Photosynthetic Gas Exchange Parameters Research Articles

Physiology and transcriptomic analysis revealed the mechanism of silicon promoting cadmium accumulation in Sedum alfredii Hance

Silicon (Si) effectively promote the yield of many crops, mainly due to its ability to enhance plants resistance to stress. However, how Si helps hyperaccumulators to extract Cadmium (Cd) from soil has remained unclear. In this study, Sedum alfredii Hance (S. alfredii) was used as material to study how exogenous Si affected biomass, Cd accumulation, antioxidation, cell ultrastructure, subcellular distribution and changes in gene expression after Cd exposure. The study has shown that as Si concentration increases (1, 2 mM), the shoot biomass of plants increased by 33.1% - 63.6%, the Cd accumulation increased by 31.9% - 96.6%, and the chlorophyll, carotenoid content, photosynthetic gas exchange parameters significantly increased. Si reduced Pro and MDA, promoted the concentrations of SOD, CAT and POD to reduce antioxidant stress damage. In addition, Si promoted GSH and PC to chelate Cd in vacuoles, repaired damaged cell ultrastructure, improved the fixation of Cd and cell wall (especially in pectin), and reduced the toxic effects of Cd. Transcriptome analysis found that genes encoding Cd detoxification, Cd absorption and transport were up-regulated by Si supplying, including photosynthetic pathways (PSB, LHCB, PSA), antioxidant defense systems (CAT, APX, CSD, RBOH), cell wall biosynthesis such as pectinesterase (PME), chelation (GST, MT, NAS, GR), Cd absorption (Nramp3, Nramp5, ZNT) and Cd transport (HMA, PCR). Our result revealed the tentative mechanism of Si promotes Cd accumulation and enhances Cd tolerance in S. alfredii, and thereby provides a solid theoretical support for the practical use of Si fertilizer in phytoextraction.

Optimizing the Mulching Pattern and Nitrogen Application Rate to Improve Maize Photosynthetic Capacity, Yield, and Nitrogen Fertilizer Utilization Efficiency.

Residual film pollution and excessive nitrogen fertilizer have become limiting factors for agricultural development. To investigate the feasibility of replacing conventional plastic film with biodegradable plastic film in cold and arid environments under nitrogen application conditions, field experiments were conducted from 2021 to 2022 with plastic film covering (including degradable plastic film (D) and ordinary plastic film (P)) combined with nitrogen fertilizer 0 (N0), 160 (N1), 320 (N2), and 480 (N3) kg·ha-1. The results showed no significant difference (p > 0.05) in dry matter accumulation, photosynthetic gas exchange parameters, soil enzyme activity, or yield of spring maize under degradable plastic film cover compared to ordinary plastic film cover. Nitrogen fertilizer is the main factor limiting the growth of spring maize. The above-ground and root biomass showed a trend of increasing and then decreasing with the increase in nitrogen application level. Increasing nitrogen fertilizer can also improve the photosynthetic gas exchange parameters of leaves, maintain soil enzyme activity, and reduce soil pH. Under the nitrogen application level of N2, the yield of degradable plastic film and ordinary plastic film coverage increased by 3.74~42.50% and 2.05~40.02%, respectively. At the same time, it can also improve water use efficiency and irrigation water use efficiency, but it will reduce nitrogen fertilizer partial productivity and nitrogen fertilizer agronomic use efficiency. Using multiple indicators to evaluate the effect of plastic film mulching combined with nitrogen fertilizer on the comprehensive growth of spring maize, it was found that the DN2 treatment had the best complete growth of maize, which was the best model for achieving stable yield and income increase and green development of spring maize in cold and cool irrigation areas.

Study on the Mechanism of Exogenous 5-Aminolevulinic Acid (ALA) in Regulating the Photosynthetic Efficiency of Pear Leaves

To provide a theoretical basis for the application of ALA in pear production, the effects of exogenous 5-aminolevulinic acid (ALA) treatment on leaf photosynthetic gas exchange parameters, chlorophyll fast fluorescence properties, and relative expression of the related genes were investigated using pear (Pyrus pyrifolia Nakai cv. ‘Whasan’) as a material in the study. The results show that exogenous ALA treatment improved the photosynthetic gas exchange parameters of pear leaves, upregulated the expression of multiple key genes which are related to ALA biosynthesis, metabolism, and transformation into chlorophylls. GUS staining in tobacco leaves showed that exogenous ALA activated the promoter activity of PypHEMA and PypCHLH genes, implying that the synthesis of endogenous ALA and chlorophylls was promoted by exogenous ALA. Furthermore, ALA promoted the expression of the genes encoding photosystem II (PSII) reaction center proteins, such as core protein D1, inner light-harvesting pigment proteins CP43 and CP47, and cytochrome b559. This led to increased PSII reaction center activity. In addition, ALA alleviated the donor side oxygen-evolving complex inhibition and reduced the closure rate on the receptor side, allowing for increased photochemical electron transfer and reduced heat dissipation while improving the photosynthetic performance index PIabs and PItotal. The findings of this study contribute to a better understanding of ALA’s promotion of plant photosynthetic efficiency, providing valuable insights for further research and potential applications in pear production.

