Response of Mediterranean Ornamental Plants to Drought Stress

Integrative leaf anatomy structure, physiology, and metabolome analyses revealed the response to drought stress in sainfoin at the seedling stage

INTRODUCTION

Sainfoin (Onobrychis viciaefolia) is a vital legume forage, and drought is the primary element impeding sainfoin growth.

OBJECTIVE

The anatomical structure, physiological indexes, and metabolites of the leaves of sainfoin seedlings with a drought-resistant line of P1 (DRL) and a drought-sensitive material of 2049 (DSM) were analyzed under drought (-1.0 MPa) with polyethylene glycol-6000 (PEG-6000).

METHODS

The leaf anatomy was studied by the paraffin section method. The related physiological indexes were measured by the hydroxylamine oxidation method, titanium sulfate colorimetric method, thiobarbituric acid method, acidic ninhydrin colorimetric method, and Coomassie brilliant blue method. The metabolomics analysis was composed of liquid chromatography tandem high-resolution mass spectrometry (LC-MS/MS).

RESULTS

The results revealed that the thickness of the epidermis, palisade tissue, and sponge tissue of DRL were significantly greater than those of DSM. The leaves of DRL exhibited lower levels of superoxide anion (O2 •-) production rate, hydrogen peroxide (H2O2) content, and malondialdehyde (MDA) content compared with DSM, while proline (Pro) content and soluble protein (SP) content were significantly higher than those of DSM. A total of 391 differential metabolites were identified in two samples. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed that the primary differential metabolites were concentrated into the tyrosine metabolism; isoquinoline alkaloid biosynthesis; ubiquinone and other terpenoid quinone biosynthesis; neomycin, kanamycin, and gentamicin biosynthesis; and anthocyanin biosynthesis metabolic pathways.

CONCLUSION

Compared with DSM, DRL had more complete anatomical structure, lower active oxygen content, and higher antioxidant level. The results improved our insights into the drought-resistant mechanisms in sainfoin.

Method for selecting ornamental species for different shading intensity in urban green spaces

In urban areas, ornamental plants face different constraints, such as the shading of buildings and trees. Therefore, the selection of suitable species and their integration or combination with pre-existing plants is very important. Trees, shrubs, and herbaceous plant species must be distributed according to plant light requirements and shading intensity. Ornamental plants are classified into two groups based on their light intensity or shade tolerance: sun and shade species. To properly position the plants, especially in the immediate vicinity of buildings, it is necessary to study the projection of shadows during the year and the most critical periods, such as July and August. The position of ornamental species with different shading tolerances can be obtained by characterizing the leaf gas exchange for each species. Among the physiological parameters, the most important is the light compensation point, which is the light intensity corresponding to a net photosynthesis equal to zero. This means that the assimilation of carbon dioxide through photosynthesis is equal to the carbon dioxide emitted by respiration. This steady state represents the most critical condition for plants to endure the summer. The distribution of species inside a green area should be determined by considering the minimum light intensity that allows sufficient photosynthesis to compensate for the respiration rate. In this context, non-destructive leaf gas exchange, chlorophyll a fluorescence, and chlorophyll content can be useful tools for selecting suitable ornamental plants under diverse shading conditions.

Hydrogen and Oxygen Isotope Fractionation Effects in Different Organ Tissues of Grapes under Drought Conditions

A study of different grapevine tissues and organs (root, stem, leaf, fruit) water isotope fractionation models from high-quality wine grapes produced in the Helan Mountains, a key wine-producing area in northwestern China, was undertaken. Results showed that δ2H values of local groundwater sources were more negative than rivers and precipitation. Soil water δ2H and δ18O values were significantly higher than those of other environmental water sources. Water from the soil surface layer (0-30 cm, δ2H and δ18O values) was more positive than the deeper layer (30-60 cm), indicating that soil water has undergone a positive fractionation effect. δ2H and δ18O values of tissues and organs from different grape varieties followed a similar pattern but were more negative than the local atmospheric precipitation line (slope between 4.1 to 5.2). The 2H and 18O fractionation relationship in grapevine organs was similar, and 18O has a higher fractionation effect than 2H. δ2H and δ18O values showed a strong fractionation effect during the transportation of water to different grape organs (trend of stem > fruit > leaf). This study showed that 18/16O fractionation in grapes is more likely to occur under drought conditions and provides a theoretical basis to improve traceability accuracy and origin protection of wine production areas.

Evaluation of Different Combinations of Ornamental Perennials for Sustainable Management in Urban Greening

Ornamental perennial plants play a strategic role in reducing green areas' management costs, keeping the ground, sparing water, and avoiding weeds. The aim of this research is to evaluate the growing performances of seven combinations of six different ornamental perennial herbaceous species and their role in weed containment under low-maintenance conditions. The experiment was performed for three years (2019-2021) in an open field. The selected species were Hemerocallis "Stella de Oro" (A), Phedimus spurius (M.Bieb.) "t Hart 'John Creech" (B), Tulbaghia violacea Harv. (C), Phlox subulata L. "Trot Pink" (D), Potentilla neumanniana Rchb. (E), and Gaillardia "Kobold" (F). Four replicates for each combination were tested (28 plots, 4 m2 each): AB, CD, EF, AB + CD, AB + EF, CD + EF; AB + CD + EF. No watering or fertilization was performed during the cultivation period. Each year, from April to November, three manual weeding activities were performed and the dry weights of the weeds' aerial parts were measured. The ground cover performance was evaluated through digital image analysis using the mobile device application Canopeo. Dry aerial perennial biomass variations between the end and beginning of the experiment were calculated. As a result, CD showed the best performance for weed containment (0.5 g m-2 weed dry weight in the third year), ground cover (63.1% and 64.3% of plot coverages during the second and third years, respectively), and producing ornamental biomass (4316.8 g m-2). The highest total dry amount of harvested weeds was shown by AB + CD + EF (1114.6 g m-2), demonstrating that combinations with a higher number of species were less efficient in avoiding weeds. The research allowed us to identify the best combinations to always keep the soil covered and to improve the ornamental and environmental values of urban green spaces under low-maintenance regimes.

