stress on declining oak trees.

Single and Combined Abiotic Stress in Maize Root Morphology

Plants are continually exposed to multiple stresses, which co-occur in nature, and the net effects are frequently more nonadditive (i.e., synergistic or antagonistic), suggesting "unique" responses with respect to that of the individual stress. Further, plant stress responses are not uniform, showing a high spatial and temporal variability among and along the different organs. In this respect, the present work investigated the morphological responses of different root types (seminal, seminal lateral, primary and primary lateral) of maize plants exposed to single (drought and heat) and combined stress (drought + heat). Data were evaluated by a specific root image analysis system (WinRHIZO) and analyzed by uni- and multivariate statistical analyses. The results indicated that primary roots and their laterals were the types more sensitive to the single and combined stresses, while the seminal laterals specifically responded to the combined only. Further, antagonistic and synergistic effects were observed for the specific traits in the primary and their laterals and in the seminal lateral roots in response to the combined stress. These results suggested that the maize root system modified specific root types and traits to deal with different stressful environmental conditions, highlighting that the adaptation strategy to the combined stress may be different from that of the individual ones. The knowledge of "unique or shared" responses of plants to multiple stress can be utilized to develop varieties with broad-spectrum stress tolerance.

Interacting effects of insect and ungulate herbivory on Scots pine growth

Most plants are subjected to damage from multiple species of herbivores, and the combined impact on plant growth can be non-additive. Since plant response to herbivores tends to be species specific, and change with repeated damage, the outcome likely depend on the sequence and number of attacks. There is a high likelihood of non-additive effects on plant growth by damage from mammals and insects, as mammalian herbivory can alter insect herbivore damage levels, yet few studies have explored this. We report the growth response of young Scots pine trees to sequential mammal and insect herbivory, varying the sequence and number of damage events, using an ungulate-pine-sawfly system. Combined sawfly and ungulate herbivory had both additive and non-additive effects on pine growth—the growth response depended on the combination of ungulate browsing and sawfly defoliation (significant interaction effect). Repeated sawfly herbivory reduced growth (compared to single defoliation) on un-browsed trees. However, on browsed trees, depending on when sawfly defoliation was combined with browsing, trees exposed to repeated sawfly herbivory had both higher, lower and the same growth as trees exposed to a single defoliation event. We conclude that the sequence of attacks by multiple herbivores determine plant growth response.

Physiological responses of rosewoods Dalbergia cochinchinensis and D. oliveri under drought and heat stresses

Dalbergia cochinchinensis and D. oliveri are classified as vulnerable and endangered, respectively, in the IUCN Red List and under continued threat from deforestation and illegal harvesting for rosewood. Despite emerging efforts to conserve and restore these species, little is known of their responses to drought and heat stress, which are expected to increase in the Greater Mekong Subregion where the species co‐occur and are endemic. In this study of isolated and combined drought and heat effects, we found that D. oliveri had an earlier stomatal closure and more constant midday water potential in response to increasing drought level, suggesting that D. oliveri is relatively isohydric while D. cochinchinensis is relatively anisohydric. Heat shock and drought had synergistic effects on stomatal closure. Our results indicate contrasting relationships in water relations, photosynthetic pigment levels, and total soluble sugars. An increase in chlorophyll a was observed in D. cochinchinensis during drought, and a concomitant increase in carotenoid content likely afforded protection against photo‐oxidation. These physiological changes correlated with higher total soluble sugars in D. cochinchinensis. By contrast, D. oliveri avoided drought by reducing chlorophyll content and compromising productivity. Anisohydry and drought tolerance in D. cochinchinensis are adaptations which fit well with its ecological niche as a pioneering species with faster growth in young trees. We believe this understanding of the stress responses of both species will be crucial to their effective regeneration and conservation in degraded habitats and in the face of climate change.

Herbivory and climate as drivers of woody plant growth: Do deer decrease the impacts of warming?

