Spatiotemporal heterogeneity of photosystem II function during acclimation to zinc exposure and mineral nutrition changes in the hyperaccumulator Noccaea caerulescens

Hormesis Responses of Photosystem II in Arabidopsis thaliana under Water Deficit Stress

Since drought stress is one of the key risks for the future of agriculture, exploring the molecular mechanisms of photosynthetic responses to water deficit stress is, therefore, fundamental. By using chlorophyll fluorescence imaging analysis, we evaluated the responses of photosystem II (PSII) photochemistry in young and mature leaves of Arabidopsis thaliana Col-0 (cv Columbia-0) at the onset of water deficit stress (OnWDS) and under mild water deficit stress (MiWDS) and moderate water deficit stress (MoWDS). Moreover, we tried to illuminate the underlying mechanisms in the differential response of PSII in young and mature leaves to water deficit stress in the model plant A. thaliana. Water deficit stress induced a hormetic dose response of PSII function in both leaf types. A U-shaped biphasic response curve of the effective quantum yield of PSII photochemistry (Φ_PSII) in A. thaliana young and mature leaves was observed, with an inhibition at MiWDS that was followed by an increase in Φ_PSII at MoWDS. Young leaves exhibited lower oxidative stress, evaluated by malondialdehyde (MDA), and higher levels of anthocyanin content compared to mature leaves under both MiWDS (+16%) and MoWDS (+20%). The higher Φ_PSII of young leaves resulted in a decreased quantum yield of non-regulated energy loss in PSII (Φ_NO), under both MiWDS (-13%) and MoWDS (-19%), compared to mature leaves. Since Φ_NO represents singlet-excited oxygen (¹O₂) generation, this decrease resulted in lower excess excitation energy at PSII, in young leaves under both MiWDS (-10%) and MoWDS (-23%), compared to mature leaves. The hormetic response of PSII function in both young and mature leaves is suggested to be triggered, under MiWDS, by the intensified reactive oxygen species (ROS) generation, which is considered to be beneficial for activating stress defense responses. This stress defense response that was induced at MiWDS triggered an acclimation response in A. thaliana young leaves and provided tolerance to PSII when water deficit stress became more severe (MoWDS). We concluded that the hormesis responses of PSII in A. thaliana under water deficit stress are regulated by the leaf developmental stage that modulates anthocyanin accumulation in a stress-dependent dose.

Toxic effects of cadmium on the physiological and biochemical attributes of plants, and phytoremediation strategies: A review

Anthropogenic activities pose a more significant threat to the environment than natural phenomena by contaminating the environment with heavy metals. Cadmium (Cd), a highly poisonous heavy metal, has a protracted biological half-life and threatens food safety. Plant roots absorb Cd due to its high bioavailability through apoplastic and symplastic pathways and translocate it to shoots through the xylem with the help of transporters and then to the edible parts via the phloem. The uptake and accumulation of Cd in plants pose deleterious effects on plant physiological and biochemical processes, which alter the morphology of vegetative and reproductive parts. In vegetative parts, Cd stunts root and shoot growth, photosynthetic activities, stomatal conductance, and overall plant biomass. Plants' male reproductive parts are more prone to Cd toxicity than female reproductive parts, ultimately affecting their grain/fruit production and survival. To alleviate/avoid/tolerate Cd toxicity, plants activate several defense mechanisms, including enzymatic and non-enzymatic antioxidants, Cd-tolerant gene up-regulations, and phytohormonal secretion. Additionally, plants tolerate Cd through chelating and sequestering as part of the intracellular defensive mechanism with the help of phytochelatins and metallothionein proteins, which help mitigate the harmful effects of Cd. The knowledge on the impact of Cd on plant vegetative and reproductive parts and the plants' physiological and biochemical responses can help selection of the most effective Cd-mitigating/avoiding/tolerating strategy to manage Cd toxicity in plants.

A Synthetic Review of Various Dimensions of Non-Destructive Plant Stress Phenotyping

Non-destructive plant stress phenotyping begins with traditional one-dimensional (1D) spectroscopy, followed by two-dimensional (2D) imaging, three-dimensional (3D) or even temporal-three-dimensional (T-3D), spectral-three-dimensional (S-3D), and temporal-spectral-three-dimensional (TS-3D) phenotyping, all of which are aimed at observing subtle changes in plants under stress. However, a comprehensive review that covers all these dimensional types of phenotyping, ordered in a spatial arrangement from 1D to 3D, as well as temporal and spectral dimensions, is lacking. In this review, we look back to the development of data-acquiring techniques for various dimensions of plant stress phenotyping (1D spectroscopy, 2D imaging, 3D phenotyping), as well as their corresponding data-analyzing pipelines (mathematical analysis, machine learning, or deep learning), and look forward to the trends and challenges of high-performance multi-dimension (integrated spatial, temporal, and spectral) phenotyping demands. We hope this article can serve as a reference for implementing various dimensions of non-destructive plant stress phenotyping.

Iron counteracts zinc-induced toxicity in soybeans

Zinc (Zn) and iron (Fe) are essential micronutrients for all living organisms and the major targets for crop biofortification. However, when acquired in excess quantities, Zn and Fe can be toxic to plants. In this study, we examined the interaction between Zn and Fe in soybean plants under various Zn and Fe treatments. While the level of Zn accumulation increased with increasing Zn supplies, Zn content greatly decreased with rising Fe supplies. Moreover, Zn uptake rates were negatively correlated with Fe supplies. However, Fe accumulation was not greatly affected by elevating Zn supplies. Excess Zn supplies were found to induce typical Fe deficiency symptoms under low Fe conditions, which can be counteracted by increasing Fe supplies. Interestingly, leaf chlorosis caused by excess Zn and low Fe supplies was not directly associated with reduced total Fe content but likely associated with deleterious effects of excess Zn. The combination of high Zn and low Fe greatly activates FRO2 and FIT1 gene expression in soybean roots. Besides, Zn-Fe interaction influences the activities of antioxidative enzymes as well as the uptake, accumulation, and homeostasis of other essential micronutrients, such as copper and manganese in soybean plants. These findings provide new perspectives on Zn and Fe interaction and on heavy metal-induced Fe deficiency-like symptoms.