Morphological performance and seasonal pattern of water relations and gas exchange in Pistacia lentiscus plants subjected to salinity and water deficit

Soil water deficit and salinity represent a major factor impacting plant survival and agricultural production. The frequency and severity of both abiotic stresses are expected to increase in a context of climate change, especially in arid and semi-arid regions. This work studied the growth pattern, biomass and mineral distribution and the seasonal pattern of water status, photosynthetic rate and stomatal conductance in plant of Pistacia lentiscus grown under different levels of water deficit and salinity. P. lentiscus plants growing under greenhouse conditions were subjected to four irrigation treatments during 11 months: control (C, 1 dS m-1), moderate water deficit (MW, 1dS m-1, 60% of the control), severe water deficit (SW, 1 dS m-1, 40% of the control) and saline (S, 4dS m-1). The results show that Pistacia lentiscus plants were more affected by deficit irrigation than salinity. Deficit irrigation and salinity inhibited plant height, with reductions of 20%, 22% and 35% for S, MW and SW, respectively. Total leaf area was not modified by effect of the treatments, with the result that plant compactness increased in MW. The salt stressed plants only showed lower relative growth rate at the end of the experiment. Plants responded to saline or drought stress by increasing their osmotic adjustment, which was more pronounced under salinity. Saline plants had the highest values in Na+ and Cl- ions and the lowest values for K+/Na+ and Ca2+/Na+ ratios in leaves and stems, which is correlated with a decrease in growth, stomatal conductance, photosynthesis and stem water potential, and can be used as a diagnostic tool to assess plant tolerance to salinity stress. As a measure of plant hydration, relative water content was more sensitive to deficit irrigation than salinity, being a good indicator of water stress. P. lentiscus plants subjected to both deficit irrigation treatments exhibited an increase in their intrinsic water use efficiency, which is an important adaptation for plants growing in environments with water scarcity.

New Strategies to Increase the Abiotic Stress Tolerance in Woody Ornamental Plants in Mediterranean Climate

The native flora of different Mediterranean countries, often woody species, was widely recognized for its ornamental potential. The shrubs, in particular, are a typology of plants very widespread in the Mediterranean environment and constituent the 'Macchia', the typical vegetation of this ecosystem. The use of native shrubs for the realization of ornamental green areas has been recently examined for their adaptability to abiotic stress. Abiotic stresses, in fact, are the major limiting growth factor in urban and peri-urban areas. The identification and use of tolerant ornamental species allow the reduction of management costs and preserve the aesthetical value of green areas. Tolerance to drought stress, for instance, in the Mediterranean climate can improve the ecosystem services of these plants in the urban environment. In this review, the possibility to early individuate different plant species' mechanisms to tolerate or avoid the stresses is analysed, as well as the possibility to increase abiotic stress tolerance through genetic and agronomic strategies. The exploration of wild or spontaneous species can be a good source for selecting tolerant plants to be used as ornamental plants in urban areas. Among agronomic strategies, biostimulants, mulching, and plant combination can provide a useful solution to counteract abiotic stress in the urban environment.

Exploring the quantitative genetics of traits associated with drought tolerance and yield in Pimpinella anisum L. under different water regimes

MAIN CONCLUSION

Genetic improvement of seed yield and drought resistance could be simultaneously gained in anise when breeding for drought resistance. Improving the water use efficiency of anise is a primary objective of anise breeding programs aimed at mitigating the impacts of drought stress. This study aimed to determine the predominant mechanisms involved in drought tolerance and investigate the genetic control of associated traits with drought tolerance and higher grain yield. According to these aims, 10 half-diallel hybrids and their five parents were evaluated in both field and greenhouse lysimetric experiments under well-watered and water deficit stress conditions. The results indicated that the inheritance of grain yield is complex and affected by water deficit stress. Similar heritability and genetic architecture were detected for flowering time and percentages of photosynthate partitioned to grain (PPPG) in both well-watered and water deficit stress treatments. Significant negative genetic correlations were observed between grain yield and flowering time, root dry mass, root diameter, root volume, root number, percentages of photosynthate partitioned to shoot, and percentages of photosynthate partitioned to root. Therefore, the selection of low values of these attributes can be used to improve grain yield under drought conditions. In contrast, a positive significant genetic linkage between grain yield and PPPG, chlorophyll content, cell membrane stability, and leaf relative water content reveal selection for high values of these attributes is favored. These attributes could be used as surrogate selection criteria in the early segregating generations. The P₁ parent (early ripening parent) contained key genes associated with PPPG and drought escape. It was concluded that improvement of drought tolerance and grain yield could be simultaneously achieved in anise breeding programs.

