Karst fissures mitigate the negative effects of drought on plant growth and photosynthetic physiology

Hard limestone substrates, which are extensively distributed, are believed to exacerbate drought and increase the difficulty of restoration in vulnerable karst regions. Fissures in such substrates may alleviate the negative effect of drought on plants, but the underlying mechanisms remain poorly understood. In a two-way factorial block design, the growth and photosynthesis of 2-year-old Phoebe zhennan seedlings were investigated in two water availabilities (high versus low) and three stimulated fissure habitat groups (soil, soil-filled fissure and non-soil-filled fissure). Moreover, the fissure treatments included both small and big fissures. Compared to the soil group, the non-soil-filled fissure group had decreased the total biomass, root biomass, total root length, and the root length of fine roots in the soil layer at both water availabilities, but increased net photosynthetic rate (Pn) and retained stable water use efficiency (WUE) at low water availability. However, there were no significant differences between the soil-filled fissure group and soil group in the biomass accumulation and allocation as well as Pn. Nevertheless, the SF group decreased the root distribution in total and in the soil layer, and also increased WUE at low water availability. Across all treatments, fissure size had no effect on plant growth or photosynthesis. Karst fissures filled with soil can alleviate drought impacts on plant root growth, which involves adjusting root distribution strategies and increasing water use efficiency. These results suggest that rock fissures can be involved in long-term plant responses to drought stress and vegetation restoration in rocky mountain environments under global climate change.

Comprehensive evaluation of carbon sequestration potential of landscape tree species and its influencing factors analysis: implications for urban green space management

BACKGROUND

Continuous increasing carbon dioxide (CO2) has aggravated global warming and promoted urban tree planting projects for many countries. So it's imperative to select high carbon sequestering landscape tree species while considering their aesthetic values of urban green space.

RESULTS

32 tree species were selected as test objects which were commonly used in landscaping in Zhengzhou, a typical northern city of China. To assess the comprehensive carbon sequestration potential of landscape tree species in different plant configuration types, we simultaneously considered their daily net carbon sequestration per unit leaf area (wCO2), daily net carbon sequestration per unit land area (WCO2) and daily net carbon sequestration of the whole plant (QCO2) through cluster analysis. Besides that, we found out the key factors affecting carbon sequestration potential of landscape tree species by redundancy analysis.

CONCLUSION

Populus, P Stenoptera, P. acerifolia among large arbors (LA), V odoratissimum, P. Serratifolia, S. oblata among small arbors (SA), and B sinica var. Parvifolia, B. Megistophylla, L quihoui among shrubs (S) were recommended for local urban green space management. Photosynthetic rate (Pn), crown area (CA) and leaf area index (LAI) were the key factors which affected the comprehensive carbon sequestration potential both for LA, SA and S.

Iron (Fe) toxicity, uptake, translocation, and physio-morphological responses in Catharanthus roseus

Iron (Fe) toxicity in plant species depends on the availability of Fe in the soil, uptake ability by the root system, and translocation rate to other parts of the plant. The aim of this study was to assess Fe uptake by root tissues of Catharanthus roseus, translocation rate to leaf tissues, and the impairment of plant physio-morphological characteristics. Fe uptake by the roots (~ 700 µg g-1 DW) of C. roseus was observed during the early exposure period (1 week), and translocation factor from root to shoot was fluctuated as an independent strategy. A high level of Fe content in the root tissues significantly inhibited root length and root dry weight. Under acidic pH condition, an enrichment of Fe in the shoots (~ 400 µg g-1 DW) led to increase in leaf temperature (> 2.5 °C compared to control) and crop stress index (> 0.6), resulting in stomatal closure, subsequently decreasing CO2 assimilation rate and H2O transpiration rate. An increment of CSI in Fe-stressed plants was negatively related to stomatal conductance, indicating stomatal closure with an increase in Fe in the leaf tissues. High Fe levels in the leaf tissues directly induced toxic symptoms including leaf bronzing, leaf spotting, leaf necrosis, leaf chlorosis, and leaf senescence in C. roseus plants. In summary, C. roseus was identified as a good candidate plant for Fe phytoextraction, depending on Fe bioaccumulation, therefore 50 mM Fe treatment was designated as an excess Fe to cause the growth inhibition, especially in the prolonged Fe incubation periods.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01379-5.