Phytotoxicity level and accumulation ability of pot marigold (Calendula officinalis L.) to zinc

The study aims to reveal the effects of different Zn levels on pot marigold. We determined some germination and young seedling properties in the first experiment, and morphological, stomatal, and physiological parameters besides uptake profiles of both Zn and other plant nutrients in the second one. We supplied the water requirement of the seeds with Zn solutions (0, 250, 500, 750, and 1,000 mg l-1) in the first experiment, and We added the same zinc doses as mg kg-1 to the soil of the pots where the plants would grow in the second one. As a result, the inhibitory effects were more prominent in the early seedling stage (especially at 1,000 mg l-1 Zn) than in the germination one. We determined plant growth retardation, decreases in leaf water contents, and increases in membrane damages with higher Zn (≥500 mg kg-1) in the experiment conducted by potting soil. We detected decreases in chlorophyll parameters parallel with the increases in Zn. The alterations in plant zinc contents revealed the accumulation capacity of pot marigold in potting conditions. That TF value >1 in Zn treatments up to 500 mg kg-1 points to the efficiency limit of pot marigold as a Zn-accumulator plant.

A Hormetic Spatiotemporal Photosystem II Response Mechanism of Salvia to Excess Zinc Exposure

Exposure of Salvia sclarea plants to excess Zn for 8 days resulted in increased Ca, Fe, Mn, and Zn concentrations, but decreased Mg, in the aboveground tissues. The significant increase in the aboveground tissues of Mn, which is vital in the oxygen-evolving complex (OEC) of photosystem II (PSII), contributed to the higher efficiency of the OEC, and together with the increased Fe, which has a fundamental role as a component of the enzymes involved in the electron transport process, resulted in an increased electron transport rate (ETR). The decreased Mg content in the aboveground tissues contributed to decreased chlorophyll content that reduced excess absorption of sunlight and operated to improve PSII photochemistry (Φ_PSII), decreasing excess energy at PSII and lowering the degree of photoinhibition, as judged from the increased maximum efficiency of PSII photochemistry (Fv/Fm). The molecular mechanism by which Zn-treated leaves displayed an improved PSII photochemistry was the increased fraction of open PSII reaction centers (qp) and, mainly, the increased efficiency of the reaction centers (Fv′/Fm′) that enhanced ETR. Elemental bioimaging of Zn and Ca by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) revealed their co-localization in the mid-leaf veins. The high Zn concentration was located in the mid-leaf-vein area, while mesophyll cells accumulated small amounts of Zn, thus resembling a spatiotemporal heterogenous response and suggesting an adaptive strategy. These findings contribute to our understanding of how exposure to excess Zn triggered a hormetic response of PSII photochemistry. Exposure of aromatic and medicinal plants to excess Zn in hydroponics can be regarded as an economical approach to ameliorate the deficiency of Fe and Zn, which are essential micronutrients for human health.

Reactive Oxygen Species Initiate Defence Responses of Potato Photosystem II to Sap-Sucking Insect Feeding

Simple Summary

Potato is one of the most universally cultivated horticultural crops and is vulnerable to a range of herbivorous insects. One of them is the brown marmorated stink bug, an invasive polyphagous sap-sucking agricultural insect pest that penetrates the phloem to sieve elements and removes sap via a specialized mouthpart, the stylet. By using the chlorophyll fluorescence imaging methodology, we examined potato photosystem II (PSII) photochemistry responses in the area of feeding on the whole leaf area. Highly increased reactive oxygen species (ROS) generation was observed as rapidly as 3 min after feeding to initiate defence responses and can be considered the primary plant defence response mechanism against herbivores. Our experimental results confirmed that chlorophyll fluorescence imaging methodology can detect spatial heterogeneity of PSII efficiency at the whole leaf surface and is a promising tool for investigating plant response mechanisms of sap-sucking insect herbivores. We suggest that PSII responses to insect feeding underlie ROS-dependent signalling. We conclude that the potato PSII response mechanism to sap-sucking insect herbivores is described by the induction of the defence response to reduce herbivory damage, instead of induction of tolerance, through a compensatory photosynthetic response mechanism that is observed after chewing insect feeding.

Abstract

Potato, Solanum tuberosum L., one of the most commonly cultivated horticultural crops throughout the world, is susceptible to a variety of herbivory insects. In the present study, we evaluated the consequence of feeding by the sap-sucking insect Halyomorpha halys on potato leaf photosynthetic efficiency. By using chlorophyll fluorescence imaging methodology, we examined photosystem II (PSII) photochemistry in terms of feeding and at the whole leaf area. The role of reactive oxygen species (ROS) in potato’s defence response mechanism immediately after feeding was also assessed. Even 3 min after feeding, increased ROS generation was observed to diffuse through the leaf central vein, probably to act as a long-distance signalling molecule. The proportion of absorbed energy being used in photochemistry (Φ_PSII) at the whole leaf level, after 20 min of feeding, was reduced by 8% compared to before feeding due to the decreased number of open PSII reaction centres (qp). After 90 min of feeding, Φ_PSII decreased by 46% at the whole leaf level. Meanwhile, at the feeding zones, which were located mainly in the proximity of the leaf midrib, Φ_PSII was lower than 85%, with a concurrent increase in singlet-excited oxygen (¹O₂) generation, which is considered to be harmful. However, the photoprotective mechanism (Φ_NPQ), which was highly induced 90 min after feeding, was efficient to compensate for the decrease in the quantum yield of PSII photochemistry (Φ_PSII). Therefore, the quantum yield of non-regulated energy loss in PSII (Φ_NO), which represents ¹O₂ generation, remained unaffected at the whole leaf level. We suggest that the potato PSII response to sap-sucking insect feeding underlies the ROS-dependent signalling that occurs immediately and initiates a photoprotective PSII defence response to reduce herbivory damage. A controlled ROS burst can be considered the primary plant defence response mechanism to herbivores.