Supplementation of Jasmonic acid Mitigates the Damaging Effects of Arsenic Stress on Growth, Photosynthesis and Nitrogen Metabolism in Rice

Experiments were conducted to evaluate the role of exogenously applied jasmonic acid (JA; 0.1 and 0.5 µM) in alleviating the toxic effects of arsenic (As; 5 and 10 µM) stress in rice. Plants treated with As showed considerable decline in growth attributes like height, fresh and dry weight of plant. Arsenic stress reduced the content of δ-amino livulenic acid (δ-ALA), glutamate 1-semialdehyde (GSA), total chlorophylls and carotenoids, with more reduction evident at higher (10 µM) As concentrations, however exogenously supplied JA alleviated the decline to considerable extent. Arsenic stress mediated decline in photosynthetic gas exchange parameters, Fv/Fm (PSII activity) and Rubisco activity was alleviated by the exogenous treatment of JA. Arsenic stress caused oxidative damage which was evident as increased lipid peroxidation, lipoxygenase activity and hydrogen peroxide concentrations however, JA treatment declined these parameters. Treatment of JA improved the activity of nitrate reductase and glutamate synthase under unstressed conditions and also alleviated the decline triggered by As stress. Activity of antioxidant enzymes assayed increased due to As stress, and the supplementation of JA caused further increase in their activities. Moreover, the content of proline, free amino acids and total phenols increased significantly due to JA application under stressed and unstressed conditions. Treatment of JA increased the content of nitrogen and potassium while as reduced As accumulation significantly.

A comparison of photosynthetic gas exchange parameters measured under in situ and in vitro conditions in Pinus nigra subsp. pallasiana and Pinus brutia trees

This study was conducted to compare the results of in situ and in vitro methods that can be used for measuring gas exchange parameters in two pine species. The study was carried out in a mixed Pinus nigra Arn. subsp. pallasiana (Lamb). Holmboe and Pinus brutia Ten. plantation in Kemer/Burdur, which has a semi-arid climate type located in the Western Mediterranean Region of Türkiye. In the first method, gas exchange parameters (net photosynthetic rate-Anet, stomatal conductance-gs and transpiration rate-E) were measured directly on the tree using a mobile scaffold to reach to the tree canopy (in situ conditions). In the second method, branches were cut at approximately 50 cm and gas exchange parameters were determined in the same needles after the branch was immediately submerged in water (in vitro conditions). Measurements were taken between June and October 2022. Student’s t-test was conducted to compare the results of in situ and in vitro photosynthesis measurement methods. No statistically significant differences were found between the results of the compared methods in terms of gas exchange parameters for both species. The results of the study showed that in vitro measurements of photosynthesis can be preferred to in situ measurements of photosynthesis in P. nigra and P. brutia under field conditions where access to the canopy of tall trees is not possible.

Response of Evapotranspiration, Photosynthetic Characteristics and Yield of Soybeans to Groundwater Depth

To clarify the physiological mechanism of different groundwater depths affecting soybean evapotranspiration, photosynthetic characteristics and yield, a field experiment with four groundwater depth levels (1 m (D1), 2 m (D2), 3 m (D3) and 4 m (D4)) was conducted through the groundwater simulation system in 2021 and 2022. In this study, a quantitative analysis was conducted on the groundwater recharge and irrigation water demand and evapotranspiration (ET) of soybean fields with different treatments, and the effects of different treatments on soybean leaf area index (LAI), chlorophyll content index (SPAD), intercepted photosynthetic active radiation (IPAR), photosynthetic gas exchange parameters, dry matter accumulation (DMA) and yield were explored. The results showed the following: (1) Groundwater depth affected soybean ET and the source of ET. With the increase in groundwater depth, groundwater recharge and its contribution to ET gradually decreased, but the amount of irrigation required gradually increased, resulting in the ET as D1 > D4 > D2 > D3. (2) Soybean LAI, SPAD and IPAR were significantly affected by the different groundwater depths, of which the D1 treatment always maintained the maximum, followed by the D4 treatment, and the D3 treatment was the minimum. The photosynthetic gas exchange parameters under different treatments changed synergistically, showing significant differences in the flowering and podding stages, notably D1 > D4 > D2 > D3. Soybean DMA and yield first decreased and then increased with the increase in groundwater depth, and the average DMA and yield under the D1 treatment increased by 27.71%, 46.80% and 22.82% and 20.29%, 29.91% and 12.83% in the two years, respectively, compared to the D2, D3 and D4 treatments. (3) The structural equation model demonstrated that the groundwater depth indirectly affected the growth of soybean leaf area by affecting groundwater recharge, which in turn regulated soybean ET and photosynthetic capacity and ultimately affected DMA and yield. The above results showed that in the case of shallow groundwater depth (D1), the largest groundwater recharge promoted the growth of soybean leaf area and chlorophyll synthesis and increased the absorption and utilization of solar radiation. And it improved the leaf stomata conditions, accelerated the gas exchange between the plant and atmosphere, enhanced the photosynthetic production capacity and ET and achieved maximum DMA and yield. Soybean leaf growth and photosynthesis diminish with the increase in groundwater depth. In the case of deep groundwater depth (D4), the maximum irrigation improved the growth and photosynthetic performance of soybean leaves, which was favorable to ET, and ultimately led to increases in DMA and yield.