Vegetation at ecotone transitions between open and forested areas is often heavily affected by two key processes: climate change and management of large herbivore densities. These both drive woody plant state shifts, determining the location and the nature of the limit between open and tree or shrub-dominated landscapes. In order to adapt management to prevailing and future climate, we need to understand how browsing and climatic factors together affect the growth of plants at biome borders. To disentangle herbivory and climate effects, we combined long-term tree growth monitoring and dendroecology to investigate woody plant growth under different temperatures and red deer (Cervus elaphus) herbivory pressures at forest-moorland ecotones in the Scottish highlands. Reforestation and deer densities are core and conflicting management concerns in the area, and there is an urgent need for additional knowledge. We found that deer herbivory and climate had significant and interactive effects on tree growth: in the presence of red deer, pine (Pinus sylvestris) growth responded more strongly to annual temperature than in the absence of deer, possibly reflecting differing plant-plant competition and facilitation conditions. As expected, pine growth was negatively related to deer density and positively to temperature. However, at the tree population level, warming decreased growth when more than 60% of shoots were browsed. Heather (Calluna vulgaris) growth was negatively related to temperature and the direction of the response to deer switched from negative to positive when mean annual temperatures fell below 6.0°C. In addition, our models allow estimates to be made of how woody plant growth responds under specific combinations of temperature and herbivory, and show how deer management can be adapted to predicted climatic changes in order to more effectively achieve reforestation goals. Our results support the hypothesis that temperature and herbivory have interactive effects on woody plant growth, and thus accounting for just one of these two factors is insufficient for understanding plant growth mechanics at biome transitions. Furthermore, we show that climate-driven woody plant growth increases can be negated by herbivory.

Insectivorous birds can see and smell systemically herbivore-induced pines

Several studies have shown that insectivorous birds are attracted to herbivore-damaged trees even when they cannot see or smell the actual herbivores or their feces. However, it often remained an open question whether birds are attracted by herbivore-induced changes in leaf odor or in leaf light reflectance or by both types of changes. Our study addressed this question by investigating the response of great tits (Parus major) and blue tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) damaged by pine sawfly larvae (Diprion pini). We released the birds individually to a study booth, where they were simultaneously offered a systemically herbivore-induced and a noninfested control pine branch. In the first experiment, the birds could see the branches, but could not smell them, because each branch was kept inside a transparent, airtight cylinder. In the second experiment, the birds could smell the branches, but could not see them, because each branch was placed inside a nontransparent cylinder with a mesh lid. The results show that the birds were more attracted to the herbivore-induced branch in both experiments. Hence, either type of the tested cues, the herbivore-induced visual plant cue alone as well as the olfactory cues per se, is attractive to the birds.

Do plants respond and recover from a combination of drought and heatwave in the same manner under adequate and deprived soil nutrient conditions?

Extreme climatic conditions with extended drought periods and heatwaves are predicted to increase in frequency and severity in many regions of the world. Aside from this, other abiotic stress factors such as nutrient deficiency could pose a serious problem to plants when combined with other stressors resulting in more complex underpinning mechanisms. In the present study, we evaluated the response of Brassica napus to single and combined impacts of drought and heatwave (HW) under adequate or deprived (N-A and N-D) soil nutrient conditions. In addition, to get better insights in the plant response to combined stress, a post-stress period, pointing out a degree of the recovery after the cessation of stress, was also included. The results showed a different manner of single drought and heatwave action. The adverse effect of drought on leaf gas exchange was lagged on the growth and became more apparent only after recovery period with no obvious difference between different nutrient levels. Contrary, the growth response of nutrient-deprived plants to single HW was weak and in most cases, insignificant. Heatwave applied simultaneously with drought highly exacerbated the adverse effect of drought both under N-A and N-D conditions. Combined drought and heatwave stress resulted in the sharper decline of A_sat and it was attributed to both stomatal and non-stomatal limitations. Interestingly, plants underwent combined drought and HW treatment under N-D conditions showed better aboveground growth recovery, compared to those grown under N-A conditions, while displayed far more diminished photochemistry of photosystem II and badly disturbed the C/N balance. This discrepancy came from the fact that soil nutrient deficiency, by itself, evoked strong stress under control climate conditions resulting in a dramatically slower aboveground growth of nutrient-deprived plant. In turn, although combined drought and HW stress had similar effect on the aboveground growth either under N-A or N-D conditions, the recovery of later one was better. These results highlight the necessity to look at plants' performance under unfavorable environmental conditions beyond the actual event, since it can be depended not only on the duration of exposure but also on the legacy effect after treatment.

Forest carbon allocation modelling under climate change

Carbon allocation plays a key role in ecosystem dynamics and plant adaptation to changing environmental conditions. Hence, proper description of this process in vegetation models is crucial for the simulations of the impact of climate change on carbon cycling in forests. Here we review how carbon allocation modelling is currently implemented in 31 contrasting models to identify the main gaps compared with our theoretical and empirical understanding of carbon allocation. A hybrid approach based on combining several principles and/or types of carbon allocation modelling prevailed in the examined models, while physiologically more sophisticated approaches were used less often than empirical ones. The analysis revealed that, although the number of carbon allocation studies over the past 10 years has substantially increased, some background processes are still insufficiently understood and some issues in models are frequently poorly represented, oversimplified or even omitted. Hence, current challenges for carbon allocation modelling in forest ecosystems are (i) to overcome remaining limits in process understanding, particularly regarding the impact of disturbances on carbon allocation, accumulation and utilization of nonstructural carbohydrates, and carbon use by symbionts, and (ii) to implement existing knowledge of carbon allocation into defence, regeneration and improved resource uptake in order to better account for changing environmental conditions.