Evaluation of Drought Responses in Two Tropaeolum Species Used in Landscaping through Morphological and Biochemical Markers

One of the most important challenges horticultural crops confront is drought, particularly in regions such as the Mediterranean basin, where water supplies are usually limited and will become even scarcer due to global warming. Therefore, the selection and diversification of stress-tolerant cultivars are becoming priorities of contemporary ornamental horticulture. This study explored the impact of water stress on two Tropaeolum species frequently used in landscaping. Young plants obtained by seed germination were exposed to moderate water stress (half the water used in the control treatments) and severe water stress (complete withholding of irrigation) for 30 days. Plant responses to these stress treatments were evaluated by determining several growth parameters and biochemical stress markers. The latter were analysed by spectrophotometric methods and, in some cases, by non-destructive measurements using an optical sensor. The statistical analysis of the results indicated that although the stress responses were similar in these two closely related species, T. minus performed better under control and intermediate water stress conditions but was more susceptible to severe water stress. On the other hand, T. majus had a stronger potential for adaptation to soil water scarcity, which may be associated with its reported expansion and naturalisation in different regions of the world. The variations in proline and malondialdehyde concentrations were the most reliable biochemical indicators of water stress effects. The present study also showed a close relationship between the patterns of variation of flavonoid and chlorophyll contents obtained by sensor-based and spectrophotometric methods.

Biochar-Mediated Control of Metabolites and Other Physiological Responses in Water-Stressed Leptocohloa fusca

We investigated biochar-induced drought tolerance in Leptocohloa fusca (Kallar grass) by exploring the plant defense system at physiological level. L. fusca plants were exposed to drought stress (100%, 70%, and 30% field capacity), and biochar (BC), as an organic soil amendment was applied in two concentrations (15 and 30 mg kg-1 soil) to induce drought tolerance. Our results demonstrated that drought restricted the growth of L. fusca by inhibiting shoot and root (fresh and dry) weight, total chlorophyll content and photosynthetic rate. Under drought stress, the uptake of essential nutrients was also limited due to lower water supply, which ultimately affected metabolites including amino and organic acids, and soluble sugars. In addition, drought stress induced oxidative stress, which is evidenced by the higher production of reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide ion (O2-), hydroxyl ion (OH-), and malondialdehyde (MDA). The current study revealed that stress-induced oxidative injury is not a linear path, since the excessive production of lipid peroxidation led to the accumulation of methylglyoxal (MG), a member of reactive carbonyl species (RCS), which ultimately caused cell injury. As a consequence of oxidative-stress induction, the ascorbate-glutathione (AsA-GSH) pathway, followed by a series of reactions, was activated by the plants to reduce ROS-induced oxidative damage. Furthermore, biochar considerably improved plant growth and development by mediating metabolites and soil physio-chemical status.

Transgressive Biochemical Response to Water Stress in Interspecific Eggplant Hybrids

In a climate change scenario, crop tolerance to drought must be urgently improved, as it represents an increasingly critical stress reducing agricultural yields worldwide. Although most crops are relatively sensitive to water stress, many of their wild relatives are more tolerant and may be used to improve drought tolerance in our crops. In this study, the response to drought of eggplant (Solanum melongena), its close wild relatives S. insanum and S. incanum and their interspecific hybrids with S. melongena was assessed. The plants were subjected to two treatments for 18 days: control, with irrigation every four days, and drought, with complete interruption of irrigation. Morphological and biomass traits were measured, and physiological and biochemical responses were analysed using stress biomarkers such as proline, flavonoids, and total phenolic compounds. Oxidative stress was quantified by measuring malondialdehyde (MDA) content. As a result of the drought treatment, plant development and tissue water content were seriously affected. Generally, water deficit also caused significant increases in MDA, proline, flavonoids, and total phenolics compounds. Our results comparing parental accessions reveal a better response to drought in one of the S. insanum accessions. The hybrid between S. melongena and S. incanum displayed a better response than the other hybrids and even its parents. The results obtained here might be helpful for future eggplant breeding programmes aimed at improving drought tolerance.

Arbuscular Mycorrhizal Fungi Promote Physiological and Biochemical Advantages in Handroanthus serratifolius Seedlings Submitted to Different Water Deficits

Climate change causes increasingly longer periods of drought, often causing the death of plants, especially when they are in the early stages of development. Studying the benefits provided by arbuscular mycorrhizal (AM) fungi to plants in different water regimes is an efficient and sustainable strategy to face climate change. Thus, this study investigated the influence of AM fungi on Handroanthus serratifolius seedlings under different water regimes, based on biochemical, and nutritional growth parameters. The experiment was carried out in H. serratifolius seedlings cultivated with mycorrhizas (+AMF) and without mycorrhizas (-AMF) in three water regimes; a severe water deficit (SD), a moderate water deficit (MD), and a well-watered (WW) condition. AM fungi provided greater osmoregulation under water deficit conditions through the accumulation of soluble sugars, total free amino acids, and proline, as well as by reducing sugar. The increase in the absorption of phosphorus and nitrate was observed only in the presence of fungi in the well-watered regimen. A higher percentage of colonization was found in plants submitted to the well-watered regimen. Ultimately, AM fungi promoted biochemical, nutritional, and growth benefits for H. serratifolius seedlings under the water deficit and well-hydrated conditions, proving that AMF can be used to increase the tolerance of H. serratifolius plants, and help them to survive climate change.