The Effects of Shading on the Photosynthetic Performance of Endangered Plant Horsfieldia hainanensis Seedlings

Shading is one of the management practices for preventing the damage or injury of plant seedlings during extreme weather and climate events, such as very high temperatures and heat stress. In this study, we investigated the effects of different shading conditions on the photosynthetic characteristics of the endangered plant Horsfieldia hainanensis in Guangxi, China. The H. hainanensis seedlings in this study underwent five shading treatments, including 20% (L1), 40% (L2), 60% (L3), 80% (L4), and 100% (control) of full sunlight. The net growth of their diameter and height, and photosynthetic gas exchange parameters including their photosynthesis rate (Pn), transpiration rate (Tr), intercellular CO2 concentration (Ci), and water use efficiency (WUE) were measured for the examined seedlings. The OJIP curve and 820 nm light absorption curve, and the osmotic substances and products of membrane lipid peroxidation were employed to assess photosynthetic capacity, identify the factors constraining photosynthetic carbon assimilation, and investigate the mechanisms influencing photosystem II (PSII) and photosystem I (PSI) in the seedlings under shade stress. The results showed that the seedlings in the L2 treatments had the highest net growth and Pn, the best photosynthetic performance, and the best coordination between PSII and PSI. The net photosynthesis (Pn) levels exhibited a declining trend in the following order: L2 > L3 > L4 > L1. In the L1 treatment, non-stomatal factors emerged as the primary determinant affecting the Pn of the seedlings. The performance index (potential) of PSII, representing the conservation of absorbed photon energy to intersystem electron acceptor reduction (PIABS and ΔI/I0) of the seedlings, decreased in the order of L2 > L3 > L4 > L1. The photosystem performance and the coordination between PSII and PSI (Φ(PSI/PSII)) of the seedlings decreased in the order of L2 > L1 > L3 > L4. Under the low and moderate shading stresses (L1–L3), more serious damages occurred in PSII than in PSI, including on the donor side of PSII and in the electron transfer from QB to the acceptor side of PSI. In contrast, more considerable injury occurred in PSI than in PSII under the stress of the heavy shading treatment (L4). Considering the alterations in their leaf osmotic regulatory substances and membrane lipid peroxidation products, our findings indicate that the L2 treatment was the most conducive to the growth of the H. hainanensis seedlings. In contrast, the L1 treatment subjected H. hainanensis seedlings to the most significant stress, resulting in substantial damage to their growth and photosynthetic mechanisms. Our research provides a scientific insight into and a practical guide for the selection of an appropriate light intensity for the conservation and cultivation of endangered plant species, such as H. hainanensis.

Individual and interactive effects of air warming and elevated O3 on carbon fixation and allocation in two urban tree species

With continuous urbanization and climate warming, increased air temperature and elevated ozone (O3) concentration often co-occur in many urban areas, but we still lack information about the interactive effects of warming and elevated O3 on urban trees. In the present experiment, the single and combined effects of increased air temperature (IT, ambient air temperature + 2 °C) and elevated O3 (EO, ambient air O3 concentrations + 80 ppb) on carbon (C) fixation and allocation in Quercus mongolica and Pinus tabuliformis, which are widely used as street tree species in urban areas of China, were investigated over two consecutive growing seasons by using 13C isotope techniques. The results showed that IT increased biomass, photosynthetic gas exchange parameters and total 13C content of both tree species. Compared to ambient temperature, IT significantly increased the total 13C content labelled by 56.6 % in Q. mongolica and by 31.2 % in P. tabuliformis in 2021. Elevated O3 induced a decrease in biomass and net photosynthetic rate (Pn) in both tree species. Compared to ambient O3, elevated O3 significantly decreased Pn by 52.6 % in Q. mongolica and by 37.4 % in P. tabuliformis in 2020. The treatment EO decreased 13C allocation to roots but increased 13C content and distribution in leaves in Q. mongolica. These findings demonstrated that EO inhibited the growth and photosynthesis of the two tree species. Our results showed that Q. mongolica was more sensitive to IT and EO than P. tabuliformis, but the former has a self-repair mechanism under increased O3 stress as it is able to invest more carbon to repair leaf damage to a certain extent. Our study also found that the total biomass, relative growth rate, Pn and total 13C content remained higher under the combination of IT and EO compared to EO alone, suggesting that moderate warming may mitigate the negative effects of elevated O3 stress to some extent.