The Effect of Insect Defoliations and Seed Production on the Dynamics of Radial Growth Synchrony among Scots Pine Pinus sylvestris L. Provenances

The intraspecific variation of climate–growth relationships observed on provenance trials results from among–provenance differences in phenotypic plasticity. Temporal variation in radial growth synchrony among provenances may be modified by adverse climatic/biotic conditions such as drought or insect defoliation. However, these factors can potentially diminish provenance–specific growth reactions and, consequently, prevent the identification of provenances with the highest adaptive potential. Thus, understanding the influence of major biotic conditions on provenance–specific climate–growth relationships seems to be important to anticipate climate change. To determine provenance–specific growth patterns in relation to climate conditions (drought), seed production (reproductive effort), and insect defoliation in a common garden of Scots pine (Pinus sylvestris L.), we applied dendroecological techniques to time–series of tree–ring widths and basal area increments. The long–term records of seed production and insect outbreaks from the local Scots pine stands were used to explain the potential effect of biotic factors on the temporal dynamics of radial growth synchrony. During a period of favorable growth conditions, Scots pine provenances showed a decline in inter–provenance synchronicity in growth patterns, while during years affected by severe soil water deficit and insect defoliation, they manifested high uniformity in growth dynamics. The long–term trend in growth synchrony among P. sylvestris provenances depend on both abiotic and biotic environmental factors. This gains significance following an introduction of the appropriate selection of tree provenances for climate–smart forestry.

Differences in isoprenoid-mediated energy dissipation pathways between coastal and interior Douglas-fir seedlings in response to drought

Plants have evolved energy dissipation pathways to reduce photooxidative damage under drought when photosynthesis is hampered. Non-volatile and volatile isoprenoids are involved in non-photochemical quenching of excess light energy and scavenging of reactive oxygen species. A better understanding of trees' ability to cope with and withstand drought stress will contribute to mitigate the negative effects of prolonged drought periods expected under future climate conditions. Therefore we investigated if Douglas-fir (Pseudotsuga menziesii(Mirb.)) provenances from habitats with contrasting water availability reveal intraspecific variation in isoprenoid-mediated energy dissipation pathways. In a controlled drought experiment with 1-year-old seedlings of an interior and a coastal Douglas-fir provenance, we assessed the photosynthetic capacity, pool sizes of non-volatile isoprenoids associated with the photosynthetic apparatus, as well as pool sizes and emission of volatile isoprenoids. We observed variation in the amount and composition of non-volatile and volatile isoprenoids among provenances, which could be linked to variation in photosynthetic capacity under drought. The coastal provenance exhibited an enhanced biosynthesis and emission of volatile isoprenoids, which is likely sustained by generally higher assimilation rates under drought. In contrast, the interior provenance showed an enhanced photoprotection of the photosynthetic apparatus by generally higher amounts of non-volatile isoprenoids and increased amounts of xanthophyll cycle pigments under drought. Our results demonstrate that there is intraspecific variation in isoprenoid-mediated energy dissipation pathways among Douglas-fir provenances, which may be important traits when selecting provenances suitable to grow under future climate conditions.

Complex Above- and Below-Ground Growth Responses of Two Urban Tree Species Following Root, Stem, and Foliage Damage-An Experimental Approach

Urban trees are subjected to numerous biotic and mechanical damages, which can affect their growth rates and health. However, for most species, a systematic analysis of tree above- and below-ground growth reactions to a variety of damages is still lacking. Under a fully factorial experimental setup, using two common urban trees (Celtis occidentalis, Fraxinus pennsylvanica), we tested the effects of various degrees of frequently occurring damage as defoliation, root reduction, and stem injuries for a total of 18 treatments. We hypothesized that (i) an increasing amount of damage would proportionally negatively affect both root and stem growth; (ii) there would be a lag or lasting effect on growth; and (iii) both species would react similarly to the treatments. Contrary to our expectation, increasing levels of single or combined damage did not have an incremental effect on either stem or root growth. Although Celtis was significantly less vigorous than Fraxinus, it did not react strongly to damage treatments compared to the control. Interestingly, Celtis that experienced stem damage alone or in combination with other damages showed higher growth rates than the control. For Celtis, root injury was the treatment having the most impact, decreasing both root and stem growth consistently throughout the 5 years following treatments, whereas defoliation decreased growth only in the first 2 years. All damage treatments negatively affected stem and root growth of Fraxinus trees. Stem growth was affected the most by defoliation in the first year following the treatment, while root injury became the driving factor in subsequent years. For both species, stem injury showed the least influence on growth rates. The control and low-level damage treatments often affected growth rates in a similar way, suggesting that low-intensity stress triggers compensatory reactions stimulating photosynthetic rates and nutrient utilization. The slower-growing tree species, Celtis, showed a less negative reaction to all damage treatments compared to Fraxinus. This study illustrates that various types of above- and below-ground injuries do not have a simple additive effect on tree growth and that trees are capable of compensating for the loss of foliage, roots, or phloem to meet their metabolic demand.