Unraveling Cadmium Toxicity in Trifolium repens L. Seedling: Insight into Regulatory Mechanisms Using Comparative Transcriptomics Combined with Physiological Analyses

Trifolium repens (T. repens) can accumulate significant amounts of heavy metal ions, and has strong adaptability to wide environmental conditions, and relatively large biomass, which is considered a potential plant for phytoremediation. However, the molecular mechanisms of T. repens involved in Cd tolerance have not yet been studied in detail. This study was conducted to examine the integrative responses of T. repens exposed to a high-level CdCl₂ by investigating the physiological and transcriptomic analyses. The results suggested that T. repens seedlings had a high degree of tolerance to Cd treatment. The roots accumulated higher Cd concentration than leaves and were mainly distributed in the cell wall. The content of MDA, soluble protein, the relative electrolyte leakage, and three antioxidant enzymes (POD, SOD, and APX) was increased with the Cd treatment time increasing, but the CAT enzymes contents were decreased in roots. Furthermore, the transcriptome analysis demonstrated that the differentially expressed genes (DEGs) mainly enriched in the glutathione (GSH) metabolism pathway and the phenylpropanoid biosynthesis in the roots. Overexpressed genes in the lignin biosynthesis in the roots might improve Cd accumulation in cell walls. Moreover, the DEGs were also enriched in photosynthesis in the leaves, transferase activity, oxidoreductase activity, and ABA signal transduction, which might also play roles in reducing Cd toxicity in the plants. All the above, clearly suggest that T. repens employ several different mechanisms to protect itself against Cd stress, while the cell wall biosynthesis and GSH metabolism could be considered the most important specific mechanisms for Cd retention in the roots of T. repens.

Photosynthesizing while hyperaccumulating nickel: Insights from the genus Odontarrhena (Brassicaceae)

Nickel-induced changes in photosynthetic activity were investigated in three Ni-hyperaccumulating Odontarrhena species with increasing Ni tolerance and accumulation capacity, O. muralis, O. moravensis, and O. chalcidica. Plantlets were grown in hydroponics at increasing NiSO₄ concentrations (0, 0.25, and 1 mM) for one week, and the effects of Ni on growth, metal accumulation, photosynthesis, and nitrogen (N) allocation to components of the photosynthetic apparatus were analysed. Nickel treatments in O. chalcidica, and O. moravensis to a lesser extent, increased not only the photochemical efficiency of photosystem II (PSII) and the CO₂ assimilation rate, but also CO₂ diffusion from the atmosphere to the carboxylation sites. These two species displayed a specific increase and/or rearrangement of the photosynthetic pigments and a higher leaf N allocation to the photosynthetic components in the presence of the metal. Odontarrhena muralis displayed a decrease in photosynthetic performance at the lowest Ni concentration due to a combination of both stomatal and non-stomatal limitations. Our data represent the first complete investigation of the effects of Ni on the photosynthetic machinery in Ni hyperaccumulating plants. Our findings clearly indicate a stimulatory, hormetic-like, effect of the metal on both biophysics and biochemistry of photosynthesis in the species with the highest hyperaccumulation capacity.

Hormetic effects of zinc on growth and antioxidant defense system of wheat plants

Although hormesis induced by heavy metals is a well-known phenomenon, the involved biological mechanisms are not fully understood. Zinc (Zn) is an essential micronutrient for wheat, an important crop contributing to food security as a main staple food; however, excessive Zn is detrimental to the growth of wheat. The aim of this study was to evaluate morphological and physiological responses of two wheat varieties exposed to a broad range of Zn concentrations (0-1000 μM) for 14 days. Hormesis was induced by Zn in both wheat varieties. Treatment with 10-100 μM Zn promoted biomass accumulation by enhancing the photosynthetic ability, the chlorophyll content and the activities of antioxidant enzymes. Increased root/shoot ratio suggested that shoot growth was severely inhibited when Zn concentration exceeded 300 μM by reducing photosynthetic ability and the content of photosynthetic pigments. Excessive Zn accumulation (Zn treatment of 300-1000 μM) in leaf and root induced membrane injuries through lipid peroxidation as malondialdehyde (MDA) content increased with increasing Zn concentration. The results show that MDA content was higher than other treatments by 16.1-151.1% and 15.0-88.3% (XN979) and 36.8-235.7% and 20.6-83.8% (BN207) in the leaves and roots under 1000 μM Zn treatment. To defend against Zn toxicity, ascorbate (AsA), glutathione (GSH), non-protein thiols (NPT) and phytochelatin (PC) content of both wheat varieties (except leaf GSH content of BN207) was increased, while, the activities of superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, and the content of soluble protein decreased by 300-1000 μM Zn. The results showed that AsA-GSH cycle and NPT and PC content of wheat seedlings play important roles in defending against Zn toxicity. This study contributes new insights into the physiological mechanisms underlying the hormetic response of wheat to Zn, which could be beneficial for optimizing plant health in changing environments and improving risk assessments.