Improving the estimation accuracy of rapeseed leaf photosynthetic characteristics under salinity stress using continuous wavelet transform and successive projections algorithm.

Soil salinization greatly restricts crop production in arid areas for salinity stress can inhibit crop photosynthesis and growth. Chlorophyll fluorescence and photosynthetic gas exchange (CFPGE) parameters are important indicators of crop photosynthesis and have been widely used to evaluate the impacts of salinity stress on crop photosynthesis and growth. Remote sensing technology can quickly and non-destructively obtain crop information under salinity stress, however, at present, the distribution of spectral features of CFPGE parameters in different regions is still unclear. In this study (2019-2020), under salinity stress conditions, the spectral data of rapeseed leaves were acquired and the CFPGE parameters were simultaneously determined. Then, continuous wavelet transformation (CWT) and standard normal variate (SNV) transformation were utilized to preprocess the raw spectral data. After that, a CFPGE parameter estimation model was constructed by using the partial least squares regression (PLSR) algorithm and the support vector machines (SVM) algorithm based on the spectral features in the red region (600-800 nm) and those in the red, blue-green (350-600 nm), and near-infrared (800-2500 nm) regions. The results showed that the spectral features of CFPGE parameters could be extracted by successive projections algorithm (SPA) based on the CWT preprocessing. The CFPGE parameter estimation model constructed based on the spectral features in the red region (675 nm, 680 nm, 688 nm, 749 nm, and 782 nm) had the highest Fv/Fm estimation accuracy on day 30, with R2c, R2p, and RPD of 0.723, 0.585, and 1.68, respectively. Based on this, the spectral features (578 nm, 976 nm, 1088 nm, 1476 nm, and 2250 nm) in the blue-green and near-infrared regions were added in the variables for modeling, which significantly improved the accuracy and stability of the model, with R2c, R2p, and RPD of 0.886, 0.815, and 2.58, respectively. Therefore, the fusion of the spectral features in the red, blue-green, and near-infrared regions could improve the estimation accuracy of rapeseed leaf CFPGE parameters. This study will provide technical reference for rapid estimation of photosynthetic performance of crops under salinity stress in arid and semi-arid areas.

Effects of Domestic Sewage on the Photosynthesis and Chromium Migration of Coix lacryma-jobi L. in Chromium-Contaminated Constructed Wetlands

To investigate the effects of domestic sewage on the photosynthesis and chromium migration of plants in chromium-contaminated constructed wetlands, small vertical flow constructed wetlands of Coix lacryma-jobi L. were set up. These wetlands were used to treat wastewater containing 0, 20, and 40 mg/L of hexavalent chromium (Cr (VI)), prepared with domestic sewage (DS), 1/2 Hoagland nutrient solution (NS), and 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN), respectively. The aim was to investigate the effects of domestic sewage on indicators, such as plant growth and chromium accumulation. The results were as follows: (1) Plant heights were significantly inhibited under 20 mg/L and 40 mg/L Cr (VI) treatments, and stem diameters were not significantly affected. The use of domestic sewage in treatment alleviated the inhibition of Cr (VI) on the growth of Coix lacryma-jobi L. (2) Indicators such as root activity, photosynthetic gas exchange, and chlorophyll fluorescence properties significantly decreased with the increase in Cr (VI) concentration. The values of these photosynthetic gas exchange parameters under the DN treatment were the greatest, followed by NS and DS. On the 70th day of Cr (VI) treatment, the net photosynthetic rate (Pn) under the DN treatment was significantly higher than that under NS and DS treatments. (3) Glutathione (GSH) content in roots, stems, and leaves of Coix lacryma-jobi L. significantly increased with the increase in Cr concentration, and it increased more significantly under the DN and DS treatments than under the NS treatment. (4) With the same Cr treatment, the Cr content in roots, stems, and leaves of Coix lacryma-jobi L. under the NS treatment was the highest, followed by DS and DN. The total Cr content in the substrate under the DN treatment was the highest, followed by DS and NS. (5) The addition of domestic sewage reduced the Cr (VI) content in the water sample and increased the organic matter content. The Cr (VI) content in the water sample under the NS treatment was the highest, followed by DS and DN. The addition of domestic sewage increased the accumulation of chromium in the substrate, decreased the absorption of chromium by plants, increased GSH content in roots, stems, and leaves, alleviated the damage of Cr (VI) to plants, and thus benefited the growth of Coix lacryma-jobi L. in the constructed wetlands and ensured the sustainable and stable operation of the wetlands.