Altered stomatal dynamics of two Euramerican poplar genotypes submitted to successive ozone exposure and water deficit

The impact of ozone (O₃) pollution events on the plant drought response needs special attention because spring O₃ episodes are often followed by summer drought. By causing stomatal sluggishness, O₃ could affect the stomatal dynamic during a subsequent drought event. In this context, we studied the impact of O₃ exposure and water deficit (in the presence or in the absence of O₃ episode) on the stomatal closure/opening mechanisms relative to irradiance or vapour pressure deficit (VPD) variation. Two genotypes of Populus nigra x deltoides were exposed to various treatments for 21 days. Saplings were exposed to 80 ppb/day O₃ for 13 days, and then to moderate drought for 7 days. The curves of the stomatal response to irradiance and VPD changes were determined after 13 days of O₃ exposure, and after 21 days in the case of subsequent water deficit, and then fitted using a sigmoidal model. The main responses under O₃ exposure were stomatal closure and sluggishness, but the two genotypes showed contrasting responses. During stomatal closure induced by a change in irradiance, closure was slower for both genotypes. Nonetheless, the genotypes differed in stomatal opening under light. Carpaccio stomata opened more slowly than control stomata, whereas Robusta stomata tended to open faster. These effects could be of particular interest, as stomatal impairment was still present after O₃ exposure and could result from imperfect recovery. Under water deficit alone, we observed slower stomatal closure in response to VPD and irradiance, but faster stomatal opening in response to irradiance, more marked in Carpaccio. Under the combined treatment, most of the parameters showed antagonistic responses. Our results highlight that it is important to take genotype-specific responses and interactive stress cross-talk into account to improve the prediction of stomatal conductance in response to various environmental modifications.

Integrated analysis of the detoxification responses of two Euramerican poplar genotypes exposed to ozone and water deficit: Focus on the ascorbate-glutathione cycle

Ozone (O₃) and drought increase tree oxidative stress. To protect forest health, we need to improve risk assessment, using metric model such as the phytotoxic O₃ dose above a threshold of y nmol·m^-2·s^-1 (PODy), while taking into account detoxification mechanisms and interacting stresses. The impact of drought events on the effect of O₃ pollution deserves special attention. Water deficit may decrease O₃ entrance into the leaves by reducing stomatal opening; however, water deficit also induces changes in cell redox homeostasis. Besides, the behaviour of the cell antioxidative charge in case of stress combination (water deficit and O₃) still remains poorly investigated. To decipher the response of detoxification mechanisms relatively to the Halliwell-Asada-Foyer cycle (HAF), we exposed poplar saplings (Populus nigra × deltoides) composed of two genotypes (Carpaccio and Robusta), to various treatments for 17 days, i.e. i) mild water deficit, ii) 120 ppb O₃, and iii) a combination of these two treatments. Ozone similarly impacted the growth of the two genotypes, with an important leaf loss. Water deficit decreased growth by almost one third as compared to the control plants. As for the combined treatment, water deficit protected the saplings from leaf ozone injury, but with an inhibitory effect on growth. The pool of total ascorbate was not modified by the different treatments, while the pool of total glutathione increased with POD₀. We noticed a few differences between the two genotypes, particularly concerning the activity of monodehydroascorbate reductase and glutathione reductase relatively to POD₀. The expression profiles of genes coding for the dehydroascorbate reductase and glutathione reductase isoforms differed, probably in link with the putative localisation of ROS production in response to water deficit and ozone, respectively. Our result would argue for a major role of MDHAR, GR and glutathione in the preservation of the redox status.