Excess Zinc Supply Reduces Cadmium Uptake and Mitigates Cadmium Toxicity Effects on Chloroplast Structure, Oxidative Stress, and Photosystem II Photochemical Efficiency in Salvia sclarea Plants

Salvia sclarea L. is a Cd2+ tolerant medicinal herb with antifungal and antimicrobial properties cultivated for its pharmacological properties. However, accumulation of high Cd2+ content in its tissues increases the adverse health effects of Cd2+ in humans. Therefore, there is a serious demand to lower human Cd2+ intake. The purpose of our study was to evaluate the mitigative role of excess Zn2+ supply to Cd2+ uptake/translocation and toxicity in clary sage. Salvia plants were treated with excess Cd2+ (100 μM CdSO4) alone, and in combination with Zn2+ (900 μM ZnSO4), in modified Hoagland nutrient solution. The results demonstrate that S. sclarea plants exposed to Cd2+ toxicity accumulated a significant amount of Cd2+ in their tissues, with higher concentrations in roots than in leaves. Cadmium exposure enhanced total Zn2+ uptake but also decreased its translocation to leaves. The accumulated Cd2+ led to a substantial decrease in photosystem II (PSII) photochemistry and disrupted the chloroplast ultrastructure, which coincided with an increased lipid peroxidation. Zinc application decreased Cd2+ uptake and translocation to leaves, while it mitigated oxidative stress, restoring chloroplast ultrastructure. Excess Zn2+ ameliorated the adverse effects of Cd2+ on PSII photochemistry, increasing the fraction of energy used for photochemistry (ΦPSII) and restoring PSII redox state and maximum PSII efficiency (Fv/Fm), while decreasing excess excitation energy at PSII (EXC). We conclude that excess Zn2+ application eliminated the adverse effects of Cd2+ toxicity, reducing Cd2+ uptake and translocation and restoring chloroplast ultrastructure and PSII photochemical efficiency. Thus, excess Zn2+ application can be used as an important method for low Cd2+-accumulating crops, limiting Cd2+ entry into the food chain.

Harnessing Chlorophyll Fluorescence for Phenotyping Analysis of Wild and Cultivated Tomato for High Photochemical Efficiency under Water Deficit for Climate Change Resilience

Fluctuations of the weather conditions, due to global climate change, greatly influence plant growth and development, eventually affecting crop yield and quality, but also plant survival. Since water shortage is one of the key risks for the future of agriculture, exploring the capability of crop species to grow with limited water is therefore fundamental. By using chlorophyll fluorescence analysis, we evaluated the responses of wild tomato accession Solanum pennellii LA0716, Solanum lycopersicum cv. Μ82, the introgression line IL12-4 (from cv. M82 Χ LA0716), and the Greek tomato cultivars cv. Santorini and cv. Zakinthos, to moderate drought stress (MoDS) and severe drought stress (SDS), in order to identify the minimum irrigation level for efficient photosynthetic performance. Agronomic traits (plant height, number of leaves and root/shoot biomass), relative water content (RWC), and lipid peroxidation, were also measured. Under almost 50% deficit irrigation, S. pennellii exhibited an enhanced photosynthetic function by displaying a hormetic response of electron transport rate (ETR), due to an increased fraction of open reaction centers, it is suggested to be activated by the low increase of reactive oxygen species (ROS). A low increase of ROS is regarded to be beneficial by stimulating defense responses and also triggering a more oxidized redox state of quinone A (QA), corresponding in S. pennellii under 50% deficit irrigation, to the lowest stomatal opening, resulting in reduction of water loss. Solanumpennellii was the most tolerant to drought, as it was expected, and could manage to have an adequate photochemical function with almost 30% water regime of well-watered plants. With 50% deficit irrigation, cv. Μ82 and cv. Santorini did not show any difference in photochemical efficiency to control plants and are recommended to be cultivated under deficit irrigation as an effective strategy to enhance agricultural sustainability under a global climate change. We conclude that instead of the previously used Fv/Fm ratio, the redox state of QA, as it can be estimated by the chlorophyll fluorescence parameter 1 - qL, is a better indicator to evaluate photosynthetic efficiency and select drought tolerant cultivars under deficit irrigation.

Leaf Age-Dependent Photosystem II Photochemistry and Oxidative Stress Responses to Drought Stress in Arabidopsis thaliana Are Modulated by Flavonoid Accumulation

We investigated flavonoid accumulation and lipid peroxidation in young leaves (YL) and mature leaves (ML) of Arabidopsis thaliana plants, whose watering stopped 24 h before sampling, characterized as onset of drought stress (OnDS), six days before sampling, characterized as mild drought stress (MiDS), and ten days before sampling, characterized as moderate drought stress (MoDS). The response to drought stress (DS) of photosystem II (PSII) photochemistry, in both leaf types, was evaluated by estimating the allocation of absorbed light to photochemistry (Φ_PSII), to heat dissipation by regulated non-photochemical energy loss (Φ_NPQ) and to non-regulated energy dissipated in PSII (Φ_NO). Young leaves were better protected at MoDS than ML leaves, by having higher concentration of flavonoids that promote acclimation of YL PSII photochemistry to MoDS, showing lower lipid peroxidation and excitation pressure (1 - q_p). Young leaves at MoDS possessed lower 1 - q_p values and lower excess excitation energy (EXC), not only compared to MoDS ML, but even to MiDS YL. They also possessed a higher capacity to maintain low Φ_NO, suggesting a lower singlet oxygen (¹O₂) generation. Our results highlight that leaves of different developmental stage may display different responses to DS, due to differential accumulation of metabolites, and imply that PSII photochemistry in Arabidopsis thaliana may not show a dose dependent DS response.