Effects of AMF Compound Inoculants on Growth, Ion Homeostasis, and Salt Tolerance-Related Gene Expression in Oryza sativa L. Under Salt Treatments

Increased soil salinization is among the main factors that limits safe rice production. Arbuscular mycorrhizal fungi (AMF) have been shown to alleviate the toxic effects of salt stress in plants. However, more studies on AMF combined with other functional microorganisms are needed to further improve salt tolerance in rice. Therefore, the compound inoculum Funneliformis mosseae (Fm) together with two functional microorganisms, Piriformospora indica (Pi) and Agrobacterium rhizogenes (Ar) was evaluated for their effect on the rice growth, photosynthetic gas exchange parameters, ion homeostasis, and the expression of salt tolerance-related genes under 0, 80, 120 and 160 mM salt stress conditions. The results showed that: (1) the rice seedling biomass of the AMF compound inoculant treatment group was significantly higher than that of the non-inoculation treatment group (P < 0.05); (2) under NaCl stress, inoculation with AMF compound inoculants can activate the rice antioxidant enzyme system and improve osmoregulation ability; (3) AMF compound inoculants can increase the concentration of K+ in the plant and inhibit the transfer of Na+ to rice leaves, maintaining a high K+/Na+; and (4) AMF compound inoculants could induce and regulate the overexpression of genes related to salt tolerance, photosynthesis and ion homeostasis in rice, and improve the tolerance of rice under salt stress. Our study showed that AMF compound inoculants could improve the adaptability of rice under NaCl stress and promote plant growth by regulating the photosynthetic gas exchange parameter, reactive oxygen species (ROS) scavenging ability, and ion homeostasis of plants. These results suggest that AMF compound inoculants may play an important role in improving rice productivity in salinized soil.Graphical

Study of Photosynthetic Gas Exchange Parameters and Relative Water Content of Flag Leaf in Soft Wheat Genotypes Under Different Water Supply Conditions

The results of the study of the relative water content (RWC) and photosynthetic gas exchange parameters of the flag leaves in 21 soft wheat genotypes under rainfed conditions of Mountain Shirvan have been presented in the paper. The research was performed with drought-exposed and irrigated variants during the grain filling phase. RWC of the flag leaves of Vostorg, Murov 2, Tale 38, and Gyrmyzy gul 1 genotypes was higher both under drought and irrigated conditions. There was a positive correlation between RWC and earing time, and a negative correlation between RWC and plant height. The average difference in RWC between irrigated and drought-exposed variants for all genotypes was 10.1%. In 12thIWWYT no. 9 and 12thIWWYT no. 20 lines, this difference was high (26.5 and 19.6%), while in Gyzyl bughda, Murov 2, and Ferrigineum 2/19 genotypes, it was low (3.5, 3.6, and 2.9%). The highest values of the rate of photosynthesis were observed in the drought-exposed genotypes Sheki 1, Aran, Tale 38, and Zirva 85 (14.2, 14.8, 14.1, and 14.3 µmol CO2m−2 s−1), and in the irrigated genotypes Aran, Vostorg, and 12thIWWYT no. 9 (24.9, 23.4, and 24.0 µmol CO2 m−2s−1). Stomatal conductance (0.115, 0.120, 0.130, 0.164 mol H2O m−2 s−1), the concentration of CO2 in intercellular spaces (146.3, 156, 5, 181.7, and 213.7 µmol CO2mol−1) and the transpiration rate (3.32, 3.58, 4.13 and 4.44 mmol H2O m−2s−1) were higher in the Sheki 1, Aran, Tale 38, and Zirva 85 varieties, which manifest higher photosynthetic rate under drought conditions than other genotypes. A significant positive correlation of RWC with the rate of photosynthesis, the stomatal conductance, the concentration of CO2in intercellular spaces, and the rate of transpiration was found under drought stress conditions.

Silicon improves the photosynthetic performance of oat leaves infected with Puccinia graminis f. sp. avenae.