Antioxidative responses of three oak species under ozone and water stress conditions

Plants are frequently exposed to adverse environmental conditions such as drought and ozone (O₃). Under these conditions, plants can survive due to their ability to adjust their metabolism. The aim of the present study was to compare the detoxification mechanisms of three oak species showing different O₃ sensitivity and water use strategy. Two-year-old seedlings of Quercus ilex, Q. pubescens and Q. robur were grown under the combination of three levels of O₃ (1.0, 1.2 and 1.4 times the ambient O₃ concentration) and three levels of water availability (on average 100, 80 and 42% of field capacity i.e. well-watered, moderate drought and severe drought, respectively) in an O₃ Free Air Controlled Exposure facility. Ozone and drought induced the accumulation of reactive oxygen species (ROS) and this phenomenon was species-specific. Sometimes, ROS accumulation was not associated with membrane injury suggesting that several antioxidative defence mechanisms inhibited or alleviated the oxidative damage. Both O₃ and drought increased total carotenoids that were able to prevent the peroxidation action by free radicals in Q. ilex, as confirmed by unchanged malondialdehyde by-product values. The concomitant decrease of total flavonoids may be related to the consumption of these compounds by the cell to inhibit the accumulation of hydrogen peroxide. Unchanged total phenols confirmed that Q. ilex has a superior ability to counteract oxidative conditions. Similar responses were found in Q. pubescens, although the negative impact of both factors was less efficiently faced than in the sympatric Q. ilex. In Q. robur, high O₃ concentrations and severe drought induced a partial rearrangement of the phenylpropanoid pathways. These antioxidative mechanisms were not able to protect the cell structure (as confirmed by ROS accumulation) suggesting that Q. robur showed a lower degree of tolerance than the other two species.

Poplar leaf rust reduces dry mass accumulation and internal nitrogen recycling more markedly under low soil nitrogen availability, and decreases growth in the following spring

Rust is one of the most important biotic stress factors that affect poplars. The aims of this work were: (i) to analyze the changes in growth and nitrogen (N) accumulation in Populus deltoides W. Bartram ex Marshall plants infected with rust (Melampsora medusae Thümen.) and to determine how internal N stores are affected by the disease, in plants growing under two N availabilities in the soil; and (ii) to evaluate the impact of rust in the early sprout in the following growing season and the cumulative effect of the disease after repeated infections. Two clones with different susceptibility to rust were analyzed. At leaf level, rust reduced gas exchange capacity, water conductance in liquid phase and photosynthetic rate in both clones. At plant level, rust reduced plant growth, accelerated leaf senescence and abscission occurred with a higher concentration of leaf N. Even though N concentration in stems and roots were not significantly reduced by rust, total N accumulation in perennial tissues was reduced in infected plants. The vigor of the early sprout of plants infected by rust in the previous season was lower than that of non-infected plants. Therefore, rust affects plant growth by reducing the photosynthetic capacity and leaf area duration, and by decreasing internal nutrient recycling. As nutrient reserves in perennial tissues are lower, rust infection reduces not only the growth of the current season, but also has a cumulative effect on the following years. The reduction of growth was similar in both clones. High availability of N in the soil had no effect on leaf physiology but increased plant growth, delayed leaf senescence and abscission, and increased total N accumulation. If fertilization increases plant growth (stem and root dry mass) it can mitigate the negative effect of the pathogen in the reduction of nutrient storages and future growth.

Combined impact of heat stress and phosphate deficiency on growth and photochemical activity of sheepgrass (Leymus chinensis)

Global climate warming has a crucial impact on many terrestrial ecosystems, including temperate steppe. In addition, phosphate deficiency is known to be the common deficiency in soils worldwide due to the low availability of the phosphate nutrient in the form of inorganic phosphate anions (Pi). Consequently, in the future, land plants are likely to simultaneously encounter heat stress and phosphate deficiency more frequently. Sheepgrass 〔Leymus chinensis (Trin.) Tzvel〕is a dominant perennial forage plant highly significant to grass productivity of Eurasian temperate grasslands. Though effects of environmental stress including drought and Pi starvation have been studied, the combined eff ;ects of phosphate deficiency and heat stress on plant physiology remain largely unclear. Here, we investigated the combined eff ;ects of heat stress and phosphate deficiency on above-ground tissue growth and photochemical properties of L. chinensis using in vivo chlorophyll fluorescence spectroscopy. We observed remarkable phenotypic alterations of reduced shoot growth and considerable leaf chlorosis in L. chinensis seedlings under the combined stress condition. Also, we compared changes in photochemical activity between the control and the corresponding stressed seedlings. Based on chlorophyll fluorescence analysis, impairment of PSI was more severe than that of PSII in the seedlings treated with the combined stress. Compared to the control, PSI and PSII activity decreased up to 35.5% and 30%, respectively, under the combined-stress condition. Moreover, our data show that the decreased photosynthetic activity is not the sum of the single-stressed conditions. These results combined with the distinction of other photochemical parameters indicate that a complex interaction between Pi-deficiency and heat stress may exist in the forage plant.

