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.

Water availability is paramount in the response of soil invertebrates towards stress situations. This study aimed to evaluate the effects of forecasted soil moisture scenarios on the avoidance behavior of two invertebrate species (the arthropod Folsomia candida and the soft-bodied oligochaete Enchytraeus crypticus) in soils degraded by different types of anthropogenic metal(loid) contamination (mining soil and agricultural soil affected by industrial chemical wastes). Different soil moisture contents (expressed as % of the soil water holding capacity, WHC) were evaluated: 50% (standard soil moisture conditions for soil invertebrates' tests); 75% (to simulate increasing soil water availability after intense rainfalls and/or floods); 40%, 30%, 25% and 20% (to simulate decreasing soil water availability during droughts). Invertebrates’ avoidance behavior and changes in soil porewater major ions and metal(loid)s were assessed after 48 h exposure. Soil incubations induced a general solubilization/mobilization of porewater major ions, while higher soil acidity favored the solubilization/mobilization of porewater metal(loid)s, especially at 75% WHC. Folsomia candida preferred soils moistened at 50% WHC, regardless the soils were contaminated or not and the changing soil porewater characteristics. Enchytraeus crypticus avoided metal(loid) contamination, but this depended on the soil moisture conditions and the corresponding changes in porewater characteristics: enchytraeids lost their capacity to avoid contaminated soils under water stress situations (75% and 20–25% WHC), but also when contaminated soils had greater water availability than control soils. Therefore, forecasted soil moisture scenarios induced by global warming changed soil porewater composition and invertebrates capacity to avoid metal(loid)-contaminated soils.

To investigate the effect of arbuscular mycorrhizal fungi (AMF) on boron (B) toxicity in plants under the combined stresses of salt and drought, Puccinellia tenuiflora was grown in the soil with the inoculation of Funneliformis mosseae and Claroideoglomus etunicatum. After three weeks of treatment, the plants were harvested to determine mycorrhizal colonization rates, plant biomass, as well as tissue B, phosphorus, sodium, and potassium concentrations. The results show that the combined stresses reduced mycorrhizal colonization. Mycorrhizal inoculation significantly increased plant biomass while reduced shoot B concentrations. Mycorrhizal inoculation also slightly increased shoot phosphorus and potassium concentrations, and reduced shoot sodium concentrations. F. mosseae and C. etunicatum were able to alleviate the combined stresses of B, salt, and drought. The two fungal species and their combination showed no significant difference in the alleviation of B toxicity. It is inferred that AMF is able to alleviate B toxicity in P. tenuiflora by increasing biomass and reducing tissue B concentrations. The increase in plant phosphorus and potassium, as well as the decrease in sodium accumulation that induced by AMF, can help plant tolerate the combined stresses of salt and drought. Our findings suggest that F. mosseae and C. etunicatum are potential candidates for facilitating the phytoremediation of B-contaminated soils with salt and drought stress.

A two-year experiment exposing Acer truncatum Bunge seedlings to elevated ozone (O3) concentrations above ambient air (AO) and drought stress (DS) was carried out using open-top chambers (OTCs) in a suburb of Beijing in north China in 2012–2013. The results suggested that AO and DS had both significantly reduced leaf mass area (LMA), stomatal conductance (Gs), light saturated photosynthetic rate (Asat) as well as above and below ground biomass at the end of the experiment. It appeared that while drought stress mitigated the expression of foliar injury, LMA, leaf photosynthetic pigments, height growth and basal diameter, due to limited carbon fixation, the O3 – induced reductions in Asat, Gs and total biomass were enhanced 23.7%. 15.5% and 8.1% respectively. These data suggest that when the whole plant was considered that drought under the conditions of this experiment did not protect the Shantung maple seedlings from the effects of O3.

We are fertilizing a thicket with 0, 10, 20 and 50kg nitrogen (N) ha⁻¹ yr⁻¹ in central Spain. Here we report changes in cover, pigments, pigment ratios and FvFm of the N-tolerant, terricolous, lichen Cladonia foliacea after 1–2 y adding N in order to study its potential as biomarker of atmospheric pollution. Cover tended to increase. Pigments increased with fertilization independently of the dose supplied but only significantly with soil nitrate as covariate. β-carotene/chlorophylls increased with 20–50kgNha⁻¹yr⁻¹ (over the background) and neoxanthin/chlorophylls also increased with N. (Neoxanthin+lutein)/carotene decreased with N when nitrate and pH seasonalities were used as covariates. FvFm showed a critical load above 40kgNha⁻¹yr⁻¹. Water-stress, iron and copper also explained variables of lichen physiology. We conclude that this tolerant lichen could be used as biomarker and that responses to N are complex in heterogeneous Mediterranean-type landscapes.

This paper summarises some of the main results of a two-year experiment carried out in an Open-Top Chambers facility in Northern Italy. Seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior have been subjected to different ozone treatments (charcoal-filtered and non-filtered air) and soil moisture regimes (irrigated and non-irrigated plots). Stomatal conductance models were applied and parameterised under South Alpine environmental conditions and stomatal ozone fluxes have been calculated. The flux-based approach provided a better performance than AOT40 in predicting the onset of foliar visible injuries. Critical flux levels, related to visible leaf injury, are proposed for P. nigra and F. sylvatica (ranging between 30 and 33 mmol O3 m-²). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold, below which may occur a complete ozone detoxification. The stomatal uptake of ozone is an important factor to evaluate visible injury appearance and evolution in plants.

