Search Results (119)

Drought is a major environmental constraint that disrupts photosynthetic processes. This study investigated the effects of foliar-applied commercial humic acid (HA) at different concentrations (1, 3 and 5 mg/mL) on the photosynthetic apparatus of sweet basil (Ocimum basilicum L. Italiano classico) under PEG-induced stress. The responses of the photosynthetic machinery were evaluated using chlorophyll a fluorescence analyses (JIP-test and PAM), leaf pigment composition, and assessments of membrane integrity. Drought stress caused pronounced alterations on both the donor and acceptor sides of photosystem II (PSII), including impaired Q_A⁻ reoxidation, reduced open PSII reaction centers (qP), diminished electron transport (ETo/RC, REo/RC), and substantial declines in performance indices (PI_ABS, PItotal). Energy dissipation increased (DI₀/RC), with regulated energy losses (Φ_NPQ) rising more strongly than non-regulated losses (Φ_NO). Drought also elevated oxidative stress markers (MDA and H₂O₂), leading to enhanced membrane injury. Among the tested concentrations, 5 mg/mL HA provided the most effective protection against drought stress. This treatment mitigated PEG-induced damage on both PSII donor and acceptor sides and increased the proportion of open reaction centers (qP). Improved PSII photochemistry corresponded with more efficient Q_A⁻ reoxidation, facilitated its interaction with plastoquinone, and caused the overall stabilization of photosynthetic functions under drought. The protective effects of HA were also evident for both PSI subpopulations. The enhanced tolerance was associated with the activation of antioxidant enzymes (CAT, SOD, APX) and the increased levels of anthocyanins and total phenolic content (TPC). In contrast, lower HA concentrations (1 and 3 mg/mL) provided insufficient protection. This study clearly demonstrates that HA enhances drought tolerance in basil in a concentration-dependent manner by protecting the structural and functional integrity of the photosynthetic apparatus, supporting its potential use as a foliar treatment to improve crop resilience under water-limited conditions. Full article

Efficient charge separation and electron transfer in Photosystem II (PSII) depend on small inorganic cofactors that maintain redox balance and catalytic stability. Chloride facilitates water-oxidizing-complex turnover and minimizes charge recombination. Bicarbonate, coordinated to the non-heme iron, facilitates electron transfer between the plastoquinones Q_A and Q_B. This work investigates cooperativity between these cofactors across PSII in the hypercarbonate-requiring cyanobacterium Limnospira maxima. Bromide-for-chloride substitution induces a distinct kinetic limitation at the water oxidizing complex. While bicarbonate depletion inhibits electron transfer at the acceptor side, bromide-substituted cells maintain a measurable level of electron flow through the intersystem chain. The presence of bromide induces structural changes that allow partial electron transfer to continue even in the absence of the bicarbonate cofactor, which is not observed in the chloride system. However, this dual anion stress results in irreversible functional impairment in some centers, whereas full recovery of activity is observed with native chloride. When the donor side is restricted by bromide, the loss of bicarbonate, which is thought to function as a proton buffer for the donor side, compromises the overall stability of the reaction center. This leads to a permanent decrease in activity of the electron transfer chain, suggesting an interdependence between the roles of chloride and bicarbonate that is essential for protecting PSII during ionic stress. Full article