Canola Responses to Drought, Heat, and Combined Stress: Shared and Specific Effects on Carbon Assimilation, Seed Yield, and Oil Composition

Photosynthetic assimilation is remarkably altered by heat and drought, and this depends on the individual or combined occurrence of stressors and their respective intensities and durations. Abiotic stressors may also alter the nutritional quality and economic value of crops. In this controlled greenhouse study, we evaluated the response of Brassica napus L., from flowering to seed development, to two temperature and water treatments and a combination of these treatments. The diffusional limitations of stomatal conductance and mesophyll conductance on photosynthesis, as well as resource-use efficiency (particularly water and nitrogen), were assessed. In addition, the effects of stressors on the seed fatty acid content and composition and the total protein content were examined. The results showed that the reduction in the net photosynthetic assimilation rate was caused by combinations of heat and drought (heat + drought) treatments, by drought alone, and, to a lesser extent, by heat alone. The stomatal conductance decreased under drought and heat + drought treatments but not under heat. Conversely, the mesophyll conductance was reduced significantly in the plants exposed to heat and heat + drought but not in the plants exposed to drought alone. The carboxylation efficiency rate and the electron transport rate were reduced under the heat treatment. The seed yield was reduced by 85.3% under the heat treatment and, to a lesser extent, under the drought treatment (31%). This emphasizes the devastating effects of hotter weather on seed formation and development. Seed oil content decreased by 52% in the plants exposed to heat, the protein content increased under all the stress treatments. Heat treatment had a more deleterious effect than drought on the seed oil composition, leading to enhanced levels of saturated fatty oils and, consequently, desaturation efficiency, a measure of oil frying ability. Overall, this study showed that except for the photosynthetic assimilation rate and stomatal conductance, heat, rather than drought, negatively affected the photosynthetic capacity, yield, and oil quality attributes when imposed during the flowering and silique-filling stages. This result highlights the necessity for a better understanding of heat tolerance mechanisms in crops to help to create germplasms that are adapted to rapid climate warming.

Whole-Plant Metabolic Allocation Under Water Stress

Trade-offs between plant growth and defense depend on environmental resource availability. Plants are predicted to prioritize growth when environmental resources are abundant and defense when environmental resources are scarce. Nevertheless, such predictions lack a whole-plant perspective-they do not account for potential differences in plant allocation above- and belowground. Such accounting is important because leaves and roots, though both critical to plant survival and fitness, differ in their resource-uptake roles and, often, in their vulnerability to herbivores. Here we aimed to determine how water availability affects plant allocation to multiple metabolic components of growth and defense in both leaves and roots. To do this, we conducted a meta-analysis of data from experimental studies in the literature. We assessed plant metabolic responses to experimentally reduced water availability, including changes in growth, nutrients, physical defenses, primary metabolites, hormones, and other secondary metabolites. Both above- and belowground, reduced water availability reduced plant biomass but increased the concentrations of primary metabolites and hormones. Importantly, however, reduced water had opposite effects in different organs on the concentrations of other secondary metabolites: reduced water increased carbon-based secondary metabolites in leaves but reduced them in roots. In addition, plants suffering from co-occurring drought and herbivory stresses exhibited dampened metabolic responses, suggesting a metabolic cost of multiple stresses. Our study highlights the needs for additional empirical studies of whole-plant metabolic responses under multiple stresses and for refinement of existing plant growth-defense theory in the context of whole plants.

Combined Drought and Heat Activates Protective Responses in Eucalyptus globulus That Are Not Activated When Subjected to Drought or Heat Stress Alone

Aiming to mimic a more realistic field condition and to determine convergent and divergent responses of individual stresses in relation to their combination, we explored physiological, biochemical, and metabolomic alterations after drought and heat stress imposition (alone and combined) and recovery, using a drought-tolerant Eucalyptus globulus clone. When plants were exposed to drought alone, the main responses included reduced pre-dawn water potential (Ψpd) and gas exchange. This was accompanied by increases in malondialdehyde (MDA) and total glutathione, indicative of oxidative stress. Abscisic acid (ABA) levels increased while the content of jasmonic acid (JA) fell. Metabolic alterations included reductions in the levels of sugar phosphates accompanied by increases in starch and non-structural carbohydrates. Levels of α-glycerophosphate and shikimate were also reduced while free amino acids increased. On the other hand, heat alone triggered an increase in relative water content (RWC) and Ψpd. Photosynthetic rate and pigments were reduced accompanied by a reduction in water use efficiency. Heat-induced a reduction of salicylic acid (SA) and JA content. Sugar alcohols and several amino acids were enhanced by the heat treatment while starch, fructose-6-phosphate, glucose-6-phosphate, and α-glycerophosphate were reduced. Contrary to what was observed under drought, heat stress activated the shikimic acid pathway. Drought-stressed plants subject to a heat shock exhibited a sharp decrease in gas exchange, Ψpd and JA, no alterations in electrolyte leakage, MDA, starch, and pigments and increased glutathione pool in relation to control. Comparing this with drought stress alone, subjecting drought stressed plants to an additional heat stress alleviated Ψpd and MDA, maintained an increased glutathione pool and reduced starch content and non-structural carbohydrates. A novel response triggered by the combined stress was the accumulation of cinnamate. Regarding recovery, most of the parameters affected by each stress condition reversed after re-establishment of control growing conditions. These results highlight that the combination of drought and heat provides significant protection from more detrimental effects of drought-stressed eucalypts, confirming that combined stress alter plant metabolism in a novel manner that cannot be extrapolated by the sum of the different stresses applied individually.