Changes in Light Energy Utilization in Photosystem II and Reactive Oxygen Species Generation in Potato Leaves by the Pinworm Tuta absoluta

We evaluated photosystem II (PSII) functionality in potato plants (Solanum tuberosum L.) before and after a 15 min feeding by the leaf miner Tuta absoluta using chlorophyll a fluorescence imaging analysis combined with reactive oxygen species (ROS) detection. Fifteen minutes after feeding, we observed at the feeding zone and at the whole leaf a decrease in the effective quantum yield of photosystem II (PSII) photochemistry (Φ_PSII). While at the feeding zone the quantum yield of regulated non-photochemical energy loss in PSII (Φ_NPQ) did not change, at the whole leaf level there was a significant increase. As a result, at the feeding zone a significant increase in the quantum yield of non-regulated energy loss in PSII (Φ_NO) occurred, but there was no change at the whole leaf level compared to that before feeding, indicating no change in singlet oxygen (¹O₂) formation. The decreased Φ_PSII after feeding was due to a decreased fraction of open reaction centers (q_p), since the efficiency of open PSII reaction centers to utilize the light energy (Fv′/Fm′) did not differ before and after feeding. The decreased fraction of open reaction centers resulted in increased excess excitation energy (EXC) at the feeding zone and at the whole leaf level, while hydrogen peroxide (H₂O₂) production was detected only at the feeding zone. Although the whole leaf PSII efficiency decreased compared to that before feeding, the maximum efficiency of PSII photochemistry (Fv/Fm), and the efficiency of the water-splitting complex on the donor side of PSII (Fv/Fo), did not differ to that before feeding, thus they cannot be considered as sensitive parameters to monitor biotic stress effects. Chlorophyll fluorescence imaging analysis proved to be a good indicator to monitor even short-term impacts of insect herbivory on photosynthetic function, and among the studied parameters, the reduction status of the plastoquinone pool (q_p) was the most sensitive and suitable indicator to probe photosynthetic function under biotic stress.

Hormetic Responses of Photosystem II in Tomato to Botrytis cinerea

Botrytis cinerea, a fungal pathogen that causes gray mold, is damaging more than 200 plant species, and especially tomato. Photosystem II (PSII) responses in tomato (Solanum lycopersicum L.) leaves to Botrytis cinerea spore suspension application were evaluated by chlorophyll fluorescence imaging analysis. Hydrogen peroxide (H₂O₂) that was detected 30 min after Botrytis application with an increasing trend up to 240 min, is possibly convening tolerance against B. cinerea at short-time exposure, but when increasing at relative longer exposure, is becoming a damaging molecule. In accordance, an enhanced photosystem II (PSII) functionality was observed 30 min after application of B. cinerea, with a higher fraction of absorbed light energy to be directed to photochemistry (Φ_PSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in a significant decrease in the dissipated non-regulated energy (Φ_NO), indicating a possible decreased singlet oxygen (¹O₂) formation, thus specifying a modified reactive oxygen species (ROS) homeostasis. Therefore, 30 min after application of Botrytis spore suspension, before any visual symptoms appeared, defense response mechanisms were triggered, with PSII photochemistry to be adjusted by NPQ in a such way that PSII functionality to be enhanced, but being fully inhibited at the application spot and the adjacent area, after longer exposure (240 min). Hence, the response of tomato PSII to B. cinerea, indicates a hormetic temporal response in terms of “stress defense response” and “toxicity”, expanding the features of hormesis to biotic factors also. The enhanced PSII functionality 30 min after Botrytis application can possible be related with the need of an increased sugar production that is associated with a stronger plant defense potential through the induction of defense genes.

Evaluation of the spatiotemporal effects of bisphenol A on the leaves of the seagrass Cymodocea nodosa

The organic pollutant bisphenol A (BPA) causes adverse effects on aquatic biota. The present study explored the toxicity mechanism of environmentally occurring BPA concentrations (0.03-3 μg L^-1) on the seagrass Cymodocea nodosa intermediate leaf photosynthetic machinery. A "mosaic" type BPA effect pattern was observed, with "unaffected" and "affected"" leaf areas. In negatively affected leaf areas cells had a dark appearance and lost their chlorophyll auto-fluorescence, while hydrogen peroxide (H₂O₂) content increased time-dependently. In the "unaffected" leaf areas, cells exhibited increased phenolic compound production. At 1 μg L^-1 of BPA exposure, there was no effect on the fraction of open reaction centers (q_P) compared to control and also no significant effect on the quantum yield of non-regulated non-photochemical energy loss in PSII (Φ_ΝΟ). However, a 3 μg L^-1 BPA application resulted in a significant Φ_ΝΟ increase, even from the first exposure day. Ultrastructural observations revealed electronically dense damaged thylakoids in the plastids, while effects on Golgi dictyosomes and the endoplasmic reticulum were also observed at 3 μg L^-1 BPA. The up-regulated H₂O₂ BPA-derived production seems to be a key factor causing both oxidative damages but probably also triggering retrograde signalling, conferring tolerance to BPA in the "unaffected" leaf areas.