The growth of transgenic canola (Brassica napus) expressing a gene for the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase was compared to non-transformed canola exposed to flooding and elevated soil Ni concentration, in situ. In addition, the ability of the plant growth-promoting bacterium Pseudomonas putida UW4, which also expresses ACC deaminase, to facilitate the growth of non-transformed and transgenic canola under the above mentioned conditions was examined. Transgenic canola and/or canola treated with P. putida UW4 had greater shoot biomass compared to non-transformed canola under low flood-stress conditions. Under high flood-stress conditions, shoot biomass was reduced and Ni accumulation was increased in all instances relative to low flood-stress conditions. This is the first field study to document the increase in plant tolerance utilizing transgenic plants and plant growth-promoting bacteria exposed to multiple stressors. Using transgenic plants and plant growth-promoting bacteria as phytoremediation methods increased plant tolerance at a metal-contaminated field site under low flood conditions.

Two different indices have been proposed for estimation of the risk caused to forest trees across Europe by ground-level ozone, (i) the concentration based AOT40 index (Accumulated Over a Threshold of 40 ppb) and (ii) the recently developed flux based AFstY index (Accumulated stomatal Flux above a flux threshold Y). This paper compares the AOT40 and AFstY indices for three forest trees species at different locations in Europe. The AFstY index is estimated using the DO3SE (Deposition of Ozone and Stomatal Exchange) model parameterized for Scots pine (Pinus sylvestris), beech (Fagus sylvatica) and holm oak (Quercus ilex). The results show a large difference in the perceived O3 risk when using AOT40 and AFstY indices both between species and regions. The AOT40 index shows a strong north-south gradient across Europe, whereas there is little difference between regions in the modelled values of AFstY. There are significant differences in modelled AFstY between species, which are predominantly determined by differences in the timing and length of the growing season, the periods during which soil moisture deficit limits stomatal conductance, and adaptation to soil moisture stress. This emphasizes the importance of defining species-specific flux response variables to obtain a more accurate quantification of O3 risk. A new flux-based model provides a revised assessment of risks of ozone impacts to European forests.

Mediterranean areas are characterized by complex hydrogeological systems, where water resources are faced with several issues such as salinity and pollution. Fifty-one water samples were gathered from the Bou-areg coastal and the Gareb aquifers to evaluate the source of water salinity and to reveal the processes of the different sources of pollution using a variety of chemical and isotopic indicators (δ²H–H2O, δ¹⁸O–H2O, δ³⁴S–SO4, and δ¹⁸O–SO4). The results of the hydrochemical analysis of water samples show that the order of dominated elements is Cl⁻ > HCO₃⁻ > SO4₂⁻ > NO₃⁻ and Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and evidenced extremely high salinity levels (EC up to 22000 μS/cm). All samples exceeded the WHO drinking water guidelines, making them unfit for human consumption. Ion ratio diagrams, isotopic results, and graphical comparing indicate that the mineralization of groundwater in the area, is controlled by carbonate dissolution, evaporite dissolution, ion exchange, and sewage invasion. The return of irrigation water plays a significant role as well in the groundwater recharge and its mineralization by fertilizers mainly. Evaporites (Gypsum), sewage, and fertilizers constitute the main sources of sulfates in the investigated water resources. These scientific results will be an added value for decision-makers to more improve the sustainable management of groundwater in water-stressed regions. The use of chemical and isotopic tracers once again shows their relevance in such zones where systematic monitoring is lacking.

Silicon is absorbed as uncharged mono-silicic acid by plant roots through passive absorption of Lsi1, an influx transporter belonging to the aquaporin protein family. Lsi2 then actively effluxes silicon from root cells towards the xylem from where it is exported by Lsi6 for silicon distribution and accumulation to other parts. Recently, it was proposed that silicon nanoparticles (SiNPs) might share a similar route for their uptake and transport. SiNPs then initiate a cascade of morphophysiological adjustments that improve the plant physiology through regulating the expression of many photosynthetic genes and proteins along with photosystem I (PSI) and PSII assemblies. Subsequent improvement in photosynthetic performance and stomatal behaviour correspond to higher growth, development, and productivity. On many occasions, SiNPs have demonstrated a protective role during stressful environments by improving plant-water status, source-sink potential, reactive oxygen species (ROS) metabolism, and enzymatic profile. The present review comprehensively discusses the crop improvement potential of SiNPs stretching their role during optimal and abiotic stress conditions including salinity, drought, temperature, heavy metals, and ultraviolet (UV) radiation. Moreover, in the later section of this review, we offered the understanding that most of these upgrades can be explained by SiNPs intricate correspondence with phytohormones, antioxidants, and signalling molecules. SiNPs can modulate the endogenous phytohormones level such as abscisic acid (ABA), auxins (IAAs), cytokinins (CKs), ethylene (ET), gibberellins (GAs), and jasmonic acid (JA). Altered phytohormones level affects plant growth, development, and productivity at various organ and tissue levels. Similarly, SiNPs regulate the activities of catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and ascorbate-glutathione (AsA-GSH) cycle leading to an upgraded defence system. At the cellular and subcellular levels, SiNPs crosstalk with various signalling molecules such as Ca²⁺, K⁺, Na⁺, nitric oxide (NO), ROS, soluble sugars, and transcription factors (TFs) was also explained.