Mediterranean moth diversity is sensitive to increasing temperatures and drought under climate change

Climate change affects ecosystems worldwide and is threatening biodiversity. Insects, as ectotherm organisms, are strongly dependent on the thermal environment. Yet, little is known about the effects of summer heat and drought on insect diversity. In the Mediterranean climate zone, a region strongly affected by climate change, hot summers might have severe effects on insect communities. Especially the larval stage might be sensitive to thermal variation, as larvae—compared to other life stages—cannot avoid hot temperatures and drought by dormancy. Here we ask, whether inter-annual fluctuations in Mediterranean moth diversity can be explained by temperature (T_Larv) and precipitation during larval development (H_Larv). To address our question, we analyzed moth communities of a Mediterranean coastal forest during the last 20 years. For species with summer-developing larvae, species richness was significantly negatively correlated with T_Larv, while the community composition was affected by both, T_Larv and H_Larv. Therefore, summer-developing larvae seem particularly sensitive to climate change, as hot summers might exceed the larval temperature optima and drought reduces food plant quality. Increasing frequency and severity of temperature and drought extremes due to climate change, therefore, might amplify insect decline in the future.

Morphological and physiological changes in Artemisia selengensis under drought and after rehydration recovery

Changes in global climate and precipitation patterns have exacerbated the existing uneven distribution of water, causing many plants to face the alternate situation of drought and water flooding. We studied the growth and physiological response of the wetland plant Artemisia selengensis to drought and rehydration. In this study, Artemisia selengensis seedlings were subjected to 32.89% (SD), 47.36 % (MD), 60.97% (MID), and 87.18 % (CK) field water holding capacity for 70 days, followed by 14 days of rehydration. The results showed that drought inhibited the increase of plant height, basal diameter, and biomass accumulation under SD and MD, but the root shoot ratio (R/S) increased. Drought stress also decreased the content of total chlorophyll (Chl), chlorophyll a (Chl-a), chlorophyll b (Chl-b), and carotenoid (Car). Soluble sugar (SS) and proline (Pro) were accumulated rapidly under drought, and the relative water content (RWC) of leaves was kept at a high level of 80%. After rehydration, the plant height, basal diameter, biomass, and R/S ratio could not be recovered under SD and MD, but these indicators were completely recovered under MID. The RWC, Chl, Chl-a, Chl-b, Car, and osmotic substances were partially or completely recovered. In conclusion, Artemisia selengensis not only can improve drought resistance by increasing the R/S ratio and osmotic substances but also adopt the compensatory mechanism during rehydration. It is predictable that A. selengensis may benefit from possible future aridification of wetlands and expand population distribution.

Assessment of biochemical compounds and antioxidant enzyme activity in barley and wheatgrass under water-deficit condition

Wheatgrass and barley grass are freshly sprouted leaves of wheat and barley seeds and are rich sources of phytochemicals. This study was conducted to investigate the effects of drought stress on the biochemical compounds and antioxidant activities of barley grass and wheatgrass extracts. The grass was cultivated in an organic soil growing medium with different levels of drought stress (a control with 100% water holding capacity (WHC), mild drought stress with 75% WHC, moderate drought stress with 50% WHC, and severe drought stress with 25% WHC) in a growth chamber by controlling temperature (20/15 °C, day/night), light (12/12 h, light/dark; intensity 150 μmol m^-2  s^-1 with quantum dot light-emitting diodes), and relative humidity (60%) for 7 days. The drought stress showed increased levels of biochemical compounds, especially phenolics, flavonoids, and vitamin C, in both barley grass and wheatgrass extracts. The wheatgrass extracts showed 1.38-1.67 times higher phenolics, flavonoids, and vitamin C contents than the barley grass extracts did. The antioxidant (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity, and nitrite-scavenging activity) and antioxidant enzymes (guaiacol peroxidase, catalase, and glutathione reductase) were the highest under severe drought stress in both barley grass and wheatgrass extracts; and the wheatgrass extracts showed 1.20-5.70 times higher antioxidant enzyme activities than the barley grass extracts did. Proper drought-stress treatment of barley grass and wheatgrass may be a convenient and efficient method to increase biochemical compounds and antioxidants in our diet to exploit the related health benefits. © 2021 Society of Chemical Industry.

Sustainable Exploitation of Greek Rosmarinus officinalis L. Populations for Ornamental Use through Propagation by Shoot Cuttings and In Vitro Cultures

Rosmarinus officinalis L. belongs to the Lamiaceae family and is cultivated worldwide due to its diverse uses as an ornamental evergreen shrub in gardening, as well as a food seasoning and a natural medicine. The present research was conducted to study the morphological traits of seven wild-grown rosemary populations in Greece, as well as the propagation of two selected populations with the most desirable ornamental characteristics, by the use of shoot cuttings and in vitro cultures. From the study of the morphological traits of the seven populations, those with interesting features were grown in the areas of Amaliada and Piperia and, thus, were chosen for studying their propagation. Rooting of the shoot cuttings was carried out during the four seasons of the year, using potassium salt Indole-3-butyric acid (K-IBA) at concentrations of 0.5 and 1 g·L−1, in various substrates under the intermittent mist or fog system. It was found that the shoot cuttings rooted easily in all four seasons, but there were better results for the population of ‘Piperia’ in autumn, with 1 g·L−1 K-IBA (80%), and ‘Amaliada’ in spring, with 0.5 g·L−1 K-IBA (82.5%), while higher rooting percentages were achieved in the fog system, on a substrate consisting of perlite and peat, in a ratio of 2:1 (85%). For the in vitro cultures, shoot tips excised from the two selected populations were successfully disinfested by pre-soaking in an antioxidant solution and then, by sterilizing them in 0.6% (w/v) NaOCl, followed by transferring them onto a Murashige and Skoog (MS) nutrient medium. ‘Amaliada’ cultures proved to be the most productive population (2.1 shoots per explant), with the highest shoot formation frequency (91.6%), when cultivated on the MS nutrient medium without plant growth regulators. For ‘Piperia’ cultures, the highest shoot formation frequency (66.6%) was achieved on the MS nutrient medium supplemented with 0.25 or 0.5 mg·L−1 6-Benzyloaminopurine (BAP) and 0.1 mg·L−1 Indole-3-acetic acid (IAA). Spontaneous root formation frequency was noticed on the MS nutrient medium, containing 0.5 mg·L−1 BAP and 0.1 mg·L−1 IAA, for both ‘Amaliada’ and ‘Piperia’ cultures (50% and 41.6%, respectively) in a single stage, with root lengths of 7.1 and 5.3 cm, respectively. Rosemary plantlets, with roots formed in vitro after transplanting them in soilless substrate, were acclimatized adequately in the greenhouse environment (~70%).