Stem rust, caused by Puccinia graminis f. sp. avenae (Pga) is a key disease affecting oat production worldwide. Silicon (Si) plays an essential role in enhancing plant resistance against pathogens. However, the scientific evidence of Si-mediated stem rust resistance of oat from the photosynthetic perspective has not been reported. The specific objective of this research was to investigate the effects of Si application on disease inhibition, photosynthetic gas exchange parameters, light response parameters, photosynthetic pigments and chlorophyll fluorescence parameters under Pga infection. Our results illustrated that Si application significantly reduced rust severity while the other parameters like net photosynthetic rate (P n), stomatal conductance (Gs), intercellular CO2 concentration (C i) and transpiration rate (T r) were significantly increased. Si application increased maximum photosynthetic rate (P nmax) and light saturation point (LSP), while reduced the dark respiration rate (Rd) and light compensation point (LCP). The results also indicated that Si application significantly increased the activities of maximum fluorescence (F m), variable fluorescence (F v), maximum quantum yield of photosystem II (F v/F m), photochemical quenching (qP), photosynthetic performance index (PI ABS), actual PSII quantum yield (ΦPSII), electron transfer rate (ETR), the absorbed light energy per unit reaction center (ABS/RC) and the dissipated energy per unit reaction center (DIo/RC), whereas it decreased the minimal fluorescence (F o), non-photochemical quenching (NPQ), the absorbed light energy used for electron transfer per unit reaction center (ETo/RC) and the absorbed light energy used for reduction of QA per unit reaction center (TRo/RC). The contents of chlorophyll a, b and carotenoids were also increased due to the change in the activity of parameters due to Si application as mentioned above. In conclusion, the results of the current study suggests that Si imparts tolerance to the stem rust possibly by the underlying mechanisms of improving gas exchange performance, and efficiency of the photochemical compounds in oat leaves.

Physiological and Transcriptomic Analyses Uncover the Reason for the Inhibition of Photosynthesis by Phosphate Deficiency in Cucumis melo L.

Phosphate (Pi) deficiency is a common phenomenon in agricultural production and limits plant growth. Recent work showed that long-term Pi deficiency caused the inhibition of photosynthesis and inefficient electron transport. However, the underlying mechanisms are still unknown. In this study, we used the physiological, histochemical, and transcriptomic methods to investigate the effect of low-Pi stress on photosynthetic gas exchange parameters, cell membrane lipid, chloroplast ultrastructure, and transcriptional regulation of key genes in melon seedlings. The results showed that Pi deficiency significantly downregulated the expression of aquaporin genes, induced an increase in ABA levels, and reduced the water content and free water content of melon leaves, which caused physiological drought in melon leaves. Therefore, gas exchange was disturbed. Pi deficiency also reduced the phospholipid contents in leaf cell membranes, caused the peroxidation of membrane lipids, and destroyed the ultrastructure of chloroplasts. The transcriptomic analysis showed that 822 differentially expressed genes (DEGs) were upregulated and 1254 downregulated by Pi deficiency in leaves. GO and KEGG enrichment analysis showed that DEGs significantly enriched in chloroplast thylakoid membrane composition (GO:0009535), photosynthesis-antenna proteins (map00196), and photosynthesis pathways (map00195) were downregulated by Pi deficiency. It indicated that Pi deficiency regulated photosynthesis-related genes at the transcriptional level, thereby affecting the histochemical properties and physiological functions, and consequently causing the reduced light assimilation ability and photosynthesis efficiency. It enriches the mechanism of photosynthesis inhibition by Pi deficiency.

Rhizophagus irregularis enhances tolerance to cadmium stress by altering host plant hemp (Cannabis sativa L.) photosynthetic properties

Arbuscular mycorrhizal fungi (AMF) are widespread and specialized soil symbiotic fungi, and the establishment of their symbiotic system is of great importance for adversity adaptation. To reveal the growth and photosynthetic characteristics of AMF–crop symbionts in response to heavy metal stress, this experiment investigated the effects of Rhizophagus irregularis (Ri) inoculation on the growth, photosynthetic gas exchange parameters, and chlorophyll fluorescence characteristics of hemp (Cannabis sativa L.) at a Cd concentration of 80 mg/kg. The results showed that (1) under Cd stress, the biomass of each plant structure in the Ri treatment was significantly higher than that in the noninoculation treatment (P < 0.05); (2) under Cd stress, the transpiration rate, stomatal conductance, net photosynthetic rate, PSII efficiency, apparent electron transport rate and photochemical quenching coefficient of the Ri inoculation group reached a maximum, with increases ranging from 1% to 28%; (3) inoculation of Ri significantly reduced Cd enrichment in leaves, which in turn significantly increased the transpiration rate, stomatal conductance, electron transfer rate, net photosynthetic rate and photosynthetic intensity, protecting PSII (P < 0.05); and (4) by measuring the light response curves of different treatments, the light saturation points of hemp inoculated with the Ri treatment reached 1448.4 μmol/m2/s, and the optical compensation point reached 24.0 μmol/m2/s under Cd stress. The Ri–hemp symbiont demonstrated high adaptability to weak light and high utilization efficiency of strong light under Cd stress. Our study showed that Ri–hemp symbiosis improves adaptation to Cd stress and promotes plant growth by regulating the photosynthetic gas exchange parameters and chlorophyll fluorescence parameters of plants. The Ri–hemp symbiosis is a promising technology for improving the productivity of Cd-contaminated soil.