Drought stress during early growth stages severely limits wheat productivity globally. Understanding varietal physiological responses to drought stress is critical for breeding climate-resilient cultivars. Two-week-old plants from two winter wheat (Triticum aestivum L.) cultivars—Katya (drought-tolerant) and Zora (drought-sensitive)—were subjected to drought for seven days, followed by rehydration. The experiments were conducted in pots in controlled conditions. The photosystem II (PSII) function was evaluated using chlorophyll a fluorescence (OJIP transients), thermoluminescence emissions and pigment content analysis. Under drought, Katya maintained functional PSII integrity with stable quantum efficiency and increased chlorophyll content, while Zora exhibited chlorophyll degradation. Fresh and dry weight declined in both genotypes but significantly only in Zora; recovery occurred after rehydration. Chlorophyll fluorescence revealed that varietal divergence was localized to the O–J phase of PSII photochemistry, indicating differences in reaction-center behavior confirmed by thermoluminescence. Katya demonstrated preserved PSII reaction-center density, balanced energy partitioning, homogeneous PSII populations, and superior recovery capacity. Conversely, Zora showed reaction-center depletion, elevated energy dissipation, impaired electron transport beyond Q_A, and persistent PSII heterogeneity even after rehydration. Drought tolerance in the studied genotypes was associated with the maintenance of PSII structural integrity, efficient photochemical function, and rapid recovery mechanisms. These physiological markers—particularly early PSII photochemistry kinetics and reaction-center stability—provide valuable selection criteria for breeding programs, targeting drought resilience under changing climate conditions. Full article

The damage caused by herbivores is generally measured as the amount of leaf tissue consumed, without accounting for the fate of the leftover tissue. As a result, the plant defense mechanisms that promote resistance to herbivore feeding by photosynthetically acclimating the rest of the plant to the feeding spot leaf area have not been well exploited. Plant-insect interactions are now becoming better defined with the development of visualization methods that permit spatial whole-leaf assessment of photosynthetic efficiency after herbivore attack. The purpose of our study was to evaluate the spatial heterogeneity of photosystem II (PSII) function at the whole-leaf level before and after herbivory by the Colorado potato beetles. Twenty minutes after Colorado potato beetle (Leptinotarsa decemlineata) feeding, the maximum efficiency of PSII photochemistry (Fv/Fm) decreased significantly, suggesting photoinhibition due to reduced efficiency of the oxygen-evolving complex (OEC). The decreased quantum yield of PSII photochemistry (Φ_PSII) after feeding, at the neighboring area of the feeding spot and at the rest of the leaf area, was attributed to the reduced efficiency of the open PSII reaction centers (Fv′/Fm′), since there was no change in the fraction of open PSII reaction centers (qp). Nevertheless, plant defense elicitation was activated by the photoprotective mechanism of non-photochemical quenching (NPQ) that reduced the singlet oxygen (¹O₂) formation in potato plants in the neighboring area of the feeding spot and at the rest of the leaf area. In addition, the increased production of hydrogen peroxide (H₂O₂) triggered by this increase suggests that it acted as a signaling molecule in the biotic stress defense response. Full article

Drought is a major environmental threat to agriculture. This study examined the role of anthocyanins in plant drought tolerance by comparing two basil varieties differing in leaf anthocyanin content: green basil (Ocimum basilicum L. Italiano Classico) and purple basil (Ocimum basilicum L. Dark Opal). The impact of the PEG-induced drought stress was assessed by monitoring changes in chlorophyll a fluorescence parameters (JIP and PAM), leaf pigment content, anthocyanin and total phenolic levels, oxidative stress markers (malondialdehyde, hydrogen peroxide and membrane integrity), as well as radical-scavenging capacity (DPPH assay). Drought stress led to a modification on both the donor (Wk) and acceptor (Vj) sides of PSII, which influences Q_A reoxidation and amounts of the closed reaction centers (1-qP). These changes inhibited photosystem II photochemistry, the rate of the electron transport (ETR), and the rate of the photosynthesis (R_Fd) and decreased performance indices (PI_ABS, PItotal), as well as the photosystem I photochemistry. The drought-induced changes were associated with an increase in the dissipated energy per reaction center (DI₀/RC). The results show that photosynthetic functions in purple basil were less affected under drought stress compared to green basil. The reason for better tolerance of purple basil is associated with elevated anthocyanin levels, which correlate with enhanced antioxidant capacity, reduced hydrogen peroxide accumulation, lower membrane lipid peroxidation, improved relative water content and membrane stability. In addition, rapid cyclic electron flow around photosystem I and a higher carotenoid to chlorophyll ratio contribute to drought tolerance in purple basil. After re-watering, purple basil recovers its photosynthetic function almost completely, unlike green basil, which shows further suppression. The increase in the anthocyanin content and radical-scavenging capacity, as well as the smaller oxidative damage under drought stress, are the main reasons for the better recovery in purple basil. Overall, the findings highlight that higher anthocyanin accumulation in purple basil confers greater drought tolerance and recovery capacity by stabilizing photosynthetic processes and reducing oxidative stress. Full article