Biochar in manure can suppress water stress of sugar beet (Beta vulgaris) and increase sucrose content in tubers

Increased soil drought events threaten the yields of sugar beet (Beta vulgaris L.) and other staples of arable production in central Europe. In this study we evaluated soil moisture and nutrients as impacted by a two and five % (wt) addition of biochar, manure and their blend to a loamy-sand Regosol. Cyclical soil drought was achieved by the controlled reduction of watering by 75% in pot experiments. Ongoing soil moisture and nutrient measurements were taken, and physiological parameters of sugar beet plants were analysed three weeks after the induced drought. At the end of the experiment (16 weeks) plants were harvested and their mass assessed, as well as their nutrient, pigment and sugar contents. In contrast to the addition of manure, soil volumetric water contents were two to three times greater after biochar amendment, compared to the control soil. Porewater analysis revealed that nutrient leaching (e.g., NO₃^-, K⁺) from manure addition to soil was reduced when biochar was blended in (by ≤86% compared to manure alone). Crop analysis showed that leaf gas exchanges were moderated during drought following soil amendment, and leaf and tuber yields were increased furthest when combined biochar-manure blends were applied (> 2-times compared to the control). Perhaps most importantly, the advantageous soil conditions induced by the combined biochar and manure addition also resulted in significantly increased sugar contents in plants (2.4-times) pointing to immediate practical applications of these results in the field.

Salt Spray and Surfactants Induced Morphological, Physiological, and Biochemical Responses in Callistemon citrinus (Curtis) Plants

The growth and aesthetic value of ornamental plant species used near coastlines are negatively influenced by salt spray. The presence of surfactants could enhance salt damage. To analyze the influences of salt spray and surfactants alone and in combination with each other, individual Callistemon plants were subjected to different treatments for 8 weeks: a solution simulating the composition of seawater (salt spray), a solution containing an anionic surfactant (surfactant), a solution with salt spray and anionic surfactant (salt plus surfactants), and deionized water (control). To study the influence of different climatic conditions, two growing periods, from January to March (I CP) and from May to July (II CP), were established. Salt spray, alone or with surfactant action, influences plants’ growth and aesthetic features in different cycle periods. The percentage of leaf damage significantly increased with salt spray and salt plus surfactants during II CP (~27%). Additionally, the Na+ and Cl− contents were enhanced in the leaves in both CPs, but the contents in the roots were only enhanced in the II CP. The gas exchanges were significantly influenced by the treatments, especially during the II CP, when a reduction in net photosynthesis due to salt spray was observed starting from the second week of stress. At the end of the experiment, in both cycle periods, the leaf proline content increased in the salt spray and salt plus surfactants treatments. In both CPs, PCA revealed that the morphological and physiological parameters were directly associated with the control and surfactants treatments, whereas the mineral contents and biochemical parameters were directly correlated with the salt and salt plus surfactants treatments. The additive effect of surfactant stress, compared to salt stress, did not appear to be significant, with the exception of CP II, and for some parameters, the solubilization action of surfactants was favored by higher temperatures.

Tree-to-Shrub Shift Benefits the Survival of Quercus mongolica Fisch. ex Ledeb. at the Xeric Timberline

Woody species are either trees or shrubs, with the exception of approximately 9.2% “trub” species exhibiting both tree and shrub growth forms. Little is known thus far about the ecological importance of plant growth-form plasticity under a drying climate. Quercus mongolica Fisch. ex Ledeb., a dominant tree species of temperate forests in East Asia, typically shows tree-to-shrub growth-form shifts at the xeric timberline and is suitable to test whether growth-form plasticity can physiologically benefit plant drought acclimation. We quantified the tree architecture, drought stress, physiological drought tolerance, and nonstructural carbohydrates of Q. mongolica trees and shrubs on the opposite slope of the same mountain at the xeric timberline in July 2018. Compared with Q. mongolica trees on shady slopes, the shrubs on sunny slopes had less available water to use and were more severely threatened by drought. Moreover, the shrubs had stronger tolerance to drought but still tended to have less nonstructural carbohydrate accumulation. The tree-to-shrub shift effectively benefits the survival of Q. mongolica under a dry climate and may strongly contribute to forest dynamics and even fire regimes under climate drying, especially for sensitive ecosystems such as the xeric timberline, which will be vulnerable under future climate change scenarios.