Effects of Bacillus amyloliquefaciens QST713 on Photosynthesis and Antioxidant Characteristics of Alfalfa (Medicago sativa L.) under Drought Stress

Drought stress is a prevalent abiotic stress that adversely affects multiple physiological processes in plants, especially their photosynthetic capacity. Application of plant growth–promoting rhizobacteria (PGPR) has been considered as an eco-friendly strategy to ameliorate the deleterious effects of drought stress on plants. The present study was carried out to investigate the effects of Bacillus amyloliquefaciens QST713 on plant growth, leaf relative water content (RWC), photosynthesis processes, photosynthetic pigment content and antioxidant enzyme activities in two alfalfa varieties, Galalxie Max (drought-tolerant) and Saidi 7 (drought-sensitive) under drought conditions. The results showed that drought stress significantly declined plant biomass production, RWC, photosynthetic pigment content (Chl a, Chl b and carotenoids) and photosynthetic gas exchange parameters (transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci)), whereas it enhanced the enzymatic activity of peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) in both cultivars. In contrast, the inoculation of the bacillus strain QST713 was more effective on plant growth, showing higher plant biomass production compared to the non-inoculated plants under drought stress. Moreover, the application of QST713 significantly promoted the content of RWC, the accumulation of chlorophyll content and the activities of antioxidant enzymes as well as enhanced the photosynthetic capacity of alfalfa seedlings under drought stress. These results suggest that QST713 could be considered as a promising bio-inoculant for plants exposed to environmental stresses.

CORRELATION BETWEEN CO2 ASSIMILATION RATES AND OTHER GAS EXCHANGE PARAMETERS OF Hevea WITH REFERENCE TO COMMENCEMENT OF TAPPING

The present study was supplement to a longer experiment to determine the feasibility of early commencement of tapping of two Hevea genotypes i.e. RRIC121 and RRISL 211. The main objective of this study was to develop a correlations between CO2 assimilation rates and other leaf gas exchange parameters of Hevea with reference to the commencement of tapping. Tapped and untapped trees of two Hevea genotypes, i.e. RRIC121 and RRISL 211 at the same age were selected for the study. All cultural practices were performed according to the Rubber Research Institute of Sri Lanka. To evaluate the significance of differences between all possible pairs of treatment means, mean separation of treatments was performed with Duncan’s Multiple Range Test (DMRT). The strength of the relationships between yield and photosynthetic gas exchange parameters was estimated by linear correlation analysis. The leaf gas exchange parameters were measured under optimal environmental conditions clearly showed the intercellular CO2 concentration (Ci) gradually decrease with increasing light intensity in both clones. The same trend was observed in the different canopy layers under both tapping treatments. The highest leaf-air vapour pressure deficit (lvpd) was shown in the leaves of the upper canopy and lowest in the bottom canopy layer. Furthermore, the water use efficiency (WUE) of clone RRISL 211 was higher than that of clone RRIC 121 under tapping. This was primarily because of the greater photosynthetic rates of RRISL 211, rather than lower transpiration rates.When considering the correlation analysis of gas exchange parameters of Hevea, CO2 assimilation rate (A) had highly significant correlations with gs, Tr and intensity of PAR. However, the absence of a correlation between PAR and WUE showed that, when PAR increases both A and Tr increase in similar magnitudes. Therefore, WUE remains approximately constant. The presence or absence of correlations in gas exchange parameters of tapped and untapped treatments were broadly similar to the overall correlation analysis. In the tapped treatments, because of tapping, the photosynthetic rate is stimulated relative to the transpiration rate. The positive latex yield response to WUE and lvpd of top leaves indicates that, in top leaves when lvpd increases transpiration also increases accordingly. Hence WUE and ivpd increase latex flow within the tree and finally increase the latex yield with increasing latex volume. On the other hand, latex yield has a negative correlation with gs. The study findings clearly revealed that, in top leaves reduced Ci increases the CO2 gradient and uptake of CO2 for more photosynthesis, which increases the latex yield. Faster utilization of absorbed Ci may enhance the yield potential.