Tobacco (Nicotiana tabacum L.) has a propensity to accumulate cadmium (Cd), especially in its leaves, which can have a detrimental impact on yield, quality, and product safety. The development of low-accumulation cultivars is a vital mitigation approach; however, the underlying mechanisms remain inadequately understood. In this study, through pot experiments, the physiological mechanisms responsible for the differential Cd accumulation between the low-accumulating tobacco line CF986 and the high-accumulating Yuyan5 were explored. A comprehensive analysis was conducted on the organ-specific Cd distribution, chemical speciation, subcellular compartmentalization, and photosynthetic responses across a gradient of Cd exposure. In comparison with Yuyan5, CF986 accumulated significantly higher amounts of Cd in the roots and stems, but substantially lower amounts in the leaves. Specifically, the Cd content in the leaves of CF986 was only 64.32–68.74% of that in Yuyan5 across different Cd exposure levels. The organ-specific Cd distribution pattern in CF986 followed the order: leaf > stem > root. Moreover, the proportion of Cd partitioned to the leaves was lower in CF986 compared to Yuyan5, while the roots and stems exhibited enhanced Cd retention, with Cd levels in stems reaching up to 2.04 times higher than those in Yuyan5. Analysis of the chemical forms and subcellular distribution of Cd indicated that the mobile Cd fractions in the stems of CF986 were significantly reduced compared to Yuyan5. A larger proportion of Cd was immobilized in the stem cell-wall fraction, which enhanced Cd retention and restricted xylem-mediated transport to the leaves. Cd exposure did not significantly affect the concentration of foliar photosynthetic pigments in CF986; however, it notably inhibited the activity of the photosystem II (PSII) reaction center. At higher Cd levels, the photoprotective thermal dissipation gradually failed, with a decrease of up to 41.36% in Φ_NO for CF986 compared to CK under Cd4.0 treatment. This research unveiled a stem barrier mechanism, whereby Cd translocation to the leaves is restricted through chemical and subcellular sequestration in the stem. This mechanism provides a novel perspective on both plant heavy metal allocation and the assurance of crop safety. Full article

Cremastra appendiculata (D. Don) Makino, a rare orchid prized for its ornamental and medicinal value, exhibits high sensitivity to light conditions during the seedling stage. To identify optimal light intensity for promoting seedling growth and elucidate the underlying physiological mechanisms, this study exposed C. appendiculata seedlings to three light treatments: low light (LL, 80% shading, 300–350 µmol·m⁻²·s⁻¹), medium light (ML, 60% shading, 600–650 µmol·m⁻²·s⁻¹), and high light (HL, 30% shading, 900–1000 µmol·m⁻²·s⁻¹). Growth and photosynthetic physiological parameters were measured to investigate the regulatory effects of light intensity. Results showed that under LL treatment, plant height, leaf area, and total biomass were significantly higher than those under HL treatment, increasing by 48%, 41%, and 50%, respectively. Leaf anatomical structure under LL displayed tightly arranged epidermal cells and intact mesophyll organization, consistent with typical shade-leaf characteristics. Chlorophyll content analysis revealed that chlorophyll a, chlorophyll b, and total chlorophyll under LL increased significantly by 75%, 35%, and 50%, respectively, compared to HL. Moreover, net photosynthetic rate peaked under LL, exceeding ML and HL by 28% and 17%, respectively. Chlorophyll fluorescence analysis further indicated that LL treatment optimized PSII performance, enhancing maximum photochemical efficiency, photosynthetic performance index, and electron transport rate per reaction center, while maintaining low thermal dissipation, indicating superior light capture and conversion efficiency. In summary, within the experimental gradient established in this study, the LL treatment represents the optimal light environment for the growth of C. appendiculata seedlings. By synergistically promoting plant morphological development, optimizing leaf structure, enhancing photosynthetic pigment content, and improving Photosystem II performance, this treatment facilitates efficient biomass accumulation. These findings provide a critical theoretical basis for the light environment management in both the conservation and artificial propagation of C. appendiculata. Full article