Aluminum uptake, translocation, physiological changes, and overall growth inhibition in rice genotypes (Oryza sativa) at vegetative stage

Aluminum (Al) contamination in acidic soil is a major problem in paddy field, causing grain yield loss, especially in central plains of Thailand. The objective of this study was to assess Al content in the root tissues, its translocation to the leaves, and Al toxicity in three genotypes of rice, RD35 (local acidic-tolerant), Azucena (positive-check Al-tolerant), and IR64 (high yielding) under 0 (control) or 1 mM AlCl₃ (Al toxicity) at pH 4.5. Al content in the root tissues of rice cv. RD35 under 1 mM AlCl₃ was peaked at 4.18 mg g^‒1 DW and significantly translocated to leaf tissues (0.35 mg g^‒1 DW), leading to reduced leaf greenness (SPAD) (by 44.9% over the control) and declined net photosynthetic rate (P_n) (by 54.5% over the control). In contrast, Al level in cvs. Azucena and IR64 was restricted in the roots (2.12 mg g^‒1 DW) with low amount of translocation in the leaf tissues (0.26 mg g^‒1 DW), resulting in maintained values of SPAD and P_n. In cv. RD35, root and shoot traits including root length, root fresh weight, shoot height, shoot fresh weight, and shoot dry weight in 1 mM Al treatment were significantly dropped by > 35% over the control, whereas these parameters in cvs. Azucena and IR64 were retained. Based on the results, RD35 rice genotype was identified as Al sensitive as it demonstrated Al toxicity in both aboveground and belowground parts, whereas Azucena and IR64 were found tolerant to 1 mM Al as they demonstrated storage of Al in the root tissues to reduce toxicity in the leaf tissues. The study suggests that root traits, shoot attributes, chlorophyll degradation, and photosynthetic reduction can be successfully employed for the screening of Al-tolerant genotypes in rice breeding programs.

Combined gas exchange characteristics, chlorophyll fluorescence and response curves as selection traits for temperature tolerance in maize genotypes

Maize is a low-temperature (LT)-sensitive plant and its physiological responses towards LT of temperate regions developed is an adaptive trait. To further our understanding about the response of maize to LT at the physiological and photosynthesis level, we conducted Infrared Gas Analysis (IRGA using LICOR6400-XT in 45-day-old grown two maize genotypes, one from temperate region (Gurez-Kashmir Himalayas), viz., Gurez local (Gz local), and another from tropics (Gujarat), viz., GM6. This study was carried out to evaluate the underlying physiological mechanisms in the two differentially temperature-tolerant maize genotypes. Net photosynthetic rate (A/P_N), 18.253 in Gz local and 25.587 (µmol CO₂ m^-2 s^-1) in GM6; leaf conductance (gs), 0.0102 in Gz local and 0.0566 (mmol H₂O m^-2 s^-1) in GM6; transpiration rate (E), 0.5371 in Gz local and 2.9409 (mmol H₂O m^-2 s^-1) in GM6; and water use efficiency (WUE), 33.9852 in Gz local and 8.7224 (µmol CO₂ mmol H₂O^-1) in GM6, were recorded under ambient conditions. Also, photochemical efficiency of photosystem II (PSII) (F_v/F_m), 0.675 in Gz local and 0.705 in GM6; maximum photochemical efficiency (F_v'/F_m'), 0.310234 in Gz local and 0.401391 in GM6; photochemical quenching (qP), 0.2375 in Gz local and 0.2609 in GM6; non-photochemical quenching (NPQ), 2.0036 in Gz local and 1.1686 in GM6; effective yield of PSII (ФPSII), 0.0789 in Gz local and 0.099 in GM6; and electron transport rate (ETR), 55.3152 in Gz local and 68.112 in GM6, were also evaluated in addition to various response curves, like light intensities and temperature. We observed that light response curves show the saturation light intensity requirement of 1600 µmol for both the genotypes, whereas temperature response curves showed the optimum temperature requirement for Gz local as 20 °C and for GM6 it was found to be 35 °C. The results obtained for each individual parameter and other correlational studies indicate that IRGA forms a promising route for quick and reliable screening of various stress-tolerant valuable genotypes, forming the first study of its kind.

Selenium improved antioxidant response and photosynthesis in fragrant rice (Oryza sativa L.) seedlings during drought stress

Drought stress substantially influences the growth and development of many crops. The present study was conducted to investigate the effects of exogenous selenium on the growth, photosynthesis and antioxidant response of fragrant rice seedlings under drought stress. In a pot experiment, fragrant rice seedlings were subjected to drought stress (soil water potential was controlled at - 0.025 ± 5 MPa) and foliar application of selenium (Se) at 0, 10, 30, and 50 μmol L^-1. Rice seedlings not exposed to drought stress and Se were used as control. Exposure of fragrant rice seedlings to drought stress resulted in significant (P < 0.05) decrease in fresh weight, dry weight, plant height and stem diameter relative to the control. Total chlorophyll, chlorophyll a, chlorophyll b and carotenoid were 20.54-27.24%, 20.82-26.83%, 19.45-29.07% and 21.49-29.17% lower with drought stress treatment compared to CK. Drought stress also significantly (P < 0.05) decreased net photosynthetic rate and soluble protein content. However, Se treatments (30 and 50 μmol L^-1) substantially improved fresh weight and dry weight of fragrant rice seedlings under drought stress. Net photosynthetic rate, activities of antioxidant enzymes (GPX, SOD and CAT) and soluble protein content in rice seedlings under drought stress improved due to Se treatment. Higher transcript levels of antioxidant-related genes (GPX1, GPX4, CATA and CATC) were also observed with Se treatment.