Exogenous melatonin promotes the growth of alfalfa (Medicago sativa L.) under NaCl stress through multiple pathways

Salinity is one of the most common factors affecting alfalfa (Medicago sativa L.), and NaCl is one of the main factors of salinity stress which can cause heavy losses in agricultural production in the world. The application of exogenous melatonin (MT) plays a major role in numerous plants against various stress environments. The effects of exogenous MT on the NaCl tolerance of alfalfa treated with the control, 100 µmol L−1 MT, 150 mmol L−1 NaCl, or 150 mmol L−1 NaCl+ 100 µmol L−1 MT were investigated. The results showed that MT increased growth parameters, inhibited chlorophyll degradation and promoted photosynthetic gas exchange parameters (photosynthetic rate, conductance to H2O, and transpiration rate) and stomatal opening under NaCl stress. Osmotic regulation substances such as soluble sugar, proline and glycine betaine were the highest in the NaCl treatment and the second in the NaCl+MT treatment. Nitrogen, phosphorus, potassium, calcium and magnesium were reduced and sodium was increased by NaCl, whereas these levels were reversed by the NaCl+MT treatment. MT inhibited cell membrane imperfection, lipid peroxidation and reactive oxygen species (ROS) accumulation caused by NaCl stress. MT up-regulated the gene expression and activity of antioxidant enzymes and increased the content of antioxidant non-enzyme substances to scavenge excessive ROS in NaCl-treated plants. In addition, all indicators interacted with each other to a certain extent and could be grouped according to the relative values. All variables were divided into PC 1 (89.2 %) and PC 2 (4 %). They were clustered into two categories with opposite effects, and most of them were significant variables. Hence, these findings reveal that exogenous MT alleviates the inhibitory effects of NaCl stress on photosynthesis, stomata opening, osmotic adjustment, ion balance and redox homeostasis, enhancing tolerance and growth of alfalfa. Furthermore, it suggests that MT could be implemented to improve the NaCl tolerance of alfalfa.

Co-over expression of Ascorbate Glutathione pathway enzymes improve mercury tolerance in tomato

The genetic modification of plants for the removal of inorganic pollutants from contaminated soil and water bodies is an emerging area for addressing environmental concerns. This approach is based on the ability of plants to take up and accumulate heavy metals, with efficiency being dependent on the underlying mechanisms of heavy metal accumulation and tolerance. A robust antioxidant pathway is determinantal for heavy metal uptake and accumulation and, therefore, in this study, we evaluated the transgenic tomato plants installed with Ascorbate Glutathione (ASA-GSH) pathway genes for uptake, accumulation, and response to mercury (Hg). We observed that ASA-GSH overexpressing lines were resilient to Hg stress as they displayed higher photosynthetic activity and increased photosynthetic gas exchange parameters with a concomitant decrease in ion leakage under Hg stress. Additionally, transgenic lines accumulated high osmolytes and showed enhanced activity of antioxidant enzymes. Moreover, the results of SEM and confocal microscopy confirmed least damage to plant tissue in ASA-GSH overexpressing lines compared to wild-type under Hg-stress which was further supported by Atomic absorption study that revealed a significant decline in Hg accumulation in the leaves of transgenic lines compared to wild-type under stress conditions. In conclusion, pyramiding of ASA-GSH pathway genes in tomato plants is an efficient approach for the development of Hg-resistant tomato plants and the reclamation of Hg-contaminated sites.

Transcriptional, secondary metabolic, and antioxidative investigations elucidate the rapid response mechanism of Pontederia cordata to cadmium

Pontederia cordata is previously demonstrated a cadmium (Cd) tolerant plant, and also a candidate for the phytoremediation of heavy-metal-contaminated wetlands. A hydroponic experiment was used to investigate variations in photosynthetic gas exchange parameters, antioxidative activities, chlorophyll and secondary metabolite contents, and transcriptome in leaves of the plant exposed to 0.44 mM Cd2+ for 0 h, 24 h, and 48 h. Under Cd2+ exposure for 24 h, the plant presented a favorable photosynthesis by maintaining relatively higher antioxidant activity. Cd2+ exposure for 48 h accelerated membrane peroxidation, declined photosynthetic pigment content, and increased polyphenol oxidase activity, thus interfering with photosynthesis. The phenylpropane pathway served as a chemical rather than physical defense against Cd2+ in the plant leaves. A total of 20,998, 4743, and 4413 differentially expressed genes (DEGs) were identified in the groups of 0 h vs 24 h, 0 h vs 48 h, and 24 h vs 48 h, respectively. The primary metabolic pathways of the DEGs were mainly enriched in nitrogen metabolism, starch and sucrose metabolism, fructose and mannose metabolism, as well as pentose-phosphate pathway, contributing to a stable cell structure and function. Flavonoid biosynthesis directly or indirectly played an antioxidative role against Cd2+ in the leaves. Forty-nine transcription factor (TF) families were identified, and 8 TF families were shared among the three groups. The present study provides a theoretical foundation for investigating tolerance mechanisms of wetland plants to Cd stress in terms of secondary metabolism and transcriptional regulation.