We used chlorophyll fluorescence techniques to investigate seasonal variations in photosystem II (PSII) quantum yield in five-year-old saplings of the sclerophyllous Peumus boldus Molina (evergreen) and Colliguaja odorifera Molina (semideciduous) planted in a semiarid site with a Mediterranean-type climate. Chlorophyll fluorescence rise kinetics (OJIP) were monitored monthly for one year (September 2024 to September 2025). With this information, we estimated the relative deviation of the performance index (PI_ABS) of each species from the average PI_ABS in each season (denoted as ∆PI_ABS). P. boldus was associated with destruction of PSII reaction centers and incapacity for electron transport, i.e., higher values of parameters ABS/RC (effective antenna size of an active reaction center) and F₀ (minimal fluorescence), whereas C. odorifera was associated with higher photosynthetic performance i.e., higher values of PI_ABS, PI_TOT (total performance index), F_V/F₀ (ratio between variable and minimal fluorescence), and F_V/F_M (maximum quantum yield of primary PSII photochemistry). P_IABS exhibited a 52 and 38% reduction (i.e., −∆PI_ABS) during spring and winter in P. boldus, but an increase (i.e., +∆PI_ABS) of 52 and 37% in the same seasons for C. odorifera. P. boldus was considerably more depressed during the winter–spring season than the summer months. This suggests that PSII function in P. boldus is more sensitive to low temperatures in winter and spring than the lack of water and high temperatures during summer. Full article

The response of tomato plants, Ailsa Craig and the carotenoid mutant tangerine, to five days of treatment by low light intensity at normal and low temperature with respect to the photosynthetic performance as well as their capacity to recover after three days under normal conditions was evaluated. Tangerine plants are characterized by defective prolycopene isomerase (CRTISO) and accumulate tetra-cis lycopene instead of all-trans lycopene. The gas exchange parameters were evaluated on intact plants and the pigment content in leaves was estimated. The photosynthetic competence of photosystem II (PSII) and photosystem I (PSI) and the effectiveness of the energy dissipation were assessed by pulse-amplitude-modulated (PAM) fluorometry. The abundance of reaction center proteins of PSII and PSI was estimated by immunoblotting. The application of low light alone or low light and low temperature reduced the chlorophyll content in both types of plants, which was more strongly expressed in Ailsa Craig. The net photosynthetic rate and photochemical activities of PSII and PSI were negatively affected by low light and much more strongly decreased when low light was applied at low temperature. The low-light-induced increase in excitation pressure on PSII and the effectiveness of non-photochemical quenching were not temperature-dependent. The negative effect of the combined treatment in tangerine was more strongly expressed in comparison with Ailsa Craig with respect to the abundance of reaction center proteins of both photosystems. Most probably, the differential photosynthetic response of the carotenoid mutant tangerine and Ailsa Craig to the combined treatment by low light and low temperature is related to the accumulation of tetra-cis-lycopene instead of all-trans-lycopene. Full article