Transcriptome comparison of different ploidy reveals the mechanism of photosynthetic efficiency superiority of triploid poplar

Triploid poplars have obvious growth advantages, especially in leaf development and photosynthetic characteristics, but the molecular mechanism has not been revealed yet. In order to better understand the regulation mechanisms of leaf and chlorophyll development in the triploid poplars, we combined the leaf phenotypic data with the transcriptomic data of the 5th, 10th, and 25th leaves from triploid and diploid poplars, using weighted gene co-expression network analysis (WGCNA), and revealed that PpnGRF5-1 had a strong correlation with leaf development and net photosynthetic rate (Pn). PpnGRF5-1 overexpression transgenic plants showed that the leaf area, Pn, and chlorophyll concentration were significantly increased. Transcriptomic data analysis of the third leaf from PpnGRF5-1 overexpression transgenic plants showed that PpnGRF5-1 could up-regulate the expression levels of chlorophyll synthesis genes and down-regulate the transcription of chlorophyll degradation enzymes. Overall, our studies have greatly expanded our understanding of the molecular mechanisms regulating triploid growth dominance.

Potential use of Helianthus tuberosus to suppress the invasive alien plant Ageratina adenophora under different shade levels

BACKGROUND

An ecological approach for managing biological invasions in agroecosystems is the selection of alternative crop species to manage the infestation of invasive alien plants through competition. In the current study, plant growth, photosynthesis, and competitive ability of the crop Helianthus tuberosus L. (Jerusalem artichoke) and the invasive alien plant Ageratina adenophora (Spreng.) R. M. King and H. Rob were compared under varying shade levels by utilizing a de Wit replacement series method. We hypothesized that H. tuberosus had higher competitive ability than A. adenophora even under shaded conditions.

RESULTS

The results showed the main stem, leafstalk length, leaf area, underground biomass, and aboveground biomass of A. adenophora were significantly lower compared to H. tuberosus in monoculture although A. adenophora had a greater number of branches that were longer on average. Under full sunlight, the total shoot length (stem + branch length), main stem length and branch length of A. adenophora were significantly suppressed (P < 0.05) by increasing proportions of H. tuberosus, and the same morphological variables of H. tuberosus were significantly higher with decreasing proportions of H. tuberosus. With increasing shade rates and plant ratios, the plant height, branch, leaf, and biomass of both plants were significantly suppressed, but to a greater degree in the case of A. adenophora. The net photosynthetic rate (Pn) of H. tuberosus and A. adenophora increased gradually from July to September, then decreased in October. The Pn of H. tuberosus was consistently higher than that of A. adenophora. Although the Pn for both species was significantly reduced with increasing shade rates and plant ratios, A. adenophora experienced greater inhibition than H. tuberosus. The relative yield (RY) of A. adenophora was significantly less than 1.0 (P < 0.05) in mixed culture under all shade levels, indicating that the intraspecific competition was less than interspecific competition. The RY of H. tuberosus was significantly less than 1.0 under 40-60% shade and greater than 1.0 (P < 0.05) under 0-20% shade in mixed culture, respectively, showing that intraspecific competition was higher than interspecific competition under low shade, but the converse was true under high shade. The relative yield total (RYT) of A. adenophora and H. tuberosus was less than 1.0 in mixed culture, indicating that there was competition between the two plants. The fact that the competitive balance (CB) index of H. tuberosus was greater than zero demonstrated a higher competitive ability than A. adenophora even at the highest shade level (60%).

CONCLUSIONS

Our results suggest that H. tuberosus is a promising replacement control candidate for managing infestations of A. adenophora, and could be widely used in various habitats where A. adenophora invades.

Increased photosynthesis and grain yields in maize grown with less irrigation water combined with density adjustment in semiarid regions

In order to design a water-saving and high-yield maize planting model suitable for semiarid areas, we conducted trials by combining supplementary irrigation with different planting densities. Three planting densities (L: 52,500, M: 75,000, and H: 97,500 plants ha^–1) and four supplementary irrigation modes (NI: no irrigation; IV: 375 m³ ha^–1 during the 11-leaf stage; IS: 375 m³ ha^–1 in the silking stage; and IVS: 375 m³ ha^–1 during both stages) were tested. The irrigation treatments significantly increased the leaf relative water content, but the high planting density significantly decreased the relative water content during the silking and filling stages. After supplementary irrigation during the 11-leaf stage, IV and IVS significantly increased the photosynthetic capacity, but decreased the leaf water use efficiency. IS and IVS significantly increased the photosynthetic capacity after supplementary irrigation in the silking stage over two years. During the filling stage, IV, IS, and IVS increased the two-year average net photosynthetic rate by 17.0%, 27.2%, and 30.3%, respectively. The intercellular CO₂ concentration increased as the density increased, whereas the stomatal conductance, transpiration rate, net photosynthetic rate, and leaf water use efficiency decreased, and the high planting density significantly reduced the leaf photosynthetic capacity. The highest grain yield was obtained using the IVS treatment under the medium planting density, but it did not differ significantly from that with the IS treatment. Furthermore, the IVS treatment used two times more water than the IS treatment. Thus, the medium planting density combined with supplementary irrigation during the silking stage was identified as a suitable water-saving planting model to improve the photosynthetic capacity and grain yield, and to cope with drought and water shortages in semiarid regions.

