Specific leaf metabolic changes that underlie adjustment of osmotic potential in response to drought by four Quercus species

Tree species and drought: Two mysterious long-standing counterparts

Around 252 million years ago (Late Permian), Earth experienced one of its most significant drought periods, coinciding with a global climate crisis, resulting in a devastating loss of forest trees with no hope of recovery. In the current epoch (Anthropocene), the worsening of drought stress is expected to significantly affect forest communities. Despite extensive efforts, there is significantly less research at the molecular level on forest trees than on annual crop species. Would it not be wise to allocate equal efforts to woody species, regardless of their importance in providing essential furniture and sustaining most terrestrial ecosystems? For instance, the poplar genome is roughly quadruple the size of the Arabidopsis genome and has 1.6 times the number of genes. Thus, a massive effort in genomic studies focusing on forest trees has become inevitable to understand their adaptation to harsh conditions. Nevertheless, with the emerging role and development of high-throughput DNA sequencing systems, there is a growing body of literature about the responses of trees under drought at the molecular and eco-physiological levels. Therefore, synthesizing these findings through contextualizing drought history and concepts is essential to understanding how woody species adapt to water-limited conditions. Comprehensive genomic research on trees is critical for preserving biodiversity and ecosystem function. Integrating molecular insights with eco-physiological analysis will enhance forest management under climate change.

Dual effect of the presence of fruits on leaf gas exchange and water relations of olive trees

The presence of fruits provokes significant modifications in plant water relations and leaf gas exchange. The underlying processes driving these modifications are still uncertain and likely depend on the water deficit level. Our objective was to explain and track the modification of leaf-water relations by the presence of fruits and water deficit. With this aim, net photosynthesis rate (AN), stomatal conductance (gs), leaf osmotic potential (Ψπ), leaf soluble sugars and daily changes in a variable related to leaf turgor (leaf patch pressure) were measured in olive trees with and without fruits at the same time, under well-watered (WW) and water stress (WS) conditions. Leaf gas exchange was increased by the presence of fruits, this effect being observed mainly in WW trees, likely because under severe water stress, the dominant process is the response of the plant to the water stress and the presence of fruits has less impact on the leaf gas exchange. Ψπ was also higher for WW trees with fruits than for WW trees without fruits. Moreover, leaves from trees without fruits presented higher concentrations of soluble sugars and starch than leaves from trees with fruits for both WW and WS, these differences matching those found in Ψπ. Thus, the sugar accumulation would have had a dual effect because on one hand, it decreased Ψπ, and on the other hand, it would have downregulated AN, and finally gs in WW trees. Interestingly, the modification of Ψπ by the presence of fruits affected turgor in WW trees, the change in which can be identified with leaf turgor sensors. We conclude that plant water relationships and leaf gas exchange are modified by the presence of fruits through their effect on the export of sugars from leaves to fruits. The possibility of automatically identifying the onset of sugar demand by the fruit through the use of sensors, in addition to the water stress produced by soil water deficit and atmosphere drought, could be of great help for fruit orchard management in the future.

Leaf trait modification in European beech trees in response to climatic and edaphic drought

Leaf morphological and physiological traits control the carbon and water relations of mature trees and are determinants of drought tolerance, but it is not well understood how they are modified in response to water deficits. We analysed five sun-canopy leaf traits (mean leaf size (LS), specific leaf area (SLA), Huber value (HV), water potential at turgor loss point (Ψ_tlp ) and foliar carbon isotope signature (δ¹³ C)) in European beech (Fagus sylvatica L.) across three precipitation gradients sampled in moist (2010), dry (2019) and very dry (2018) summers, and tested their response to short-term water deficits (climatic water balance (CWB) preceding sample collection) and long-term water availability (mean annual precipitation (MAP), plant-available soil water capacity (AWC) and neighbourhood competition). Across the 34 sites, LS varied seven-fold (3.9-27.0 cm² ), SLA four-fold (77.1-306.9 cm²·g^-1 ) and HV six-fold (1.0-6.65 cm² ·m^-2 ). In the 2018 dataset, LS showed a negative and HV a positive relationship to MAP, which contradicts relations found in multi-species samples. Average Ψ_tlp ranged from -1.90 to -2.62 MPa and decreased across the sites with decreasing CWB in the month prior to measurement, as well as with decreasing MAP and AWC in 2019. Studied leaf traits varied considerably between years, suggesting that mast fruiting and the severe 2018 drought caused the formation of smaller leaves. We conclude that sun-canopy leaf traits of European beech exhibit considerable plasticity in response to climatic and edaphic aridity, and that osmotic adjustment may be an important element in the drought response strategy of this anisohydric tree species.