Cadmium toxicity in Salvia sclarea L.: An integrative response of element uptake, oxidative stress markers, leaf structure and photosynthesis

The herbal plant Salvia sclarea L. (clary sage) is classified to cadmium (Cd) accumulators and considered as a potential plant for phytoremediation of heavy metal polluted soil. However, the effect of Cd only treatment on the function of the photosynthetic apparatus of S. sclarea, as well as the mechanisms involved in Cd tolerance have not yet been studied in detail. This study was conducted to examine the integrative responses of S. sclarea plants exposed to a high Cd supply (100 µM) for 3 and 8 days by investigating element nutrient uptake, oxidative stress markers, pigment composition, photosynthetic performance and leaf structure. Measurements of the functional activities of photosystem I (PSI, by P700 photooxidation), photosystem II (PSII, by chlorophyll fluorescence parameters), the oxygen-evolving complex (oxygen evolution by Joliot- and Clark-type electrodes), as well as the leaf pigment and phenolic contents, were used to evaluate the protective mechanisms of the photosynthetic apparatus under Cd stress. Data suggested that the molecular mechanisms included in the photosynthetic tolerance to Cd toxicity involve strongly increased phenolic and anthocyanin contents, as well as an increased non-photochemical quenching and accelerated cyclic electron transport around PSI up to 61%, which protect the function of the photosynthetic apparatus under stress. Furthermore, the tolerance of S. sclarea to Cd stress is also associated with increased accumulation of Fe in leaves by 25%. All the above, clearly suggest that S. sclarea plants employ several different mechanisms to protect the function of the photosynthetic apparatus against Cd stress, which are discussed here.

Past and Future of Plant Stress Detection: An Overview From Remote Sensing to Positron Emission Tomography

Plant stress detection is considered one of the most critical areas for the improvement of crop yield in the compelling worldwide scenario, dictated by both the climate change and the geopolitical consequences of the Covid-19 epidemics. A complicated interconnection of biotic and abiotic stressors affect plant growth, including water, salt, temperature, light exposure, nutrients availability, agrochemicals, air and soil pollutants, pests and diseases. In facing this extended panorama, the technology choice is manifold. On the one hand, quantitative methods, such as metabolomics, provide very sensitive indicators of most of the stressors, with the drawback of a disruptive approach, which prevents follow up and dynamical studies. On the other hand qualitative methods, such as fluorescence, thermography and VIS/NIR reflectance, provide a non-disruptive view of the action of the stressors in plants, even across large fields, with the drawback of a poor accuracy. When looking at the spatial scale, the effect of stress may imply modifications from DNA level (nanometers) up to cell (micrometers), full plant (millimeters to meters), and entire field (kilometers). While quantitative techniques are sensitive to the smallest scales, only qualitative approaches can be used for the larger ones. Emerging technologies from nuclear and medical physics, such as computed tomography, magnetic resonance imaging and positron emission tomography, are expected to bridge the gap of quantitative non-disruptive morphologic and functional measurements at larger scale. In this review we analyze the landscape of the different technologies nowadays available, showing the benefits of each approach in plant stress detection, with a particular focus on the gaps, which will be filled in the nearby future by the emerging nuclear physics approaches to agriculture.

Tolerance Mechanisms of the Aromatic and Medicinal Plant Salvia sclarea L. to Excess Zinc

In recent years, due to the development of industrial and agricultural production, heavy metal contamination has attracted increasing attention. Aromatic and medicinal plant Salvia sclarea L. (clary sage) is classified to zinc (Zn) accumulators and considered as a potential plant for the phytoremediation of heavy metal polluted soils. In this study, an adaptation of clary sage to 900 µM (excess) Zn exposure for eight days in a hydroponic culture was investigated. The tolerance mechanisms under excess Zn exposure were assessed by evaluating changes in the nutrient uptake, leaf pigment and phenolic content, photosynthetic activity and leaf structural characteristics. The uptake and the distribution of Zn, as well as some essential elements such as: Ca, Mg, Fe, Mn and Cu, were examined by inductively coupled plasma mass spectrometry. The results revealed that Salvia sclarea is a Zn-accumulator plant that tolerates significantly high toxic levels of Zn in the leaves by increasing the leaf contents of Fe, Ca and Mn ions to protect the photosynthetic function and to stimulate the photosystem I (PSI) and photosystem II (PSII) activities. The exposure of clary sage to excess Zn significantly increased the synthesis of total phenolics and anthocyanins in the leaves; these play an important role in Zn detoxification and protection against oxidative stress. The lipid peroxidation and electrolyte leakage in leaves, used as clear indicators for heavy metal damage, were slightly increased. All these data highlight that Salvia sclarea is an economically interesting plant for the phytoextraction and/or phytostabilization of Zn-contaminated soils.

Rapid Hormetic Responses of Photosystem II Photochemistry of Clary Sage to Cadmium Exposure

Five-day exposure of clary sage (Salvia sclarea L.) to 100 μM cadmium (Cd) in hydroponics was sufficient to increase Cd concentrations significantly in roots and aboveground parts and affect negatively whole plant levels of calcium (Ca) and magnesium (Mg), since Cd competes for Ca channels, while reduced Mg concentrations are associated with increased Cd tolerance. Total zinc (Zn), copper (Cu), and iron (Fe) uptake increased but their translocation to the aboveground parts decreased. Despite the substantial levels of Cd in leaves, without any observed defects on chloroplast ultrastructure, an enhanced photosystem II (PSII) efficiency was observed, with a higher fraction of absorbed light energy to be directed to photochemistry (ΦPSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in an important decrease in the dissipated non-regulated energy (ΦNO), modifying the homeostasis of reactive oxygen species (ROS), through a decreased singlet oxygen (1O2) formation. A basal ROS level was detected in control plant leaves for optimal growth, while a low increased level of ROS under 5 days Cd exposure seemed to be beneficial for triggering defense responses, and a high level of ROS out of the boundaries (8 days Cd exposure), was harmful to plants. Thus, when clary sage was exposed to Cd for a short period, tolerance mechanisms were triggered. However, exposure to a combination of Cd and high light or to Cd alone (8 days) resulted in an inhibition of PSII functionality, indicating Cd toxicity. Thus, the rapid activation of PSII functionality at short time exposure and the inhibition at longer duration suggests a hormetic response and describes these effects in terms of "adaptive response" and "toxicity", respectively.