Expression levels of vacuolar ion homeostasis-related genes, Na+ enrichment, and their physiological responses to salt stress in sugarcane genotypes

Sugarcane is a sugar-producing crop widely grown in tropical regions in over 120 countries of the world. Salt-affected soil is one of the most significant abiotic constraints that inhibit growth and crop productivity, and, consequently, reduce sucrose concentration in the stalk. The present study investigated vacuolar ion homeostasis, Na⁺ accumulation, and physiological and morphological adaptations under salt stress in two different sugarcane genotypes (salt-tolerant K88-92 and salt-sensitive K92-80) under greenhouse conditions. Na⁺ was rapidly absorbed by the root tissues of both sugarcane genotypes within 3-7 days of 150 mM NaCl treatment, as confirmed by the results of CoroNa Green fluorescence staining. In addition, the rate of Na⁺ translocation from roots to shoots was evidently reduced, leading to lower amount of Na⁺ in the leaf tissues. At the cellular level, expression of ShNHX1 (vacuolar Na⁺/H⁺ antiporter), ShV-PPase (vacuolar H⁺-pyrophosphatase), and ShV-ATPase (vacuolar H⁺-ATPase) was upregulated in salt-stressed plants for the compartmentation of Na⁺ into the vacuoles of root cells. Interestingly, sucrose, glucose, and fructose in root tissues of salt-stressed sugarcane cv. K88-92 were increased by 10.61, 5.58, and 1.81 folds, respectively, over the control. Total soluble sugars in the roots and free proline in the leaves of sugarcane cv. K88-92 (salt-tolerant) were enriched by 3.08 and 1.99 folds, respectively, when plants were exposed to 150 mM NaCl, leading to maintain better photosynthetic abilities, net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (E), and water use efficiency (WUE) in sugarcane cv. K88-92 than those in cv. K92-80. The study concludes that Na⁺ compartmentation in the root tissue acts as a major defense mechanism in sugarcane, especially in salt-tolerant genotype.

Expression levels of the Na+/K+ transporter OsHKT2;1 and vacuolar Na+/H+ exchanger OsNHX1, Na enrichment, maintaining the photosynthetic abilities and growth performances of indica rice seedlings under salt stress

Salt affected soil inhibits plant growth, development and productivity, especially in case of rice crop. Ion homeostasis is a candidate defense mechanism in the salt tolerant plants or halophyte species, where the salt toxic ions are stored in the vacuoles. The aim of this investigation was to determine the OsNHX1 (a vacuolar Na⁺/H⁺ exchanger) and OsHKT2;1 (Na⁺/K⁺ transporter) regulation by salt stress (200 mM NaCl) in two rice cultivars, i.e. Pokkali (salt tolerant) and IR29 (salt susceptible), the accumulation of Na⁺ in the root and leaf tissues using CoroNa Green^® staining dye and the associated physiological changes in test plants. Na⁺ content was largely increased in the root tissues of rice seedlings cv. Pokkali (15 min after salt stress) due to the higher expression of OsHKT2;1 gene (by 2.5 folds) in the root tissues. The expression of OsNHX1 gene in the leaf tissues was evidently increased in salt stressed seedlings of Pokkali, whereas it was unchanged in salt stressed seedlings of IR29. Na⁺ in the root tissues of both Pokkali and IR29 was enriched, when subjected to 200 mM NaCl for 12 h and easily detected in the leaf tissues of salt stressed plants exposed for 24 h, especially in cv. Pokkali. Moreover, the overexpression of OsNHX1 gene regulated the translocation of Na⁺ from root to leaf tissues, and compartmentation of Na⁺ into vacuoles, thereby maintaining the photosynthetic abilities in cv. Pokkali. Overall growth performance, maximum quantum yield (F_v/F_m), photon yield of PSII (Φ_PSII) and net photosynthetic rate (P_n) was improved in salt stressed leaves of Pokkali than those in salt stressed IR29.