Effects of Water Availability on Leaf Trichome Density and Plant Growth and Development of Shepherdia ×utahensis

Many arid lands across the globe are experiencing more frequent and extreme droughts due to warmer temperatures resulting from climate change, less predictable precipitation patterns, and decreased soil moisture. Approximately 60-90% of household water is used for urban landscape irrigation in the western United States, necessitating the establishment of landscapes using drought-tolerant plants that conserve water. Shepherdia ×utahensis (hybrid buffaloberry) is a drought-tolerant plant with dense leaf trichomes (epidermal appendages) that may limit excessive water loss by transpiration. However, little is known about how S. ×utahensis regulates leaf heat balance when transpirational cooling is limited. The objective of this research was to investigate the effects of substrate water availability on plant growth and development and trichome density of S. ×utahensis. Ninety-six clonally propagated plants were grown using an automated irrigation system, and their substrate volumetric water contents were controlled at 0.05-0.40 m³·m^-3 for 2 months. Results showed that water stress impaired plant growth and increased the proportion of visibly wilted leaves. Shepherdia ×utahensis acclimates to drought by reducing cell dehydration and canopy overheating, which may be accomplished through decreased stomatal conductance, smaller leaf development, leaf curling, increased leaf thickness, and greater root-to-shoot ratio. Leaf trichome density increased when stem water potential decreased, resulting in greater leaf reflectance of visible light. Cell and leaf expansion were restricted under water stress, and negative correlations were exhibited between epidermal cell size and trichome density. According to our results, plasticity in leaves and roots aids plants in tolerating abiotic stresses associated with drought. Acclimation of S. ×utahensis to water stress was associated with increased trichome density due to plasticity in cell size. Dense trichomes on leaves reflected more lights which appeared to facilitate leaf temperature regulation.

Interactive Effects of Drought and Saline Aerosol Stress on Morphological and Physiological Characteristics of Two Ornamental Shrub Species

Effects of drought and aerosol stresses were studied in a factorial experiment based on a Randomized Complete Design with triplicates on two ornamental shrubs. Treatments consisted of four levels of water container (40%, 30%, 20%, and 10% of water volumetric content of the substrate) and, after 30 days from experiment onset, three aerosol treatments (distilled water and 50% and 100% salt sea water concentrations). The trial was contextually replicated on two species: Callistemon citrinus (Curtis) Skeels and Viburnum tinus L. ‘Lucidum’. In both species, increasing drought stress negatively affected dry biomass, leaf area, net photosynthesis, chlorophyll a fluorescence, and relative water content. The added saline aerosol stress induced a further physiological water deficit in plants of both species, with more emphasis on Callistemon. The interaction between the two stress conditions was found to be additive for almost all the physiological parameters, resulting in enhanced damage on plants under stress combination. Total biomass, for effect of combined stresses, ranged from 120.1 to 86.4 g plant−1 in Callistemon and from 122.3 to 94.6 g plant−1 in Viburnum. The net photosynthesis in Callistemon declined by the 70% after 30 days in WC 10% and by the 45% and 53% in WC 20% and WC 10% respectively after 60 days. In Viburnum plants, since the first measurement (7 days), a decrease of net photosynthesis was observed for the more stressed treatments (WC 20% and WC 10%), by 57%. The overall data suggested that Viburnum was more tolerant compared the Callistemon under the experimental conditions studied.

Morpho-Physiological Classification of Italian Tomato Cultivars (Solanum lycopersicum L.) According to Drought Tolerance during Vegetative and Reproductive Growth

Irrigation is fundamental for agriculture but, as climate change becomes more persistent, there is a need to conserve water and use it more efficiently. It is therefore crucial to identify cultivars that can tolerate drought. For economically relevant crops, such as tomatoes, this purpose takes on an even more incisive role and local agrobiodiversity is a large genetic reservoir of promising cultivars. In this study, nine local Italian cultivars of tomatoes plus four widely used commercial cultivars were considered. These experienced about 20 d of drought, either at vegetative or reproductive phase. Various physio-morphological parameters were monitored, such as stomatal conductance (gs), photosynthesis (A), water use efficiency (WUE), growth (GI) and soil water content (SWC). The different responses and behaviors allowed to divide the cultivars into three groups: tolerant, susceptible, and intermediate. The classification was also confirmed by a principal component analysis (PCA). The study, in addition to deepening the knowledge of local Italian tomato cultivars, reveals how some cultivars perform better under stress condition than commercial ones. Moreover, the different behavior depends on the genotype and on the growth phase of plants. In fact, the Perina cultivar is the most tolerant during vegetative growth while the Quarantino cultivar is mostly tolerant at reproductive stage. The results suggest that selection of cultivars could lead to a more sustainable agriculture and less wasteful irrigation plans.

Thiamin stimulates growth and secondary metabolites in turnip (Brassica rapa L.) leaf and root under drought stress

Thiamin, an important member of the vitamin B family, is believed to play a significant role in mitigating environmental stresses including drought stress. In turnip, drought stress causes a reduced growth, biomass yield, pigment content, total phenolics and ascorbic acid (AsA), particularly at 50% field capacity (F.C.) in the two cultivars (cv) studied. However, a significant enhancement was observed in the contents of leaf proline, glycinebetaine (GB), malondialdehyde (MDA), hydrogen peroxide (H₂ O₂ ) and the activities of catalase (CAT) and superoxide dismutase (SOD) as well as root proline, GB, total phenolics, AsA, H₂ O₂ , MDA and the activities of peroxidase (POD) and SOD. However, foliar-applied thiamin significantly improved (particularly 100 mM) all the growth attributes, photosynthetic pigments, leaf and root osmoprotectants (GB and proline), AsA, total phenolics and the activities of enzymatic antioxidants such as SOD and POD as well as root CAT in both turnip cultivars under drought stress conditions. Foliar application of thiamin was effective in decreasing the leaf and root H₂ O₂ and MDA content in both cultivars particularly at 50% F.C. Thiamin-induced growth of both turnip cultivars, particularly of cv. Purple Top, was found to be associated with increased photosynthetic pigments, proline and GB contents and antioxidant capacity, but reduced levels of reactive oxygen species (ROS) under water deficit conditions. So, it is suggested that exogenous application of thiamin can be effective in improving drought tolerance of plants.