Differential regulation of volatile emission from Eucalyptus globulus leaves upon single and combined ozone and wounding treatments through recovery and relationships with ozone uptake

Both ozone and wounding constitute two key abiotic stress factors, but their interactive effects on plant constitutive and stress-elicited volatile (VOC) emissions are poorly understood. Furthermore, the information on time-dependent modifications in VOC release during recovery from a combined stress is very limited. We studied the modifications in photosynthetic characteristics and constitutive and stress-induced volatile emissions in response to single and combined applications of acute ozone (4, 5, and 6 ppm) and wounding treatments through recovery (0.5-75 h) in a constitutive isoprene and mono- and sesquiterpene emitter Eucalyptus globulus. Overall, the photosynthetic characteristics were surprisingly resistant to all ozone and wounding treatments. Constitutive isoprene emissions were strongly upregulated by ozone and combined ozone and wounding treatments and remained high through recovery phase, but wounding applied alone reduced isoprene emission. All stress treatments enhanced emissions of lipoxygenase pathway volatiles (LOX), mono- and sesquiterpenes, saturated aldehydes (C7-C10), benzenoids, and geranylgeranyl diphosphate (GGDP) pathway volatiles. Once elicited, GGDP volatile, saturated aldehyde and benzenoid emissions remained high through the recovery period. In contrast, LOX emissions, and total mono- and sesquiterpene emissions decreased through recovery period. However, secondary rises in total sesquiterpene emissions at 75 h and in total monoterpenes at 25-50 h were observed. Overall, acute ozone and wounding treatments synergistically altered gas exchange characteristics and stress volatile emissions. Through the treatments and recovery period, stomatal ozone uptake rate and volatile emission rates were poorly correlated, reflecting possible ozone-scavenging effect of volatiles and thus, reduction of effective ozone dose and elicitation of induced defense by the acute ozone concentrations applied. These results underscore the important role of interactive stresses on both constitutive and induced volatile emission responses.

Stress acclimation and particulate matter accumulation in Pinus sylvestris saplings affected by moderate combinations of urban stressors

To predict how the function of urban vegetation and the provision of ecosystem services respond to combinations of natural and anthropogenic drivers, a better understanding of multiple stress interactions is required. This study tested combined effects of moderate levels of drought, soil salinity and exposure to diesel exhaust on parameters of physiology, metabolism, morphology and growth of Pinus sylvestris L. saplings. We found that plant responses were primarily dominated by single stressors and a few two-way interactions. Stressor combinations did not have considerable additional negative effects on plant performance compared to single stressors. Hence, synergistic and antagonistic interactions were rare and additive effects frequent. Drought cycles caused most negative effects, from chlorophyll a fluorescence and epicuticular wax content to growth responses, while soil salinity caused fewer negative effects but contributed to reduction in fine root growth and fluorescence parameters at low air contamination. Interestingly, the air contamination alone had only marginal effects on plant morphology and growth, but contributed an antagonistic effect, dampening the negative effect of drought and salinity on the maximum quantum efficiency of PSII photochemistry (F_v/F_m) and fine root biomass. Although, these effects were moderate, it appears that exhaust exposure had a cross-acclimation effect on plant responses to drought and salinity. We also found that salinity had a negative effect on the accumulation of particulate matter on shoots, illustrating that the plant stress situation can affect the provisioning of certain ecosystem services like pollution attenuation. These findings have implications for the understanding of the complex natural and anthropogenic stress situation of urban, and how to maintain the ecological functions and delivery of ecosystem services.