Promoting water deficit tolerance and anthocyanin fortification in pigmented rice cultivar (Oryza sativa L. subsp. indica) using arbuscular mycorrhizal fungi inoculation

Drought or water deficit is a major abiotic stress that can reduce growth and productivity in the rice crop especially in the rain-fed areas, which face long-term water shortage. The objective of this investigation was to promote the drought tolerant abilities in pigmented rice cv. 'Hom Nil' at booting stage using arbuscular mycorrhizal fungi (AMF)-inoculation, mixed spores of Glomus geosporum, G. etunicatum and G. mosseae in the soil before rice seedling transplantation. At booting stage, the AMF-inoculated (+AMF) and AMF-uninoculated plants (-AMF) were subjected to control (well-watering; 46.6% SWC) and water deficit condition (14 days water withholding; 13.8% SWC). Colonization percentage in the AMF-inoculated root tissues were evidently proved in both with and without water deficit conditions, leading to elevate total phosphorus in root and leaf tissues. Interestingly, sucrose and total soluble sugar concentration in the flag leaf were increased by 5.0 folds and 1.5 folds, respectively in the plants under water deficit (WD). Free proline was accumulated in flag leaf when exposure to water deficit, subsequently regulated by AMF-inoculation. Total soluble sugar and free proline enrichment in 'Hom Nil' was a major mode of osmotic adjustment to control osmotic potential in the cellular level when exposed to water deficit, leading to maintained photosynthetic abilities and growth performances. Concentration of chlorophyll b in AMF-inoculated plants under water deficit stress was retained, causing to improve chlorophyll fluorescence and net photosynthetic rate. Shoot height and number of tillers were significantly declined by 12.5% and 11.6%, respectively, when subjected to WD. At the harvest, grain yield, panicle dry weight and fertility percentage of AMF-inoculated rice from WD were greater than those without AMF by 1.5, 3.9 and 2.4 folds, respectively. Cyanidin-3-glucoside and peonidin-3-glucoside concentrations in pericarp were enriched in the grain derived from AMF-inoculation with water deficit stress. Overall growth characters and physiological adaptations in 'Hom Nil' grown under water deficit condition were retained by AMF inoculation, resulting in enhanced yield attributes and anthocyanin fortification in rice grain.

Titanium dioxide nanoparticles impaired both photochemical and non-photochemical phases of photosynthesis in wheat

Titanium dioxide nanoparticles (TiO₂-NP) are increasingly being proposed for nanoagriculture but their effect on photosynthesis is limited and contradictory, mostly regarding putative chronical effects associated to the exposure to the commercial P25 formulation (anatase:rutile). This research aims at evaluating how chronical exposure to P25-NP affect photosynthetic processes in Triticum aestivum. Wheat plants were exposed (from the germination stage) to 0, 5, 50, and 150 mg L^-1 P25-NP for 20 days. P25-NP impaired both light-dependent and -independent phases of photosynthesis, decreased chlorophyll a content, maximal and effective efficiency of PSII, net photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO₂ concentration, and starch content. On the other hand, no effects were observed in photochemical and in non-photochemical quenching values, on total soluble sugar (TSS) content or in RuBisCO activity. Our results support that the induced decay in chlorophyll a content compromised the electron transport through PSII and that stomatal limitations impaired CO₂ assimilation. The decline of starch content seems to be a consequence of its degradation as a mechanism to maintain the TSS levels. Consequently, we propose that photosynthetic related endpoints are sensitive and valuable biomarkers to assess TiO₂-NP toxicity.

Comparative effect of 28-homobrassinolide and 24-epibrassinolide on the performance of different components influencing the photosynthetic machinery in Brassica juncea L

BRs are polyhydroxylated sterol derivatives, classified as phytohormones. Plants of Brassica juncea var. Varuna were grown in pots and an aqueous solution (10^-8 M) of two brassinosteroid isomers 28-homobrassinolide (HBL) and 24-epibrassinolide (EBL) of same concentration (10^-8 M) was applied to their leaves. The treatment up-regulated the photosynthetic machinery directly by enhancing water splitting activity, photochemical quenching, non-photochemical quenching, maximum PSII efficiency, actual PSII efficiency, electron transport rate, stomatal movement, stomatal conductance, internal CO₂ concentration, transpiration rate, net photosynthetic rate and carbohydrate synthesis. Moreover, the level of biochemical enzymes (carbonic anhydrase and nitrate reductase), reactive oxygen species (superoxide and hydrogen peroxide) generation, antioxidant enzyme activity and mineral status (C, N, Mg, P, S, K), which indirectly influence the rate of photosynthesis, also improved in the treated plants. Out of the two BR analogues tested, EBL excelled in its effects over HBL.

Application of a coupled model of photosynthesis and stomatal conductance for estimating plant physiological response to pollution by fine particulate matter (PM2.5)

Fine particulate matter (PM_2.5) is a current environmental issue that has an impact on the global ecology. Vegetation is a known sink for PM_2.5 deposition but the effects of these particles on plant growth, and specifically on plant photosynthesis by changing their leaf water potential, are still not well understood. This study aimed to determine and characterize possible relationships between PM_2.5 and plant photosynthesis under different PM_2.5 concentrations. Both indoor and outdoor measurements were carried out to evaluate the variation dynamics of net photosynthetic rate and stomatal conductance of four plant species with different leaf characteristics under different PM_2.5 levels. A calibrated coupled model of photosynthesis and stomatal conductance was developed to estimate the relationship between plant photosynthesis and PM_2.5 reliably. Net photosynthetic rate and stomatal conductance declined over time at elevated PM_2.5, with large variations with PM_2.5 concentrations. Using a calibrated model of photosynthesis coupled to stomatal conductance, we show that PM_2.5 can influence plant photosynthesis that primarily occurs through the stomata on leaves. Although the effect of particles on plant photosynthesis was not as high as that of photosynthetically active radiation, temperature, and CO₂ concentration around the leaf, the effect from PM_2.5 can be significant, in particular, in highly polluted atmospheres.