Photosystem II (PSII) is very sensitive to both biotic and abiotic stress conditions, mirroring global climate changes. Crop production worldwide faces rising hazards from the increased duration, frequency, and intensity of drought stress episodes as a result of climate change, and its effects, when combined with biotic stress, are becoming more noticeable. In the present work, we examined PSII responses of well-watered (WW) tomato plants or mildly drought-stressed (MDS) plants to 20 min of Tuta absoluta larvae feeding. The effective quantum yield of PSII photochemistry (Φ_PSII) of the whole leaf in WW plants, after 20 min of larvae feeding, compensated for the reduction in Φ_PSII observed at the feeding area. In contrast, the reduced Φ_PSII at the feeding areas of MDS plants, after 20 min of larvae feeding, was not compensated at the whole-leaf level because of the drought stress. The increased Φ_PSII and electron transport rate (ETR) at the whole-leaf level in WW plants was attributed to the increased fraction of open PSII reaction centers (qp), since there was no difference in the efficiency of the open PSII reaction centers (Fv′/Fm′) before and after feeding. Therefore, the response of PSII photochemistry in WW plants to short-term biotic stress resulted in an overcompensation reaction, which developed a whole-leaf photosynthetic enhancement. However, short-term biotic stress in combination with mild abiotic stress resulted in decreased PSII photochemistry. It is concluded that increased crop damage is likely to occur due to the global climate-change-induced drought episodes, influencing insect herbivory. Full article

Low inner leaves in the thick canopy of dense Chinese fir plantations frequently show premature senescence and dieback regardless of age. To elucidate the underlying mechanisms, a 28-day growth chamber experiment was conducted under dark conditions to induce leaf senescence. Changes in leaf area, photosynthetic performance, and the responses of carbon metabolism and the antioxidant defense system were analyzed. Leaf area decreased significantly with time in darkness. The photosystem II reaction center was damaged, and fluorescence parameters and chlorophyll contents decreased, resulting in reduced light energy capture and conversion efficiencies. Photosynthetic rate, apparent quantum yield, stomatal conductance, transpiration rate, and light use efficiency all decreased, while the light compensation point and intercellular CO₂ concentration increased. Antioxidant enzyme activities initially increased but eventually collapsed as the stress continued and H₂O₂ and malondialdehyde accumulated, causing membrane conductivity, i.e., membrane permeability, to increase by 122%. Meanwhile, reduced non-structural carbohydrates, especially total non-structural carbohydrates content, decreased by 45.32%, triggering sugar starvation and accelerating aging. Our study provided new physiological evidence for light-stress response mechanisms in Chinese fir. Specifically, it revealed that dark-induced leaf senescence was mainly caused by irreversible damage to the photosynthetic apparatus and oxidative stress, which together led to carbon starvation and ultimately death. Full article

The fraction of open Photosystem II (PSII) reaction centers (q_L) is critical for connecting broadband PSII fluorescence (ChlF_PSII) with the actual electron transport from PSII to Photosystem I. Accurately estimating q_L is fundamental for determining ChlF_PSII, which, in turn, is vital for mechanistically estimating the actual electron transport rate and photosynthetic CO₂ assimilation. Chlorophyll fluorescence provides direct physiological insights, offering a robust foundation for q_L estimation. However, uncertainties in the ChlF_PSII–q_L relationship across different plant functional types (PFTs) limit its broader application at large spatial scales. To address this issue, we developed a leaf-level instrument capable of simultaneously measuring actively and passively induced chlorophyll fluorescence. Using this system, we measured light response, CO₂ response, and temperature response curves across 52 species representing seven PFTs. Our findings reveal the following: (1) a strong linear correlation between ChlF_PSII derived from passively induced fluorescence and that from actively induced fluorescence (R² = 0.85), and (2) while the parameters of the ChlF_PSII–q_L relationship varied among PFTs, ChlF_PSII reliably modeled q_L within each PFT, with the R² ranging from 0.85 to 0.96. This study establishes quantitative ChlF_PSII–q_L relationships for various PFTs by utilizing passively induced fluorescence to calculate ChlF_PSII. The results demonstrate the potential for remotely sensed chlorophyll fluorescence data to estimate q_L and strengthen the use of fluorescence-based approaches for mechanistic GPP estimation at large spatial scales. Full article