The competitive status of trees determines their responsiveness to increasing atmospheric humidity - a climate trend predicted for northern latitudes

The interactive effects of climate variables and tree-tree competition are still insufficiently understood drivers of forest response to global climate change. Precipitation and air humidity are predicted to rise concurrently at high latitudes of the Northern Hemisphere. We investigated whether the growth response of deciduous trees to elevated air humidity varies with their competitive status. The study was conducted in seed-originated silver birch and monoclonal hybrid aspen stands grown at the free air humidity manipulation (FAHM) experimental site in Estonia, in which manipulated stands (n = 3 for both species) are exposed to artificially elevated relative air humidity (6-7% over the ambient level). The study period included three growing seasons during which the stands had reached the competitive stage (trees were 7 years old in the final year). A significant 'treatment×competitive status' interactive effect on growth was detected in all years in birch (P < 0.01) and in one year in aspen stands (P = 0.015). Competitively advantaged trees were always more strongly affected by elevated humidity. Initially the growth of advantaged and neutral trees of both species remained significantly suppressed in humidified stands. In the following years, dominance and elevated humidity had a synergistic positive effect on the growth of birches. Aspens with different competitive status recovered more uniformly, attaining similar relative growth rates in manipulated and control stands, but preserved a significantly lower total growth yield due to severe initial growth stress. Disadvantaged trees of both species were never significantly affected by elevated humidity. Our results suggest that air humidity affects trees indirectly depending on their social status. Therefore, the response of northern temperate and boreal forests to a more humid climate in future will likely be modified by competitive relationships among trees, which may potentially affect species composition and cause a need to change forestry practices.

Moderate salinity reduced phenanthrene-induced stress in the halophyte plant model Thellungiella salsuginea compared to its glycophyte relative Arabidopsis thaliana: Cross talk and metabolite profiling

It was shown that halophytes experience higher cross-tolerance to stresses than glycophytes, which was often associated with their more powerful antioxidant systems. Moreover, salinity was reported to enhance halophyte tolerance to several stresses. The aim of the present work was to investigate whether a moderate salinity enhances phenanthrene stress tolerance in the halophyte Thellungiella salsuginea. The model plant Arabidopsis thaliana, considered as its glycophyte relative, was used as reference. Our study was based on morpho-physiological, antioxidant, and metabolomic parameters. Results showed that T. salsuginea was more tolerant to phenanthrene stress as compared to A. thaliana. An improvement of phenanthrene-induced responses was recorded in the two plants in the presence of 25 mM NaCl, but the effect was significantly more obvious in the halophyte. This observation was particularly related to the higher antioxidant activities and the induction of more adapted metabolism in the halophyte. Gas Chromatography coupled with Mass Spectrometry (GC-MS) was used to quantify alcohols, ammonium, sugars, and organic acids. It showed the accumulation of several metabolites, many of them are known to be involved in signaling and abiotic stress tolerance. Moderate salinity and phenanthrene cross-tolerance involved in these two stresses was discussed.

An ecoclimatic framework for evaluating the resilience of vegetation to water deficit

The surge in global efforts to understand the causes and consequences of drought on forest ecosystems has tended to focus on specific impacts such as mortality. We propose an ecoclimatic framework that takes a broader view of the ecological relevance of water deficits, linking elements of exposure and resilience to cumulative impacts on a range of ecosystem processes. This ecoclimatic framework is underpinned by two hypotheses: (i) exposure to water deficit can be represented probabilistically and used to estimate exposure thresholds across different vegetation types or ecosystems; and (ii) the cumulative impact of a series of water deficit events is defined by attributes governing the resistance and recovery of the affected processes. We present case studies comprising Pinus edulis and Eucalyptus globulus, tree species with contrasting ecological strategies, which demonstrate how links between exposure and resilience can be examined within our proposed framework. These examples reveal how climatic thresholds can be defined along a continuum of vegetation functional responses to water deficit regimes. The strength of this framework lies in identifying climatic thresholds on vegetation function in the absence of more complete mechanistic understanding, thereby guiding the formulation, application and benchmarking of more detailed modelling.

Assessing the Effectiveness of Local Management of Coral Reefs Using Expert Opinion and Spatial Bayesian Modeling

Multiple stressors are an increasing concern in the management and conservation of ecosystems, and have been identified as a key gap in research. Coral reefs are one example of an ecosystem where management of local stressors may be a way of mitigating or delaying the effects of climate change. Predicting how multiple stressors interact, particularly in a spatially explicit fashion, is a difficult challenge. Here we use a combination of an expert-elicited Bayesian network (BN) and spatial environmental data to examine how hypothetical scenarios of climate change and local management would result in different outcomes for coral reefs on the Great Barrier Reef (GBR), Australia. Parameterizing our BN using the mean responses from our experts resulted in predictions of limited efficacy of local management in combating the effects of climate change. However, there was considerable variability in expert responses and uncertainty was high. Many reefs within the central GBR appear to be at risk of further decline based on the pessimistic opinions of our expert pool. Further parameterization of the model as more data and knowledge become available could improve predictive power. Our approach serves as a starting point for subsequent work that can fine-tune parameters and explore uncertainties in predictions of responses to management.