Different strategies of Cd tolerance and accumulation in Arabidopsis halleri and Arabidopsis arenosa

Pseudometallophytes are commonly used to study the evolution of metal tolerance and accumulation traits in plants. Within the Arabidopsis genus, the adaptation of Arabidopsis halleri to metalliferous soils has been widely studied, which is not the case for the closely related species Arabidopsis arenosa. We performed an in-depth physiological comparison between the A. halleri and A. arenosa populations from the same polluted site, together with the geographically close non-metallicolous (NM) populations of both species. The ionomes, growth, photosynthetic parameters and pigment content were characterized in the plants that were growing on their native site and in a hydroponic culture under Cd treatments. In situ, the metallicolous (M) populations of both species hyperaccumulated Cd and Zn. The NM population of A. halleri hyperaccumulated Cd and Zn while the NM A. arenosa did not. In the hydroponic experiments, the NM populations of both species accumulated more Cd in their shoots than the M populations. Our research suggests that the two Arabidopsis species evolved different strategies of adaptation to extreme metallic environments that involve fine regulation of metal homeostasis, adjustment of the photosynthetic apparatus and accumulation of flavonols and anthocyanins.

Arbuscular Mycorrhizal Symbiosis Enhances Photosynthesis in the Medicinal Herb Salvia fruticosa by Improving Photosystem II Photochemistry

We investigated the influence of Salvia fruticosa colonization by the arbuscular mycorrhizal fungi (AMF) Rhizophagus irregularis on photosynthetic function by using chlorophyll fluorescence imaging analysis to evaluate the light energy use in photosystem II (PSII) of inoculated and non-inoculated plants. We observed that inoculated plants used significantly higher absorbed energy in photochemistry (Φ_PSII) than non-inoculated and exhibited significant lower excess excitation energy (EXC). However, the increased Φ_PSII in inoculated plants did not result in a reduced non-regulated energy loss in PSII (Φ_NO), suggesting the same singlet oxygen (¹O₂) formation between inoculated and non-inoculated plants. The increased Φ_PSII in inoculated plants was due to an increased efficiency of open PSII centers to utilize the absorbed light (Fv'/Fm') due to a decreased non-photochemical quenching (NPQ) since there was no difference in the fraction of open reaction centers (q_p). The decreased NPQ in inoculated plants resulted in an increased electron-transport rate (ETR) compared to non-inoculated. Yet, inoculated plants exhibited a higher efficiency of the water-splitting complex on the donor side of PSII as revealed by the increased Fv/Fo ratio. A spatial heterogeneity between the leaf tip and the leaf base for the parameters Φ_PSII and Φ_NPQ was observed in both inoculated and non-inoculated plants, reflecting different developmental zones. Overall, our findings suggest that the increased ETR of inoculated S. fruticosa contributes to increased photosynthetic performance, providing growth advantages to inoculated plants by increasing their aboveground biomass, mainly by increasing leaf biomass.

Hydrogen Peroxide Production by the Spot-Like Mode Action of Bisphenol A

Bisphenol A (BPA), an intermediate chemical used for synthesizing polycarbonate plastics, has now become a wide spread organic pollutant. It percolates from a variety of sources, and plants are among the first organisms to encounter, absorb, and metabolize it, while its toxic effects are not yet fully known. Therefore, we experimentally studied the effects of aqueous BPA solutions (50 and 100 mg L^-1, for 6, 12, and 24 h) on photosystem II (PSII) functionality and evaluated the role of reactive oxygen species (ROS) on detached leaves of the model plant Arabidopsis thaliana. Chlorophyll fluorescence imaging analysis revealed a spatiotemporal heterogeneity in the quantum yields of light energy partitioning at PSII in Arabidopsis leaves exposed to BPA. Under low light PSII function was negatively influenced only at the spot-affected BPA zone in a dose- and time-dependent manner, while at the whole leaf only the maximum photochemical efficiency (Fv/Fm) was negatively affected. However, under high light all PSII photosynthetic parameters measured were negatively affected by BPA application, in a time-dependent manner. The affected leaf areas by the spot-like mode of BPA action showed reduced chlorophyll autofluorescence and increased accumulation of hydrogen peroxide (H₂O₂). When H₂O₂ was scavenged via N-acetylcysteine under BPA exposure, PSII functionality was suspended, while H₂O₂ scavenging under non-stress had more detrimental effects on PSII function than BPA alone. It can be concluded that the necrotic death-like spots under BPA exposure could be due to ROS accumulation, but also H₂O₂ generation seems to play a role in the leaf response against BPA-related stress conditions.

Spatial Heterogeneity of Cadmium Effects on Salvia sclarea Leaves Revealed by Chlorophyll Fluorescence Imaging Analysis and Laser Ablation Inductively Coupled Plasma Mass Spectrometry

In this study, for a first time (according to our knowledge), we couple the methodologies of chlorophyll fluorescence imaging analysis (CF-IA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in order to investigate the effects of cadmium (Cd) accumulation on photosystem II (PSII) photochemistry. We used as plant material Salvia sclarea that grew hydroponically with or without (control) 100 μM Cd for five days. The spatial heterogeneity of a decreased effective quantum yield of electron transport (Φ_PSΙΙ) that was observed after exposure to Cd was linked to the spatial pattern of high Cd accumulation. However, the high increase of non-photochemical quenching (NPQ), at the leaf part with the high Cd accumulation, resulted in the decrease of the quantum yield of non-regulated energy loss (Φ_NO) even more than that of control leaves. Thus, S. sclarea leaves exposed to 100 μM Cd exhibited lower reactive oxygen species (ROS) production as singlet oxygen (¹O₂). In addition, the increased photoprotective heat dissipation (NPQ) in the whole leaf under Cd exposure was sufficient enough to retain the same fraction of open reaction centers (q_p) with control leaves. Our results demonstrated that CF-IA and LA-ICP-MS could be successfully combined to monitor heavy metal effects and plant tolerance mechanisms.