Biochemical, Physiological, and Molecular Aspects of Ornamental Plants Adaptation to Deficit Irrigation

There is increasing concern regarding global warming and its severe impact on the farming sector and food security. Incidences of extreme weather conditions are becoming more and more frequent, posing plants to stressful conditions, such as flooding, drought, heat, or frost etc. Especially for arid lands, there is a tug-of-war between keeping high crop yields and increasing water use efficiency of limited water resources. This difficult task can be achieved through the selection of tolerant water stress species or by increasing the tolerance of sensitive species. In this scenario, it is important to understand the response of plants to water stress. So far, the response of staple foods and vegetable crops to deficit irrigation is well studied. However, there is lack of literature regarding the responses of ornamental plants to water stress conditions. Considering the importance of this ever-growing sector for the agricultural sector, this review aims to reveal the defense mechanisms and the involved morpho-physiological, biochemical, and molecular changes in ornamental plant’s responses to deficit irrigation.

An insight into the role of silicon on retaliation to osmotic stress in finger millet (Eleusine coracana (L.) Gaertn)

Finger millet, a vital nutritional cereal crop provides food security. It is a well-established fact that silicon (Si) supplementation to plants alleviates both biotic and abiotic stresses. However, precise molecular targets of Si remain elusive. The present study attempts to understand the alterations in the metabolic pathways after Si amendment under osmotic stress. The analysis of transcriptome and metabolome of finger millet seedlings treated with distilled water (DW) as control, Si (10 ppm), PEG (15%), and PEG (15%) + Si (10 ppm) suggest the molecular alterations mediated by Si for ameliorating the osmotic stress. Under osmotic stress, uptake of Si has increased mediating the diversion of an enhanced pool of acetyl CoA to lipid biosynthesis and down-regulation of TCA catabolism. The membrane lipid damage reduced significantly by Si under osmotic stress. A significant decrease in linolenic acid and an increase of jasmonic acid (JA) in PEG + Si treatment suggest the JA mediated regulation of osmotic stress. The relative expression of transcripts corroborated with the corresponding metabolites abundance levels indicating the activity of genes in assuaging the osmotic stress. This work substantiates the role of Si in osmotic stress tolerance by reprogramming of fatty acids biosynthesis in finger millet.

Drought Stress Impacts on Plants and Different Approaches to Alleviate Its Adverse Effects

Drought stress, being the inevitable factor that exists in various environments without recognizing borders and no clear warning thereby hampering plant biomass production, quality, and energy. It is the key important environmental stress that occurs due to temperature dynamics, light intensity, and low rainfall. Despite this, its cumulative, not obvious impact and multidimensional nature severely affects the plant morphological, physiological, biochemical and molecular attributes with adverse impact on photosynthetic capacity. Coping with water scarcity, plants evolve various complex resistance and adaptation mechanisms including physiological and biochemical responses, which differ with species level. The sophisticated adaptation mechanisms and regularity network that improves the water stress tolerance and adaptation in plants are briefly discussed. Growth pattern and structural dynamics, reduction in transpiration loss through altering stomatal conductance and distribution, leaf rolling, root to shoot ratio dynamics, root length increment, accumulation of compatible solutes, enhancement in transpiration efficiency, osmotic and hormonal regulation, and delayed senescence are the strategies that are adopted by plants under water deficit. Approaches for drought stress alleviations are breeding strategies, molecular and genomics perspectives with special emphasis on the omics technology alteration i.e., metabolomics, proteomics, genomics, transcriptomics, glyomics and phenomics that improve the stress tolerance in plants. For drought stress induction, seed priming, growth hormones, osmoprotectants, silicon (Si), selenium (Se) and potassium application are worth using under drought stress conditions in plants. In addition, drought adaptation through microbes, hydrogel, nanoparticles applications and metabolic engineering techniques that regulate the antioxidant enzymes activity for adaptation to drought stress in plants, enhancing plant tolerance through maintenance in cell homeostasis and ameliorates the adverse effects of water stress are of great potential in agriculture.

Drought-hardening improves drought tolerance in Nicotiana tabacum at physiological, biochemical, and molecular levels

BACKGROUND

Drought stress is the most harmful one among other abiotic stresses with negative impacts on crop growth and development. Drought-hardening is a feasible and widely used method in tobacco seedlings cultivation. It has gained extensive interests due to its role in improving drought tolerance. This research aimed to investigate the role of drought-hardening and to unravel the multiple mechanisms underlying tobacco drought tolerance and adaptation.