Evaluation of zinc oxide nanoparticles on lettuce (Lactuca sativa L.) growth and soil bacterial community

The wide spread of nanoparticles (NPs) has caused tremendous concerns on agricultural ecosystem. Some metallic NPs, such as zinc oxide (ZnO), can be utilized as a nano-fertilizer when used at optimal doses. However, little is known about the responses of plant development and concomitant soil bacteria community to ZnO NPs. The present pot experiment studied the impacts of different doses of ZnO NPs and bulk ZnO (0, 1, 10, 100 mg ZnO/kg), on the growth of lettuce (Lactuca sativa L.) and the associated rhizospheric soil bacterial community. Results showed that at a dose of 10 mg/kg, ZnO NPs and bulk ZnO, enhanced the lettuce biomass and the net photosynthetic rate; whereas, the Zn content in plant tissue was higher in NPs treatment than in their bulk counterpart at 10 mg/kg dose or higher. For the underground observations, 10 mg/kg treatment doses (NPs or bulk) significantly changed the soil bacterial community structure, despite the non-significant variations in alpha diversity. Taxonomic distribution revealed that some lineages within Cyanobacteria and other phyla individually demonstrated similar or different responses to ZnO NPs and bulk ZnO. Moreover, some lineages associated with plant growth promotion were also influenced to different extents by ZnO NPs and bulk ZnO, suggesting the distinct microbial processes occurring in soil. Collectively, this study expanded our understanding of the influence of ZnO NPs on plant performance and the associated soil microorganisms.

Physiological characteristics, dry matter, and active component accumulation patterns of Changium smyrnioides in response to a light intensity gradient

CONTEXT

Changium smyrnioides Wolff (Apiaceae) is an endangered medicinal plant with numerous pharmacological uses.

OBJECTIVE

To investigate the effect of light intensity levels on the growth and accumulation of secondary metabolites of C. smyrnioides, cultivated seedlings were subjected to different relative light intensities via sun-shading.

MATERIALS AND METHODS

Changium smyrnioides seedlings were subjected to five irradiance treatments (100, 60.54, 44.84, 31.39, and 10.56% sunlight) in glasshouse for 9 months. Enzymatic and non-enzymatic antioxidants with spectrophotometric method, photosynthetic parameters with Li-6400XT, dry matter accumulation and active component contents in the root with spectrophotometric and HPLC method were analyzed.

RESULTS

With an increase in relative light intensity levels, activities of enzymatic and non-enzymatic antioxidants, and malondialdehyde (MDA) contents were increased overall, while net photosynthetic rate (P_n) and dry matter accumulation patter first increased and then declined. The highest net photosynthetic rate (30.68 μmol/m²·s) and dry root weight (5.07 g) were achieved under 60.54% sunlight. Lower relative light intensity levels stimulated the accumulation levels of bioactive compounds in the roots so that the highest contents of mannitol (1.35%) and choline (405.58 μg/g) were recorded under 31.39% sunlight, and the highest polysaccharide content (10.80%) were achieved under 44.84% sunlight. With a decrease in the relative light intensity levels, the water-soluble component content increased first and then decreased.

DISCUSSION AND CONCLUSION

The results revealed that 31.39-60.54% sunlight serve as appropriate relative light intensity conditions for cultivated C. smyrnioides.

Photosynthesis of alfalfa (Medicago sativa) in response to landfill leachate contamination

Thousands of unlined landfills and open dumpsites have put great threat on the security of soil and ground water due to leachate leakage. Alfalfa is believed potential as a phytoremediation plant for leachate contamination based on strong root system and the excellent capacity of removing various kinds of pollutants. A lab-scale investigation was conducted to figure out the sensitiveness of alfalfa photosynthesis in response to leachate contamination. The results demonstrated that both of the maximum photosynthetic efficiency (F_v/F_m) and net photosynthetic rate (P_n) were slightly inhibited in the high-dosage group. Based on statistical analysis, higher sensitivity of P_n to leaching parameters than F_v/F_m was observed. There were significant correlations between most of leaching parameters (pH, ammonium and COD) and P_n with correlation coefficients of 0.530, -0.580 and -0.578 (p < 0.01), respectively. Therefore, P_n is potential for acting as an effective indicator for staple leaching characteristics of leachate contaminated soils.