Mitigation of salinity stress in barley genotypes with variable salt tolerance by application of zinc oxide nanoparticles

Salinity has become a major environmental concern of agricultural lands, impairing crop production. The current study aimed to examine the role of zinc oxide nanoparticles (ZnO NPs) in reducing the oxidative stress induced by salinity and the overall improvement in phytochemical properties in barley. A total of nine different barley genotypes were first subjected to salt (NaCl) stress in hydroponic conditions to determine the tolerance among the genotypes. The genotype Annora was found as most sensitive, and the most tolerant genotype was Awaran 02 under salinity stress. In another study, the most sensitive (Annora) and tolerant (Awaran 02) barley genotypes were grown in pots under salinity stress (100 mM). At the same time, half of the pots were provided with the soil application of ZnO NPs (100 mg kg-1), and the other half pots were foliar sprayed with ZnO NPs (100 mg L-1). Salinity stress reduced barley growth in both genotypes compared to control plants. However, greater reduction in barley growth was found in Annora (sensitive genotype) than in Awaran 02 (tolerant genotype). The exogenous application of ZnO NPs ameliorated salt stress and improved barley biomass, photosynthesis, and antioxidant enzyme activities by reducing oxidative damage caused by salt stress. However, this positive effect by ZnO NPs was observed more in Awaran 02 than in Annora genotype. Furthermore, the foliar application of ZnO NPs was more effective than the soil application of ZnO NPs. Findings of the present study revealed that exogenous application of ZnO NPs could be a promising approach to alleviate salt stress in barley genotypes with different levels of salinity tolerance.

Comparative Metabolic Study of Two Contrasting Chinese Cabbage Genotypes under Mild and Severe Drought Stress

Chinese cabbage (Brassica rapa L. ssp. pekinensis) is an important leafy vegetable crop cultivated worldwide. Drought is one of the most important limiting factors for the growth, production and quality of Chinese cabbage due to its weak drought tolerance. In order to deepen the understanding of drought stress response in Chinese cabbage, metabolomics studies were conducted in drought−tolerant (DT) and drought−susceptible (DS) genotypes of Chinese cabbage under water deficit−simulated mild and severe drought stress conditions. A total of 777 metabolites were detected, wherein 90 of them were proposed as the drought−responsive metabolites in Chinese cabbage, with abscisic acid (ABA), serine, choline alfoscerate, and sphingosine as potential representative drought stress biomarkers. We also found that drought−tolerant and drought−susceptible genotypes showed differential metabolic accumulation patterns with contrasting drought response mechanisms. Notably, constitutively high levels of ABA and glutathione were detected in drought−tolerant genotype in all tested and control conditions. In addition, proline, sucrose, γ−aminobutyric acid, and glutathione were also found to be highly correlated to drought tolerance. This study is the first metabolomic study on how Chinese cabbage responds to drought stress, and could provide insights on how to develop and cultivate new drought−resistant varieties.

Aerial and underground organs display specific metabolic strategies to cope with water stress under rising atmospheric CO2 in Fagus sylvatica L

Beech is known to be a moderately drought-sensitive tree species, and future increases in atmospheric concentrations of CO₂ ([CO₂ ]) could influence its ecological interactions, also with changes at the metabolic level. The metabolome of leaves and roots of drought-stressed beech seedlings grown under two different [CO₂ ] (400 (aCO₂ ) and 800 (eCO₂ ) ppm) was analyzed together with gas exchange parameters and water status. Water stress estimated from predawn leaf water potential (Ψ_pd ) was similar under both [CO₂ ], although eCO₂ had a positive impact on net photosynthesis and intrinsic water use efficiency. The aerial and underground organs showed different metabolomes. Leaves mainly stored C metabolites, while those of N and P accumulated differentially in roots. Drought triggered the proline and N-rich amino acids biosynthesis in roots through the activation of arginine and proline pathways. Besides the TCA cycle, polyols and soluble sugar biosynthesis were activated in roots, with no clear pattern seen in the leaves, prioritizing the root functioning as metabolites sink. eCO₂ slightly altered this metabolic acclimation to drought, reflecting mitigation of its effect. The leaves showed only minor changes, investing C surplus in secondary metabolites and malic acid. The TCA cycle metabolites and osmotically active substances increased in roots, but many other metabolites decreased as if the water stress was dampened. Above- and belowground plant metabolomes were differentially affected by two drivers of climate change, water scarcity and high [CO₂ ], showing different chemical responsiveness that could modulate the tree adaptation to future climatic scenarios.

Untargeted MS-Based Metabolomics Analysis of the Responses to Drought Stress in Quercus ilex L. Leaf Seedlings and the Identification of Putative Compounds Related to Tolerance

The effect and responses to drought stress were analyzed in Quercus ilex L. seedlings using a nontargeted metabolomic approach, implementing the approaches of previous studies in which other -omics platforms, transcriptomics, and proteomics were employed. This work aimed to characterize the Q. ilex leaf metabolome, determining possible mechanisms and molecular markers of drought tolerance and identifying putative bioactive compounds. Six-month-old seedling leaves subjected to drought stress imposed by water withholding under high-temperature and irradiance conditions were collected when leaf fluorescence decreased by 20% (day 17) and 45% (day 24) relative to irrigated seedlings. A total of 3934 compounds were resolved, with 616 being variable and 342 identified, which belonged to five chemical families. Out of the identified compounds, 33 were variable, mostly corresponding to amino acids, carboxylic acids, benzenoids, flavonoids and isoprenoids. Epigallocatechin, ellagic acid, pulegone, indole-3-acrylic acid and dihydrozeatin-O-glucoside were up-accumulated under drought conditions at both sampling times. An integrated multi-omics analysis of phenolic compounds and related enzymes was performed, revealing that some enzymes involved in the flavonoid pathways (chalcone synthase, anthocyanidin synthase and anthocyanidin reductase) were up-accumulated at day 24 in non-irrigated seedlings. Some putative markers of tolerance to drought in Q. ilex are proposed for assisting breeding programs based on the selection of elite genotypes.