RESULTS

This study was designed in which various drought-hardening treatments (CK (no drought-hardening), T1 (drought-hardening for 24 h), T2 (drought-hardening for 48 h), and T3 (drought-hardening for 72 h)) were applied to two tobacco varieties namely HongHuaDaJinYuan (H) and Yun Yan-100 (Y). The findings presented a complete framework of drought-hardening effect at physiological, biochemical, and gene expression levels of the two tobacco varieties under drought stress. The results showed that T2 and T3 significantly reduced the growth of the two varieties under drought stress. Similarly, among the various drought-hardening treatments, T3 improved both the enzymatic (POD, CAT, APX) and non-enzymatic (AsA) defense systems along with the elevated levels of proline and soluble sugar to mitigate the negative effects of oxidative damage and bringing osmoregulation in tobacco plants. Finally, the various drought-hardening treatments (T1, T2, and T3) showed differential regulation of genes expressed in the two varieties, while, particularly T3 drought-hardening treatment-induced drought tolerance via the expression of various stress-responsive genes by triggering the biosynthesis pathways of proline (P5CS1), polyamines (ADC2), ABA-dependent (SnRK2, AREB1), and independent pathways (DREB2B), and antioxidant defense-related genes (CAT, APX1, GR2) in response to drought stress.

CONCLUSIONS

Drought-hardening made significant contributions to drought tolerance and adaptation in two tobacco variety seedlings by reducing its growth and, on the other hand, by activating various defense mechanisms at biochemical and molecular levels. The findings of the study pointed out that drought-hardening is a fruitful strategy for conferring drought tolerance and adaptations in tobacco. It will be served as a useful method in the future to understand the drought tolerance and adaptation mechanisms of other plant species. Drought-hardening improved drought tolerance and adaptation of the two tobacco varieties. T1 indicates drought-hardening for 24 h, T2 indicates drought-hardening for 48 h, T3 indicates drought-hardening for 72 h.

An Evaluation of Different Parameters to Screen Ornamental Shrubs for Salt Spray Tolerance

In the context of seaside landscaping, one of the greatest challenges for practitioners and scientists is to select suitable species that are able to tolerate salt spray. This is the key aspect for a wide number of potentially suitable species. The objectives of this study were (1) to identify plant traits associated with species tolerance to salt stress and (2) to evaluate the responses of different shrub species to salt spray. For this purpose, a study was conducted to determine the effects of salt spray on twenty-four ornamental shrubs using rapid and easy-to-use methodology. The species were subjected twice a week to nebulization treatment with simulated seawater solution for 60 days. Every 20 days, net photosynthesis, stomatal conductance, transpiration rate, and chlorophyll a fluorescence were determined. Furthermore, dry biomass of the different organographic portions, leaf number, leaf area, Specific Leaf Area, chlorophyll content, and leaf damage were determined at the end of the experiment. The species exposed to seawater solution showed different physiological and morphological responses. Based on the above indices, these ornamental shrubs were screened and categorized as tolerant, moderately tolerant, or susceptible. The results suggest that Convolvulus, Ceratonia, and Ligustrum are more tolerant to salt spray than numerous other genotypes; L. langmaniae, Cascabela, and L. frutescens, conversely, are more sensitive. Among the plant traits, the morphological parameters thoroughly characterized the effects of the salt spray, but they were destructive, with the only exception being the leaf damage percentage. This last non-destructive parameter is interesting considering the aesthetic value that ornamental plants must have. The physiological parameters, and in particular photosynthesis activity, can instead be used as a non-destructive screening method to select species suitable for ornamental green spaces near the sea.

Evaluation of Antioxidant Systems and Ascorbate-Glutathione Cycle in Feijoa Edible Flowers at Different Flowering Stages

Background: Feijoa (Acca sellowiana (O. Berg)) was initially introduced as an ornamental plant, but nowadays, it is widely cultivated for the numerous beneficial properties of its edible fruits. Feijoa flowers have been included in the list of edible flowers, but despite this, they are still considered niche products due to consumer skepticism and lack of publicity. Methods: This study evaluated the physicochemical traits, antioxidant system, and ascorbate-glutathione cycle in feijoa flowers at five flowering stages based on the Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie (BBCH) scale. Results: The results showed that the optimal stage to harvest feijoa flower is the F2 stage characterized by high bioactive compounds content. Furthermore, the enzymes involved in oxidative stress and the ascorbate-glutathione cycle showed different trends during the flowering stages. Conclusions: This study provides new evidence to understand the possible role of bioactive compounds and ascorbate-glutathione cycle in the regulation of flower development, defining the optimal stage to harvest flowers.

Abiotic Stress Effects on Performance of Horticultural Crops

The yield and quality of horticultural crops mainly depend on genotype, environmental conditions, and cultivation management. Abiotic stresses, such as adverse environmental conditions, can strongly reduce crop performance, with crop yield losses ranging from 50% to 70%. The most common abiotic stresses are represented by cold, heat, drought, flooding, salinity, nutrient deficiency, and high and low light intensities, including ultraviolet radiation. These abiotic stresses affect multiple physiological and biochemical processes in plants. The ability of plants to face these stresses depends on their adaptation aptitude, and tolerant plants may express different strategies to adapt to or avoid the negative effects of abiotic stresses. At the physiological level, photosynthetic activity and light-use efficiency of plants may be modulated to enhance tolerance against the stress. At the biochemical level, several antioxidant systems can be activated, and many enzymes may produce stress-related metabolites to help avoid cellular damage, including such compounds as proline, glycine betaine, amino acids, etc. This special issue gathers eight papers; three are reviews and five are research papers. Two reviews are focused on the application of appropriate agronomic strategies for counteracting the negative effects of abiotic stresses. The third review is based on ornamental plant production under drought stress conditions and the effect on their ornamental quality. The research papers report the effect of climate change on crop development, yield, and quality. Abiotic stresses have been proven to reduce crop performance and yield. Research studies are essential for understanding the key adaptation strategies of plants that can be exploited for improving the crop stress tolerance.