Stomatal and non-stomatal factors regulated the photosynthesis of soybean seedlings in the present of exogenous bisphenol A

Bisphenol A (BPA) is an emerging environmental endocrine disruptor that has toxic effects on plants growth. Photosynthesis supplies the substances and energy required for plant growth, and regulated by stomatal and non-stomatal factors. Therefore, in this study, to reveal how BPA affects photosynthesis in soybean seedlings (Glycine max L.) from the perspective of stomatal and non-stomatal factors, the stomatal factors (stomatal conductance and behaviours) and non-stomatal factors (Hill reaction, apparent quantum efficiency, Rubisco activity, carboxylation efficiency, the maximum Rubisco carboxylation velocity, ribulose-1,5-bisphospate regeneration capacities mediated by maximum electron transport rates, and triose phosphate utilization rate) were investigated using a portable photosynthesis system. Moreover, the pollution of BPA in the environment was simulated. The results indicate that low-dose BPA enhanced net photosynthetic rate (Pn) primarily by promoting stomatal factors, resulting in increased relative growth rates and accelerated soybean seedling growth. High-dose BPA decreases the Pn by simultaneously inhibiting stomatal and non-stomatal factors, and this inhibition decreases the relative growth rates further reducing soybean seedling growth. Following the withdrawal of BPA, all of the indices were restored to varying degrees. In conclusion, low-dose BPA increased the Pn by promoting stomatal factors while high-dose BPA decreased the Pn by simultaneously inhibiting stomatal and non-stomatal factors. These findings provide a model (or, hypothesis) for the effects of BPA on plant photosynthesis.

Brassinosteroids improve photosystem II efficiency, gas exchange, antioxidant enzymes and growth of cowpea plants exposed to water deficit

Water deficit is considered the main abiotic stress that limits agricultural production worldwide. Brassinosteroids (BRs) are natural substances that play roles in plant tolerance against abiotic stresses, including water deficit. This research aims to determine whether BRs can mitigate the negative effects caused by water deficiency, revealing how BRs act and their possible contribution to increased tolerance of cowpea plants to water deficit. The experiment was a factorial design with the factors completely randomised, with two water conditions (control and water deficit) and three levels of brassinosteroids (0, 50 and 100 nM 24-epibrassinolide; EBR is an active BRs). Plants sprayed with 100 nM EBR under the water deficit presented significant increases in ΦPSII, qP and ETR compared with plants subjected to the water deficit without EBR. With respect to gas exchange, PN, E and gs exhibited significant reductions after water deficit, but application of 100 nM EBR caused increases in these variables of 96, 24 and 33%, respectively, compared to the water deficit + 0 nM EBR treatment. To antioxidant enzymes, EBR resulted in increases in SOD, CAT, APX and POX, indicating that EBR acts on the antioxidant system, reducing cell damage. The water deficit caused significant reductions in Chl a, Chl b and total Chl, while plants sprayed with 100 nM EBR showed significant increases of 26, 58 and 33% in Chl a, Chl b and total Chl, respectively. This study revealed that EBR improves photosystem II efficiency, inducing increases in ΦPSII, qP and ETR. This substance also mitigated the negative effects on gas exchange and growth induced by the water deficit. Increases in SOD, CAT, APX and POX of plants treated with EBR indicate that this steroid clearly increased the tolerance to the water deficit, reducing reactive oxygen species, cell damage, and maintaining the photosynthetic pigments. Additionally, 100 nM EBR resulted in a better dose-response of cowpea plants exposed to the water deficit.

Is foliar spray of proline sufficient for mitigation of salt stress in Brassica juncea cultivars?

The effects of foliar application of proline (20 mM) on growth, physio-biochemical, and yield parameters were assessed in two Brassica juncea (L.) Czern & Coss cultivars, namely, Varuna and RH-30, at different levels (2.8, 4.2, or 5.6 dsm(-1)) of NaCl in soil. At 29 days after sowing (DAS), plants were sprayed with either 20 mM proline or water in the presence or absence of NaCl stress. The NaCl negatively affected parameters related to growth, photosynthesis, and yield in both varieties but more in RH-30 than in Varuna. Exogenous application of proline counteracted the effects of salt stress in Varuna only, by increasing the antioxidative capacity of the plants. Moreover, proline was not effective in alleviating the detrimental effects of higher salt concentrations on the studied parameters. Proline application to unstressed plants increased growth, photosynthesis, and yield parameters in both varieties; however, the effects were more prominent in Varuna than in RH-30.