Aspirin (Asp) is extensively used in human health as an anti-inflammatory, antipyretic, and anti-thrombotic drug. In this study, we investigated if the foliar application of Asp on tomato plants has comparable beneficial effects on photosynthetic function to that of salicylic acid (SA), with which it shares similar physiological characteristics. We assessed the consequences of foliar Asp-spray on the photosystem II (PSII) efficiency of tomato plants, and we estimated the reactive oxygen species (ROS) generation and the chloroplast ultrastructural changes. Asp acted as an osmoregulator by increasing tomato leaf water content and offering antioxidant protection. This protection kept the redox state of plastoquinone (PQ) pull (qp) more oxidized, increasing the fraction of open PSII reaction centers and enhancing PSII photochemistry (Φ_PSII). In addition, Asp foliar spray decreased reactive oxygen species (ROS) formation, decreasing the excess excitation energy on PSII. This resulted in a lower singlet oxygen (¹O₂) generation and a lower quantum yield for heat dissipation (Φ_NPQ), indicating the photoprotective effect provided by Asp, especially under excess light illumination. Simultaneously, we observed a decrease in stomatal opening by Asp, which reduced the transpiration. Chloroplast ultrastructural data revealed that Asp, by offering a photoprotective effect, decreased the need for the photorespiration process, which reduces photosynthetic performance. It is concluded that Asp shares similar physiological characteristics with SA, having an equivalent beneficial impact to SA by acting as a biostimulant of the photosynthetic function for an enhanced crop yield. Full article

The success of establishing new trees in cities and their subsequent growth depend, among others, on the proper selection of tree species which can easily tolerate the post-planting stress. In the spring of 2023, young Italian alder (Alnus cordata (Loisel.) Duby) and common lime (Tilia × europaea L. ‘Pallida’) trees were planted in a street of heavy traffic in Warsaw. In the summer of 2023, leaf samples were collected during the growing season for chlorophyll a fluorescence measurements and chemical analyses. Additionally, the autumn phenological phases were monitored. Chlorophyll a fluorescence measurements revealed higher values of F_v/F_m, density of reaction centers per cross-section, and electron transport chain efficiency between photosystems II and I, as well as lower energy dissipation rate per active reaction center of photosystem II in A. cordata. Moreover, A. cordata revealed higher chlorophyll a, chlorophyll b, and carotenoid content. The flavonoid and proline content in both species was the highest by the end of July and then decreased. In T. × europea ‘Pallida’, the contents of these stress biomarkers increased in the late growing season. Our results showed that T. × europaea ‘Pallida’ is less resistant to post-planting stress in urban conditions, while A. cordata showed higher resistance to variable weather conditions, high photosynthetic efficiency, and long foliage lifespan. Full article

1-Methyl cyclopropene (1-MCP) is known as an ethylene antagonist, yet its mechanisms in regulating photosynthetic electron transport and energy dissipation in chrysanthemum under heat stress are not well understood. Here, the chlorophyll a fluorescence and modulated 820 nm reflection transients were analyzed in heat-tolerant and heat-sensitive chrysanthemum plants. This study demonstrates that 1-MCP pre-treatment helps maintain the net photosynthetic rate (P_n) and the reaction center activity of photosystems I and II (PSI and PSII) during heat stress. Specifically, 1-MCP treatment significantly increases the fraction of active oxygen-evolving complex (OEC) centers and reduces relative variable fluorescence intensity at the J step (V_J) as well as the efficiency of electron transfer at the PSI acceptor side (δ_Ro). These effects mitigate damage to the photosynthetic electron transport chain. Additionally, 1-MCP-treated plants exhibit decreased quantum yield of energy dissipation (φ_Do) and reduced energy flux per reaction center (DI_o/RC). Overall, 1-MCP enhances light utilization efficiency and excitation energy dissipation in the PSII antennae, alleviating heat stress-induced damage to PSI and PSII structures and functions. This study not only advances our understanding of 1-MCP’s regulatory role in photosynthetic processes under heat stress but also provides a basis for using exogenous substances to improve chrysanthemum heat resistance. Full article