Leaf Age-Dependent Effects of Foliar-Sprayed CuZn Nanoparticles on Photosynthetic Efficiency and ROS Generation in Arabidopsis thaliana

Young and mature leaves of Arabidopsis thaliana were exposed by foliar spray to 30 mg L-1 of CuZn nanoparticles (NPs). The NPs were synthesized by a microwave-assisted polyol process and characterized by dynamic light scattering (DLS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). CuZn NPs effects in Arabidopsis leaves were evaluated by chlorophyll fluorescence imaging analysis that revealed spatiotemporal heterogeneity of the quantum efficiency of PSII photochemistry (ΦPSΙΙ) and the redox state of the plastoquinone (PQ) pool (qp), measured 30 min, 90 min, 180 min, and 240 min after spraying. Photosystem II (PSII) function in young leaves was observed to be negatively influenced, especially 30 min after spraying, at which point increased H2O2 generation was correlated to the lower oxidized state of the PQ pool.. Recovery of young leaves photosynthetic efficiency appeared only after 240 min of NPs spray when also the level of ROS accumulation was similar to control leaves. On the contrary, a beneficial effect on PSII function in mature leaves after 30 min of the CuZn NPs spray was observed, with increased ΦPSΙΙ, an increased electron transport rate (ETR), decreased singlet oxygen (1O2) formation, and H2O2 production at the same level of control leaves.An explanation for this differential response is suggested.

Zinc Uptake, Photosynthetic Efficiency and Oxidative Stress in the Seagrass Cymodocea nodosa Exposed to ZnO Nanoparticles

We characterized zinc oxide nanoparticles (ZnO NPs) by dynamic light scattering (DLS) measurements, and transmission electron microscopy (TEM), while we evaluated photosystem II (PSII) responses, Zn uptake kinetics, and hydrogen peroxide (H₂O₂) accumulation, in C. nodosa exposed to 5 mg L⁻¹ and 10 mg L⁻¹ ZnO NPs for 4 h, 12 h, 24 h, 48 h and 72 h. Four h after exposure to 10 mg L⁻¹ ZnO NPs, we noticed a disturbance of PSII functioning that became more severe after 12 h. However, after a 24 h exposure to 10 mg L⁻¹ ZnO NPs, we observed a hormetic response, with both time and dose as the basal stress levels needed for induction of the adaptive response. This was achieved through the reduced plastoquinone (PQ) pool, at a 12 h exposure, which mediated the generation of chloroplastic H₂O₂; acting as a fast acclimation signaling molecule. Nevertheless, longer treatment (48 h and 72 h) resulted in decreasing the photoprotective mechanism to dissipate excess energy as heat (NPQ) and increasing the quantum yield of non-regulated energy loss (Φ_NO). This increased the formation of singlet oxygen (¹O₂), and decreased the fraction of open reaction centers, mostly after a 72-h exposure at 10 mg L⁻¹ ZnO NPs due to increased Zn uptake compared to 5 mg L⁻¹.

Hormesis: A Compelling Platform for Sophisticated Plant Science

The field of dose-response has received attention from the early modern period in the history of science. While it was thought that linear dose-response is the rule of thumb, significant efforts revealed that biphasic dose-response commonly occurs when the experimental design permits its detection. This phenomenon is called hormesis and suggests that a basal stress level is needed for optimum health. Extensive evidence has accumulated showing the occurrence of hormesis in numerous plant species and the induction of adaptive responses by low stress doses that precondition plants for a following massive environmental challenge. However, the ecological consequences of low-level stress remain underexplored. In this Opinion article, we propose that hormesis can provide a compelling platform for sophisticated, next-generation plant science.

Toxic Effects of Cd and Zn on the Photosynthetic Apparatus of the Arabidopsis halleri and Arabidopsis arenosa Pseudo-Metallophytes

Hyperaccumulation and hypertolerance of Trace Metal Elements (TME) like Cd and Zn are highly variable in pseudo-metallophytes species. In this study we compared the impact of high Cd or Zn concentration on the photosynthetic apparatus of the Arabidopsis arenosa and Arabidopsis halleri pseudo-metallophytes growing on the same contaminated site in Piekary Slaskie in southern Poland. Plants were grown in hydroponic culture for 6 weeks, and then treated with 1.0 mM Cd or 5.0 mM Zn for 5 days. Chlorophyll a fluorescence and pigment content were measured after 0, 1, 2, 3, 4, and 5 days in plants grown in control and exposed to Cd or Zn treatments. Moreover, the effect of TME excess on the level of oxidative stress and gas-exchange parameters were investigated. In both plant species, exposure to high Cd or Zn induced a decrease in chlorophyll and an increase in anthocyanin contents in leaves compared to the control condition. After 5 days Cd treatment, energy absorbance, trapped energy flux and the percentage of active reaction centers decreased in both species. However, the dissipated energy flux in the leaves of A. arenosa was smaller than in A. halleri. Zn treatment had more toxic effect than Cd on electron transport in A. halleri compared with A. arenosa. A. arenosa plants treated with Zn excess did not react as strongly as in the Cd treatment and a decrease only in electron transport flux and percentage of active reaction centers compared with control was observed. The two species showed contrasting Cd and Zn accumulation. Cd concentration was almost 3-fold higher in A. arenosa leaves than in A. halleri. On the opposite, A. halleri leaves contained 3-fold higher Zn concentration than A. arenosa. In short, our results showed that the two Arabidopsis metallicolous populations are resistant to high Cd or Zn concentration, however, the photosynthetic apparatus responded differently to the toxic effects.