Promotion of photosynthesis in transgenic rice over-expressing of maize C4 phosphoenolpyruvate carboxylase gene by nitric oxide donors

We determined the effects of exogenous nitric oxide on photosynthesis and gene expression in transgenic rice plants (PC) over-expressing the maize C4pepc gene, which encodes phosphoenolpyruvate carboxylase (PEPC). Seedlings were subjected to treatments with NO donors, an NO scavenger, phospholipase inhibitors, a Ca(2+) chelator, a Ca(2+) channel inhibitor, and a hydrogen peroxide (H2O2) inhibitor, individually and in various combinations. The NO donors significantly increased the net photosynthetic rate (PN) of PC and wild-type (WT), especially that of PC. Treatment with an NO scavenger did inhibit the PN of rice plants. The treatments with phospholipase inhibitors and a Ca(2+) chelator decreased the PN of WT and PC, and photosynthesis was more strongly inhibited in WT than in PC. Further analyses showed that the NO donors increased endogenous levels of NO and PLD activity, but decreased endogenous levels of Ca(2+) both WT and PC. However, there was a greater increase in NO in WT and a greater increase in PLD activity and Ca(2+) level in PC. The NO donors also increased both PEPC activity and pepc gene expression in PC. PEPC activity can be increased by SNP alone. But the expression of its encoding gene in PC might be regulated by SNP, together with PA and Ca(2+).

CPPU elevates photosynthetic abilities, growth performances and yield traits in salt stressed rice (Oryza sativa L. spp. indica) via free proline and sugar accumulation

Application of N-2-(chloro-4-pyridyl)-N-phenyl urea (CPPU) to salt susceptible cultivar of indica rice (cv. PTT1) effected on free proline and soluble sugar accumulation and regulated the physio-morphological responses when subjected to salt stress condition was firstly demonstrated in this study. Soluble sugars, including sucrose, glucose and fructose, in leaf blade and leaf sheath were enriched in 0.1 mM CPPU pretreated plants subsequently exposed to 16.6 dS m(-1) NaCl for 10 days. In the long period (15 days) salt stress, free proline content in the leaf blade and leaf sheath were evidently peaked to act as osmotic adjustment in the salt-stressed plants. In addition, the photosynthetic pigments, including chlorophyll a, chlorophyll b, total chlorophyll and total carotenoids, were maintained by the functional regulation of soluble sugar and free proline in the cellular levels, thereby leading to higher net photosynthetic rate. Further, the stomatal closure and transpiration rate in CPPU pretreated plants were retained under salt stress, thereby resulting in alleviation of growth performance and yield traits. This study suggested that exogenous application of CPPU may alternatively play effective role to improve the salt tolerant abilities of salt susceptible rice crop.

Cloning and expression analysis of transketolase gene in Cucumis sativus L

Transketolase (TK, EC 2.2.1.1) is a key enzyme in the photosynthetic carbon reduction cycle (Calvin cycle). A full-length cDNA encoding transketolase (TK, designated as CsTK) was isolated from cucumber leaves (Cucumis sativa L. cv 'Jinyou 3') by RT-PCR and RACE. The cDNA contained 2368 nucleotides with a complete open reading frame (ORF) of 2229 nucleotides, which was predicted to encode a peptide of 742 amino acids. Expression analysis by real-time PCR and western blot revealed that TK mRNA was abundant in cucumber leaves and detectable in stems, fruits and roots. TK activity and the gene expression at the mRNA and protein levels was higher in mid-position leaves (4th apical leaves) than in upper position leaves (1st) and base position leaves (12th). The diurnal variation of CsTK expression and TK activity in the optimal functional leaf (4th leaf) was a single-peak curve, and the peak appeared at 14:00 on a sunny day. Low temperature (5 °C) and low light (100 μmol m(-2) s(-1)) induced CsTK expression, but the expression level decreased after 24 h of chilling stress. Over-expression of CsTK increased the TK activity, mRNA abundance and activities of other main enzymes in Calvin cycle, and net photosynthetic rate (Pn) in transgenic cucumber leaves. Transgenic plants showed a higher ratio of female flower and yield relative to the wild type (WT) plants. The decreases in Pn and carboxylation efficiency (CE) were less in transgenic plants than that in WT during low temperature and low light intensity.

Transcriptional regulations of the genes of starch metabolism and physiological changes in response to salt stress rice (Oryza sativa L.) seedlings

The aim of this investigation was to compare the transcriptional expression of starch metabolism, involving genes and physiological characters, in seedlings of two contrasting salt-tolerant rice genotypes, in response to salt-stress. The soluble sugar content in rice seedlings of both salt-tolerant and salt-sensitive genotypes was enriched, relating to starch degradation, in plants subjected to 200 mM NaCl. In the salt-tolerant cultivar Pokkali, a major source of carbon may be that derived from the photosynthetic system and starch degradation. In starch degradation, only Pho and PWD genes in Pokkali were upregulated in plants subjected to salt stress. In contrast, the photosynthetic abilities of IR29 salt-susceptible cultivar dropped significantly, relating to growth reduction. The major source of carbohydrate in salt-stressed seedlings of the IR29 cultivar may be gained from starch metabolism, regulated by ADP-glucose pyrophosphorylase (AGP), starch synthase (SS), starch branching enzyme (SBE), starch debranching enzyme (ISA), glucan-water dikinase (GWD), dispropotionating enzyme (DPE), phospho glucan-water dikinase (PWD) and starch phosphorylase (Pho). Also, the major route of soluble sugar in salt-stressed Pokkali seedlings was derived from photosynthesis and starch metabolism. This was identified as novel information in the present study.